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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.
Laser powder bed fusion (LPBF) of Ni-based superalloys shows great potential for high temperature applications, for example, as a burner repair application for gas turbines where the thin-walled structure is important. It motivates this work to investigate the evolution of microstructure and the anisotropic mechanical behavior when plate-like specimens are built with a thickness from 4 mm down to 1 mm. By performing texture analysis using neutron diffraction, a clear transition in fiber texture from <011> to <001> is indicated when the specimen becomes thinner. The residual stress shows no thickness dependence, and at the subsurface the residual stress reaches the same level as the yield strength. Due to the rough as-built surface, a roughness compensation method for mechanical properties of thin-walled structures is outlined and demonstrated. Tensile tests from room temperature up to 700 ◦C have been carried out. Anisotropic mechanical behavior is found at all temperatures, which is strongly related to the anisotropic texture evolution. Stronger texture evolution and grain rotations are discovered when the tensile loading is applied along the building direction. The mechanical behavior has been compared to a wrought material, where the high dislocation density and the subgrain structure of the LPBF material result in a higher yield strength. Combining the statistical texture analysis by neutron diffraction with mechanical testing, EBSD grain orientation mapping and the investigation of dislocation structures using transmission electron microscopy, this work illustrates the significance of texture for the thin-wall effect and anisotropic mechanical behavior of LPBF materials.
The influence of the support structure on residual stress and distortion in SLM Inconel 718 parts
(2018)
The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.
Although layer-based additive manufacturing methods such as laser powder bed fusion (PBF-LB) offer an immense geometrical freedom in design, they are typically subject to a build-up of internal stress (i.e. thermal stress) during manufacturing. As a consequence, significant residual stress (RS) is retained in the final part as a footprint of these internal stresses. Furthermore, localized melting and solidification inherently induce columnar-type grain growth accompanied by crystallographic texture. Although diffraction-based methods are commonly used to determine the RS distribution in PBF-LB parts, such features pose metrological challenges in their application. In theory, preferred grain orientation invalidates the hypothesis of isotropic material behavior underlying the common methods to determine RS. In this work, more refined methods are employed to determine RS in PBF-LB/M/IN718 prisms, based on crystallographic texture data. In fact, the employment of direction-dependent elastic constants (i.e. stress factors) for the calculation of RS results in insignificant differences from conventional approaches based on the hypothesis of isotropic mechanical properties. It can be concluded that this result is directly linked to the fact that the {311} lattice planes typically used for RS analysis in nickel-based alloys have high multiplicity and less strong texture intensities compared with other lattice planes. It is also found that the length of the laser scan vectors determines the surface RS distribution in prisms prior to their removal from the baseplate. On removal from the baseplate the surface RS considerably relaxes and/or redistributes; a combination of the geometry and the scanning strategy dictates the sub-surface RS distribution.
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.
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.
Residual stresses in Laser Beam Melting (LBM) – Critical Review and outlook of activities at BAM
(2019)
Additive manufacturing (AM) technologies have experienced an exceedingly rapid growth, which is coupled with the knowledge about the resulting material properties and performance. In particular, residual stress (RS) was soon recognized as an important issue in AM parts, such that parts are usually subjected to a post build-heat-treated. Significant effort has been spent on simulations of RS in AM, especially using finite element methods. As a consequence, the experimental determination of RS has thereby become increasingly important as a validation tool for simulations, as well as a method for assessing the influence of process parameters. In particular, diffraction methods, which are fundamentally non-destructive, offer enormous possibilities to gain knowledge on the residual stress state in real components, since synchrotron radiation and neutrons can penetrate even heavy metals up to several millimeters or centimeters, respectively. Indeed, significant progress has been achieved, in the understanding of the origins of the RS fields as a function of process parameters, as well as their stability under thermal and/or mechanical exposure.
In this paper, a few success stories will be outlined. It will be shown how the determination of RS in metallic parts (with the focus on those produced by laser powder bed fusion) has even revealed that process parameters that were previously considered unimportant (e.g. the position and orientation on the base plate) play a major role in the onset of residual stress accumulation.
However, while RS characterization is starting to be considered in the component design, deposition strategy (e.g. build plate temperature), and even in the definition of the relevant metric to assess the quality of a part, much is still to be investigates about the hypotheses underlying its experimental determination. Therefore, some aspects to be aware of, or even those which to date are unclear, will also be discussed. These include the determination of the stress-free reference and of the principal axes of stress. All of these aspects will lead towards a comprehensive understanding of the process-structure-performance relationships in AM materials and parts.
Residual stresses in Laser Beam Melting (LBM) – Critical Review and outlook of activities at BAM
(2019)
Additive manufacturing (AM) technologies have experienced an exceedingly rapid growth, which is coupled with the knowledge about the resulting material properties and performance. In particular, residual stress (RS) was soon recognized as an important issue in AM parts, such that parts are usually subjected to a post build-heat-treated. Significant effort has been spent on simulations of RS in AM, especially using finite element methods. As a consequence, the experimental determination of RS has thereby become increasingly important as a validation tool for simulations, as well as a method for assessing the influence of process parameters. In particular, diffraction methods, which are fundamentally non-destructive, offer enormous possibilities to gain knowledge on the residual stress state in real components, since synchrotron radiation and neutrons can penetrate even heavy metals up to several millimeters or centimeters, respectively. Indeed, significant progress has been achieved, in the understanding of the origins of the RS fields as a function of process parameters, as well as their stability under thermal and/or mechanical exposure.
In this paper, a few success stories will be outlined. It will be shown how the determination of RS in metallic parts (with the focus on those produced by laser powder bed fusion) has even revealed that process parameters that were previously considered unimportant (e.g. the position and orientation on the base plate) play a major role in the onset of residual stress accumulation.
However, while RS characterization is starting to be considered in the component design, deposition strategy (e.g. build plate temperature), and even in the definition of the relevant metric to assess the quality of a part, much is still to be investigates about the hypotheses underlying its experimental determination. Therefore, some aspects to be aware of, or even those which to date are unclear, will also be discussed. These include the determination of the stress-free reference and of the principal axes of stress. All of these aspects will lead towards a comprehensive understanding of the process-structure-performance relationships in AM materials and parts.
Additive manufacturing (AM) technologies are experiencing an exceedingly rapid growth, driven by their potential through layer wise deposition for transformational improvements of engineering design, leading to efficiency and performance improvements. Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) method which permits the fabrication of complex structures that cannot otherwise be produced via conventional subtractive manufacturing methods. Nevertheless, the rapid cooling rates associated with this process results in the formation of significant and complex residual stress (RS) fields. A large body of both experimental and simulation research has been dedicated in recent years to the control and mitigation of RS in AM. In order to validate simulations with the end goal of being able to model the residual stress state in AM components and to devise strategies for their reduction during manufacturing, experimental methods need to be able to accurately determine 3D residual stresses fields in complex geometries. Several destructive and non-destructive methods can be used to analyze the RS state, the choice of which depends on the geometry and the information required. Diffraction-based methods using penetrating neutron and synchrotron X-rays at large scale facilities offer the possibility to non-destructively spatially resolve both surface and bulk residual stresses in complex components and track their changes following applied thermal or mechanical loads. This presentation will overview the success stories of using large scale facilities by the BAM for the characterization of residual stresses in additively manufactured metallic alloys. In particular, the study of the influence of process parameters on the residual stress state and the relaxation of these stresses through heat treatment will be presented. However there remains challenges to overcome particularly of the hypotheses underlying the experimental determination of residual stresses, which will be discussed.
Metal Additive manufacturing (AM) technologies such as Laser Powder Bed Fusion (LPBF) enable the fabrication of complex structures, giving rise to potential improvements in component and manufacturing efficiency. However, the processes are typically characterized by the generation of high magnitude residual stress (RS) which can have detrimental consequences for subsequent applications. Therefore, the characterization of these RS fields and the understanding of their formation and mitigation through optimized processing is crucial for the wider uptake of the technology. Due to the potential complex nature and high value of components manufactured by LPBF, it is important to have suitable characterisation methods which can determine the spatial variations of RS in a non-destructive manner. Neutron diffraction is considered to be the best suited for these requirements. However, the microstructures developed in the complex thermal cycles experience in the production can pose challenges to the ND method for RS analysis. The BAM has conducted significant research over the past years to overcome these obstacles, enabling higher confidence in the RS determined in LPBF materials by neutron diffraction. This contribution will overview some of these advancements made recently at European neutron sources including on Stress-Spec at FRM2/MLZ.
Residual stresses Analysis in Additively Manufactured alloys using neutron diffraction (L-PBF)
(2023)
An overview of recent progress at BAM of residual stress analysis in additively manufactured, in particular Laser Powder Bed Fusion of metallics materials, using neutron diffraction will be presented. This will cover important topics of the stress-free reference, the diffraction elastic moduli and principal stress determination.
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.
Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
Submerged arc welded (SAW) components of creep-resistant low-alloyed Cr-Mo-V steels are used for thick-walled heavy petrochemical reactors (wall-thickness up to 475 mm) as well as employed in construction of modern high-efficient fossil fired power plants. These large components are accompanied by significant restraints during welding fabrication, especially at positions of different thicknesses like welding of nozzles. As a result, residual stresses occur, playing a dominant role concerning so-called stress relief cracking (SRC) typically during post weld heat treatment (PWHT). Besides specific metallurgical factors (like secondary hardening due to reprecipitation), high tensile residual stresses are a considerable influence factor on SRC.
For the assessment of SRC susceptibility of certain materials mostly mechanical tests are applied which are isolated from the welding process. Conclusions regarding the influence of mechanical factors are rare so far. The present research follows an approach to reproduce loads, which occur during welding of real thick-walled components scaled to laboratory conditions by using Tests designed on different measures. A large-scale slit specimen giving a high restraint in 3 dimensions by high stiffness was compared to a medium-scale multi-pass welding U-Profile specimen showing a high degree of restraint in longitudinal direction and a small-scale TIG-remelted specimen. The small-scale specimens were additionally subjected to mechanical bending to induce loads that are found during fabrication on the real-scale in heavy components. Results show for all three cases comparable high tensile residual stresses up to yield strength with high gradients in the weld metal and the heat affected zone. Those high tensile stresses can be significant for cracking during further PWHT.
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.
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.
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.
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.