Mitigation Of Residual Stress Research Articles

Residual Stress Mitigation in Perovskite Solar Cells via Butterfly-Inspired Hierarchical PbI2 Scaffold.

Residual stress in metal halide perovskite films intimately affects the photovoltaic figure of merit and longevity of perovskite solar cells. A delicate management of the crystallization kinetics is critical to the preparation of high-quality perovskite films. Only very limited methods, however, are available to regulate the residual stress of a perovskite film in a controllable manner, particularly for a perovskite film fabricated by a two-step method. Here, we demonstrate the construction of a hierarchical PbI2 scaffold inspired by Archaeoprepona demophon butterfly by combining an interlayer guided growth of porous structure and nanoimprinting. The hierarchically structured PbI2 that emulates the physical structure of the butterfly wing scale permits unimpeded permeation of organic amine salts and sufficient space for volume expansion during the crystallization process, accompanied by preferential perovskite growth of a defectless (001) crystal plane. The optimized perovskite film outperforms the control with reduced residual stress and defect density. Consequently, perovskite solar cells with a respectable power conversion efficiency reaching 23.4% (certified 23%) and an impressive open-circuit voltage of 1.184 V can be achieved. The target device can maintain 80% of initial efficiency after maximum power point tracking under illumination for 700 h. This work expands the range of engineering toward PbI2 by exploring a simultaneously tailored morphology and crystallinity and highlights the significance of a hierarchical PbI2 scaffold as an alternative choice to mitigate residual stress in a two-step processed perovskite active layer and boost the longevity of perovskite solar cells.

Elimination of Low-Angle Grain Boundary Networks in FeCrAl Alloys with the Electron Wind Force at a Low Temperature

Low-angle grain boundaries (LAGBs) accommodate residual stress through the rearrangement and accumulation of dislocations during cold rolling. This study presents an electron wind force-based annealing approach to recover cold-rolling induced residual stress in FeCrAl alloy below 100 °C in 1 min. This is significantly lower than conventional thermal annealing, which typically requires temperatures around 750 °C for about 1.5 h. A key feature of our approach is the athermal electron wind force effect, which promotes dislocation movement and stress relief at significantly lower temperatures. The electron backscattered diffraction (EBSD) analysis reveals that the concentration of low-angle grain boundaries (LAGBs) is reduced from 82.4% in the cold-rolled state to a mere 47.5% following electropulsing. This level of defect recovery even surpasses the pristine material’s initial state, which exhibited 54.8% LAGBs. This reduction in LAGB concentration was complemented by kernel average misorientation (KAM) maps and X-ray diffraction (XRD) Full Width at Half Maximum (FWHM) measurements, which further validated the microstructural enhancements. Nanoindentation tests revealed a slight increase in hardness despite the reduction in dislocation density, suggesting a balance between grain boundary refinement and dislocation dynamics. This proposed low-temperature technique, driven by athermal electron wind forces, presents a promising avenue for residual stress mitigation while minimizing undesirable thermal effects, paving the way for advancements in various material processing applications.

Mitigation of Residual Stress and Distortion of AlSi10Mg Parts Produced by Laser Powder Bed Fusion through a Proper Selection of Support Geometry

Laser beam powder bed fusion (LB-PBF) is an edge additive manufacturing technology that allows complex near-net shape components to be produced. The freedom of design of the LB-PBF process makes it possible to produce optimized geometries, driving the application of this process in sectors in which high performance is fundamental such as aerospace and automotive. However, the building process inherently generates residual stresses in the part and the use of support structures become essential to anchor the part to the building platform and avoid problems in the downfacing surfaces that may warp or collapse during the production process. In this paper, different support densities obtained using different geometries of support structures are investigated to evaluate how they affect the residual stress distribution in the supported part. Two families of support structures were considered, linear-type and volumetric-type, including also a hybrid support structure that combined a massive volume topped by a linear support structure. Results highlighted that the combined choice of support density and geometry influences the magnitude and the distribution of sub-superficial residual stresses near the support-part interface and that appropriate design is essential to prevent excessive distortion or failure.

Efficient residual stress mitigation in additively manufactured 18Ni300 maraging steel

This study explored an effective post-heat treatment (PHT) strategy for achieving optimal residual stress relief in 18Ni300 maraging steel. The primary focus was to investigate and enhance the mechanisms underlying residual stress relief in distinct PHT processes, including aging and annealing-aging. The investigation considered various parameters, including PHT conditions, component size, and initial residual stress field. Components with cubic geometry were fabricated through laser powder bed fusion (LPBF) and subjected to both aging and annealing-aging processes. The residual stresses were extracted from these specimen variants using the contour method. The simulation involved generating the initial residual stress in the LPBF using the flash heating method and simulating the PHT process using the Arrhenius equation-based creep model. Simulations complemented and elucidated the experimental results. The validated model was employed to comprehensively investigate the intricate interrelationships among the various PHT parameters. The results indicate that the annealing-aging process leads to a substantial reduction in the residual stress compared to the aging process. Furthermore, a process map was established by considering the PHT conditions and component size. This map aids in determining the optimal PHT conditions for achieving the desired levels of stress reduction. Therefore, the present study comprehensively explored the PHT effect and its interrelated parameters, suggesting an effective PHT strategy for residual stress mitigation in LPBF, and particularly a process map that is beneficial for practical applications.

Evolution, Control, and Mitigation of Residual Stresses in Additively Manufactured Metallic Materials: A Review

Powder bed fusion (PBF) and directed energy deposition (DED) are the two major types of metal additive manufacturing (AM) processes, in which significant residual stresses could be generated in the as‐deposited materials and in turn affect part quality and performance. Therefore, how to control and mitigate residual stress has been a most pressing challenge for metal AM. Herein, the significant efforts made in literature to address this challenge for both PBF and DED processes are reviewed. The extensive survey covers the formation of residual stress in AM, influence of various AM process parameters, modeling techniques for predicting residual stresses, and post and in situ treatments for mitigating unfavorable residual stress distributions. More importantly, a critical analysis is provided to discuss the research gaps and opportunities, such that more exciting research will be stimulated to address the outstanding issues in this area and improve the quality and service life of AM‐produced components. As a result, once being better equipped with residual stress control knowledge and tools, the industry will be more confident in adopting metal AM techniques.

Residual stress and strain mitigation in direct laser deposition through adjustment buildup geometry or addition of ductile transition layer

Residual stress and strain mitigation in direct laser deposition through adjustment buildup geometry or addition of ductile transition layer

Effects of Stress-Relieving Temperature on Residual Stresses, Microstructure and Mechanical Behaviour of Inconel 625 Processed by PBF-LB/M

Inconel 625 (IN625) superalloys can be easily fabricated by the laser-based powder bed fusion (PBF-LB/M) process, allowing the production of components with a high level of design freedom. However, one of the main drawbacks of the PBF-LB/M process is the control over thermally induced stresses and their mitigation. A standard approach to prevent distortion caused by residual stress is performing a stress-relieving (SR) heat treatment before cutting the parts from the building platform. Differently from the cast or wrought alloy, in additively manufactured IN625, the standard SR at 870 °C provokes the early formation of the undesirable δ phase. Therefore, this unsuitable precipitation observed in the PBF-LB/M material drives the attention to develop a tailored SR treatment to minimise the presence of undesirable phases. This work investigates SR at lower temperatures by simultaneously considering their effects on residual stress mitigation, microstructural evolution, and mechanical properties. A multiscale approach with cantilever and X-ray technologies was used to investigate how the residual stress level is affected by SR temperature. Moreover, microstructural analyses and phase identifications were performed by SEM, XRD, EBSD, and DSC analyses. Finally, mechanical investigations through microhardness and tensile tests were performed as well. The results revealed that for the additively manufactured IN625 parts, an alternative SR treatment able to mitigate the residual stresses without a massive formation of δ phase could be performed in a temperature range between 750 and 800 °C.

Residual stress mitigation in directed energy deposition

Directed Energy Deposition (DED), a class of additive manufacturing techniques, has seen rapid growth over the last decade for potential applications in aerospace, medical devices, and energy systems. Despite notable progress in the research and development of AM, control and mitigation of residual stress during DED remains a challenge. In this work, we propose a novel approach that can be used for the mitigation of residual stresses in additively manufactured components. Specifically, we propose to mitigate the residual stress state of as-deposited components using alloy design, engineering of solid-state transformations, and the introduction of both hard and soft metallic phases. We demonstrate this strategy with a model system consisting of pure Fe and Fe–Cu. Experimental results indicate that residual stresses can be successfully manipulated by adjusting the alloy composition as a soft metallic phase can accommodate plastic deformation. Moreover, our findings suggest that the solid-state transformations experienced by the Fe and Fe-rich phases contribute to the observed differences in magnitude and location of residual stresses. This study is the first to suggest using residual stress as an engineering criterion in the design of alloys for metal additive manufacturing.

Finite element modelling for mitigation of residual stress and distortion in macro-scale powder bed fusion components

Powder bed fusion (PBF) has attracted significant attention in many applications due to its capability of fabricating complex and customized metal parts. However, the potential for high inherent residual stresses that produce distortion in additive manufacturing (AM) components prevents more widespread application of the AM technique. Efficient and accurate prediction of residual stress and distortion at component-level (macro-scale) is a complex task. Although process-level (meso-scale) thermo-mechanical simulations have resulted in accurate predictions for small-scale parts, the computational times (typically weeks) and memory requirements for application of such methods to component-level are prohibitive. The main goal of the current study therefore is to present an efficient and accurate finite element (FE) simulation method with detailed validation for PBF manufacture of a complex 3D Inconel 625 benchmark bridge component (macro-scale). The simulation results are successfully validated against the published benchmark experimental measurements from neutron diffraction, X-ray diffraction (XRD), contour method and coordinate measurement machine (CMM) by the National Institute of Standards and Technology (NIST) laboratory. A key additional novelty of the present work is the investigation of the effects of substrate removal and preheating on mitigation of residual stresses and distortions using the validated model. Ultimately, these results will guide the selection of optimal manufacturing protocols and integration of the FE-based AM modelling for industrial application with complex geometries. The ultimate aim of the present work is to facilitate fatigue life prediction of complex geometry AM components including residual stress effects, for example, conformally-cooled injection moulding dies (for different material than Inconel 625).

Mitigation of Porosity and Residual Stress on Carbody Aluminum Alloy Vibration Welding: A Systematic Literature Review

Fatigue resistance is influenced by porosity and residual stress in welded joints. Fatigue failure in some means of transportation is caused by the inability to withstand the load received from the car body and passengers while operating. This study uses a systematic literature review (SLR) method to identify the effect of vibration welding on porosity and residual stress. Vibration can reduce the empty cavity (porosity) and increase the density of the weld. The ultrasonic vibration spot resistance (UVSR) method with 20 kHz on AA6082 is able to reduce residual stress up to 53% and is effective for homogenization of concentrated residual stress up to 57%.

X-ray diffraction measurements and computational prediction of residual stress mitigation scanning strategies in powder bed fusion additive manufacturing

The repeated localised heating-melting-cooling-solidification processes during laser beam powder bed fusion (PBF-LB) additive manufacturing (AM) induce intricate thermal residual stress (RS) in manufactured parts. Non-destructive characterisation using X-ray diffraction was used to measure the RS of Ti-6Al-4V square plates that were manufactured by using six different scanning strategies. Computational modelling was used to interpret the experimental stress measurement results. It was revealed that an inclined scanning strategy is beneficial for reducing the average through-thickness RS because the inclined scanning strategy can mitigate the non-uniform thermal profile and corresponding residual thermal stresses in successive layers of material. Among all the different scanning strategies that were analysed in this work, the 45° inclined 90° rotation scanning resulted in the lowest RS. The thicker parts have a greater gradient of RS than the thinner parts, after base plate removal. This research outcome can help the AM industry to design or optimise the process parameters of the PBF-LB aiming to minimise the RS of metal parts.

Experimental and Calphad Methods for Evaluating Residual Stresses and Solid-State Shrinkage after Solidification

Laser powder bed fusion is an additive manufacturing method that is based on melting and solidification of powder material. Due to the local heating above the melting point, thermal stresses are usually formed in the final part. Mitigation of residual stresses is usually assessed by laser scan strategies and not by alloy tailoring. In this paper a segregation-based residual stress formation mechanism is proposed and assessed computationally. Additionally, an experimental setup for rapid screening of residual stress formation in various alloys is proposed. The results should ease material development of metal alloys tailored for additive manufacturing by allowing the comparison of residual stress formation tendency (e.g., solid state shrinkage) between alloys. The proposed computational method is comparative in nature and forecasting absolute residual stress values would require known temperature dependent elastoplastic properties for the alloys as well as exact thermal history. The proposed experimental method is quantitative but its reliability depends on material properties such as yield strength.

Topology optimization of supports with imperfect bonding in additive manufacturing

Supports are an important ingredient of the building process of structures by additive manufacturing technologies. They are used to reinforce overhanging regions of the desired structure and/or to facilitate the mitigation of residual thermal stresses due to the extreme heat flux produced by the source term (laser beam). Very often, supports are, on purpose, weakly connected to the built structure for easing their removal. In this work, we consider an imperfect interface model for which the interaction between supports and the built structure is not ideal, meaning that the displacement is discontinuous at the interface while the normal stress is continuous and proportional to the jump of the displacement. The optimization process is based on the level set method, body-fitted meshes and the notion of shape derivative using the adjoint method. We provide 2-d and 3-d numerical examples, as well as a comparison with the usual perfect interface model. Completely different designs of supports are obtained with perfect or imperfect interfaces.

Fatigue performance of rib-to-deck joints in orthotropic steel deck with PWHT

Post-weld heat treatment (PWHT) displays potential for enhancing the fatigue strength of rib-to-deck (RD) welded joints in orthotropic steel decks (OSDs) by mitigating welding residual stress. In this study, residual stress measurements, fatigue tests, and fracture analyses were performed to investigate the influence of PWHT on the welding residual stress and fatigue strength of RD joints. Eighteen full-scale specimens were fabricated, i.e. ten PWHT-processed specimens and eight as-welded specimens without additional treatment. Hole drill (HD) and X-ray diffraction (XRD) were used to measure the residual stress in four PWHT specimens and four as-welded specimens. The PWHT specimen showed a reduction in the peak residual stress by 76% and 78% in the HD and XRD results, respectively. Fatigue tests were carried out on the six PWHT specimens and four as-welded specimens. The comparison was made between the two specimen types using the mean value of equivalent fatigue strength derived at 2 million cycles. The results indicated that PWHT improved the mean equivalent fatigue strength of RD joints by 23% and 33% under nominal stress and hot-spot stress, respectively. Parametric investigations were carried out using finite element-based fracture analysis to elucidate the influence of residual stress mitigation on fatigue crack growth in RD joints. The result indicated a significant decrease in the crack growth rate owing to residual stress mitigation. Such a decrease enhances the fatigue strength of RD joints.

Attainment of favorable microstructure for residual stress reduction through high-temperature heat treatment on additive manufactured inconel 718 alloy

Mitigation of residual stress in additive manufactured metal parts through post-heat treatment helps enhancement of life during service. The present work aims to investigate the effect of stress-relieving heat treatment (SRHT) at 1065 °C on microstructural changes, residual stress patterns, and mechanical properties of Inconel 718 (IN718) blocks fabricated through use of the laser-based powder bed fusion (L-PBF) process. The X-ray diffraction technique known as cos α method was used to capture the Debye ring for the measurement of surface residual stress. Dissolution of the columnar dendrites along with laves phases in the interdendritic region was seen after SRHT. The resultant microstructure showed partially recrystallized grains along with the formation of annealing twins and strengthening phases such as γ’, γ’’, and tiny metal carbides. The residual stress on the top surface of as-built sample was found to be 77% higher on the corners compared to the center region. The distribution of residual stress was seen as not uniform in the as-built condition. The SRHT sample showed uniform distribution of compressive residual stresses in all the locations. This phenomenon was due to the diffusion of atoms present in the high-stress regions migrated to low-stress regions until attaining their equilibrium. The combination of face centered cubic (FCC) and hexagonal lattices in as-built microstructure led to misfit strain between the grains. After SRHT, microstructure of IN718 experienced relaxation of residual stresses due to the transformation of hexagonal structured laves phase into the body-centered tetragonal (BCT) γ’’and FCC-structured γ’ phases. Furthermore, SRHT significantly alters the mechanical properties of IN718 alloy causing a 23.03% increase in average hardness and a 21.04% increase in tensile strength.

3D printing and post-curing optimization of photopolymerized structures: Basic concepts and effective tools for improved thermomechanical properties

The final thermo-mechanical properties of structural parts fabricated by masked stereolithography (MSLA) are highly determined not only by the processing parameters, but also by the post-processing methods. Improper implementation of post-treatment often leads to underperforming printouts. A novel tool for complex characterization of 3D printed bodies was developed and systematically demonstrated on a commercial free-radical photopolymerization (FRP) resin. The method relies on superimposed static and oscillatory mechanical test combining the heat deflection temperature (HDT) measurement together with the dynamic mechanical analysis (DMA) in a single test for fast and reliable characterization of parameters determining the curing behaviour of the photopolymer. The influence of post-curing time was addressed with a special focus on network density. Furthermore, the print orientation, having a high impact on mechanical properties, is discussed with a particular regard on the residual stress mitigation in future applications, such as 3D-printed cellular bodies.

Mitigation of residual stresses and microstructure homogenization in directed energy deposition processes

In additive manufacturing (AM), residual stresses and microstructural inhomogeneity are detrimental to the mechanical properties of as-built AM components. In previous studies, the reduction of the residual stresses and the optimization of the microstructure have been treated separately. Nevertheless, the ability to control both them at the same time is mandatory for improving the final quality of AM parts. This is the main goal of this paper. Thus, a thermo-mechanical finite element model is firstly calibrated by simulating a multi-track 40-layer Ti–6Al–4V block fabricated by directed energy deposition (DED). Next, the numerical tool is used to study the effect of the baseplate dimensions and the energy density on both residual stresses and microstructure evolution. On the one hand, the results indicate that the large baseplate causes higher residual stresses but produces more uniform microstructures, and contrariwise for the smaller baseplate. On the other hand, increasing the energy density favors stress relief, but its effect fails to prevent the stress concentration at the built basement. Based on these results, two approaches are proposed to control both the stress accumulation and the metallurgical evolution during the DED processes: (i) the use of a forced cooling suitable for small baseplates and, (ii) the adoption of grooves when large baseplates are used. The numerical predictions demonstrated the effectiveness of the proposed manufacturing strategies.

Residual Stress Mitigation in Directed Energy Deposition

Residual Stress Mitigation in Directed Energy Deposition

Room-temperature stress reduction in welded joints through electropulsing

Conventional residual stress mitigation techniques involve long processing times at high temperatures and/or mechanical loading to build plastic compressive stress below the surface. In this study, we present a new residual stress mitigation methodology at near ambient temperature in less than a minute. This is demonstrated on a welded joint of 316 L stainless steel, where low-frequency DC current pulses are shown to recrystallize the specimen and reduce residual stress. We present experimental evidence of ∼30 % reduction in electrical resistance, which corresponded to ∼40 % decrease in both microhardness and residual stress, measured by the X-ray diffraction tests. Similar improvement was qualitatively observed through significant decrease in the low-angle grain boundary density, which also reflects the decrease of the residual stress. The technique can be applied to relieve residual stress in conditions difficult for the conventional processing, such as locations with extreme space constraints or objects that cannot be heat treated.

X-Ray Diffraction Measurements and Computational Prediction of Residual Stress Mitigation Scanning Strategies in Powder Bed Fusion Additive Manufacturing

X-Ray Diffraction Measurements and Computational Prediction of Residual Stress Mitigation Scanning Strategies in Powder Bed Fusion Additive Manufacturing