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Eingeladener Vortrag
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Influence of a 265 °C heat treatment on the residual stress state of a PBF-LB/M AlSi10Mg alloy
(2022)
Laser Powder Bed Fusion (PBF-LB/M) additive manufacturing (AM) induces
high magnitude residual stress (RS) in structures due to the extremely heterogeneous cooling and heating rates. As the RS can be deleterious to the fatigue resistance of engineering components, great efforts are focused on understanding their generation and evolution after post-process heat treatments. In this study, one of the few of its kind, the RS relaxation induced in an as-built PBF-LB/M AlSi10Mg material by a low-temperature heat treatment (265 °C for 1 h) is studied by means of X-ray and neutron diffraction. Since the specimens are manufactured using a baseplate heated up to 200 °C, low RS are found in the as-built condition. After heat treatment a redistribution of the RS is observed, while their magnitude remains constant. It is proposed that the redistribution is induced by a repartition of stresses between the a-aluminium matrix and the silicon phase, as the morphology of the silicon phase is affected by the heat treatment. A considerable scatter is observed in the neutron diffraction RS profiles, which is principally correlated to the presence (or absence) of pockets of porosity developed at the borders of the chessboard pattern.
Additive manufacturing methods such as laser powder bed fusion (LPBF) allow geometrically complex parts to be manufactured within a single step. However, as an aftereffect of the localized heat input, the rapid cooling rates are the origin of the large residual stress (RS) retained in as-manufactured parts. With a view on the microstructure, the rapid directional cooling leads to a cellular solidification mode which is accompanied by columnar grown grains possessing crystallographic texture. The solidification conditions can be controlled by the processing parameters and the scanning strategy. Thus, the process allows one to tailor the microstructure and the texture to the specific needs. Yet, such microstructures are not only the origin of the mechanical anisotropy but also pose metrological challenges for the diffraction-based RS determination. In that context the micromechanical elastic anisotropy plays an important role: it translates the measured microscopic strain to macroscopic stress. Therefore, it is of uttermost importance to understand the influence of the hierarchical microstructures and the texture on the elastic anisotropy of LPBF manufactured materials.
This study reveals the influence of the build orientation and the texture on the micro-mechanical anisotropy of as-built Inconel 718. Through variations of the build orientation and the scanning strategy, we manufactured specimens possessing [001]/[011]-, [001]-, and [011]/[111]-type textures. The resulting microstructures lead to differences in the macroscopic mechanical properties. Even further, tensile in-situ loading experiments during neutron diffraction measurements along the different texture components revealed differences in the microstrain response of multiple crystal lattice planes. In particular, the load partitioning and the residual strain accumulation among the [011]/[111] textured specimen displayed distinct differences measured up to a macroscopic strain of 10 %. However, the behavior of the specimens possessing [001]/[011]-and [001]-type texture was only minorly affected. The consequences on the metrology of RS analysis by diffraction-based methods are discussed.
Al-Si alloys produced by Laser Powder Bed Fusion (PBF-LB/M) techniques allow the fabrication of lightweight free-shape components. Due to the extremely heterogeneous cooling and heating, PBF-LB/M induces high magnitude residual stress (RS) and a fine Si microstructure. As the RS can be deleterious to the fatigue resistance of engineering components, great efforts are focused on understanding their evolution before and after post-process heat treatments (HT).
Laser Powder Bed Fusion (L-PBF) allow the fabrication of lightweight near net shape AlSi10Mg components attractive to the aerospace, automotive, biomedical and military industries. During the build-up process, high cooling rates occur. Thus, L-PBF AlSi10Mg alloys exhibit a Si-nanostructure in the as-built condition, which leads to superior mechanical properties compared to conventional cast materials. At the same time, such high thermal gradients generally involve a deleterious residual stress (RS) state that needs to be assessed during the design process, before placing a component in service. To this purpose post-process heat treatments are commonly performed to relieve detrimental RS. In this contribution two low-temperature stress-relief heat treatments (SRHT) are studied and compared with the as-built state: a SRHT at 265°C for 1 hour and a SRHT at 300°C for 2 hours. At these temperatures microstructural changes occur. In the as-built state, Si atoms are supersaturated in the α-aluminium matrix, which is enveloped by a eutectic Si-network. At 265°C the Si precipitation from the matrix to the pre-existing network is triggered. Thereafter, above 295°C the fragmentation and spheroidization of the Si branches takes place, presumably by Al–Si interdiffusion. After 2 hours the original eutectic network is completely replaced by uniformly distributed blocky particles. The effect of the heat and the microstructure modification on the RS state and the fatigue properties is investigated. Energy dispersive x-ray and neutron diffraction are combined to investigate the near-surface and bulk RS state of a L-PBF AlSi10Mg material. Differences in the endurance limit are evaluated experimentally by high cycle fatigue (HCF) tests and cyclic R-curve determination.
Metal Additive Manufacturing (AM) technologies such as Laser Powder Bed Fusion (LPBF) are characterized by layer wise construction, which enable advancements of component design, with associated potential gains in performance and efficiency. However, high magnitude residual stresses (RS) are often a product of the rapid thermal cycles typical of the layerwise process. Therefore, a deep understanding of the formation of RS, the influence of process parameters on their magnitude and the impact on mechanical performance is crucial for widespread application. The experimental characterisation of these RS is essential for safety related engineering application and supports the development of reliable numerical models. Diffraction-based methods for RS analysis using neutrons and high energy X-rays enable non-destructive spatially resolved characterisation of both surface and bulk residual stresses in complex components. This presentation will provide an overview of recent research by the BAM at large scale facilities for the characterization of residual stresses in LPBF metallic alloys as a function of process parameters. In addition, the challenges posed by the textured and hierarchical microstructures of LPBF materials on diffraction-based RS analysis in AM materials will be discussed. This will include the question of the d0 reference lattice spacing and the appropriate choice of the diffraction elastic constants (DECs) to calculate the level of RS in LPBF manufactured alloys.
The present work seeks to extend the level of understanding of the stress field evolution during direct laser deposition (DLD) of a 3.2 mm thick multilayer wall of Ti-6Al-4V alloy by theoretical and experimental studies. The process conditions were close to the conditions used to produce large-sized structures by the DLD method, resulting in specimens having the same thermal history. A simulation procedure based on the implicit finite element method was developed for the theoretical study of the stress field evolution. The accuracy of the simulation was significantly improved by using experimentally obtained temperature-dependent mechanical properties of the DLD-processed Ti-6Al4V alloy. The residual stress field in the buildup was experimentally measured by neutron diffraction. The stress-free lattice parameter, which is decisive for the measured stresses, was determined using both a plane stress approach and a force-momentum balance. The influence of the inhomogeneity of the residual stress field on the accuracy of the experimental measurement and the validation of the simulation procedure are analyzed and discussed. Based on the numerical results it was found that the non-uniformity of the through-thickness stress distribution reaches a maximum in the central cross-section, while at the buildup ends the stresses are distributed almost uniformly. The components of the principal stresses are tensile at the buildup ends near the substrate. Furthermore, the calculated equivalent plastic strain reaches 5.9% near the buildup end, where the deposited layers are completed, while the plastic strain is practically equal to the experimentally measured ductility of the DLD-processed alloy, which is 6.2%. The experimentally measured residual stresses obtained by the force-momentum balance and the plane stress approach differ slightly from each other.
This study reports on the stress relaxation potential of stress-relieving heat treatments for laser powder bed fused 316L. The residual stress is monitored non-destructively using neutron diffraction before and after the heat treatment. Moreover, the evolution of the microstructure is analysed using scanning electron microscopy. The results show, that a strong relaxation of the residual stress is obtained when applying a heat treatment temperature at 900°C. However, the loss of the cellular substructure needs to be considered when applying this heat treatment strategy.
An unusual microstructure, inherent residual stresses and void formation are the three key aspects to control when assessing metallic parts made by LPBF. This talk explains an experiment to unravel the interlinked influence of the two mechanisms for the formation of residual stresses in LPBF: the temperature gradient mechanism and constricted solidification shrinkage. The impact of each mechanism on the shape and magnitudes of the residual stress distribution is described. Combined results from neutron diffraction, X-ray diffraction, computed tomography and in-situ thermography are presented.
Also, influence of scan strategies as well as surface roughness of subjacent layers on void formation is shown. Results from computed tomography and in-situ thermography of a specimen dedicated to study the interaction of the melt pool with layers of powder underneath the currently illuminated surface are presented.
Scanning Manufacturing Parameters Determining the Residual Stress State in LPBF IN718 Small Parts
(2021)
The influence of scan strategy on the residual stress (RS) state of an as-built IN718 alloy produced by means of laser powder bed fusion (LPBF) is investigated. Two scan vector rotations (90°-alternation and 67°-rotation), each produced following two different scan vector lengths (long and short), are used to manufacture four rectangular prisms. Neutron diffraction (ND) and laboratory X-ray diffraction (XRD) techniques are used to map the bulk and surface RS state, respectively. The distortion induced upon removal from the baseplate is measured via profilometry. XRD measurements show that the two long scan vector strategies lead to higher RS when compared with the equivalent short scan vector strategies. Also, the 67°-rotation strategies generate lower RS than their 90°-alternation counterparts. Due to the lack of reliable stress-free d0 references, the ND results are analyzed using von Mises stress. In general, ND results show significant RS spatial non-uniformity. A comparison between ND and distortion results indicates that the RS component parallel to the building direction (Z-axis) has a predominant role in the Z-displacement. The use of a stress balance scheme allows to discuss the d0 variability along the length of the specimens, as well as examine the absolute RS state.
The determination of residual stresses becomes more complicated with increasing complexity of the structures investigated. Additive manufacturing techniques generally allow the production of ‘lattice structures’ without any additional manufacturing step. These lattice structures consist of thin struts and are thus susceptible to internal stress-induced distortion and even cracks. In most cases, internal stresses remain locked in the structures as residual stress.
The determination of the residual stress in lattice structures through nondestructive neutron diffraction is described in this work. It is shown how two difficulties can be overcome: (a) the correct alignment of the lattice structures within the neutron beam and (b) the correct determination of the residual stress field in a representative part of the structure. The magnitude and the direction of residual stress are discussed. The residual stress in the strut was found to be uniaxial and to follow the orientation of the strut, while the residual stress in the knots was more hydrostatic. Additionally, it is shown that strain measurements in at least seven independent directions are necessary for the estimation of the principal stress directions. The measurement directions should be chosen according to the sample geometry and an informed choice on the possible strain field. If the most prominent direction is not measured, the error in the calculated stress magnitude increases considerably.
Micromechanical response of multi-phase Al-alloy matrix composites under uniaxial compression
(2020)
Aluminum alloys are extensively used in the automotive industry. Particularly, squeeze casting production of Al-Si alloys is employed in the conception of metal matrix composites (MMC) for combustion engines. Such materials are of a high interest since they allow combining improved mechanical properties and reduced weight and hence improve efficiency. Being a multiphase material, most MMCs show complex micromechanical behavior under different load conditions. In this work we investigated the micromechanical behavior of two MMCs, both consisting of a near-eutectic cast AlSi12CuMgNi alloy, one reinforced with 15%vol. Al2O3 short fibers and the other with 7%vol. Al2O3 short fibers + 15%vol. SiC particles. Both MMCs have complex 3D microstructure consisting of four and five phases: Al-alloy matrix, eutectic Si, intermetallics, Al2O3 fibers and SiC particles.
The in-situ neutron diffraction compression experiments were carried out on the Stress-Spec beamline and disclosed the evolution of internal phase-specific stresses in both composites. In combination with the damage mechanism revealed by synchrotron X-ray computed tomography (SXCT) on plastically pre-strained samples, this allowed understanding the role of every composite’s phase in the stress partitioning mechanism. Finally, based on the Maxwell scheme, a micromechanical model was utilized. The model perfectly rationalizes the experimental data and predicts the evolution of principal stresses in each phase.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses.
Wire Arc Additive Manufacturing (WAAM) features high deposition rates and, thus, allows production of large components that are relevant for aerospace applications. However, a lot of aerospace parts are currently produced by forging or machining alone to ensure fast production and to obtain good mechanical properties; the use of these conventional process routes causes high tooling and material costs. A hybrid approach (a combination of forging and WAAM) allows making production more efficient. In this fashion, further structural or functional features can be built in any direction without using additional tools for every part. By using a combination of forging basic geometries with one tool set and adding the functional features by means of WAAM, the tool costs and material waste can be reduced compared to either completely forged or machined parts. One of the factors influencing the structural integrity of additively manufactured parts are (high) residual stresses, generated during the build process. In this study, the triaxial residual stress profiles in a hybrid WAAM part are reported, as determined by neutron diffraction. The analysis is complemented by microstructural investigations, showing a gradient of microstructure (shape and size of grains) along the part height. The highest residual stresses were found in the transition Zone (between WAAM and forged part). The total stress range showed to be lower than expected for WAAM components. This could be explained by the thermal history of the component.
We show that the equation proposed by Takeuchi and Argon to explain the creep behavior of Al–Mg solid solution can be used to describe also the creep behavior of pure aluminum. In this frame, it is possible to avoid the use of the classic pre-exponential fitting parameter in the power law equation to predict the minimum creep strain rate. The effect of the fractal arrangement of dislocations, developed at the mesoscale, must be considered to fully explain the experimental data. These ideas allow improving the recently introduced SSTC model, fully describing the primary and secondary creep regimes of aluminum alloys without the need for fitting. Creep data from commercially pure Al99.8% and Al–Mg alloys tested at different temperatures and stresses are used to validate the proposed ideas.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured (AM) parts and important factors for residual stress formation which have implications on structural integrity. This study examined the influence of heat input on the distribution of residual stresses in two prisms produced by laser powder bed fusion (L-PBF) of austenitic stainless steel 316L.
The layers of the prisms were exposed using two distinct helix scanning strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. Residual stresses were characterised at one plane perpendicular to the building direction at half of its build height using neutron diffraction. In addition, the defect distribution was analysed via micro X-ray computed tomography (µCT) in a twin specimen.
Both scanning strategies reveal residual stress distributions typical for AM: compressive stresses in the bulk and tensile stresses at the surface. However, temperature gradients and maximum stress levels differ due to the different heat input. Regarding the X-ray µCT results, they show an accumulation of defects at the corners where the laser direction turned through 90°.
The results demonstrate that neutron diffraction and X-ray µCT can be successfully used as non-destructive methods to analyse through-thickness residual stress and defect distribution in AM parts, and in the presented case, illustrate the influence of scanning strategies. This approach contributes to deeper assessment of structural integrity of AM materials and components.
Aluminum alloys are extensively used in the automotive industry. Particularly, squeeze casting production of Al-Si alloys is employed in the conception of metal matrix composites (MMC) for combustion engines. Such materials are of a high interest since they allow combining improved mechanical properties and reduced weight and hence improve efficiency. We investigate two types of MMCs, which can be potentially used for production of combustion engine pistons: 1) a near-eutectic cast AlSi12CuMgNi alloy reinforced with 15%vol. Al2O3 random planar oriented short fibers and 2) the same alloy reinforced with 7%vol. Al2O3 random planar oriented short fibers + 15%vol. SiC particles.
Complex 3D microstructure of the samples in as-cast condition, consisting of four and five phases (Al matrix, eutectic Si, intermetallics, Al2O3 fibers and SiC particles) was investigated by synchrotron computed tomography (CT). Advanced methods based on machine learning were applied for segmentation of all phases. This allowed extracting quantitative information such as volume fraction, shape and interconnectivity of every phase.
In-situ compression tests during neutron diffraction experiments were used to track the load transfer among phases, while CT on pre-strained samples to monitor and quantify damage. A micromechanical model was developed to simulate the evolution of the internal stress of each phase during uniaxial compression. We showed that in composites with the Al2O3 fiber mat plane perpendicular to the load axis the Al-alloy matrix presents a large hydrostatic stress component, i.e. undergoes compression also in the direction transverse to the external load. This feature holds with and without the addition of SiC ceramic reinforcement and is absent in the case when the Al2O3 fiber mat plane is parallel to the load axis. We show, that the intermetallics play a decisive role at very high loads, when all other reinforcement phases suffer (extensive) damage. The addition of SiC particles does alleviate the load on the Al2O3 fibers, on the eutectic Si, and on the intermetallic phases in both cases of parallel and orthogonal (to the load axis) Al2O3 fiber orientation. Apart from the beneficial addition of ceramic reinforcement, the presence of intermetallic phase and eutectic silicon, forming an interconnected network even at high loads, when single particles break, confers peculiar properties to these multi-phase composites.
Combination of high-resolution time-of-flight (TOF) Neutron diffraction, scanning electron microscopy (SEM) and small angle neutron scattering (SANS) have been used to study hardening precipitation in 2014Al and 2124Al alloys. Neutron diffraction experiment revealed presence of tetragonal CuAl2 Phase (θ/θ´ precipitates) in 2014Al alloy and orthorhombic CuMgAl2 Phase (S-precipitates) in 2124Al alloy. SEM image analysis revealed that the θ/θ´-precipitates have platelet morphology while the S-precipitates exhibit rod-like shapes. From SANS data analysis the characteristic dimensions of precipitate particles and their polydisperse distributions were estimated.
The residual stress distribution in IN718 elongated prisms produced by Selective Laser Melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction.
Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed (RE) from the base plate were characterized.
While surface stress fields seem constant for AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements.
Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e. with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the baseplate.
Near eutectic cast Al–Si alloys are the basis of all Al-alloys used for pistons due to their high fluidity and relatively high strength-to-weight ratio. Their microstructure is characterized by the presence of eutectic-Si embedded in an age hardenable -Al matrix. During solution heat treatment, the eutectic Si network undergoes a process of gradual disintegration, reducing the load bearing capability and the strength of the alloy. The improvement of the high temperature strength is achieved by addition of transition elements such as Cu, Mg or Ni through the formation of stiff intermetallic (IM) phases.
In present study a near-eutectic AlSi12CuMgNi alloy produced by squeeze casting was investigated in as-cast condition. The in-situ neutron diffraction compression tests revealed the stress partitioning mechanism between phases of the alloy. Large stresses were found in IMs, showing their significant role as a reinforcement elements. After the failure of IM and Si phases the load is transferred back to the Al matrix, inducing further plastic deformation in it, yet without creating microcracking. The neutron diffraction experiments were coupled with assessment of internal damage after ex-situ compression tests by synchrotron radiation computed tomography. The quantitative analysis of CT data revealed that local tensile stresses in IM and Si phases induce microcracks with preferential orientation, parallel to the axis of applied load.
Altogether, present experimental data was used as an input for the developed micromechanical model based on Maxwell homogenization scheme. In contrast to the neutron diffraction experiment, which can only determine stress differences between the axial and radial sample directions, the model allows predicting the principal stresses in each phase of the alloy.
Load partitioning between phases in a cast AlSi12CuMgNi alloy was investigated by in-situ compression test during neutron diffraction experiments. Computed tomography (CT) was used to determine volume fractions of eutectic Si and intermetallic (IM) phases, and to assess internal damage after ex-situ compression tests. The CT reconstructed volumes showed the interconnectivity of IM phases, which build a 3D network together with eutectic Si. Large stresses were found in IMs, revealing their significant role as a reinforcement for the alloy. An existing micromechanical model based on Maxwell scheme was extended to the present case, assuming the alloy as a three-phase composite (Al matrix, eutectic Si, IM phases). The model agrees well with the experimental data.
Moreover, it allows predicting the principal stresses in each phase, while experiments can only determine stress differences between the axial and radial sample directions. Finally, we showed that the addition of alloying elements not only allowed developing a 3D interconnected network, but also improved the strength of the Al matrix, and the ability of the alloy constituents to bear mechanical load.
This study examines the relationship between the magnetic mesostructure with the microstructure of low carbon steel tungsten inert gas welds. Optical microscopy revealed variation in the microstructure of the parent material, in the heat affected and fusion zones, correlating with distinctive changes in the local magnetic stray fields measured with high spatial resolution giant magneto resistance sensors. In the vicinity of the heat affected zone high residual stresses were found using neutron diffraction.
Notably, the gradients of von Mises stress and triaxial magnetic stray field modulus follow the same tendency transverse to the weld. In contrast, micro-X-ray fluorescence characterization indicated that local changes in element composition had no independent effect on magnetic stray fields.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in the bulk of IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2017)
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts in comparison to the traditional subtractive manufacturing strategies. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
The residual stress distribution of IN718 parts produced by Selective Laser Melting (SLM) technique was studied by means of neutron diffraction.
Two deposition hatching lengths were considered in the fabrication. Both lateral (building direction) and top (finishing) near-surface regions were characterized. Measurements on samples in as-built condition and after release from the plate proved the presence of stress gradients both in-plane and along the building direction.
As-built samples presented in top region a longitudinal stress relief for large hatching, whereas small one showed tensile stresses around (200MPa) in the middle, evolving towards compression at the tip of the sample (down to -300MPa). Towards the lateral edge, longitudinal stresses shifted also to compression. The transverse stresses for large hatching were relief in the middle, whereas for small hatching shifted to compression at the edge. As for the normal component, this was more homogenous: stress-relief was proved for large hatching and, in contrast, was in compression (-200MPa) for small hatching. In the building direction (lateral region from base plate to top) of the sample with large hatching all stress components showed tensile values near the base plate (particularly high for normal component around 300MPa), decreasing towards compression to the top, where they were almost released.
After release, in the top region, the longitudinal stress component for small hatching showed high compressive stresses in the central part (down to -250MPa). In contrast, for large hatching a stress relief was found. In the transversal direction, this behavior was inverted: a small hatching released stresses more effectively, while a large hatching presented high tensile stresses (around 200MPa). As for the normal component (i.e., building direction), the sample with small hatching was found in compression, while that with large hatching was stress-released or slightly in tension (around 100MPa). In the lateral surface region, all components showed similar behavior: a small hatching promoted high compressive stresses along the building direction, whereas a large hatching showed small tensile values at the bottom, which balance towards the top region. There is an overall shift of stresses in 3 directions towards tension when compared with as-built condition for top region. In contrast, the lateral region is stress-relief or shifted towards compression after cutting from baseplate.
In conclusion, hatching length parameter strongly influenced the 3D distribution of residual stress in SLM produced parts.
In the present study, samples fabricated by varying the deposition hatch length during selective laser melting of nickel based superalloy Inconel 718 were investigated. Microstructure and texture of these samples was characterized using scanning electron microscopy, combined with electron back-scattered diffraction, and residual stress assessment, using neutron diffraction method. Textured columnar grains oriented along the sample building direction were observed in the shorter hatch length processed sample. A ten-fold increase in the hatch length reduced the texture intensity by a factor of two attributed to the formation of finer grains in the longer hatch length sample. Larger gradients of transverse residual stress in the longer hatch length sample were also observed. Along the build direction, compressive stresses in the shorter hatch length and negligible stresses for the longer hatch length specimen were observed. Changes to the temperature gradient (G) in response to the hatch length variation, influenced the G to growth rate (R) ratio and the product G × R, in agreement with the microstructures and textures formed. For the residual stress development, geometry of the part also played an important role. In summary, tailored isotropy could be induced in Inconel 718 by a careful selection of parameters during selective laser melting.
The evolution of the microstructure of multiphase metal matrix composite AlSi12CuNiMg with 7% vol. Al2O3 + 15% vol. SiC (short fibers and whiskers, respectively) was studied by synchrotron computed tomography. It comprehended as cast and after heat treatment conditions, as well as damage evaluation after ex-situ compression tests at room temperature. The volume fraction of different phases, their distribution, their orientation, and damage events are studied.
The influence on mechanical properties of the orientation of the planar random short fibres Al2O3 towards loading was investigated. Phase-specific load partition analysis for samples with fiber plane parallel and orthogonal to load, respectively, was performed by means of neutron diffraction (ND) during in-situ compression tests at room temperature. ND results proved that damage occurrence in the fillers strongly depends on the preferential orientation of those, playing a crucial role in the failure of the sample. The computed tomography observations confirm the damage observations from curves of load partition analysis of each phase.
The combination of tomographic, microstructural data with other experimental techniques and with modeling is paramount, if we want to extract the maximum amount of information on material and component properties. In particular, quantitative image analysis, statistical approaches, direct discretization of tomographic reconstructions represent concrete possibilities to extend the power of the tomographic 3D representation to insights into the material and component performance. This logic thread equally holds for industrial and academic research, and valorizes expensive experiments such as those carried out at synchrotron sources, which cannot be daily repeated.
I will show a few examples of possible use of X-ray tomographic data for quantitative assessment of damage evolution and microstructural properties, as well as for non-destructive testing. Examples of micro-structured inhomogeneous materials will be given, such as Composites, Ceramics, Concrete, and Additively manufactured parts. I will also show how X-ray refraction computed tomography (CT) can be highly complementary to classic absorption CT, being sensitive to internal interfaces.
Additionally, I will present a new technique in our portfolio, Neutron Diffraction, which is extremely well suited to the study of internal stresses, both residual and under external load.
Stress-induced damage evolution in cast AlSi12CuMgNi alloy with one- and two ceramic reinforcements
(2017)
Two composites, consisting of an as-cast AlSi12CuMgNi alloy reinforced with 15%vol. Al2O3 short fibres and with 7%vol. Al2O3 short fibres + 15%vol. SiC particles were studied. Synchrotron computed tomography disclosed distribution, orientation, and volume fraction of the different phases. In-situ compression tests during neutron diffraction in direction parallel to the fibres plane revealed the load partition between phases. Internal damage (fragmentation) of the Si phase and Al2O3 fibres was directly observed in CT reconstructions. Significant debonding between Al-matrix and SiC particles was also found. Finally, based on the Maxwell scheme, a micro-mechanical model was utilized for the new composite with two ceramic reinforcements; it rationalizes the experimental data, and predicts the evolution of all internal stress components in each phase.
Evaluation of high spatial resolution imaging of magnetic stray fields for early damage detection
(2017)
The paper discusses the evaluation of elastic and plastic strain states in two low-carbon steels of the same steel group with high spatial resolution GMR (giant magneto resistance) sensors. The residual stress distributions of tungsten inert gas welded plates were determined by means of neutron diffraction as a reference. The normal component of local residual magnetic stray fields arise in the vicinity of the positions of maximum stress. The experiments performed on flat tensile specimen indicate that the boundaries of plastic deformations are a source of stray fields. The spatial variations of magnetic stray fields for both the weld and the tensile samples are in the order of the earths magnetic field.
The residual stress distribution of tungsten inert gas welded S235JRC+C plates was determined by means of neutron diffraction(ND). Large longitudinal residual stresses with maxima around 600 MPa were found. With these results as reference, the evaluation of residual stress with high spatial resolution GMR (giant magneto resistance) sensors was discussed. The experiments performed indicate a correlation between changes in residual stresses (ND) and the normal component of local residual magnetic stray fields (GMR). Spatial variations in the magnetic field strength perpendicular to the welds are in the order of the magnetic field of the earth.
Characterization of residual stress state by neutron diffraction and residual magnetic field mapping
(2016)
Based on the residual stress characterization of tungsten inert gas welded S235JRC+C plates by means of neutron diffraction, the evaluation of residual stress with high spatial resolution GMR (giant magneto resistance) sensors is discussed. The experiments performed indicate a correlation of residual stress changes and local residual magnetic stray fields.
Neutron diffraction: the forgotten non-destructive technique for residual stress analysis … and more
(2016)
3-D Stress Analysis
(Bulk) Stress mapping
Thick (and thin) films & Interfaces
Bulk high temperature
Real time
In-situ testing: Large sample environment (Stress rigs, Furnaces, …)
Neutrons and Synchrotron Radiation allow all this because they are FASTER , DEEPER and MORE PRECISE than lab equipment
(Flux)(Energy)(Parallel Beam)
Characterization of multiphase metal matrix composites by means of CT and neutron diffraction
(2016)
The present study examines the relationship between the microstructure of multiphase MMC and their damage mechanisms. The matrix AlSi12CuMgNi was combined with 15% vol. Al2O3 (short fibres) and with 7% vol. Al2O3 + 15% vol. SiC (short fibres and whiskers, respectively). The experimental approach encompasses 3D microstructure characterization by means of computed tomography of samples (a) as-received, (b) after heat treatment, and (c) after compression tests at room temperature. The volume fraction of different phases, their distribution, their orientation and the presence of defects and damage are studied.
Influence of addition of SiC whiskers on mechanical properties of composite was investigated. Phase-specific load partition analysis for samples with fibre plane parallel to load was perform by using neutron diffraction measurements during in-situ compression. It shows damage in the Si phase, while Al2O3 short fibres carry load without damage until failure. The computed tomography observations confirm the load partition analysis.
Magnetic testing methods are frequently applied in non-destructive evaluation of ferromagnetic materials. In the past decade, metal magnetic memory (MMM) technique according to ISO 24497 is gaining considerable interest in the magnetic NDT community. In contrast to traditional Magnetic Flux Leakage (MFL) testing, the inspection objects are not intentionally magnetized by an external magnetic field. Due to physical coupling between mechanical stress and magnetization in ferromagnetic materials, it is assumed that the distribution of the “natural” MFL (self-magnetic-leakage field, SMLF) indicate zones of different remanent magnetization, which in turn, correspond to the internal stress of specimen or in the most general sense to material degradation.
Usually, MMM measurements are performed by relatively bulky magnetic inspection sensors providing a spatial resolution in millimetre range. High precision GMR (Giant Magneto Resistance) measurements in the micrometer regime along with image based representation and evaluation can provide a higher degree of information. We present a concise summary of a broader research project aimed at studying the correlation of magnetic structure and microstructure of steels. Particularly, we compare residual stress measurements in S235JR steel by means of neutron diffraction with high resolution magnetic field measurements. In addition, we discuss the influence of deformation-induced magnetization in plastically deformed specimens with and without notches due to various quasi-static and cyclic load levels. Furthermore, comparative measurements with common non-destructive testing methods are presented.
Despite of a quantitative evaluation of material degradation, the in the field inspection by MMM remains problematic due to substantial influences on such as external magnetization, anisotropy of internal magnetization and material degradation, as well as geometry and surface effects of the inspection objects on SMFL signals. The prospects and restrictions of the MMM technique are discussed in this contribution.
In the present study, internal damage to an AlSi12CuMgNi alloy reinforced with planar random Al2O3 short fibres was investigated after compression testing. Due to the alloy composition, this composite contains a second reinforcement phase in the form of eutectic Si, which builds interpenetrated networks in the volume and increases the creep resistance and load-bearing capacity of the material. Materials with their fibre plane parallel and transversal to the load direction were characterized in order to investigate the dependence of load partition and damage on fibre plane orientation. In-situ compression testing during neutron diffraction measurements showed that internal damage is strongly influenced by the load partition between matrix and reinforcement. Moreover, micro-computed tomography was performed in the same material after ex-situ compression for damage analysis. In the case of a fibre plane perpendicular to the applied load, breakage and interconnected cracks appeared in a significantly higher volume fraction than with a fibre plane parallel to load.
The program PowderCell is a crystallographic tool for visualization of Crystal structures. However, it offers also non-conventional features like the fulautomatical generation of subgroups or the consideration of non-standard settings of space-group types. On the one Hand the program is very useful for non-crystallographers who like to get an impression of the atomic arrangement within the unit cell. But also for crystallographers it is recommendable because it contains a lot of additional information which can be extracted and used like data given in the International Tables for Crystallography, Vol. A. However, the most important advantage of the program is the simultaneous calculation of the X-ray or neutron diffraction powder patterns for a mixture of up to 10 crystalline phases. Between more than 7 different characteristic radiations can be chosen and their influence on the resulting powder pattern can be studied. Furthermore, experimental diffractograms can be analysed using a refinement procedure. The implemented LeBail-algorithm allows the investigation of unknown phases. In combination with the refinement algorithm an interface to Shelx offers the possibility for a step by step ab initio structure analysis. Certainly, the user-friendly shell is one reason that especially users who are not so familar with space-group symmetry, crystal structure data or diffractometry use this powerful tool for the solution of scientific or analytical problems as well as in teaching.
Observation of martensite formation by combined use of synchrotron diffraction and dilatometry
(2016)
Welding residual stress engineering by means of an adjusted martensite phase transformation would be highly attractive as detrimental tensile residual stresses may be prevented already during welding without time and cost intensive post processing. The present study shows a synchrotron diffraction analysis of a martensitic steel subjected to thermo-mechanical load cycles. Experiments were conducted regarding the microstructural strain response during the austenite to martensite transformation. The strains are a function of the temperature and the specific loads applied during cooling. The relation between the transformation plasticity of the material, the amount of martensite formed and the arising strains can thus be assessed. The lattice plane specific strains were compared to experimental findings from (macro) dilatation tests. It is shown that the microscopic material behavior differs remarkably from the one observed on the macroscopic scale, what leads to characteristic residual stresses in the material.