Undesirable Residual Stresses Research Articles

Thermomechanical modelling of residual stresses and distortion in laser powder bed fusion: Assessment of the effect of build plate preheating on the behaviour of Inconel 718

The fabrication of complex metallic parts via laser powder bed fusion (L–PBF) is becoming more mainstream to overcome the limitations of traditional manufacturing methods. However, L–PBF induces undesirable residual stresses (RS) and possible geometric distortion of the component. This study aims to show the capabilities of build plate preheating in mitigating both defects. A thermomechanical finite element model is developed to investigate the outcomes of various preheating conditions when producing Inconel 718 samples consisting in a base and an overhanging beam restricted with support structures. Initially, the model is calibrated against the deflection result of an academic work employing a resemblant geometry. Our study is then performed. The as–built von Mises stress evolution along two paths as well as part distortion are assessed at 100, 300, and 500°C, then compared to the reference “no preheating” case results. A peak top surface RS of 803MPa was reached without preheating and reduced by 48.69% at 500°C. Along the vertical path, the mean stress value went from 897MPa at 20°C to 474MPa at 500°C. Distortion of the as–built part is essentially a non–uniform contraction of the beam along the x–direction due to progressive bending of the supports about y–axis. Once the beam is unrestricted from the substrate, it deforms as a cantilever structure. Increasing the preheating temperature decreases the maximum build direction deflection by 14.56% at 100°C, up to 76.94% at 500°C in comparison to the 4.12mm reached at 20°C. In conclusion, preheating the substrate is a very effective way to mitigate RS and distortion in L–PBF of Inconel 718. Even the relatively low 100°C showed a notable improvement in this regard.

Bragg edge imaging characterization of multi-material laser powder-bed fusion specimens

Multi-material laser powder-bed fusion (M2LPBF) is a novel additive manufacturing approach that makes it possible to print different materials along the built direction and within a single layer of a component. At the interface between the different materials, the deposited powders melt, mix and solidify very rapidly, than can produce a range of desired and undesired phases, residual stresses and defects. Here we applied Bragg edge imaging to characterize M2LPBF specimens of stainless steel and CuCrZr with vertical and horizontal interfaces. A diffuse interface is observed in the samples with both vertical and horizontal interfaces. The analysis of the (111) and (200) Bragg edges height across the samples demonstrated a clear difference between the crystallographic texture of both alloys, with a strong alignment of the (002) planes along one of the transversal directions in the steel and a random texture within the copper alloy.

Experimental Tribological Study on Additive Manufactured Inconel 718 Features Against the Hard Carbide Counter Bodies

Abstract In past years, machining processes have been required when fabricating the complex Inconel 718 parts, and these processes cause undesired tensile residual stresses. Inconel 718 also exhibits extreme work hardening throughout the machining process. To avoid these issues, recently, Inconel 718 parts with high geometric complexity and dimensional accuracy, the laser powder bed fusion (LPBF) process, which belongs to additive manufacturing, has been extensively used. These Inconel 718 parts with LPBF processing are frequently utilized in various industries, including aerospace, automotive, pharmaceutical, and food processing, because of their high strength, biocompatibility, and corrosion resistance. Wear resistance is essential in addition to these properties for designing and crushing applications. In this paper, tribological tests were conducted on the LPBF-processed Inconel 718 parts and compared to casted Inconel 718 parts against the four types of counter bodies, namely boron carbide, silicon carbide, tungsten carbide, and titanium carbide. The studies were carried out for 30 min with a constant load of 5 N, frequency of 10 Hz, and stroke length of 1 mm. In comparison to casted samples, LPBF-processed samples showed low coefficient of friction (COF) values. The highest COF was observed on the cast Inconel 718 against the tungsten carbide counter body. The wear mechanisms were studied using scanning electron microscopy.

High-resolution 3D strain and orientation mapping within a grain of a directed energy deposition laser additively manufactured superalloy

The industrialization of Laser Additive Manufacturing (LAM) is challenged by the undesirable microstructures and high residual stresses originating from the fast and complex solidification process. Non-destructive assessment of the mechanical performance controlling deformation patterning is therefore critical. Here, we use Dark Field X-ray Microscopy (DFXM) to map the 3D subsurface intragranular orientation and strain variations throughout a surface-breaking grain within a directed energy deposition nickel superalloy. DFXM results reveal a highly heterogenous 3D microstructure in terms of the local orientation and lattice strain. The grain comprises ≈ 5 μm-sized cells with alternating strain states, as high as 5 ×10−3, and orientation differences <0.5°. The DFXM results are compared to Electron Backscatter Diffraction measurements of the same grain from its cut-off surface. We discuss the microstructure developments during LAM, rationalising the development of the deformation patterning from the extreme thermal gradients during processing and the susceptibility for solute segregation.

A Methodology to Reduce Thermal Gradients Due to the Exothermic Reactions in Resin Transfer Molding Applications

Resin transfer molding (RTM) is among the most used manufacturing processes for composite parts. Initially, the resin cure is initiated by heat supply to the mold. The supplementary heat generated during the reaction can cause thermal gradients in the composite, potentially leading to undesired residual stresses which can cause shrinkage and warpage. In the present numerical study of these processes, a one-dimensional finite difference method is used to predict the temperature evolution and the degree of cure in the course of the resin polymerization; the effect of some parameters on the thermal gradient is then analyzed, namely: the fiber nature, the use of multiple layers of reinforcement with different thermal properties and also the temperature cycle variation. The validity of this numerical model is tested by comparison with experimental and numerical results in the existing literature.

Effect of preheating during laser metal deposition on the properties of laminated bending dies

Metal-laminated tooling provides a fast and cheap manufacturing concept. In this study, laser metal deposition (LMD) is used for reducing and eliminating the stair step effect in a metal-laminated bending die. Preheating could decrease the undesired residual stresses in additive manufacturing, thus a systematical analysis of the effect of preheating of the laminae on the surface quality and mechanical properties of the bending die is performed. Ferritic steel sheets (S355 MC) with a thickness of 2 mm are laser cut and stacked up to manufacture the laminated bending die with a radius of 6 mm. The sheets are joined and the stair steps are filled with LMD with stainless steel powder 316L-Si. The initial temperature of the tool sheets (substrates), beside room temperature, is elevated up to 300 °C. The effect of the preheating on the surface roughness, shape deviation, hardness, and residual stresses of the die are investigated. The mean height of the surface increases by 59% at elevated temperatures. However, the tensile residual stress parallel to the weld direction at the middle of the deposited area decreases only around 25%. The functionality of the forming tools manufactured by this method is proven by bending of DC06 and HC380LA sheets.

Residual Stress Distribution in Selective Laser Melting of SS316L Parts

Additive manufacturing is one of the fastest-growing fields in materials engineering. This is because there is a new trend for custom, high-precision, and on-demand manufacturing. The undesired residual stress induced in the components during layer-by-layer melting and solidification of the metal powder is an important issue related to the selective laser melting (SLM) process that needs to be studied deeply. These stresses may impair mechanical performance and potentially result in premature failure. As a result, a thorough knowledge of residual stress is crucial for improved component dependability. By keeping constant process parameters, samples were produced with a difference in the scanning method. Results indicate that the defect-free parts are manufactured in all the four patterns used, and the self-balanced residual stresses are within the safe limits of yield strength.

Discontinuous Galerkin FEM with Hot Element Addition for the Thermal Simulation of Additive Manufacturing

Despite its promising advantages, the application of directed energy deposition (DED) to produce large metal parts is hindered by challenges inherent to the process. Undesired residual stresses, distortions and heterogeneous material properties mainly originate from a part’s thermal history. Fast part-scale thermal models therefore facilitate improved applicability of DED by enabling the prediction and mitigation of these unwanted effects. In this work, the efficiency of a discontinuous Galerkin-based thermal model with heat input by hot element addition, is evaluated and improved to allow such fast simulations. It is found that the model permits the use of a coarse discretization around the heat source, which significantly reduces simulation time while maintaining accurate solutions. It is also shown that the model naturally facilitates the use of local time stepping, which can considerably improve the efficiency of thermal additive manufacturing simulations.

Innovative Assistance System for Setting up a Mechatronic Straightening Machine

High-strength wire materials are usually available as strip material which is further processed in a forming process (e.g. punch-bending). For storage and transport of the semi-finished wire to the customer, the material is wound onto coils. The manufacturing and coiling process introduces plastic deformations into the wire, which lead to undesirable residual stresses and wire curvature of the semi-finished product. These residual stresses and curvatures cause variations in the material properties of the semi-finished product, which have a negative impact on the subsequent product quality. Straightening machines are used to compensate the residual stresses and the curvature in the wire. At the beginning of the straightening process, the straightening machines must be set up in such a way that residual stresses and curvatures are optimally compensated. This setup process is usually a manual and iterative process, where a lot of material is wasted until the optimal settings for the straightening machine are found.In order to reduce the amount of material waste, the operator must be supported in the setup process. In this context, a new and innovative setup assistance system was developed to support the operator during the setup process. The setup assistant system automatically detects the wire curvature by means of an optical measuring system. Based on the optically detected measuring points, the wire curvature is determined by a robust calculation algorithm. Based on a database built up through the carried out experimental and numerical research work, the optimum setting parameters for the straightening machine are suggested to the operator without lengthy trial and error. After confirmation by the operator, the roller settings are automatically adjusted by the mechatronic straightening machine. With the presented method, the conventional iterative setup procedure can be made more resource-efficient and a high straightening quality can be reproducibly achieved.

ESTIMATION OF RESIDUAL STRESSES IN QUENCHED 4140 LOW ALLOY STEEL USING X-RAY DIFFRACTION (XRD)

During the quenching process, coolants, heating temperature, and holding time must be carefully adjusted to avoid large temperature differences that may lead to undesirable high residual stresses that may limit the surface life of the material. The objectives of this project are to estimate the values of residual stresses in quenched 4140 low alloy steel using X-ray diffraction (XRD), to study the effect of holding time and heating temperature on the behavior and values of residual stresses, and to determine an empirical equation for residual stresses in terms of heating temperature and holding time based on the experimental results. The method and analysis for this research include both experimental and theoretical aspects. The experimental work involves the selection of 4140 low alloy steel, preparation of the material (including specimen design and specimen preparation), pretreatment (annealing) to remove any unknown history of the selected low alloy steel, hardening, hardness testing, and XRD testing. The theoretical work is the determination of an empirical equation, based on the data obtained from the experiments, that describes the behavior of residual stresses in terms of heating temperature and holding time. This research contributes a better understanding of the best working conditions to yield the best hardness and, more importantly, the conditions that affect the value of residual stresses. The results show that the holding time is the primary determinant of the values of residual stresses and that XRD is an effective method for measuring these values.

Effect of post welding heat treatment on the weld quality of micro plasma arc welded SS-316L thin sheet

High thermal gradient formed during the fusion welding process results in development of undesirable residual stress in the weldments. This stress is developed due to restraint by the parent metal during weld solidification. The high heat input results in non-uniform heat distribution across the weld region in other words non-uniform microstructural development across the weld region and hence the mechanical properties of the joint are often not uniform. In order to avoid inhomogeneity in the mechanical properties and also to reduce/eliminate undesirable residual stress, the welded samples are given post welding heat treatment. In this research, 500 µm thin SS-316L sheets are welded using micro plasma arc welding process and then the welded specimens are heat treated. The welding experiments are conducted by varying welding speed, welding current and stand-off distance. Weld bead microstructure, micro-hardness, ultimate tensile strength (UTS), yield strength and percentage elongation are determined before and after heat treatment. The effects of welding heat input and process parameters on the measured weld qualities are studied. Analysis of variance is also performed to estimate the influence of factors and their interaction on the weld quality. The post weld heat treatment results in an increase in the grain size of the HAZ and is found in the range of 38.96 µm to 56.22 µm whereas for as-welded samples it is in the range of 29.88 µm to 50.40 µm. The average UTS value of the heat treated samples is increased by 9.9% compared to the as-welded samples. The hardness of the fusion zone varies in the range of 175-215 HV.

Computational crack propagation modeling of welded structures under as‐welded and high frequency mechanical impact (HFMI) treatment conditions

AbstractThe fatigue damage assessment of welded structures is of critical importance in the design of many engineering structures. The thermomechanical nature of the welding process leads to undesired tensile residual stresses, which results in fatigue performance degradation of welded structures. High‐frequency mechanical impact (HFMI) technique offers significant potentials to induce compressive residual stresses into the welded joints, thus improving fatigue life of welded parts. In this paper, a new crack propagation modeling approach considering effects of residual stress fields and the crack closure based on a modification of the Forman model is proposed to assess fatigue crack propagation performance of welded joints under as‐welded and HFMI treatment conditions. Experimental residual stress and fatigue data sets of 5083‐H321 aluminum, CSA 350W, ASTM A514, S355, and S960 steel welded joints are used to validate the proposed approach. Both predicted and experimental results show that the fatigue life improvement by the HFMI treatment depends on the applied stress level and material type. The fatigue life improvement is greater at the lower stress levels with a factor of more than 10–30 times, and the fatigue life increase becomes less at the higher stress levels with a factor of two to three times depending on the material type of welded joints. The fatigue life improvement by the HFMI is more significant with a higher material yield strength. Compared results showed that the proposed modeling approach provides efficient and accurate life predictions of the welded joints of five different materials under both as‐weld and HFMI conditions. The proposed modeling framework can be used as an effective analysis technique for fatigue crack propagation life of welded structures under pre‐ and post‐weld treatment conditions to account for residual stress fields.

Simultaneously minimizing residual stress and enhancing strength of selective laser melted nano-TiB2 decorated Al alloy via post-uphill quenching and ageing

Selective laser melting (SLM) of aluminium alloys is of research interest to produce customized or complex-shaped metal components for functional or structural applications. In order to palliate the undesirable residual stress developed in the SLM process, traditional post-annealing heat treatment has been widely used to lower residual stress in selective laser melted (SLMed) Al alloys. However, the traditional post-annealing method usually leads to a decrease in strength which is another concern for applications. In this study, we developed and validated a new strategy combining post-uphill quenching and subsequent ageing to simultaneously minimise residual stress and increase the strength of SLMed nano-TiB2 decorated Al alloy parts. The results show that the high as-built residual stress was partly counteracted by the newly-produced residual stress in the opposite direction during the uphill quenching stage and synergistically reduced by dislocation recovery during the ageing stage. The microstructural features of grains and cell structures remain unchanged after the treatment. While substantial new nano-Si particles with dot-like or needle-like morphology precipitate inside the cells. The tensile strength was improved mainly due to effective precipitation strengthening by dispersed nanosized Si precipitates. The proposed strategy is expected to be useful in other materials and components fabricated by SLM in which residual stress is also unwanted.

Characterization of single- and multilayer cold-spray coating of Zn on AZ31B

Abstract Zinc, a soft material with a low melting point and high corrosion resistance, was coated onto AZ31B Mg alloy using different cold spraying process parameters. The physical and mechanical properties of the resulting Zn/AZ31B samples were then investigated to explore the effect of the process parameters on the microstructural and mechanical characteristics. The results obtained via X-ray diffraction show the formation of an intermetallic material at the interface of Zn/AZ31B even at low process temperatures. In addition, spherical droplets of Zn were observed at the surface, confirming the partial melting of Zn particles during the impact. This partial melting is believed to lead to the formation of intermetallic compounds during solidification. To engineer the residual stress induced in the cold spraying process, a thin layer of dense Zn was then used as an intermediate layer before coating with Al7075, forming a multilayered surface of Al7075/Zn/AZ31B. Because of the higher thermal expansion coefficient of Zn compared with those of Al7075 and AZ31B, beneficial compressive residual stress could be created in all three layers of this novel multilayer deposition. Without the Zn interlayer, Al7075/AZ31B under the same coating parameters exhibited undesirable tensile residual stress in the substrate.

Effect of constitutive model on residual stress development in the pressure tube rolled joint

Being one of the most important joints in a pressurized heavy water reactor (PHWR), pressure tube rolled joint is critical during the construction and operation. This joint is formed through the roll expansion process that leads to an interference fit between the Zr 2.5%Nb pressure tube and stainless-steel end fitting. For integrity of this joint, a sufficient pull-out strength and leak tightness must be maintained, while having lower undesirable residual stresses, especially in the transition zone. The roll expansion process parameters are controlled to achieve optimum contact pressure and limit the tensile residual stress. The parameters of this roll expansion process are designed either through experiments or through numerical simulations. One important aspect that is often ignored in the numerical simulations is the effect of material constitutive behavior of the Zr 2.5%Nb alloy, particularly the anisotropic yield behavior and the cyclic stress-strain response. In this paper, these two aspects of constitutive behavior are investigated, using three-dimensional finite element simulations, to understand their influence on the residual stress development in the roll expansion process. The cyclic behavior was modelled using Chaboche nonlinear kinematic hardening model. The model parameters were extracted from low cycle fatigue experiments with the strain amplitude equivalent to that seen in roll expansion process. The results show that constitutive behavior, especially the cyclic behavior, has a significant impact on the residual stress. Anisotropic yielding of the pressure tube was also seen to affect the tensile residual stress and the contact pressure, although to a lesser extent. Finally, after comparison between four combinations of constitutive behavior, a simple isotropic yield with isotropic hardening model is suggested to predict the tensile residual hoop stress in the rolled joint, conservatively.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Summary The development of mixed tin-lead (Sn-Pb)-based perovskite solar cells (PSCs) with low band gap (1.2–1.4 eV) has become critical not only for pushing single-junction devices toward the maximum efficiency given by the Shockley-Queisser limit, but also for enabling all-perovskite tandem devices beyond this limit. However, achieving high power-conversion efficiency (PCE) and long-term device operation stability simultaneously remains a significant challenge for Sn-Pb-based PSCs. Here, we demonstrate near ideal-band-gap (∼1.34 eV) methylammonium-free Sn-Pb-based PSCs with high efficiency (∼20%) and promising operational stability of maintaining >80% of initial PCE over 750 h under maximum-power-point tracking. The key to this success is the use of a SnCl2⋅3FACl complex additive that improves the microstructure and reduces the development of residual stress in the Sn-Pb perovskite thin films, which in turn enhances the efficiency and stability of the Sn-Pb-based ideal-band-gap PSCs.

Efficient thermal simulation of large-scale metal additive manufacturing using hot element addition

Directed energy deposition processes can reduce material waste and manufacturing time of large metal parts through near net-shape production at high deposition rates. However, the localised high heat input gives rise to undesired heat accumulation, residual stresses and distortions. In this work, a fast thermal model is developed to aid in predicting and preventing these drawbacks by providing insight in the relation between process settings, deposition strategy and thermal response. Material addition and heat input are efficiently combined by adding new elements at elevated temperature. Newly deposited elements are assigned an artificially enhanced heat capacity to match the process heat input. The discontinuous Galerkin finite element method is used for spatial discretisation. The resulting numerical scheme is fully explicit and can be solved element-wise. Unlike previous prescribed-temperature heat input models, the proposed method correctly captures the process heat input, irrespective of substrate temperature and element size and without calibration. Comparison with experimental data shows that the thermal history of a large additively manufactured part can be accurately predicted.

Thermal Conductivity and Stability Studies of Cooking and Waste Cooking Oil as a Based Fluid of TiO2 Nanofluid for Carbon Steel Quenching Process

Selection of quench media is important as it depends on the hardenability of the metal alloys, component’s thickness and geometry. Recently, oil and water are frequently used as a quench media in heat treatment industries. Improper quench media will cause the material to become brittle, suffers from geometric distortion, or having a high undesirable residual stress in the components. Oil was used as the fluid base in this research as the main observation focus. To obtain nanofluids and explore thermal conductivity and stability of nanofluids, the two-step method was introduced to prepare the nanofluids with 4 types of different cooking and waste cooking oil as a base fluid (Palm oil, sunflower oil, canola oil and corn oil). TiO2 powder will be mixed with oil fluid by using magnetic stirrer for 1 hour and ultrasonic bathing. Observation method used in 30 days with thermal conductivity and zeta potential to evaluate stability of each nanofluid specimens. Heat treatment onto carbon steel applied and the sample were heated at 950°C for 90 minutes and then quenched in difference oils. Microstructure and hardness analysis applied to get the result. It can conclude, adding nanoparticles in oil base fluid will enhance thermal conductivity and improve thermal performance of heat transfer of the base fluid. Quenching process with oil-based nanofluid will produce martensite structure and waste sunflower and corn oil with TiO2 it's not stable after 30 days observation but the palm and canola oil with TiO2 can achieve the highest hardness test and thermal conductivity.

Impacts of Machining and Heat Treating Practices on Residual Stresses in Alpha-Beta Titanium Alloys

Machining and thermal processing can introduce undesirable residual stresses and distortion in titanium alloy components, and although the distribution and magnitude of these residual stresses is highly relevant for component and process design in the aerospace industry, the relationships between processing variables, processing steps, residual stress signature, and subsurface microstructures are not well understood. The current study reports on the preliminary results of experiments designed to mimic typical machining and thermal processing practices for aerospace alpha-beta Ti alloys. Traditional climb cutting and high-speed peel cutting operations are included in CNC machining experiments, and both solution treating and aging heat treatments are considered for thermal processing experiments. Characterization of samples includes strain measurement using the sin2ψ and cosine α methods with x-ray diffraction as well as microstructural characterization using traditional metallographic techniques. The results of this study show a large tool-path dependence for residual stresses in machined surfaces as well as a significant difference in the residual stress behavior for solution treated and quenched compared to solution treated and aged samples of Ti 6Al-4V alloy.

Effects of finish turning on an austenitic weld investigated using diffraction methods

Arc welding generally introduces undesired local residual stress states on engineering components hindering high-quality performance in service. Common procedures to reduce the tensile residual stresses are post-heat treatments or mechanical surface treatments like hammering or shot-peening. Assessments of residual stress profiles of post-weld treatments underneath the weld surface are essential, especially in high safety exigency systems like pressure vessels or piping at power plants. In this study, neutron diffraction is used to determine the stress profile after finish milling of an austenitic steel weld in order to verify a chained finite element simulation predicting the final residual stress fields including milling and welding contributions. Non-destructive measurements with spatial resolutions of less than 0.2 mm within the first 1 mm from the surface were mandatory to confirm the finite element simulations of the coarse-grained austenitic material. In the data analysis procedure, the obtained near-surface data have been corrected for spurious strain effects whenever the gauge volume was partially immersed in the sample. Moreover, constraining the surface data to values obtained by x-ray diffraction and data deconvolution within the gauge volume enabled access of the steep residual stress profile within the first 1 mm.