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AbstractThe sustainable and resource-efficient production of wind energy plants requires the use of modern high-strength fine-grain structural steels. This applies to both foundation and erection structures, like mobile or ship cranes. During the assembly of steel structures, unacceptable defects can occasionally be found in the weld area. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Due to the high shrinkage restraint of the joint groove in the overall structure, the superposition of global and local welding-induced stresses may lead to crack formation and component failure, particularly in interaction with the degradation of the microstructure and mechanical properties of high-strength steels during the repair process. However, manufacturers hardly have any information about these issues and there is a lack of recommendations and guidelines to take these safety-relevant aspects into account in adequate repair concepts. The aim of this research is to derive recommendations for repair concepts appropriate to the stresses and materials involved providing a basis for standards and guidelines to avoid cold cracking, damage and expensive reworking especially for high-strength steels. Part 1 of this study involves systematic investigations of influences of shrinkage restraint during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures. The quantification of the shrinkage restraint of repair weld joints was achieved by means of experimental and numerical restraint intensity analysis. In welding experiments with self-restrained slot specimens, restraint intensity and introduction of hydrogen via the welding arc using anti spatter spray were varied systematically to analyse the effect on welding result, residual stresses and cold cracking. It could be shown that increasing restraint intensities result in significantly higher transverse residual stress levels. In the case of hydrogen introduction S500MLO showed no cold cracking independent of the restraint conditions. However, S960QL was found to be considerably cold cracking sensitive if hydrogen is introduced. With increasing restraint intensity length and number of cold cracks increases significantly. Part 2 [1] of this study is focussed on microstructure and residual stresses due to gouging and stress optimization via adequate heat control parameters in repair welding.
The high potential of hydrogen as a key factor on the pathway towards a climate neutral economy, leads to rising demand in technical applications, where gaseous hydrogen is used. For several metals, hydrogen-metal interactions could cause a degradation of the material properties. This is especially valid for low carbon and highstrength structural steels, as they are commonly used in natural gas pipelines and analyzed in this work.
This work provides an insight to the impact of hydrogen on the mechanical properties of an API 5L X65 pipeline steel tested in 60 bar gaseous hydrogen atmosphere. The analyses were performed using the hollow specimen technique with slow strain rate testing (SSRT). The nature of the crack was visualized thereafter utilizing μCT imaging of the sample pressurized with gaseous hydrogen in comparison to one tested in an inert atmosphere.
The combination of the results from non-conventional mechanical testing procedures and nondestructive imaging techniques has shown unambiguously how the exposure to hydrogen under realistic service pressure influences the mechanical properties of the material and the appearance of failure.
The present Ph.D. thesis provides a comprehensive experimental and theoretical study of the bulging-cracking relationship in laser beam welding of thick unalloyed steel sheets. It focuses on experimentally proving the existence of the bulging region and on developing a coupled multiphysics mathematical framework to analyze its influence on the three critical factors controlling the susceptibility to solidification cracking, namely the thermal, metallurgical, and mechanical factors.
The research employs a novel experimental setup, utilizing a combination of transparent quartz glass and thick unalloyed steel sheet, enabling real-time visualization of the weld pool geometry and confirming the existence of a distinctive bulging region. To deepen the understanding of these experimental insights, an extensive multiphysics mathematical framework was developed and rigorously verified and validated. This framework introduces an innovative approach using Lamé curves for accurately describing complex three-dimensional weld pool geometries, including the bulging region's characteristics. Through analytical solutions and numerical procedures, it facilitates the computation of solidification parameters, which are crucial for understanding the metallurgical aspects of crack formation. The framework also incorporates a mechanical model to assess and evaluate the local stress distribution within the bulging region.
The findings indicate that an elongated, sharply shaped bulging region significantly increases the susceptibility to solidification cracking. This is attributed to its adverse impact on the distribution and local dwell time of liquid metal residing at grain boundaries during solidification, combined with the localized tensile stresses identified in the bulging region.
In essence, this research contributes to the broader understanding of solidification cracking in laser beam welding of thick unalloyed steel sheets, with a particular focus on the bulging region. The insights and methodologies developed in this thesis are valuable for future research and advancements in the application of the laser beam welding technology for joining high-thickness unalloyed steel components.
This study investigates the room‐ and high‐temperature (650 °C) tensile and low‐cycle‐fatigue behavior of Inconel 718 produced by laser powder bed fusion (PBF‐LB/M) with a four‐step heat treatment and compares the results to the conventional wrought material. The microstructure after heat treatment is characterized on different length scales. Compared to the wrought variant, the elastic and yield properties are comparable at both test temperatures while tensile strength, ductility, and strain hardening capacity are lower. The fatigue life of the PBF‐LB/M variant at room temperature is slightly lower than that of the wrought material, while at 650 °C, it is vice versa. The cyclic stress response for both material variants is characterized by cyclic softening, which is more pronounced at the higher test temperature. High strain amplitudes (≥0.7%) at room temperature and especially a high testing temperature result in the formation of multiple secondary cracks at the transitions of regions comprising predominantly elongated grain morphology and columns of stacked grains with ripple patterns in the PBF‐LB/M material. This observation and pronounced crack branching and deflection indicate that the cracks are controlled by sharp micromechanical gradients and local crystallite clusters.
The presentation gives an overview of BAM's activities on processing influences and application properties of MPEAs in the form of joined and machined high and medium entropy alloys (CoCrFeMnNi and CoCrNi). In the case of welding, the focus is on defect-free welded joints with sufficient mechanical properties. In the case of machining, the focus is on the possible influence on the surface quality of the materials through adequate milling parameters. In addition, the hydrogen absorption and diffusion properties as well as the electrochemical corrosion behavior are fundamentally examined.
Despite the advances in hardware and software techniques, standard numerical methods fail in providing real-time simulations, especially for complex processes such as additive manufacturing applications. A real-time simulation enables process control through the combination of process monitoring and automated feedback, which increases the flexibility and quality of a process. Typically, before producing a whole additive manufacturing structure, a simplified experiment in the form of a beadon-plate experiment is performed to get a first insight into the process and to set parameters suitably. In this work, a reduced order model for the transient thermal problem of the bead-on-plate weld simulation is developed, allowing an efficient model calibration and control of the process. The proposed approach applies the proper generalized decomposition (PGD) method, a popular model order reduction technique, to decrease the computational effort of each model evaluation required multiple times in parameter estimation, control, and optimization. The welding torch is modeled by a moving heat source, which leads to difficulties separating space and time, a key ingredient in PGD simulations. A novel approach for separating space and time is applied and extended to 3D problems allowing the derivation of an efficient separated representation of the temperature.
The results are verified against a standard finite element model showing excellent agreement. The reduced order model is also leveraged in a Bayesian model parameter estimation setup, speeding up calibrations and ultimately leading to an optimized real-time simulation approach for welding experiment using synthetic as well as real measurement data.
Stresses in repair welding of high-strength steels—part 2: heat control and stress optimization
(2024)
In welding of high-strength steels, e.g. for foundations and erection structures of wind energy plants, unacceptable defects can occasionally be found in the weld area, which should be removed by thermal gouging and subsequent re-welding. High shrinkage restraint of repair welds may lead to crack formation and component failure, predominantly in interaction with degraded microstructures and mechanical properties due to repair cycles. This study aims for elaboration of recommendations for repair concepts appropriate to the stresses and materials involved to avoid cold cracking, damage and expensive reworking. In part 1 [1] of this study, systematic investigations of influences of shrinkage restraint on residual stresses and cold cracking risk during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures were focussed. In this part 2, the microstructure, particularly hardness, and residual stresses due to gouging and influences of heat control parameters in repair welding are analysed. A clear reduction in residual stress after gouging can be observed, especially for the specimens with restrained transverse shrinkage. Gouging to a depth of approx. 2/3 of the seam height does not lead to a complete relaxation of the observed reaction forces. Particularly for the higher strength steel S960QL, there are pronounced areas influenced by the gouging process in which a degradation of the microstructure and properties should be assumed. Overall, the repair welds show a significant increase in the width of the weld and HAZ compared to the original weld, especially in the case of S960QL/G89. The repair welds show higher welding-induced stresses than the original welds, especially in the areas of the HAZ and the base metal close to the weld seam. This behaviour can be attributed overall to increased restraint conditions due to the remaining root weld or shorter gouge grooves. In good agreement with earlier investigations, the residual stresses transverse to the weld can be significantly reduced by upwardly limited working or interpass temperatures, and the reaction stresses resulting from high restraint conditions can be effectively counteracted. The influence of the heat input on the stress formation is low compared to the interpass temperature for both test materials.
In this presentation, the results of the determination of the diffraction and single-crystal elastic constants of laser powder bed fused Inconel 718 are presented. The analysis is based on high-energy synchrotron diffraction experiments performed at the Deutsches Elektronen-Synchrotron. It is shown that the characteristic microstructure of laser powder bed fused Inconel 718 impacts the elastic anisotropy and therefore the diffraction and single-crystal elastic constants. Finally, the consequences on the diffraction-based residual stress determination of laser powder bed fused Inconel 718 are discussed.
Based on the state-of-the-art research and regulations, the application of the fracture mechanics master curve (MC) concept to ferritic ductile cast iron (DCI) is being investigated in a joint research project between MPA Stuttgart and BAM Berlin. This paper outlines the research approach, the planned investigations and first results of the experimental program.
For the hydrogen-based energy economy of tomorrow, the construction of the necessary infrastructure will play a central role. Most materials used to date, such as welded steels, can be prone to hydrogen embrittlement under certain conditions. This includes the classic delayed cold cracking during welding as well as degradation phenomena during service of components in hydrogen-containing environment. For the evaluation of any hydrogen effect, for example, on the mechanical properties of a welded metallic material, the hydrogen content must be precisely determined. In the case of weld seams, the carrier gas hot extraction (CGHE) according to ISO 3690 is meanwhile state-of-the-art. CGHE is based on accelerated hydrogen degassing due to the thermal activation of hydrogen at elevated temperatures. In addition to the quantification of hydrogen, thermal desorption analysis (TDA) with varying heating rates can be used to determine and evaluate the hydrogen trapping at microstructural defects in the material. For both techniques, experimental and metrological influences must be considered, which have a major effect on the result. For example, ISO 3690 suggests different sample geometries and minimum extraction times for CGHE. This study summarizes the results and experiences of numerous investigations at the Federal Institute for Materials Research and Testing (BAM) with different sample temperatures and geometries (ISO 3690 type B and cylindrical TDA samples) regarding the influence of the sample surface (polished/welded), measurement accuracy depending on the sample volume and the insufficient monitoring of the effect of PI control on the extraction temperature. A deviating extraction temperature from the target temperature can significantly falsify the measurement results. Based on the results, methods are shown which allow the desired extraction temperature to be reached quickly without physically interfering with the measuring equipment. This serves to significantly improve the reliability of the hydrogen measurement through increased signal stability and accelerated hydrogen desorption. In general, an independent temperature measurement with dummy samples is recommended for the heating procedure of choice to exclude possible undesired temperature influences before the measurement. The methods described can be transferred directly to industrial applications
In this contribution, we present a physically motivated heat source model for the numerical modeling of laser beam welding processes. Since the calibration of existing heat source models, such as the conic or Goldak model, is difficult, the representation of the heat source using so-called Lamé curves has been established, relying on prior Computational Fluid Dynamics (CFD) simulations.
Lamé curves, which describe the melting isotherm, are used in a subsequent finite-element (FE) simulation to define a moving Dirichlet boundary condition, which prescribes a constant temperature in the melt pool. As an alternative to this approach, we developed a physically motivated heat source model, which prescribes the heat input as a body load directly. The new model also relies on prior CFD simulations to identify the melting isotherm. We demonstrate numerical results of the new heat source model on boundary-value problems from the field of laser beam welding and compare it with the prior CFD simulation and the results of the Lamé curve model and experimental data.
Electromagnetic stirring is known to promote material flow, reduce porosity, uniform elements distribution, and refine grain in laser beam welding (LBW), which enhances the applicability of LBW in various industries. In this study, a phase-field model of dendrite growth in AA5754 Al alloy electromagnetic stirring laser beam welding was established. The model considered the thermal electromagnetic Lorentz force resulting from the interaction between the electric field generated by the Seebeck effect and the magnetic field, as well as the temperature gradient and solidification rate of the solidification interface obtained from the computational fluid dynamics electromagnetic stirring LBW model. The variation rules of dendrite growth with different magnetic parameters and effects are analyzed. Comprehensively, the magnetic field promotes the solidification rate, thus promoting interfacial instability and a large magnetic flux density leads to a faster interface instability. The solidification rate as well as the temperature gradient affect the growth rate, and the accelerated growth caused by the so lidification rate with a high frequency and a large magnetic flux density effectively inhibits the slow growth caused by the temperature gradient. The thermal electromagnetic Lorentz force is the main factor for the branch increment at low frequencies, while both thermal electromagnetic Lorentz force and temperature gradient in crease the number of branches at high frequencies. The calculated average branch numbers considering various factors in the stable stage under different magnetic parameters were consistent with the results of the scanning electron microscope tests.
Next to chemical composition, metallurgy and welding parameters, the intensity of restraint is one of the variables influencing solidification cracking. Tests like the Houldcroft Test, rate the hot cracking susceptibility indirectly on the amount of restraint the welding can endure without cracking. Modern lightweight steel construction welds can be subject to a larger amount of restraint due to their high-strength nature compared to classical use cases. By varying the plate thickness of Controlled Thermal Severity (CTS) tests produced out of S1100 QL, it was possible to vary the intensity of restraint on fillet welds at a high level. Testing was conducted with four different filler wires for Metal Active Gas (MAG) welding, including three solid and one metal-cored wire. In addition, two sets of welding parameters were tested. The first set with high heat input and high welding speed was shown to be more prone to solidification cracking compared to the second set with lower heat input and welding speed. The results show an increase in solidification cracking with increasing restraint severity.
This article focuses on the selective placement of additional LTT layers to generate compressive residual stresses in fatigue-critical areas of conventional weld joints. This enables an economical solution without effecting the integrity of welded joints. For this, longitudinal stiffeners made of high-strength steel were gas metal arc welded using conventional welding consumable in the first layer. Afterwards, a chromium-nickel alloyed LTT welding consumable was deposit on front sides of the stiffeners. By varying the welding parameters, different weld geometries of the LTT filler metal could be analyzed. The effects of additional LTT layers were investigated with regards to residual stresses, microstructure and strength. X-ray residual stresses measurements show that the residual stresses at the failure critical weld toe are significantly reduced by using additional LTT layers. While the conventional weld is characterized by tensile residual stresses, compressive residual stresses can be detected at the LTT weld. The level of residual stresses is influenced by the geometry of the LTT layer. Additional LTT layers with a high offset to the conventional weld generate more compressive residual stress in the HAZ than with a low offset. Therefore, the weld geometry has a considerable impact on the residual stress profile.
Influence of machining on residual stresses in additive manufactured high-strength steel components
(2024)
This study focuses on the effect of machining on the residual stresses and distortion of WAAM specimens. Defined specimens were welded fully automatically with a special WAAM solid wire (yield strength >820 MPa) with different geometric designs. The residual stresses state before and after cutting of the AM structure from the substrate plate were analyzed by means of X-ray diffraction on the specimen surface and complementary by 3d deformation analyses using photogrammetry. The results reveal significant influences of the geometry on the relaxation and redistribution of residual stresses.
MVT testing was conducted in Transvarestraint mode in multiple welding conditions and under a range of bending speeds. The materials observed were high strength filler metals used in GTAW namely DIN EN ISO 16834-A G 69 4 M21 Mn3Ni1CrMo (G69), 16834-A G 89 6 M21 Mn4Ni2CrMo (G89 6), 16834-A G 89 5 M21 Mn4Ni2,5CrMo (G89 5) and a filler wire 18276-A T 89 4 ZMn2NiCrMo M M21 1 H5 (T89). For evaluation light optical microscope pictures were used. Pixels containing cracks were manually segmented using a self-written program. Out of the segmented images data including crack length, position and area can be calculated for every crack. The results show dependencies of solidification cracking on the test parameters.
This presentation describes the usability of the hollow tensile specimen technique for in-situ material testing in a hydrogen atmosphere.
In addition, the presentation provides an outlook on the methodology for investigating the suitability of pipeline steels and their weld seams for hydrogen operation.
The weldability of stainless steels is largely controlled by the chemical composition, and alloys with ferritic or ferritic-austenitic solidification show the highest resistance to hot cracking. As the resulting phase balance also
affects the final properties, it may be beneficial to both foresee and measure the weld metal ferrite content. The WRC ’92 constitution diagram is currently the most accurate prediction tool available, but it does not take the cooling rate into consideration and the precision may be less accurate for stainless steels with high ferrite numbers (FN). This study aims to validate the reliability of the WRC ’92 diagram for weld metals with FN > 50. The chemical composition was altered through gas tungsten arc welding (GTAW) of UNS S32205 with ER347 filler wire that had been PVD-coated with either niobium (Nb), copper (Cu), nickel (Ni), manganese (Mn), carbon (C), or silicon (Si). The actual ferrite content was assessed using image analysis, FERITSCOPE® and X-ray diffraction (XRD). While the WRC ’92 diagram predictions were deemed accurate to acceptable for Ni, Si, and Mn, notable deviations were observed for Nb, Cu, and C. The FeriteScope exhibited a consistent trend with image analysis, though the values were higher, and scatter was wider, and the conversion factor is open for discussion. The lowest accuracy and largest spread were obtained using non-contact XRD, rendering it unsuitable for ferrite measurements of welds.
Effect of high-pressure hydrogen environment on the physical and mechanical properties of elastomers
(2024)
This study presents the influence of high-pressure hydrogen environment on the physical and mechanical properties of two types of cross-linked hydrogenated acrylonitrile butadiene rubbers. Based on the CSA/ANSI standard, static exposures in hydrogen experiments were performed up to 100 MPa at 120 °C. Characterization before and after exposure was conducted by means of density and hardness measurements, dynamic mechanical analysis (DMA), tensile tests, compression set, FT-IR and AFM analyses to assess effects after decompression. While the effect of high-pressure exposure is significant immediately after exposure, most of the physical and mechanical properties recover after 48 hours. FT-IR, AFM, SEM and compression set results indicate, however, permanent effects.
Controlled Thermal Severity (CTS) tests are known for applicatying high restraint intensity to fillet welds. Due to two different variations of the test it was possible to apply different amounts of restraint intensity to high-strength GMAW fillet welds at a high level of restraint. The results show the effect of heat input, filler material and restraint intensity on solidification cracking susceptibility.
The Varestraint test and its variant Transvarestraint are one of the most widely used techniques for evaluating a material solidification cracking sensitivity during welding. The result of such tests is a crack length which is proportional to the material’s cracking susceptibility. Nevertheless, the welding and load parameters can unintentionally influence the crack length, which in some cases can distort the material evaluation. An approach is described as to how these effects can be assessed with the aid of a digital crack analysis. The crack lengths are compared position-dependently with their possible propagation due to the weld pool movement during continuous loading. The index derived from this can be used by the operator to evaluate his test parameters. In addition, a comparison of the results of different Varestraint setups is made possible. Alongside experimental results, a numerical sensitivity analysis is presented on how individual welding and loading parameters can affect the crack lengths.
Multiple principal element alloys encompass the well-known high entropy alloys (HEA). The alloy system represents a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.%. Thus, this alloying concept differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. However, in the last 20 years, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding processes. The weldability of HEAs has received very little attention so far. The experience with dissimilar metal welds is completely lacking but is essential for the application of these materials in combination with conventional materials. The present study presents comprehensive experimental results on the weldability of an equimolar CoCrFeMnNi-HEA in cold-rolled and heat-treated condition, which was joined by tungsten inert gas welding to an austenitic steel AISI 304. The mechanical properties of the dissimilar metal welds were characterized by cross-weld tensile samples, whereas the local deformation in the weld of the different welding zones was measured by digital image correlation. In accordance with the respective initial HEA condition (cold-rolled vs. heat-treated), the local strain behavior was divergent and influenced the global mechanical properties of both DMW types. Nonetheless, the experiments provided proof in principle of the weldability for dissimilar joints of the CoCrFeMnNi-HEA welded to conventional materials like austenitic stainless steels ensuring a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.
Numerical and experimental assessment of liquid metal embrittlement in externally loaded spot welds
(2024)
Zinc-based surface coatings are widely applied with high-strength steels in automotive industry. Some of these base materials show an increased brittle cracking risk during loading. It is necessary to examine electrogalvanized and uncoated samples of a high strength steel susceptible to liquid metal embrittlement during spot welding with applied external load. Therefore, a newly developed tensile test method with a simultaneously applied spot weld is conducted. A fully coupled 3D electrical, thermal, metallurgical and mechanical finite element model depicting the resistant spot welding process combined with the tensile test conducted is mandatory to correct geometric influences of the sample geometry and provides insights into the sample’s time dependent local loading. With increasing external loads, the morphology of the brittle cracks formed is affected more than the crack depth. The validated finite element model applies newly developed damage indicators to predict and explain the liquid metal embrittlement cracking onset and development as well as even ductile failure.
Hydrogen has great potential on the path towards decarbonization of the energy and transport sectors and can mitigate the urgent issue of global warming. It can be sustainably produced through water electrolysis with potentially zero emissions, and efficiently used (e.g., in fuel cell systems). Despite its environmental advantages, hydrogen-metal interactions could result in the degradation of the mechanical properties of several structural materials. In order to determine the magnitude of the material degradation in relation to hydrogen exposure, extensive material testing is required. The standardized procedure for in-situ testing for the quantification of the impact of compressed gaseous hydrogen (CGH2) relies on the utilization of an autoclave around the tested specimen. Such test set-up is complex, expensive, time-consuming and requires special equipment, trained personnel, and strict safety procedures. A relatively recent method to circumvent these issues and provide affordable results consists of using hollow specimens, thus applying the hydrogen pressure inside rather than outside the specimen. It allows to reduce the volume of hydrogen by several orders of magnitude and to perform the tests more efficiently and in a safer manner. This study focuses on evaluating the tensile properties of X65 vintage pipeline steel tested in a high-pressure hydrogen environment using hollow specimens. Tests are performed in 6 MPa H2 and Ar at the nominal strain rate of 10−6 s−1 to evaluate the reduced area at fracture and the elongation loss. The effect of surface finishing on crack initiation and propagation is investigated by comparing two different manufacturing techniques. In this way, this study provides insights into the applicability of a novel, reliable, and safe testing method which can be used to assess the hydrogen-assisted ductility loss in metallic materials.
The hydrogen economy is one of the main solutions for achieving climate neutrality in Europe. Metallic materials, predominantly steels, are the most common structural materials in the various components along the hydrogen supply chain. Ensuring their sustainable and safe use in hydrogen technologies is a key factor in the ramp-up of the hydrogen economy. This requires extensive materials qualification, however, most of the accepted, and standardised test methods for determining the influence of gaseous hydrogen on metallic materials describe complex and costly procedures that are only available to a very limited extent worldwide (e.g., autoclave technique).
The hollow specimen technique is presented as an alternative method that can overcome the limitations of current techniques and complement them. To standardise the technique, a process has been initiated by ISO in 2021. Knowledge gaps for tests with the technique in hydrogen have been identified by DIN. The H2HohlZug project, which falls under the umbrella of TransHyDE, aims to address the identified knowledge gaps and provide a foundation for a comprehensive standardisation of the hollow specimen technique.
As energy carrier of the future, green hydrogen can make a decisive contribution to a sustainable energy supply. Particularly important is the decarbonization of heavy industry. Therefore, a reliable supply of hydrogen must be guaranteed. A hydrogen pipeline grid can achieve this purpose. In Europe concrete plans for a hydrogen pipeline grid already exist, as the so called “European Hydrogen backbone”. As building a completely new pipeline grid is economically not justifiable, a dual-way strategy of repurposing natural gas (NG) pipelines and newly built pipelines is intended. Long distance pipeline grids require regular maintenance and repairs. For natural gas (NG) pipelines in-service welding is state of the art. Concepts such as hot-tapping (drilling into pressurized pipelines) and “stoppling” (sealing off pipeline sections after installing bypasses or pressure relieves) allow the repair of damaged pipeline sections or the installation of new branches without shutting the pipeline down, which in most cases isn’t possible from an economic standpoint. The EIGA 121/14 guideline already pointed out in 2014 that “a hydrogen hot-tap shall not be considered a routine procedure”. This shows that the determination of the transferability of these repair concepts to hydrogen pipelines is of utmost importance. Due to the degrading effects of hydrogen on mechanical proper-ties, commonly referred to as “hydrogen embrittlement” it is necessary to understand and investigate the occurring effects. In the special case of repair welding on in-service hydrogen pipelines an increased hydro-gen uptake due to a large heat input and microstructural changes in the material needs to be considered. Therefore, material degradation must be investigated to determine whether modifications of repair procedures are necessary to ensure a reliable and safe hydrogen transportation via pipelines. For this reason, this paper gives an overview on existing weld repair concepts and its risks and limitations in terms of hydrogen absorption and possible embrittlement effects when transferred to hydrogen pipelines. The complexity lies in a large variety of materials (low alloyed steels), pipeline diameters and corresponding welded joints. The material compatibility of materials in used (repurposed NG-pipelines) and new condition must be investigated. Particularly for repurposed/used pipelines the effect of a pre-matured/corroded inner surface and sometimes insufficient documentation of the material “history” needs to be considered. An overview on ongoing and completed R&D-projects with respect to repair concepts for hydrogen pipelines and hydrogen effects on pipeline materials is given. The focus hereby lies on possible methods of material testing and modeling. Its current difficulties, limits and possible solution will be discussed.
Contemporary steels creep-resistant steels like the 13CrMoV9-10 are used as thick plates e.g., for applications in hydrocrackers. The large plate thickness requires high efficiency welding processes like submerged arc welding (SAW) and a mandatory post weld heat treatment (PWHT). The PWHT-parameters stem from a combination of empirical knowledge and traditional experiments on free shrinking (FS) welds. In that connection, the formation of so-called stress-relief cracking (SRC) must be anticipated. The SRC susceptibility of a welded component is a complex combination and interaction of com-ponent specific, thermal, and metallurgical factors. Many of which have to date not been conclusively researched. For example, it is well known that SRCs develop during PWHT due to formation and growth of carbides on prior austenite grain boundaries, resulting in a significant hardness increase (secondary hardening) and toughness decrease. This leads to a high SRC susceptibility of the coarse grain heat-affected zone (CGHAZ) of the last weld bead during PWHT. This is intensified in case of high residual stresses from the component-specific, structurally induced stiffness. In combination with multi-layer welding this leads to a multiaxial stress distribution. Nonetheless, the combined effect of PWHT and high-residual stresses on the SRC formation is merely unknown. For that reason, this study presents the effect of different heating rates in conjunction with a certain component stiffness. For that reason, SAW joints of 13CrMoV9-10 steel were investigated under FS and defined external restraint condition.
A fundamental study of physical mechanisms of wineglass-shaped fusion zone profile in laser melting
(2024)
The fusion zone geometry in laser melting processes e.g., laser welding and laser-based additive manufacturing, of metallic materials has commonly a wineglass-shaped profile which is critical to the grain orientation and stress distribution. Hereby, we adopt for the first time a decoupling work through a combination of multi-physics modelling and experiments to reveal the fundamental mechanisms of this special morphology. Two physicsbased easy-to-use metal vapour models are proposed to consider the vapour’s momentum and thermal effects separately. It is found that the direct laser energy absorption and Marangoni shear stress which are widely hypothesised to dominate the wineglass-shape formation show only a minor influence. The additional heating from the metallic vapour plume rather than its momentum impact contributes predominantly to the enlarging of the molten pool top region, resulting directly in the formation of the wineglass-shaped fusion zone. The generality of the plume heating effect is also validated in two types of materials (steel and Al) in a wide range of parameters.
Microalloying elements such as Nb and Ti play a decisive function in achieving the desired mechanical strength of quenched and tempered, high-strength fine-grain structural steels with a nominal yield strength ≥ 690 MPa. The current specifications for the chemical composition only provide manufacturers with upper limits. However, even minor deviations in the alloy concept can have a significant impact on the mechanical properties. Consequently, accurate prediction of weldability and the integrity of welded joints becomes difficult or even impossible due to differences in composition and the resulting microstructures. Undesirable consequences include a possible softening of the heat-affected zone (HAZ) or, conversely, hardening effects. In view of these challenges, various microalloying strategies with different Ti and Nb contents are being systematically investigated for the first time using specially developed laboratory casting alloys. Each alloying route is based on the common S690QL, maintaining both the chemical composition and the heat treatment parameters. To analyse the weldability, three-layer welds were performed using gas metal arc welding (GMAW) and critical microstructures, such as areas of the heat-affected zone (HAZ) that exhibit significant softening or hardening, were identified. The effect of the softened HAZ region on failure was evaluated using transverse tensile specimens. Digital image correlation (DIC) is used to image changes in local strains in different HAZ regions in situ. Using a specially developed mirror system, the local strains of the microstructure zones on the top and bottom of the weld are recorded simultaneously. This makes it possible to analyse how the weld seam geometry (e.g., V-seam) influences the strain gradients. In addition, the analysis of the local deformation helps to understand the effects of the softened HAZ on the global strain, the fracture constriction, the fracture position, and the overall fracture behavior.
Joining and welding technologies are of high importance for the manufacturing of components and parts used in sustainable energy generation, conversion, and transport. In that connection, offshore and on-shore installed wind turbines are of high interest for the generation of electrical energy as well as photo-voltaic systems (solar cells). The electricity can be either directly transported or conversed via power-to-gas e.g., to hydrogen. In that scope, electrolyzer up to MW-range are of interest as well as the conver-sion back to electricity via fuel cells. In addition, hydrogen is a key element of the decarburization of in-dustries as well as the mobility sector encompassing sea, air and land transportation driven by hydrogen or its derivates. Well-known examples cover the direct reduction of iron ore to replace the conventional blast furnace process up to gas turbines or fuel cells for home-end use. All mentioned technologies re-quire reliable components, which are to a high extend dependent on joining and especially welding pro-cessing of materials. Especially, the (petro-) chemical industry has many years of experience with both materials used in hydrogen applications. The challenge is e.g., the transition to mass production of sys-tem components of electrolyzers/fuel cells and for distribution grids. At this point, the scalability of cur-rently applied joining processes often limits the economic efficiency, whereas especially laser welding or additive manufacturing will be of high interest. In that connection, it is very important to provide answers by joint research of universities, institutes, and industrial companies. Very often, solutions are already available and “just” have to be investigated and adapted for the new application, like repair welding of NG pipelines. For that reason, we want to set up a series of comprehensive papers with the aforementioned title. The idea is to get an in-depth but manageable overview of the importance of joining technologies in sustaina-ble energy generation, conversion, and transport encompassing current processes, limitations, and fur-ther perspectives. In that connection, the additive manufacturing is gaining more and more attention. If applicable, current challenges in the adaption or creation of new standards/regulations shall be addressed.
The ISO 3690 standard “Determination of hydrogen content in arc weld metal” requires a thermal activation of the diffusible hydrogen in a piece of weld metal for the subsequent ex situ concentration measurement by carrier gas hot extraction CGHE or thermal desorption spectroscopy (TCD). Laser-induced breakdown spectroscopy (LIBS) offers a time and spatially resolved, almost non-destructive, in situ measurement of hydrogen at surfaces without sample preparation. We measured hydrogen in steels, which were charged either electrochemically or by high-pressure hydrogen gas, and compared the results. Further, the feasibility of quantitative hydrogen line scan measurements with LIBS was demonstrated by measuring hydrogen at water jet cut surfaces. The hydrogen concentrations measured with the help of LIBS were compared with CGHE measurements. It was observed that hydrogen can be reliably measured with LIBS for concentrations larger than 2 wt.-ppm. The maximum hydrogen concentration achieved using electrochemical charging was 85.1 ppm. The results show that LIBS is a promising technique for time- and spatially resolved measurements of hydrogen in steels.
Within fracture mechanics safety assessment for steels in nuclear technology, the probabilistic master curve (MC) concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. However, for ductile cast iron (DCI), a systematic review of potential modifications to the assumptions and the procedure according to ASTM E1921 and an associated validation are still lacking. For this reason, the application of the fracture mechanics MC concept to ferritic ductile cast iron is being investigated in a joint research project between MPA Stuttgart and BAM Berlin. According to Hojo et al. (2008) the MC concept is already in use for the brittle fracture safety assessment of ferritic forged steel containers, which would suggest an extension to ductile cast iron containers. In compliance with IAEA SSG-26, the safety assessment of DCI containers for transport as well as storage of radioactive materials is based on the fracture mechanics criterion of general exclusion of crack initiation. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods. Therefore, a methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using only a low number of small fracture mechanics test specimens extracted from a component. For the experimental program, a DCI material of grade GJS-400 (GGG-40) has been produced as it is generally used for transport and storage containers for nuclear material. A basic mechanical-technological material characterization consisting of tensile, Charpy, and Pellini tests has been planned and a comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens at various loading rates and temperatures. The specimen testing is divided between the project partners based on specimen geometry. MPA Stuttgart has been tasked with the testing of the C(T), DC(T) and Pellini specimens, while BAM Berlin was assigned the SE(B), Charpy and tensile tests. These tests allow for the determination of the loading rate relevant for brittle fracture via C(T)25 specimens and an assessment of size effects by transferring the relevant loading rate to other specimen geometries and sizes. Numerical analyses of different specimen geometries and test setups are planned to investigate the influence of stress triaxiality and loading rate. Furthermore, the time-dependent course of the Weibull stress is assessed via these numerical analyses. Throughout the experimental program, the microstructure-property relationship will also be investigated based on quantitative metallographic and fractographic analyses like scanning electron microscope images and metallographic sections. With the acquired experimental database fundamental assumptions of the MC concept, like the weakest link model and the Weibull distribution, will be assessed for the application to DCI. In the end, the applicability of the MC concept will be evaluated and if necessary, modifications to the MC concept for DCI will be proposed. This paper will present the preliminary results of the GJS400 material characterization and the related selection of test specimen extraction points. First results of the C(T)25 and SE(B)140 specimen testing will also be discussed supported by first metallographic analyses.
High-strength steels offer potential for weight optimization due to reduced wall thicknesses in modern constructions. Additive manufacturing processes such as Wire Arc Additive Manufacturing (WAAM) enable the resource-efficient production of structures. In the case of defects occurring in weld seams orWAAM components due to unstable process conditions, the economical solution is local gouging or machining and repair welding. It is important to understand the effects of machining steps on the multiaxial stress state in conjunction with the design-related shrinkage restraints. Research into how welding and slot milling of welds andWAAM structures affects residual stresses is still lacking. For this reason, component-related investigations with high-strength steels with yield strengths ≥790 MPa are carried out in our research. In-situ digital image correlation (DIC) and ex-situ X-ray diffraction (XRD) were used to analyze the stresses and strains induced on specimens during and after milling. The systematic analyses revealed a significant interaction of the stiffness and microstructure of the specimens with the initial residual stresses induced by welding. Subsequent repair welds can result in significantly higher residual stresses.
Within fracture mechanics safety assessment for steels in nuclear technology, the probabilistic master curve (MC) concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. However, for ferritic ductile cast iron (DCI), a systematic review of potential modifications to the assumptions and the procedure according to ASTM E1921 and an associated validation are still lacking. For this reason, the application of the fracture mechanics MC concept to ferritic ductile cast iron is being investigated in a joint research project between MPA Stuttgart and BAM Berlin. According to IAEA SSG-26, the safety assessment of DCI containers for transport as well as storage of radioactive materials is based on the fracture mechanics criterion of general crack initiation exclusion. Within this context, the focus of the research project is on brittle fracture. The goal is the establishment of a methodology to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using samples extracted from a component.
The experimental program uses a DCI material of grade GJS-400, which is generally used for transport and storage containers of radioactive materials. Firstly, a basic mechanical-technological material characterization will be performed consisting of tensile, Charpy and Pellini tests. Following the material characterization, the loading rate relevant for brittle fracture will be determined using instrumented C(T)25-fracture tests at a temperature of -40°C. To be able to transfer the relevant loading rate to other specimen geometries or sizes, the time-dependent course of the Weibull stress is assessed via numerical analyses. With this information an extensive experimental program consisting of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25 and SE(B)140 specimens is performed. These experimental results will be further supported by numerical and fractographic analyses. The empirical experimental database developed for DCI materials will be used to assess the applicability of the MC concept for DCI materials under dynamic loading.
At the current time manufacturing and precracking of test specimens is ongoing. The authors provide first results concerning the mechanical-technological material characterization, the relevant loading rate for brittle fracture determined via C(T)25 tests, and some early results of other specimen types. These experimental results are also supported by preliminary numerical and statistical analyses.
AbstractPowder Bed Fusion with Laser Beam of Metals (PBF-LB/M) has gained more industrial relevance and already demonstrated applications at a small series scale. However, its widespread adoption in various use cases faces challenges due to the absence of interfaces to established Manufacturing Execution Systems (MES) that support customers in the predominantly data-driven quality assurance. Current state-of-the-art PBF-LB/M machines utilize communication architectures, such as OPC Unified Architecture (OPC UA), Message Queuing Telemetry Transport (MQTT) and Representational State Transfer Application Programming Interface (REST API). In the context of the Reference Architecture Model Industry 4.0 (RAMI 4.0) and the Internet of Things (IoT), the assets, particularly the physical PBF-LB/M machines, already have an integration layer implemented to communicate data such as process states or sensor values. Missing is an MES component acting as a communication and information layer. To address this gap, the proposed Extract Transform Load (ETL) pipeline aims to extract relevant data from the fabrication of each build cycle down to the level of scan vectors and additionally to register process signals. The suggested data schema for archiving each build cycle adheres to all terms defined by ISO/TC 261—Additive Manufacturing (AM). In relation to the measurement frequency, all data are reorganized into entities, such as the AM machine, build cycle, part, layer, and scan vector. These scan vectors are stored in a runtime-independent format, including all metadata, to be valid and traceable. The resulting machine log represents a comprehensive documentation of each build cycle, enabling data-driven quality assurance at process level.
Residual stress formation during repeated gouging and repair welding cycles of high-strength steels
(2024)
The construction of foundation and erection structures for wind power plants requires the use of modern, sustainable and resource-efficient high-strength fine-grained structural steels. Weld defects due to the welding process are unacceptable. To overcome this issue, local thermal gouging followed by re-welding is a common and cost-effective method. The high shrinkage restraint of the gouge by the surrounding structure can cause crack initiation when design and re-weld induced residual stresses are superimposed. This risk is intensified by the progressive degradation of the microstructure and mechanical properties of high-strength steels during the weld repair process.
This investigation focuses on high-strength steels S500MLO for offshore applications and S960QL for mobile crane applications. The reduction and development of residual stresses caused by local thermal gouging and re-welding was investigated. Digital Image Correlated (DIC) stress-strain analysis was performed during preheating, welding and cooling. The results of the global DIC analysis and local longitudinal and transverse residual stresses of the weld determined by X-ray diffraction were found to be in good agreement. Furthermore, different stress levels were identified during gouging and welding. Repeated repair cycles led to an increase of longitudinal and transverse residual stresses in the weld metal as well as a hardness increase in the heat affected zone.
We investigated the phase transformations during butt-welding of supermartensitic steel plates with help of Neutron-Bragg-Edge Imaging (NBEI). Gas tungsten arc welding (GTAW) was used with a motorized torch allowing for automated weldments. The austenitization in the heat affected zone (HAZ) could be clearly visualized at λ = 0.39 nm, a wavelength smaller than the Bragg edge wavelengths of both austenite and martensite. The re-transformation into the martensitic phase during cooling was clearly detected. However, we observed an unexpected additional change in transmission at λ = 0.44 nm, a wavelength larger than the wavelength of the Bragg edges of both the martensitic and austenitic phases. We attribute this change to the Deybe-Waller-Factor that describes the temperature dependence of coherent scattering at a crystal lattice. The observed two-dimensional attenuation map corresponds well with a temperature distribution modelling by software macros in ANSYS [3]. Here, the absolute temperature values could be achieved by calibrating the modelled attenuation with help of a thermocouple placed at the steel plate. This allows in return for a direct two-dimensional temperature reading based on the Debye-Waller-relation between neutron attenuation and sample temperature.
Direct energy deposition additive manufacturing technologies that utilize an electric arc have great potential for generating large volume metal components. However, selecting process parameters that yield the desired near net shape design and requested mechanical component behavior is not a trivial task due to the complex relationship between all process parameters and material characteristics. This presentation exemplifies the application of a newly developed solid welding wire doped with TiB to enhance grain refinement in the deposited metal for additive manufacturing based on DED-Arc of high-strength precipitation hardening AlMgSi-aluminum alloys. It is worth noting that the solid wire is the result of our preliminary metallurgical studies on grain refinement in aluminum weld metal.
Consequently, research focuses on the correlation between process parameters and component quality to understand the underlying mechanisms. This is crucial for evaluating a robust process parameter space that yields component quality in line with corresponding standards which are mainly taken from welding technology.
Specifically, we examine component quality by analyzing pore size and distribution, as well as grain morphology. To enhance the mechanical properties of the deposited metal, a post-weld heat treatment was conducted, comprising of solution treatment, quenching, and artificial aging. The study also evaluates the effects of various heat treatment strategies on the final mechanical properties of the material.
To demonstrate the applicability of 3D metal printing of high-strength aluminium alloys, a more complex demonstrator was created. It has been shown that DED-Arc can produce high-volume aluminium parts with the same quality as the corresponding subtractive processing strategy.
Additionally, the entire additive manufacturing chain has been digitally integrated, enabling traceability of all relevant process steps, which is essential for reliable subsequent quality assessment.
A steel pipeline segment of 2.5 m length was subjected to quasi-static four-point bending load in three steps for studying the initial cracking and damage accumulation based on the Acoustic Emission (AE) technique and by the direct current potential drop (DCPD) technique. For the latter, a new post-test analysis method was established.
AE is found more sensitive to crack initiation than DCPD. Formation of mesoscopic and macroscopic cracks as well as their closure and the resulting friction generate weighted peak frequencies below 400 kHz, whereas microscopic cracking produces AE with broad band spectra identifiable by weighted peak frequencies above 400 kHz. Critical states alike the maximum load level and the leak opening were accompanied by peak amplitudes above 85 dBAE. This rather fundamental study provides a data base for possibly developing advanced strategies of detection and alarm systems based on acoustic monitoring of pipelines, or at least, steel structures.
This work explores the feasibility of producing bead-on-plate welds of a CrCoNi medium entropy alloy and a CrMnFeCoNi high entropy alloy using electron beam welding. The alloys were welded in two states: one in an as-cold-rolled condition and the other in an annealed condition. In addition, the materials are welded with two different parameters. The FCC microstructure of the welds is investigated using scanning electron microscopy assisted by energy-dispersive X-ray spectroscopy and electron-backscattered diffraction. The impact of the weld on the microstructure is discussed. The heat-affected zone is negligible for the annealed condition of both medium and high entropy alloys since there is no driving force for recrystallisation and the exposure time to high temperature is insufficient for grain coarsening. The texture formed in the fusion zone is also discussed and compared to the texture in the base metal and the heat-affected zone. Although the grain growth along the (100) crystallographic direction is preferential in all cases, the crystallographic texture type differs from each weld. Higher hardness values are measured in the medium entropy alloy’s base metal and fusion zone than in the high entropy alloy.
The combination of high corrosion resistance and good mechanical properties of duplex steels (DSS) is due to their chemical composition and the balanced phase ratio of ferrite (α) and austenite (γ).
Many industrial applications require a material joint of DSS. Tungsten inert gas (TIG) welding is relatively easy to use, requires little space and allows automated welding, with very high reproducibility and is therefore excellent for welding DSS.
During solidification of these steels, critical phase ratios of α and γ can occur, leading to solidification cracking, susceptibility to corrosion, lower ductility, and critical strength values. Therefore, to obtain the desired material properties, the α/γ distribution must be reliably predicted. This is usually done using the WRC1992 diagram. However, the prediction accuracy of the ferrite content in this diagram is usually not accurate enough and must therefore be optimized. It is therefore necessary to monitor even the smallest changes in the chemical composition of the weld metal, ideally during welding. This is done in these experiments using Laser-induced Breakdown Spectroscopy (LIBS). A major advantage of this technique is the highly accurate time- and spatially-resolved measurement of chemical composition during welding. Previous work has quantified the chemical composition in the weld metal and heat affected zone. In the presented study, the influence of individual elements, such as Nb and Cu, on the resulting weld microstructure is investigated.
Large scale hydrogen assisted cracking test for thick walled SAW joints for offshore applications
(2023)
Offshore wind turbines continuously increase in size and weight and demand adequate offshore foundations concepts like monopiles, tripods or jackets. These components are typically submerged arc welded (SAW) high-strength thick steel plates like the S420ML. During welding, the occurrence of delayed hydrogen assisted cracking (HAC) must be anticipated. HAC is a critical combination of the local hydrogen concentration within a susceptible microstructure under certain mechanical load, i.e., the occurring (welding) residual stresses. The welding sequence of the thick-walled plates complicate the residual stress distribution due to the necessary repeated thermal cycling, i.e., welding seam / layer deposition to fill the joint. For that purpose, SAW with two-wire-technique was used to weld a specially designed and prototype-like mock-up of a real component with a thickness of 50 mm, filled more than 20 passes and a seam length of 1,000 mm. Additional welded stiffeners simulated the effect of a high restraint, to stimulate critical HAC conditions. The necessity of a minimum waiting time (MWT) before the NDT can be conducted (to exclude HAC) was critically verified by the application of ultrasonic testing of the welded joint at different time-steps of the NDT of 48 h after welding. The residual stresses were determined by a robot XRD goniometer. Tensile residual stresses up to the yield limit are found both in the weld metal and in the heat-affected zone. Numerical modelling allowed the qualitative estimation of the hydrogen diffusion in the weld. A remarkable HAC occurrence was not identified and proves both, a certain resistance of the weld joint to HAC and the (questionable) duration of the MWT.
This presentation summarizes the latest results on the microalloying influence on the preciptitation behavior on the mechanical properties of welded high-strength steel with a design yield strength of 690 MPa and above. For that reason, Charpy tests were performed for the pure weld metal and different heat-affected zones. Thermodynamic phase field modeling showed the usefulness of calculations in Thermocalc to predict the preciptiation behavior and (finally) to conclude the mechanistic behavior of the welded joint.
High entropy alloys (HEA) are a new class of materials that have been investigated since the early 2000s and offer great potential to replace conventional alloys. However, since they sometimes have significant contents of expensive alloying elements such as Co or Ni, their use is only conceivable in highly stressed areas of components. For this purpose, the weldability with conventional alloys such as high-alloy austenitic steels must be investigated. In addition to the resulting microstructure, the mechanical properties are also fundamental for the usability of HEAs in DMWs. For this purpose, TIG welds of CoCrFeMnNi HEA (cold rolled and recrystallized state) with AISI 304 austenitic steel are investigated. These mechanical properties are analyzed in this work by means of tensile tests and local hardness measurement. The local strain behavior of the welded joints is also characterized by means of Digital Image Correlation (DIC). The results of the local hardness measurement show a clear influence of the initial condition of the HEA on the HAZ. Thus, the HEA in the cold-rolled condition shows a clear softening because of recrystallization processes in the HAZ. On the other hand, there is no influence on the hardness of the weld metal, which is approx. 200 HV0.1 in both cases. The tensile tests show a consistent failure of the weld in the weld metal. However, regardless of the HEA condition, strengths in the range of the recrystallized HEA (RM ~ 550–600 MPa) are achieved, although with significantly reduced fracture elongations.
Alloy 36 (1.3912), also known as “Invar,” is an alloy with 36% nickel. The alloy has a remarkably low thermal expansion coefficient in certain temperature ranges. This peculiarity is called the invar effect, which was discovered in 1896 by the Swiss physicist Charles Édouard Guillaume. Therefore, it is used in applications in which dimensional stability is critical, such as molding tools for composite materials in aerospace, automotive applications, or liquified natural gas (LNG) cargo tanks. Moreover, increasingly complex structures and the optimization of resource efficiency also require additive manufacturing steps for the production or repair of components. Additively manufactured components have a heterogeneous microstructure and anisotropic mechanical properties. In addition, the manufactured components require subsequent machining surface finishing, like finish milling, to achieve their final contour. Nickel iron alloys are difficult to machine. Additionally, inhomogeneous microstructure may lead to unstable cutting forces and conditions. In part I of this investigation, the initial alloy 36 is modified with the elements Ti, Zr, and Hf up to a maximum of 0.33 wt.-%. The influence of the modification elements on the microstructure as well as on the hardness of the AM components is examined. Furthermore, one modification is applied to metal arc welding process and investigated. Part II focuses on the effect of the alloy modifications on machinability as well as on the surface integrity of plasma-transferred-arc-welded (PTA) and finish milled invar components.
Duplex stainless steels (DSS) are frequently used, especially in applications requiring high strength combined with high corrosion resistance in aggressive media. Examples include power plant components and maritime structures. During welding of these steels, local variations in chemical composition can occur. This results in ferritization of the material and negatively affects the mechanical properties of the components. In this work, tungsten inert gas (TIG) welding experiments were performed with DSS. Chemical composition analysis was realized in situ by using Laser Induced Breakdown Spectroscopy (LIBS). The aim of the work is to quantitatively measure the chemical composition in the weld seam of various DSS and to identify possible influences of welding parameters on the microstructure of the material. The chemical concentrations of the main alloying elements Cr, Ni, Mn on the surface of the sample during the welding process and the cooling process were measured. Mn and Ni are austenite stabilizers and their content increases during welding by using certain high alloyed filler material. Spectra were recorded every 1.3 s at a spacing of approximately 2 mm. During the cooling process the location of the measurement was not changed. The LIBS method is proofed to be suitable for the quantitative representation of the chemical compositions during the welding process.
It was found that there is a systematic dependence of the crack growth on the welding parameters as well as on the load. Decreasing welding speeds with simultaneously higher heat input promote crack formation with increasing strain rate.
Based on the results, a new crack criterion was finally selected which allows an extended assessment of the material behavior during the MVT-test. The threshold introduced can quantify the dependencies of the crack formation on the welding and testing parameters, which could not be comprehensively described with the conventional evaluation. In principle, the evaluation routine can also be transferred to other Varestraint or externally loaded hot cracking tests.
This presentation summarizes the latest research results of the influence of the post weld heat treatment (PWHT) rate and
the global mechanical stress conditions on the stress relief cracking (SRC) susceptibility of low-alloyed and creep-resistant CrMoV steel submerged arc weld joints. It was found that a certain effect of very low heating rates could be confirmed. In addition, the interlinking of SRC susceptibility increasing effects during the PWHT and the ex-post indentification in metallographic cross-section via microstructure characterization is very complex.
Since there is a continuously growing demand for complex, frequently heavy-sectioned spheroidal graphite cast iron (SGI) castings it is worth paying attention to the chunky graphite (CHG) degeneration which may occur under certain technological circumstances. Although a reference line for preventive actions in terms of general metallurgical and process measures could be drawn to avoid CHG in heavy-sectioned ferritic SGI castings, a broad majority of experts claim the avoidance of CHG in heavy sections cannot yet be rated a hundred percent process safe. A major reason may be seen in the fact that a universal, generally accepted explanation of CHG formation and growth has not yet been established, although several theories have been proposed. Nevertheless, metallurgical aspects are not in the focus of this paper.
This paper is about the current state of methods to detect CHG in SGI on the laboratory and component scales. Capabilities and limits of different metallographic, fractographic and non-destructive computer tomographic methods to recognize and quantify CHG are discussed. With respect to the characteristic fili-gree three-dimensional string-like, multi-branched CHG structure, which is non-isometric and non-dispersed, serious implications on the possibility to quantitatively characterize the amount of CHG must be considered.
In contrary to the metallurgical aspects, the knowledge about the impact of CHG on the materials and com-ponents properties is still surprisingly limited. Therefore, special emphasis of this paper is on the impact of CHG degeneration on the properties of ferritic SGI. Experimental results are reviewed to illustrate the effect of CHG on mechanical strength and ductility properties as well as fracture mechanics properties in terms of crack resistance and fracture toughness.
The present situation is characterized by discussions and uncertainty about the acceptance or rejection of SGI components containing CHG. Addressing this, conclusions from the materials engineering point of view are drawn for quality control, a safe operational strategy in the foundry and component safety.
Since there is a continuously growing demand for complex, frequently heavy-sectioned spheroidal graphite cast iron (SGI) castings it is worth paying attention to the chunky graphite (CHG) degeneration which may occur under certain technological circumstances. Although a reference line for preventive actions in terms of general metallurgical and process measures could be drawn to avoid CHG in heavy-sectioned ferritic SGI castings, a broad majority of experts claim the avoidance of CHG in heavy sections cannot yet be rated a hundred percent process safe. A major reason may be seen in the fact that a universal, generally accepted explanation of CHG formation and growth has not yet been established, although several theories have been proposed. Nevertheless, metallurgical aspects are not in the focus of this paper.
This paper is about the current state of methods to detect CHG in SGI on the laboratory and component scales. Capabilities and limits of different metallographic, fractographic and non-destructive computer tomographic methods to recognize and quantify CHG are discussed. With respect to the characteristic fili-gree three-dimensional string-like, multi-branched CHG structure, which is non-isometric and non-dispersed, serious implications on the possibility to quantitatively characterize the amount of CHG must be considered.
In contrary to the metallurgical aspects, the knowledge about the impact of CHG on the materials and com-ponents properties is still surprisingly limited. Therefore, special emphasis of this paper is on the impact of CHG degeneration on the properties of ferritic SGI. Experimental results are reviewed to illustrate the effect of CHG on mechanical strength and ductility properties as well as fracture mechanics properties in terms of crack resistance and fracture toughness.
The present situation is characterized by discussions and uncertainty about the acceptance or rejection of SGI components containing CHG. Addressing this, conclusions from the materials engineering point of view are drawn for quality control, a safe operational strategy in the foundry and component safety.
Eight commercial austenitic stainless steels were tensile tested in reference atmosphere, in gaseous high-pressure hydrogen and in gaseous hydrogen precharged condition using conventional (CS) and tubular specimens (TS). For all configurations, 0.2 yield strength and ultimate tensile strength were comparable. In reference atmosphere, reduction of area of CS was higher compared to TS, whereas in gaseous high pressure hydrogen reduction of area of CS was lower compared to TS. In gaseous hydrogen precharged condition reduction of area of CS and TS were comparable for the severely affected grades. The differences in necking behavior between CS and TS are explained by different competitions between necking and hydrogen assisted crack initiation and growth especially for the tests in high pressure hydrogen gas.
Measurement of hydrogen concentration in steel using Laser-induced Breakdown spectroscopy (LIBS)
(2023)
Currently, the ISO 3690 standard requires a thermal activation of the diffusible hydrogen in weld metal for the subsequent ex situ concentration measurement by TDS. Laser-induced breakdown spectroscopy (LIBS) would offer a time and spatially resolved, almost non-destructive, measurement of hydrogen at surfaces. We could already show that this analysis technique allows for highly accurate in situ measurements of chemical compositions of e.g. Mn, Cr and Ni during welding e.g. in the melt pool, the weld metal or the heat affected zone. For quantitative LIBS measurements of hydrogen in steel, certified reference materials with different hydrogen concentrations were used for calibration.
There is a significant problem in welding steels: A critical combination of diffusible hydrogen, hardness microstructure and weld residual stresses can cause cold cracks to form in the weld microstructure. We investigated hydrogen in steels, which was loaded either electrochemically or with high-pressure hydrogen gas. Subsequently, various experiments were conducted to verify the feasibility of hydrogen measurements with LIBS. For example, depth profiles of hydrogen distribution could be obtained. Furthermore, samples were water jet cut and a line scan was made over the cross section to obtain a spatially resolved representation of the hydrogen. Total hydrogen concentrations were determined using carrier gas hot extraction. Our results show that LIBS is a promising technique for time and spatially resolved measurement of hydrogen in steels.
Microalloying elements, such as Nb and Ti, are essential for significantly increasing the strength of quenched and tempered, high-strength structural steels with a nominal yield strength ≥ 690 MPa and their welded joints. The standard specifications (e.g., EN 10025-6) for the chemical composition are only tolerated limit contents within which the steel manufacturers operate. The standard composition, however, says nothing per se about the properties of the material. Even small deviations in the alloy route can have a drastic effect on the mechanical properties. This makes it difficult or even impossible to adequately predict the weldability and integrity of the welded joint. An undesirable side effect is the possible softening of the heat-affected zone (HAZ), as well as the opposite hardening.Mechanical-technological investigations of the notched impact strength confirm the results of the simulation regarding the development of different microalloy routes in the welded state. From this, the influence of the heat effect of welding on the microstructure formation in the HAZ and the corresponding mechanical properties can be described qualitatively.
For a significant increase in the strength of high-strength fine-grained structural steels with a nominal yield strength ≥690 MPa, the addition of microalloying elements such as Nb and Ti is required. The standard specifications for the chemical composition of these steels (e.g., in EN 10025-6) often only give the manufacturer limit contents to achieve the defined properties. The effect of the alloying elements in the heat affected zone (HAZ) is sometimes completely contrary.
This makes it difficult to adequately predict the batch dependency regarding weldability and the load-bearing behaviour of the welded joint. Three different micro-alloyed steels of the grade S690QL were produced on a laboratory scale, focusing on different Nb and Ti contents. To investigate the tempering effect, these were gas metal arc welded in three layers. In addition to metallographic investigations of individual HAZ areas, thermodynamic phase calculations were carried out using Thermo-Calc, following variations in the chemical composition. This provides an understanding of phase transformation, precipitation growth, and dissolution during welding as a function of temperature and cooling conditions. The results show
a divergent metallurgical behaviour in the HAZ of the three different micro-alloyed steels. Thereby, the Ti micro-alloyed grade showed a strong softening of the HAZ in contrast to the Nb micro-alloyed grade. This can be attributed to a contrary precipitation behaviour during welding.
In the course of tomorrow's hydrogen-based energy transition, the construction of the corresponding infrastructure will play a central role. In that context, large diameter long-distance transmission pipelines for hydrogen will be the backbone in the European Union with service pressures from 70 to 90 bar (e.g., de-pending on national regulations). It is a major goal to repurposing the existing natural gas (NG) infrastructure despite the necessity of new pipelines. From that point of view repair welding or further welding of branch pipe etc. can be necessary during in-service, i.e., permanent flow of pressurized hydrogen. The reason is that a shut-down of large diameter pipelines is not easy or sometimes merely impossible. At the moment, it is entirely open if current repair welding procedures for NG pipe-lines can be transferred to pure hydrogen pipelines. For that reason, a collaborative project between BAM, DVGW (German Association for Gas and Water Professions) and a large number of gas grid operators, pipeline manufacturers and construction companies was initiated in 2023 to answer questions on: (1) How many hydrogen is additionally absorbed during the preheating and maintaining at interpass temperature under remaining operational pressures? (2) Is the hydrogen concentration sufficient to reach a critical condition? (3)Which material and weld microstructure are the most susceptible? (4) Is there a difference in the repair welding behavior of NG pipelines with materials in “used” condition? (5) Which welding parameters and joint dimensions must be ensured for safe repair welding? The final aim of this project is the publication of a recommended practice for repair welding of in-service hydrogen pipelines. For that reason, the present study gives an overview on: (A) current practice in repair welding of in-service pipelines and (b) plans for hydrogen pipelines and first results of international research projects.
This contribution briefly summarizes the standardization activities in accordance with the "Nationale Roadmap Wasserstofftechnologien" and presents selected results on the activities in Germany with scope on hydrogen transport in pipelines. The talk was given during a panel discussion to set-up a steering committee for standardization for hydrogen pipelines and welding , coordinated by the International Institute of Welding.
The role of defects in the fatigue strength of Wire Arc Additively Manufactured (WAAMed) Ti-6Al-4V is analysed by means of the IBESS model, a fracture mechanics short crack growth approach based on the cyclic R-curve. Pores and crack-like defects are analysed. Estimations of the role of pore shape and size agree well with published fatigue data of WAAM Ti-6Al-4V with pores. The model is also used to explain the effect of fabrication defects on the scatter of experimental data. This demonstrates that short crack growth models represent a suitable engineering tool for the fatigue assessment of defective AM materials.
The application of anti-corrosion coated, high-strength steels in the automotive industry has increased in recent years. In combination with various zinc-based surface coatings, liquid metal embrittlement cracking can be observed in some of these materials. A high-quality, crack-free spot-welded joint is essential to realize the lightweight potential of the materials. In this work, the LME susceptibility of different coatings, which will be determined by the crack length and the occurrence rate, will be investigated using a welding under external load setup. The uncoated specimens did not show any LME. EG, GI and GA showed significantly less LME than ZM coatings. The latter coatings showed much larger crack lengths than the EG, GI and GA coatings. Furthermore, two mechanisms regarding the LME occurrence rate were observed: the occurrence of LME in zinc–magnesium coatings was theorized to be driven by the material properties of the coatings, whereas the occurrence of LME at EG, GI and GA samples was forced mainly by the application of the external tensile load.
In the experimental setup of this work, the materials were exposed to unusually high mechanical loads (up to 80% of their yield strength) to evoke LME cracks.
The elastic properties (Young's modulus, shear modulus) of Ni-based alloy Inconel IN718 were investigated between room temperature and 800 °C in an additively manufactured variant (laser powder bed fusion, PBF‑LB/M) and from a conventional process route (hot rolled bar). The moduli were determined using the dynamic resonance method. The data set includes information on processing parameters, heat treatments, grain size, specimen dimensions and weight, Young’s and shear modulus as well as their measurement uncertainty.
The dataset was generated in an accredited testing lab using calibrated measuring equipment. The calibrations meet the requirements of the test procedure and are metrologically traceable. The dataset was audited as BAM reference data.
The elastic properties (Young's modulus, shear modulus) of austenitic stainless steel AISI 316L were investigated between room temperature and 900 °C in an additively manufactured variant (laser powder bed fusion, PBF‑LB/M) and from a conventional process route (hot rolled sheet). The moduli were determined using the dynamic resonance method. The data set includes information on processing parameters, heat treatments, grain size, specimen dimensions and weight, Young’s and shear modulus as well as their measurement uncertainty.
The dataset was generated in an accredited testing lab using calibrated measuring equipment. The calibrations meet the requirements of the test procedure and are metrologically traceable. The dataset was audited as BAM reference data.
This article reports temperature-dependent elastic properties (Young’s modulus, shear modulus) of three alloys measured by the dynamic resonance method. The alloys Ti-6Al-4V, Inconel IN718, and AISI 316 L were each investigated in a variant produced by an additive manufacturing processing route and by a conventional manufacturing processing route. The datasets include information on processing routes and parameters, heat treatments, grain size, specimen dimensions, and weight, as well as Young’s and shear modulus along with their measurement uncertainty. The process routes and methods are described in detail. The datasets were generated in an accredited testing lab, audited as BAM reference data, and are hosted in the open data repository Zenodo. Possible data usages include the verification of the correctness of the test setup via Young’s modulus comparison in low-cycle fatigue (LCF) or thermo-mechanical fatigue (TMF) testing campaigns, the design auf VHCF specimens and the use as input data for simulation purposes.
Additive manufacturing (AM) has revolutionized the manufacturing industry, offering a new paradigm to produce complex geometries and parts with customized properties. Among the different AM techniques, the wire arc additive manufacturing (WAAM) process has gained significant attention due to its high deposition rate and low equipment cost. However, the process is characterized by a complex thermal history, dynamic metallurgy, and mechanical behaviour that make it challenging to simulate it in real-time for online process control and optimization.
In this context, a reduced order model (ROM) using the proper generalized decomposition (PGD) method is proposed as a powerful tool to overcome the limitations of conventional numerical methods and enable the real-time simulation of the temperature field of WAAM processes. Though, the simulation of a moving heat source leads to a hardly separable parametric problem, which is handled by applying a novel mapping approach. Using this procedure, it is possible to create a simple separated representation of the model, also allowing to simulate multiple layers.
In this contribution, a PGD model is derived for the WAAM procedure simulating the temperature field. A good agreement with a standard finite element method is shown. The reduced model is further used in a stochastic model parameter estimation using Bayesian inference, speeding up calibrations and ultimately leading to a calibrated real-time simulation.
The spatial laser energy absorption inside the keyhole is decisive for the dynamic molten pool behaviors and the resultant weld properties in high-power laser beam welding (LBW). In this paper, a numerical simulation of the LBW process, considering the 3D transient heat transfer, fluid flow, and keyhole dynamics, is implemented, in which the free surface is tracked by the volume-of-fluid algorithm. The underlying laser-material interactions i.e., the multiple reflections and Fresnel absorption, are considered by an advanced ray-tracing method based on a localized Level-Set strategy and a temperature-dependent absorption coefficient. The laser energy absorption is analyzed from a time-averaged point of view for a better statistical representation. It is found for the first time that a noticeable drop of the time-averaged laser energy absorption occurs at the focus position of the laser beam, and the rest region of the keyhole has relatively homogenous absorbed energy. This unique absorption pattern may lead to a certain keyhole instability and have a strong correlation with the detrimental bulging and narrowing phenomena in the molten pool. The influence of the different focus positions of the laser beam on the keyhole dynamics and molten pool profile is also analyzed and compared. The obtained numerical results are compared with experimental measurements to assure the validity of the proposed model.
The amount of absorbed energy in the keyhole as well as its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops because of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. In this paper, we discuss manyfold different improvements on the laser-material interaction, namely, the ray tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine condition in the determination algorithm for the intersection of reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2, thus ignoring around 14% of the total laser beam energy. In the third step, the changes in the laser radiation distribution in the vertical direction were adapted by using different approximations for the converging and the diverging regions of the laser beam, thus mimicking the beam caustic. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.
State-of-the-art laser powder bed fusion (PBF-LB/M) machines allow pre-heating of the substrate plate to reduce stress and improve part quality. However, two major issues have been shown in the past: First, with increasing build height the apparent pre-heat temperature at the surface can deviate drastically from the nominal pre-heat temperature in the substrate plate. Second, even within a single layer the local surface pre-heat temperature can show large gradients due to thermal bottlenecks in the part geometry underneath the top surface. Both lead to unwanted changes in microstructure or defects in the final parts. In this study, a first attempt is taken to show the feasibility of pre-heating the top surface with the onboard laser beam to overcome the mentioned issues. A single layer of a group of three parts built from IN718 to a height of 33.5 mm is pre-heated in a commercially available PBF-LB/M machine to an average steady state surface temperature of 200 °C using the onboard laser beam. The parts are continuously heated, omitting powder deposition and melting step. Temperatures are measured by thermocouples underneath the surface. The experiments are supported by a thermal finite element (FE) model that predicts the temperature field in the parts. When heating the parts uniformly with the laser beam, differences in surface temperatures as large as 170 K are observed. To overcome this inhomogeneity, the heat flux supplied by the laser beam is modulated. An optimized, spatial heat flow distribution is provided by the thermal FE model and translated into a scan pattern that reproduces the optimized heat distribution on the PBF-LB/M machine by locally modulating hatch distance and scan velocity. This successfully reduces the differences in surface temperature to 20 K. Thermographic imaging shows that a homogeneous surface temperature can be achieved despite the localized heat input by the beam. The potential for industrial application of the optimized laser-heating technique is discussed.
Structural alloys are largely employed in key industrial sectors and their demand is predicted to rise rapidly for the next decades. Most of these materials require a large amount of energy for extraction and manufacturing, which causes the emission of greenhouse gases and other pollutants. Therefore, strategies for improving the sustainability of structural metallic alloys are urgently needed.
Additive Manufacturing (AM), in particular Laser Powder Bed Fusion (PBF-LB/M), aims to be a sustainable manufacturing process, as it allows the build-up of complex geometry in near net-shape from 3D models, while minimizing material waste and the energy required for the process and post-process treatments.
Nevertheless, the application of additively manufactured parts in structural safety-relevant applications is still hindered by the poor fatigue performance. The cause of this has been mainly attributed to the presence of manufacturing defects and surface roughness. Therefore, a huge effort has been made to optimize the process parameters and to introduce post-process treatments to minimize the defect content. However, material flaws cannot be fully eliminated, but these can be considered in a damage tolerance framework for the prediction of the fatigue performance of additively manufactured metallic materials, which is essential for part design and qualification.
This work aims at presenting different modelling strategies for the prediction of the fatigue limit of AM metals. Simple empirical models and more complex models based on fatigue short crack propagation are proposed. The investigated material is an AlSi10Mg alloy fabricated by PBF-LB/M and subjected to two different low-temperature heat-treatments (265°C for 1 h and 300°C for 2h). The results show that the models can provide good approximation of the fatigue limits and help in the interpretation of the scatter of fatigue data.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.
Magnetohydrodynamic technology is increasingly recognized as a promising approach for enhancing the quality of deep penetration laser beam welding. This study employs experimental and numerical methods to investigate the effects of an auxiliary oscillating magnetic field on reducing porosity in the laser beam welding of 5754 aluminum alloy. The experimental results clearly demonstrate a significant reduction in the porosity ratio, thereby validating the efficacy of applying MHD technology in mitigating porosity during the laser beam welding process. In addition, a transient 3D multi-physical model has been developed, integrating the magnetohydrodynamic and metal vapor plume, to gain a more comprehensive understanding of the porosity suppression mechanism. The introduction of an oscillating magnetic field generates a time-averaged downward Lorentz force. This Lorentz force, in turn, induces an electromagnetic expulsive force, which effectively increases the upward escape velocity of bubbles in the molten pool. Furthermore, the molten pool shape is significantly enlarged, which further facilitate the escape of bubbles. The simulation results agree well with the experimental results.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.
Numerical study on the formation of a bulging region in partial penetration laser beam welding
(2023)
A transient three-dimensional thermo-fluid dynamics numerical model was developed to study the formation of a bulging region in partial penetration laser beam welding. The model accounts for the coupling between the fluid flow, the heat transfer, and the keyhole dynamics by considering the effects of multiple reflections and Fresnel absorption of the laser beam in the keyhole, the phase transitions during melting and evaporating, the thermo-capillary convection, the natural convection, and the phase-specific and temperature-dependent material properties up to the evaporation temperature. The validity of the model was backed up by experimentally obtained data, including the drilling time, the weld pool length, the local temperature history outside the weld pool, the process efficiency, and a range of metallographic crosssections.
The model was applied for the cases of partial penetration laser beam welding of 8 mm and 12 mm thick unalloyed steel sheets. The obtained experimental and numerical results reveal that the bulging region forms transiently depending on the penetration depth of the weld, showing a tendency to transition from a slight bulging to a fully developed bulging region between penetration depths of 6 mm and 9 mm, respectively.
A three-dimensional numerical model was developed to accurately predict the steady-state weld pool shape in full penetration laser beam welding. The model accounts for the coupling between the heat transfer and the fluid dynamics by considering the effects of solid/liquid phase transition, thermo-capillary convection, natural convection, and phase-specific and temperature-dependent material properties up to the evaporation temperature. A fixed right circular cone was utilized as a keyhole geometry to consider the heat absorbed from the laser beam. The model was used to analyze the influence of the thermodynamic activity of solutes and turbulence on the weld pool shape. A mesh sensitivity analysis was performed on a hybrid mesh combining hexahedral and tetrahedral elements. For the case of full penetration laser beam welding of 8 mm thick unalloyed steel sheets, the dependence of the weld pool shape on the surface-active element sulfur was found to be negligible. The analysis of the results showed that a laminar formulation is sufficient for accurately predicting the weld pool shape since the turbulence has a minor impact on the flow dynamics in the weld pool. The validity of the numerical results was backed up by experimental measurements and observations, including weld pool length, local temperature history, and a range of metallographic crosssections.
The contactless magnetohydrodynamic technology has been considered as a potential and promising method to improve the weld qualities of deep penetration laser beam welding. In this paper, numerical investigations are conducted to study the influence of the auxiliary oscillating magnetic field on the porosity suppression in laser beam welding of 5754 aluminum alloy. To obtain a deeper insight into the suppression mechanism, a three-dimensional transient multi-physical model is developed to calculate the heat transfer, fluid flow, keyhole dynamic, and magnetohydrodynamics. A ray tracing algorithm is employed to calculate the laser energy distribution on the keyhole wall. A time-averaged downward Lorentz force is produced by an oscillating magnetic field. This force acts in the molten pool, leading to a dominant downward flow motion in the longitudinal section, which blocks the bubble migration from the keyhole tip to the rear part of the molten pool. Therefore, the possibility for the bubbles to be captured by the solidification front is reduced. The electromagnetic expulsive force provides an additional upward escaping speed for the bubbles of 1 m/s ~ 5 m/s in the lower and middle region of the molten pool. The simulation results are in a good agreement with experimental measurements. Based on the results obtained in this study, a better understanding of the underlying physics in laser beam welding enhanced by an auxiliary oscillating magnetic field can be provided and thus the welding process can be further optimized reducing the porosity formation.
Numerical analysis of ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys
(2023)
The ultrasonic vibration enhanced friction stir welding (UVeFSW) process has unique advantages in joining dissimilar Al/Mg alloys. While there are complex coupling mechanisms of multi-fields in the process, it is of great significance to model this process, to reveal the influence mechanism of ultrasonic vibration on the formation of Al/Mg joints.
In this study, the acoustic-plastic constitutive equation was established by considering the influence of both ultrasonic softening and residual hardening on the flow stress at different temperatures and strain rates. And the ultrasonic induced friction reduction (UiFR) effect on friction coefficient in different relative directions at the FSW tool-workpiece interface was quantitatively calculated and analyzed.
The Al/Mg UVeFSW process model was developed through introducing the above acoustic effects into the model of Al/Mg friction stir welding (FSW). The ultrasonic energy is stronger on the aluminum alloy side. In the stirred zone, there is the pattern distribution of ultrasonic sound pressure and energy. The heat generation at the tool-workpiece contact interface and viscous dissipation were reduced after applying ultrasonic vibra-tion. Due to the UiFR effect, the projection of friction coefficient and heat flux distributions at the tool-workpiece interface present a "deformed" butterfly shape. The calculated results show that ultrasonic vibra-tion enhanced the material flow and promoted the mixing of dissimilar materials.
The weld quality and the possible defect formation are directly determined by the weld pool shape and the thermo-fluid dynamics therein. In this paper, an untypical weld pool profile, i.e., elongated at its top and bottom but narrowed at the middle, is found experimentally and numerically in the wire feed laser beam welding. The detrimental influence of the weld pool narrowing on the element transport is analyzed and discussed. A magnetohydrodynamic technique is utilized to suppress the narrowing, aiming at a more homogenous element distribution. It is found that a low-temperature region is formed in the middle of the weld pool due to the interaction of the two dominant circulations from the top and bottom regions. The weld pool is significantly narrowed due to the untypical growth of the mushy zone in the low-temperature region, which results in a direct blocking effect on the downward flow and the premature solidification in the middle region. The Lorentz force produced by a transverse oscillating magnetic field shows the potential to change the flow pattern into a single-circulation type and the low-temperature-gradient region is mitigated.
Therefore, the downward transfer channel is widened, and its premature solidification is prevented. The numerical results are well validated by experimental measurements of metal/glass observation and X-ray fluorescence element mapping.
Through experimental observation and auxiliary numerical simulation, this investigation studies the different types of grain refinement of 5754 aluminum alloy laser beam welding by applying a transverse oscillating magnetic field. Scanning electron microscope results have proved that the application of a magnetic field can reduce the average crystal branch width and increase its number. The interaction between the induced eddy current generated by the Seebeck effect and the applied external magnetic field produces a Lorentz force, which is important for the increase of the number of crystal branches. Based on the theory of dendrite fragmentation and the magnetic field-induced branches increment, the grain size reduction caused by the magnetic field is studied. Furthermore, the effects of the magnetic field are analyzed by combining a phase field method model and simulations of nucleation and grain growth. The grain distribution and average grain size after welding verifies the reliability of the model. In addition, the introduction of a magnetic field can increase the number of periodic three-dimensional solidification patterns. In the intersection of two periods of solidification patterns, the metal can be re-melted and then re-solidified, which prevents the grains that have been solidified and formed previously from further growth and generates some small cellular grains in the new fusion line. The magnetic field increases the building frequency of these solidification structures and thus promotes this kind of grain refinement.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by the experimental and numerical method. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer-Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that the additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the molten pool shape. The probability of keyhole collapse at the bottom increases significantly to 72% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and the keyhole collapse are obtained. The developed model has been validated by the experiments, which shows a good agreement.
Fracture mechanics-based methods can be applied to the fatigue assessment of mechanical components containing defects or cracks subjected to cyclic loading. The knowledge of the crack growth behavior for short and long cracks enables the determination of both finite and infinite life regimes. Nevertheless, a first problem arises from the empirical determination of the fatigue data, which vary according to the test conditions, e.g., the test method, frequency and the stress ratio R. This is due to crack closure and further environmental phenomena affecting crack propagation behavior. A second issue regards the lack of standard experimental procedures for the determination of the fatigue crack propagation threshold ΔKth dependency on the crack size. This is problematic, since ΔKth increases with the build-up of the closure effects in the short crack regime and its inaccurate experimental determination leads to large deviations in the assessment and, even more critical, to non-conservative predictions. Aiming at providing more reliable and safer assessment procedures, the present work proposes a method based on effective crack growth data obtained at high stress ratios (R ≈ 0.8). A short-crack model based on the cyclic R-curve and CPLR data is employed for describing the development of the closure effects. The predictions are compared to established methods and validated by experimental tests.
The Kitagawa-Takahashi (KT) diagram is a well-established tool in the fatigue community for the prediction of the fatigue limit of metallic materials in presence of defects. Nevertheless, its determination is associated with a large number of fatigue tests carried out on specimens with artificial defects (notches) to describe the fatigue limit-defect size relationship in the short crack regime. The preparation of the specimens and the execution of the tests is time expensive. Therefore, few phenomenological models have been introduced in the past to provide a first approximation of the KT diagram based solely on the fatigue limit for smooth specimen and the fatigue crack propagation threshold for long cracks. Despite the use of such models is widely spread, these suffer from problems related to the uncertainties of the material parameters. Furthermore, the mechanics of short cracks, which is the physical mechanism behind the concept of the fatigue limit, is not considered. This work discusses the advantages and drawbacks of using short fatigue crack propagation models for predicting the fatigue limit of flawed metallic materials. The KT diagrams for a EA4T railway axle steel in as-received and full-hardened condition are approximated by means of the cyclic R-curve method.
Soft polymers such as the investigated polyurethane, characterized by low Young’s moduli and prone to high shear deflection, are frequently applied in pneumatic cylinders. Their performance and lifetime without external lubrication are highly determined by the friction between seal and shaft and the wear rate. FEM simulation has established itself as a tool in seal design processes but requires input values for friction and wear depending on material, load, and velocity. This paper presents a tribological test configuration for long stroke, reciprocating movement, allowing the generation of data which meet the requirements of input parameters for FEM simulations without the geometrical influences of specific seal profiles. A numerical parameter study, performed with an FEM model, revealed the most eligible sample geometry as a flat, disc-shaped sample of the polymer glued on a stiff sample holder. At the same time, the study illustrates that the sensitivity of the contact pressure distribution to Poisson’s ratio and CoF can be minimized by the developed and verified setup. It ensures robust, reliable, and repeatable experimental results with uniform contact pressures and constant contact areas to be used in databases and FEM simulations of seals, enabling upscaling from generically shaped samples to complex seal profiles.
The strain field can reflect the initiation time of solidification cracks during the welding process. The traditional strain measurement is to first obtain the displacement field through digital image correlation (DIC) or optical flow and then calculate the strain field. The main disadvantage is that the calculation takes a long time, limiting its suitability to real-time applications. Recently, convolutional neural networks (CNNs) have made impressive achievements in computer vision.
To build a good prediction model, the network structure and dataset are two key factors. In this paper, we first create the training and test sets containing welding cracks using the controlled tensile weldability (CTW) test and obtain the real strain fields through the Lucas–Kanade algorithm. Then, two new networks using ResNet and DenseNet as encoders are developed for strain prediction, called StrainNetR and StrainNetD. The results show that the average endpoint error (AEE) of the two networks on our test set is about 0.04, close to the real strain value. The computation time could be reduced to the millisecond level, which would greatly improve efficiency.
The use of Low Transformation Temperature (LTT) filler materials is an innovative way to reduce welding residual stresses. The generation of compressive residual stresses in the weld and heat-affected zone can lead to an improvement of the fatigue life of high-strength welds. However, high-alloy filler metals can be prone to solidification cracking. Two important LTT alloys were evaluated under variable welding and loading parameters using the Modified Varestraint/Transvarestraint (MVT) test. Initially conflicting results were interpreted using a newly developed image-based crack detection routine [1]. Based on the melt pool dimensions, the shape of the isotherms is reconstructed, and the theoretical crack growth along numerically determined crystallization paths is considered in relation to the cracks observed. Decreasing welding speeds with simultaneously higher heat input promote crack formation with increasing strain rate.
A technique for calculating the main solidification parameters for a two-dimensional columnar crystal growth during complete penetration laser beam welding of thin steel sheets was developed. Given that the weld pool interface is described by Lamé curves (superellipses) within the horizontal plane of growth, general analytical solutions were derived for the geometry of the crystal axis and the corresponding growth rate and cross-sectional area of the crystal. A dimensionless analysis was performed to provide insights on the dependence of the solidification parameters on the shape and dimensions of the rear part of the weld pool boundary. The derived solutions were applied for the case of complete penetration laser beam keyhole welding of 2 mm thick 316L austenitic chromium-nickel steel sheets. It was shown that the reconstruction of the weld pool boundary with Lamé curves provides higher accuracy and flexibility compared to results obtained with elliptical functions. The validity of the proposed technique and the derived analytical solutions was backed up by a comparison of the obtained solutions to known analytical solutions and experimentally determined shapes and sizes of the crystals on the top surface of the sheet. The dimensions of the calculated crystal axis correlated well with the experimentally obtained results.
The fatigue assessment of welded joints requires several input data, which can be subdivided into three categories: geometry, material and loading. The number of input data depends essentially on the complexity of the models employed and on the level of accuracy of the analysis. It is common practice to use safety factors in design to account for the scatter of the input parameters. Nevertheless, overly-conservative factors lead often to unrealistic estimations of fatigue life. This work presents a fracture mechanics-based model for the structural integrity assessment of welded joints under constant amplitude fatigue loading, in which the local geometry at the weld toe and the fatigue crack growth properties are considered statistically distributed. The approach is validated against a large number of experimental data.
Fraucture mechanics-based probablistic structural integrity assessment for aero-engine turbine disk
(2023)
Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The aim of this work is to present part of a fracture mechanics-based probabilistic assessment procedure under development which aims at calculating the critical rotational speed of the turbine disk based on the numerical-analytical solutions and regulations for the failure probability. In particular, the rim-peeling failure mode is considered as case study. A semi-circular surface crack is modelled at the most stressed region at the diaphragm of a turbine disk. In order to design a lab representative specimen, beside the crack driving force, expressed in terms of J-integral, also the constraint to plastic deformation e.g., stress triaxiality, at the crack-tip must be similar for the same crack in the specimen and in the disk. The analytical solutions to calculate the crack driving force for the lab representative specimen are used for the Monte Carlo simulations, the result of which has been assessed in the form of a Failure Assessment Diagram (FAD). The results of the probabilistic structural integrity assessment show good agreement between Monte Carlo simulations and certification values for the disk in terms of expected failure mode and value of the critical speed.
The essence of dynamic failure is closely linked to the formation of adiabatic shear bands (ASB), which result from the localization of shear strain under high deformation speeds accompanied by a rapid temperature increase. Understanding this phenomenon is crucial in view of safety issues when impacts of fast rotating machine components (i.e. aircraft turbine blades) may occur. Our contribution addresses both the experimental evidence and characterization of ASBs due to high-speed impact tests at the Split HOPKINSON pressure bar (SHPB) setup and the finite element analysis to determine the parameters of the underlying constitutive model, which is closely related to JOHNSON-COOK (JC) material model.
Experimental investigations were performed on notched shear specimens made of the fine -grained structural steel S690QL and the displacements in the regions affected by shear localization were measured with subset-based local Digital Image Correlation (DIC). The displacement fields, obtained in the SHPB tests, were considered as an objective to validate and to identify the constitutive parameters with. The JC model could reasonably reproduce the displacement distribution. In order to overcome the issues with mesh dependency we provide a nonlocal extension based on the implicit gradient model approach.
The high cooling rates (~106 K/s) occurring during Laser Powder Bed Fusion (PBF-LB/M) of AlSi10Mg induce to the formation of a fine nanometric silicon network in the as-built condition. Such unprecedented microstructure enhances the mechanical strength when compared to equivalent as-cast materials. Nevertheless, PBF-LB/M also leads to high magnitude residual stress (RS) due to the extreme localized temperature gradients. The presence of RS can be detrimental to the fatigue life of engineering components, and great efforts are focused on understanding their generation and evolution after post-process heat treatments. Typically, T6 heat treatments are used to mitigate RS and improve mechanical performances by Mg2Si precipitation during ageing at 160-180°C. Nevertheless, the solutionizing at 500-540°C vanishes the fine silicon network, leading to the formation of micrometric (average of ~2-5 µm) polygonal Si particles, similar to those observed in T6 heat-treated Al-Si cast materials. Therefore, the aim of this work is to evaluate the ability of two so-called low temperature heat treatments (i.e., at 265°C and 300°C) to mitigate RS while retaining the fine as-built microstructure inherent to PBF-LB/M AlSi10Mg. The fatigue behavior of the as-built material is subsequently compared to the two low temperature conditions.
One of the main advantages of metal additive manufacturing (MAM) techniques is their ability to produce components with site-specific microstructural features. Nevertheless, microstructural defects and lack of repeatability are still major concerns in MAM. In this study, a laser powder bed fusion (PBF-LB/M) IN718 material, produced using two different scan length vectors, is investigated using Bragg-edge neutron 2D imaging (BENI) combined with electron backscatter diffraction (EBSD) analysis. BENI is able to detect, on a macroscopic scale, process-induced changes in texture in a large field of view covering the entire sample (20×80 mm2). In addition, high-resolution BENI (HR-BENI), with a pixel size of 12.8 µm, provides a micro-scale examination of the local variations of texture and grain morphology, otherwise undistinguishable using the standard resolution. As such, HR-BENI offers a straightforward and detailed way of screening the integrity of MAM parts at cm-length scales.
The microstructure has a great influence on short fatigue crack growth in metallic materials. Laser-based Powder Bed Fusion AlSi10Mg alloys exhibit in the as-built condition a fine fibrous Si structure and a supersaturated solid solution of Si in the α-Al matrix, which is significantly modified by heat treatments starting already at temperatures under 260 °C. This study focuses on the influence of post-process heat treatments on the microstructural evolution and the resulting fatigue crack growth resistance. As compared to the as-built condition, two heat treatments at 265 °C/1 h and at 300 °C/2 h are found to be beneficial to the fatigue crack growth resistance of the investigated material.
Based on state-of-the-art research and regulations, the application of the fracture mechanics master curve (MC) concept to ferritic ductile cast iron (DCI) is being investigated in a recently started German joint research project between BAM Berlin and MPA Stuttgart. This paper outlines the research approach and the planned investigations and provides first results.
According to the IAEA guidelines SSG 26, the safety assessment of DCI containers for transport as well as interim and final storage of radioactive materials is based on the fracture mechanics criterion of general crack initiation exclusion. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods under dynamic loading conditions. Within fracture mechanics safety assessments for steels in nuclear technology, the probabilistic MC concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. The MC concept allows for an effective statistical consideration of the scatter of the material toughness in the transition regime. Furthermore, the MC can also be used for elevated loading rates as they occur in accident or crash scenarios. However, for DCI, a systematic review of potential modifications to the MC assumptions and procedure according to ASTM E1921 and an associated validation are still lacking. Therefore, a corresponding methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using samples extracted from a component.
For the experimental program, a DCI material was produced as it is generally used for transport and storage containers. A basic mechanical-technological material characterization consisting of tensile, Charpy, and Pellini tests is planned. A comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens. Moreover, numerical analyses of different specimen geometries and test setups are planned to investigate the influences of stress triaxiality and loading rate. Throughout the experimental program, the microstructure-property relationship will be investigated based on quantitative metallographic and fractographic analyses. Finally, different MC assessment strategies will be considered, and modifications will be made if necessary.
Based on state-of-the-art research and regulations, the application of the fracture mechanics master curve (MC) concept to ferritic ductile cast iron (DCI) is being investigated in a recently started German joint research project between BAM Berlin and MPA Stuttgart. This paper outlines the research approach and the planned investigations and provides first results.
According to the IAEA guidelines SSG 26, the safety assessment of DCI containers for transport as well as interim and final storage of radioactive materials is based on the fracture mechanics criterion of general crack initiation exclusion. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods under dynamic loading conditions. Within fracture mechanics safety assessments for steels in nuclear technology, the probabilistic MC concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. The MC concept allows for an effective statistical consideration of the scatter of the material toughness in the transition regime. Furthermore, the MC can also be used for elevated loading rates as they occur in accident or crash scenarios. However, for DCI, a systematic review of potential modifications to the MC assumptions and procedure according to ASTM E1921 and an associated validation are still lacking. Therefore, a corresponding methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using samples extracted from a component.
For the experimental program, a DCI material was produced as it is generally used for transport and storage containers. A basic mechanical-technological material characterization consisting of tensile, Charpy, and Pellini tests is planned. A comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens. Moreover, numerical analyses of different specimen geometries and test setups are planned to investigate the influences of stress triaxiality and loading rate. Throughout the experimental program, the microstructure-property relationship will be investigated based on quantitative metallographic and fractographic analyses. Finally, different MC assessment strategies will be considered, and modifications will be made if necessary.
Components in aircrafts are usually replaced when critical defects are present.
An alternative approach is repairing using gas dynamic cold spraying: metal particles are shot at a surface at supersonic speeds to selectively rebuild damaged material. Compared to other material-deposition techniques, its advantage is the small thermal impact on the component, preserving its mechanical properties.
Component-repair can save considerable amounts of energy and resources. However, its industrial application at large scale needs reproducible, good repair-material properties to guarantee a safe component life. The aim of this project is the development of safe, automatized repair-procedures considering the mechanical fatigue properties of the repair.
Understanding the void defect formation mechanism and simultaneous predicting the tool service life in friction stir welding are critical for optimizing the welding parameters. However, the void defect formation mechanism in friction stir welding is not yet elucidated. In this study, a novel integrated thermal-fluid-structure coupling model of the friction stir welding process was proposed for simultaneous prediction of the weld formation and tool service life. A new non-uniform distribution model of the tool-workpiece contact pressure was proposed to describe the interaction between the tool and the workpiece. The void defect formation mechanism was quantitatively studied using the proposed integrated thermal-fluid-structure coupling model. The results show that the plastic material flows in the horizontal direction and can completely fill the cavity behind the tool for the welding condition of forming a sound weld. While the tool-workpiece contact interfacial frictional shear stress in the rear of the tool is decreased significantly which leads to a severe decrease in the plastic material flow velocity. Therefore, after bypassing the tool from the retreating side, the plastic material at the bottom of the weld stagnates, and void defect forms in the middle and lower part of the weld at the advancing side. The difference between the maximum and the minimum tool-workpiece contact pressure could serve as a numerical criterion to predict void defects. A sound joint is formed when the difference is lower than the critical value of 15 MPa, while a void defect is formed in the weld if it is higher than this critical value. The maximum equivalent stress acting on the tool is located at the pin root with severe stress concentration at a high welding speed. The front of the tool is subjected to tensile stress while its rear is subjected to compressive stress, therefore the tool is apt to fracture at its root under an inappropriate welding condition. The average normal stress of the tool varies periodically with its period consistent with the rotation period of the tool. The service life of the tool is decreased with the increase in welding speed and the decrease in rotation speed. The model is validated by experimental results.
Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The actual part qualification and certification procedures make use of spin-tests conducted on production-similar disks. While these tests provide, on the one hand, a reliable definition of the critical conditions for real components, on the other hand they represent a relevant cost item for engine manufacturers. The aim of this work is to present two alternative burst speed assessment methods under development based on the Failure Assessment Diagram (FAD) and a global stability criterion, respectively. In the scope of the fracture mechanics assessment, the failure modes hoop-burst and rim-peeling are investigated with semicircular surface cracks modelled at the critical regions on the turbine disk. The comparison of the predicted critical rotational speed shows good agreement between the assessment methods.
Additive manufacturing (AM) in general and laser powder bed fusion (PBF-LB/M) in particular are becoming increasingly important in the field of production technologies. Especially the high achievable accuracies and the great freedom in design make PBF-LB/M interesting for the manufacturing and repair of gas turbine blades. Part repair involves building AM-geometries onto an existing component. To minimise the offset between component and AM-geometry, a precise knowledge of the position of the component in the PBF-LB/M machine is mandatory. However, components cannot be inserted into the PBF-LB/M machine with repeatable accuracy, so the actual position will differ for each part. For an offset-free build-up, the actual position of the component in the PBF-LB/M machine has to be determined. In this paper, a camera-based position detection system is developed considering PBF-LB/M constraints and system requirements. This includes finding an optimal camera position considering the spatial limitations of the PBF-LB/M machine and analysing the resulting process coordinate systems. In addition, a workflow is developed to align different coordinate systems and simultaneously correct the perspective distortion in the acquired camera images. Thus, position characteristics can be determined from images by image moments. For this purpose, different image segmentation algorithms are compared. The precision of the system developed is evaluated in tests with 2D objects. A precision of up to 30μm in translational direction and an angular precision of 0.021∘ is achieved. Finally, a 3D demonstrator was built using this proposed hybrid strategy. The offset between base component and AM-geometry is determined by 3D scanning and is 69μm.
In 2015, the University of Manchester hosted a workshop (‘Mind the Gap’) aimed at identifying gaps in a number of structural integrity fitness-for-service procedures, including R5, R6, BS 7910 and API/ASME. The findings were subsequently summarised in a journal paper and shared with the relevant stakeholders. A second workshop, this time hosted by the University of Bristol in 2017, was intended to build on the findings of the earlier event, identifying which gaps had been filled, which remain and whether new ones have been identified in the meantime. ‘Mind the Gap 2’ was wide-ranging, including consideration of failure by fracture, fatigue crack growth, hightemperature creep and environmentally assisted crack growth, along with the use of innovative techniques to follow the progress of crack growth from the atomic to the macroscopic scale. A summary of the whole event is thus outside the scope of a single paper, so here we concentrate mainly on advances in fracture assessment, on the interface between inspection and ECA, and on how developments are being incorporated into structural integrity procedures. There is a particular emphasis on the energy transition in the UK, where the planned energy mix will include both nuclear power and offshore wind.