Large Residual Stresses Research Articles

Understanding the impact of texture on the micromechanical anisotropy of laser powder bed fused Inconel 718

The manufacturability of metallic alloys using laser-based additive manufacturing methods such as laser powder bed fusion has substantially improved within the last decade. However, local melting and solidification cause hierarchically structured and crystallographically textured microstructures possessing large residual stress. Such microstructures are not only the origin of mechanical anisotropy but also pose metrological challenges for the diffraction-based residual stress determination. Here we demonstrate the influence of the build orientation and the texture on the microstructure and consequently the mechanical anisotropy of as-built Inconel 718. For this purpose, we manufactured specimens with [001]/[011]-, [001]- and [011]/[overline{1} {text{11}}]-type textures along their loading direction. In addition to changes in the Young’s moduli, the differences in the crystallographic textures result in variations of the yield and ultimate tensile strengths. With this in mind, we studied the anisotropy on the micromechanical scale by subjecting the specimens to tensile loads along the different texture directions during in situ neutron diffraction experiments. In this context, the response of multiple lattice planes up to a tensile strain of 10% displayed differences in the load partitioning and the residual strain accumulation for the specimen with [011]/[overline{1} {text{11}}]-type texture. However, the relative behavior of the specimens possessing an [001]/[011]- and [001]-type texture remained qualitatively similar. The consequences on the metrology of residual stress determination methods are discussed.

In situ neutron diffraction analysis of microstructural evolution-dependent stress response in austenitic stainless steel under cyclic plastic deformation

Understanding fatigue phenomena in metals is of great significance for mechanical performance in engineering systems. Microstructural evolution in austenitic stainless steels during cyclic plastic deformation has been studied via diffraction line profile analysis; however, their microstructure-dependent mechanical response upon stress partitioning in the matrix (austenite) and deformation-induced martensite has remained largely unexplored. In this study, the stress response analysis of austenitic stainless steel was performed using neutron diffraction. The phase stress in the austenite correlated well with the dislocation density in the phase. The actual stress in the martensite was nearly half of the assumed stress and the phase stress in the austenite. However, the apparent stress (the residual stress subtracted from the actual stress) was similar to the assumed stress as the martensite contains a fairly large compressive residual stress (approximately 1 GPa). Overall, the loading stresses at peak loads can be explained by sharing stress on the austenite and martensite.

Microscopic residual stress evolution at the SiC/Al interface during nanoindentation via molecular dynamics simulation

We investigated the evolution of structure and microscopic residual stress at the SiC/Al interface at the atomic scale during nanoindentation by using molecular dynamics simulations. We find that microscopic residual stresses mainly distribute in the interface region, especially the SiC side ranging from 50 to 60 GPa after relaxation. During nanoindentation, there is a large microscopic residual stress concentration at the indentation area. The microscopic residual stress of the SiC side at the indentation area is more serious than that of the Al side. It is found that some SiC atoms in the indentation region are extruded from the SiC side by the indenter, and the fractured SiC atoms are subjected continuously by the indenter, which accelerates the evolution of structure and microscopic residual at the SiC/Al interface. The fractured SiC atoms aggravate the microscopic residual stress concentration at the SiC/Al interface. Al may release some of the microscopic residual stresses by plastic deformation, while the SiC can only reduce the microscopic residual stress by fracture. Therefore, it is necessary to remove the fractured SiC atoms in time to prevent more complex microscopic residual stresses and to improve the machining process of SiC/Al composites.

Effect of Heat Treatment on Microstructure and Properties of GH3536 Fabricated by Selective Laser Melting

Selective laser melting (SLM) forming technology to prepare nickel-based superalloy parts can significantly save costs and solve bottleneck problems. The extremely high-temperature gradient and large residual stress during SLM lead to structural defects and compositional segregation. The parts formed by SLM urgently need heat treatment to control the microstructure composition and improve mechanical properties. Results showed that the heat treatment did not significantly change the microcracks and pores in the SLM sample, but the carbides in the grain boundary gradually changed from a granular distribution to a continuous strip distribution. After heat treatment, the elongation increased significantly, but the yield strength decreased. The tensile fracture of the SLM samples changed from a transgranular fracture to a ductile fracture, and obvious plastic deformation occurred, confirming that heat treatment can improve the benefits of the SLM sample.

Effect of Residual Stress on Hydrogen Diffusion in Thick Butt-Welded High-Strength Steel Plates

Thick high-strength steel plates are increasingly being used for ship structures. Moreover, hydrogen enters the process of manufacturing and service, and large residual tensile stress occurs near the weld. Such stress can facilitate the diffusion and accumulation of hydrogen in the material, leading to hydrogen embrittlement fracture of the shell. Therefore, residual-stress-induced diffusion and accumulation of hydrogen in the stress concentration region of thick butt-welded high-strength steel plate structures need to be studied. In this study, manual metal arc welding was realized by numerical simulation of residual stress in a thick butt-welded high-strength steel plate model using the thermoelastic–plastic theory and a double ellipsoidal heat source model. To analyze residual stress, a set of numerical simulation methods was obtained through comparative analysis of the test results of relevant literature. Residual and hydrostatic stress distributions were determined based on these methods. Then, hydrogen diffusion parameters in each region of the model were obtained through experimental tests. Finally, the results of the residual stress field were used as the predefined field of hydrogen diffusion to conduct a numerical simulation analysis. The distribution of hydrogen diffusion under the influence of residual stress was obtained based on the theory of stress-induced hydrogen diffusion. The weak area of the welding joint was found to be near the weld toe, which exhibited high hydrostatic stress and hydrogen concentration. Further, the maximum hydrogen concentration value of the vertical weld path was approximately 6.1 ppm, and the maximum value of the path parallel to the weld centerline and 31 mm away from the weld centerline was approximately 6.22 ppm. Finally, the hydrostatic tensile stress in the vertical weld path was maximized (~345 MPa), degrading the material properties and causing hydrogen-related cracking. Hence, a reliable method for the analysis of hydrogen diffusion according to residual stress in thick high-strength steel plates was obtained. This work could provide a research basis for controlling and eliminating the adverse effects of hydrogen on the mechanical properties of ship structures and ensuring the safe service of marine equipment.

Mapping of three-dimensional residual stresses by neutron diffraction in nickel-based superalloy discs prepared under different quenching conditions

Large interior residual stresses, generated during various thermomechanical treatments, could greatly affect not only the machining dimension precision during subsequent machining processes but also the structural stability under service in superalloy components. In this paper, neutron diffraction measurements have been carried out to characterize the three-dimensional residual stress distributions in IN718 superalloy discs quenched with different media, including water, oil, air and insulation. Three-dimensional residual stress maps have been derived in these specimens. It was found that, in the water-quenched, oil-quenched and air-cooled specimens, the residual stresses are compressive near the surface balanced by tension within the interior, with the peak magnitude stresses of 560 MPa, 480 MPa and 120 MPa, respectively. It was evidenced that there is no obvious residual stress in the insulation-cooled specimen. The corresponding Vickers hardness maps show a larger hardness at the core and smaller hardness near the surface in the water-quenched, oil-quenched and air-cooled specimens, whereas there is a larger hardness near the cylindrical surface and smaller hardness at the central axis in the insulation-cooled specimen, achieved by the differential cooling rate control. Our investigations show that the coordinated regulation of residual stress and microstructure and properties can be realized by reasonably selecting cooling media.

Structural characterization of Au/Cr bilayer thin films using combined X‐ray reflectivity and grazing incidence X‐ray fluorescence measurements

Gold thin‐film mirrors are widely used as X‐ray reflective optics to manipulate X‐rays especially in synchrotron beamlines. Application of a thin underlying buffer layer of chromium enhances the adhesion of gold film to the substrate material and provides long term structural stability. In the present work, Au/Cr bilayers deposited using the DC magnetron sputtering technique with different thickness values on a Si substrate are studied to better understand the correlation between micro‐structure properties and residual stress. In order to obtain precise structural properties of Cr binding layer and gold layer, as well as in‐between nanoscaled interfacial structure, nondestructive probing is carried out by simultaneously utilizing synchrotron‐based combined X‐ray reflectivity (XRR)–grazing incidence X‐ray fluorescence (GIXRF) techniques. It has been clearly observed that when thickness of Cr layer is relatively small, a composite interlayer structure is formed at the Au/Cr boundary and at the Cr/Si interface boundary due to the interdiffusion of two layer mediums. However, when the thickness of the Cr layer is relatively larger (~90 Å), emergence of such an interlayer medium does not take place. The grazing incidence X‐ray diffraction (GIXRD) measurements have also been performed to investigate crystalline structure and residual stress of the gold layer medium. Our results show that relatively large residual stress exists in the Au layer if thickness of Cr layer is small. The surface roughness morphology of the top gold layer obtained from the atomic force microscopy measurements showed the partial agreement with the results obtained from the XRR‐GIXRF technique.

Investigation of the Shot Size Effect on Residual Stresses through a 2D FEM Model of the Shot Peening Process

Shot peening is a surface treatment process commonly used to enhance the fatigue properties of metallic engineering components. In industry, various types of shots are used, and a common strategy is to regenerate a portion (approximately up to 35% of the total shot mix weight) of used and worn shots with new ones of the same type. Shots of the same type do not have a constant diameter, as it is concluded by experience that the diameter variation is beneficial for fatigue life. The process of stochasticity raises the difficulty for the application of computational methods, such as finite elements analysis, for the calculation of pivotal parameters, for instance, the development of the residual stress field. In the present work, a recently developed plane strain 2D FEM model is used, which has the capability to consider various shot size distributions. With the aid of this model, it became feasible to study the effect of the shot-size distribution, its sensitivity, and to draw conclusions considering the industrial practice of using a mixture with new and worn shots. The diameter of these shot types differs significantly, and a used shot may have a diameter three times smaller than a new one. As concluded from the finite element results, which are verified from experimental measurements, a shot type with a larger diameter causes a wider valley in the stress profile, and the peak stress depth increases. Alongside the peak stress depth movement, with smaller shots, larger residual stresses are observed closer to the surface. Thus, the superimposition of many shots with variable diameters causes the development of a residual stress field with enhanced characteristics. Furthermore, this residual stress field may be further enhanced by adjusting or increasing the percentage weight of the used shots, up to ~50%.

Gradient microstructure and foreign-object-damaged fatigue properties of Ti6Al4V titanium alloy processed by the laser shock peening and subsequent shot peening

A dislocation-gradient microstructure layer with large depth and high compressive residual stress (CRS) value on the surface of Ti6Al4V titanium alloy is processed by laser shock peening and subsequent shot peening (LSP + SP). The microstructure evolution and its foreign-object-damaged fatigue properties were investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction, microhardness tests, finite element numerical simulation and vibration fatigue tests. Foreign object damage (FOD) was introduced through the indentations of hardness testers, which can lead to the increase of stress concentration and redistribution of surface residual stress around craters. However, the dislocation-gradient microstructure layer can prolong crack initiation and propagation lives by 184% and 45% for specimens with FOD, respectively. The median fatigue life of (LSP + SP) specimens with FOD even shows longer than that of untreated specimens without FOD. Furthermore, a synergistic strengthening mechanism, involving dislocation strengthening, grain refinement and CRS, was clearly revealed.

Research on calculation of contact fatigue life of rough tooth surface considering residual stress

Grinding and shot peening produce large residual stresses on the gear teeth. Research on the calculation of contact stress and fatigue life of rough gear teeth considering the residual stress has not been published in the literature. The influence of residual stress and rough surface on contact fatigue is difficult to be ignored, so the research of the paper is of significance. Based on the finite element modeling method of rough surface contact and multiaxial fatigue criterion, the gear contact stress and fatigue life under the action of rough surface and residual stress is obtained. The study found that residual compressive stress can reduce the high stress area of the near-surface and lead to a low-stress area between the near-surface and the sub-surface. In addition, residual compressive stress can reduce the near-surface fatigue failure depth (NFFD) and make the sub-surface fatigue failure area tend to be deeper while increasing the contact stiffness of the tooth surface. The logarithmic value log10(2Nf) of the sub-surface fatigue life is quantitatively related to the roughness Sq and the residual stress amplitude σrs, and the fitting polynomial of the three variables is obtained, which provides a theoretical reference for the prediction and calculation of the contact fatigue life of the tooth sub-surface.

Revealing the deformation mechanisms of the heterogeneous structured CrMnFeCoNi high entropy alloy with ameliorated mechanical and corrosion resistance properties

Developing high entropy alloy (HEA) with both high strength and ductility is a longstanding challenge as potential structural materials. The strength and ductility trade-off can achieve via tailoring heterogeneous structures. Herein, a gradient structure with grain size variation from surface to center was successfully introduced into the CrMnFeCoNi HEA by ultrasonic nanocrystal surface modification (UNSM) process. More importantly, the deformation mechanisms and strengthening mechanisms of the heterogeneous structured CrMnFeCoNi HEA during tension were also investigated. Experimental results demonstrated that the heterogeneous structured CrMnFeCoNi HEA shows excellent strength and ductility combination properties ascribed to the grain refinement strengthening, dislocation density strengthening, and twinning strengthening. Besides, the corrosion resistance properties are also simultaneously improved, resulting from the low surface roughness, small grain size, and large compressive residual stress value. During tensile deformation process, the deformation mode is the dislocation slip at low strain and the slip system of the alloy is mainly {111}〈110〉. With the increase of deformation amount, twin crossing occurs, and grain size gradually decreases. The deformation mode changes from dislocation slip to twinning at high strain. In conclusion, the UNSM process ameliorates the comprehensive mechanical and corrosion resistance properties of CrMnFeCoNi HEA.

Rapid method for comparative studies on stress relief heat treatment of additively manufactured 316L

The additive manufacturing method laser powder bed fusion (L-PBF) is known to introduce large residual stresses in the built component. Optimization of process parameters and subsequent heat treatment is crucial to relieve these residual stresses. However, many of the available tools used to analyze these residual stresses are either prohibitively expensive, or too time consuming for initial prototyping stages.A qualitative method for rapid evaluation of the effectiveness of stress relief heat treatment of L-PBF manufactured 316L has been tested. Residual stress induced distortion has been measured with contact and non-contact methods to study the effect of different stress relief heat treatment temperatures (600–950 °C, fixed holding time: 1 h). Over the examined temperature interval, at which deformation was measured, distinct differences were observable at each temperature with both methods. Based on the distortion, shape stability was considered reached after subjecting the test geometry to a heat treatment temperature of 900 °C for 1 h. Complementary mechanical testing and microstructural characterization were carried out to provide a more general understanding of the implications of each heat treatment temperature. Microstructural characterization revealed that complete dissolution of the cellular sub-grain features occurred at the same temperature as where the minimum magnitude of distortion was obtained.

Stress-induced Néel vector reorientation in γ -FeMn antiferromagnetic thin films

The relationship between stresses and the orientation of the Néel vector were studied by varying the residual stresses in magnetron sputtered FeMn thin films by adjusting Argon working pressures. Quasistatic magnetization and AC susceptibility measurements reveal that the FeMn film with compressive stress (−27 MPa/−0.015% strain) possesses an out-of-plane Néel vector orientation with a 44 kOe spin-flop field, as contrasted to the FeMn film with tensile stress (25 MPa/0.014% strain) showing an in-plane orientation with a 34 kOe spin-flop field. An energy formulation for the films estimates a magnetostriction value of 109 ppm following an effective anisotropy of −8 kJ/m3. The film with the larger residual stress (77 MPa/0.043% strain) displayed a strain-induced phase transition from γ-FeMn to α-FeMn. These results show the dependency of the Néel vector on the stress state indicative of relatively large magnetostriction.

Residual stress evolution and tailoring of cold pilgered Ti-3Al-2.5V tube

Cold pilgering has been considered as one of the preferred fabrication technologies for high-strength titanium alloy tube. However, the complex multi-pass thermo-mechanical loading histories in whole cold pilgering, viz., multi-stroke rolling and annealing as well as surface strengthening treatment might significantly influence the inhomogeneous deformation and residual stress which closely related to the fatigue and other service performances of tube products. The objective of this manuscript was to clarify the residual stress evolution characteristics in cold pilgering of high-strength titanium alloy tube while to propose an efficient approach for the tailoring of the residual stress. The residual stress distribution features at different deformation zones during cold pilgering were numerically investigated base on the verified finite element model. On account of the coordination of inhomogeneous deformation along the tube wall, the cold pilgered tube exhibited residual tensile and compressive stress on outer and inner surface, respectively. The effects of key process parameters on the residual stress of the cold pilgered tube were explored as well as the relevant inhomogeneous deformation features were discussed, the results showed that the value of residual stress could be affected by the variation of the Q value and area reduction but independent with other parameters. By combining the experiment, it can be also concluded that the residual stress states of the cold pilgered tube were maintained with the variation of the process parameters of cold pilgering and annealing. Furthermore, aiming at the residual stress tailoring and service performances improvement of the tube, the magnetic abrasive finishing (MAF) was introduced while the measurement results indicated that large residual compressive stress could be generated on the outer and inner surface of the tube simultaneously.

Effect of surface nanocrystallization produced by laser shock processing on the corrosion fatigue behavior of 300M steel

The effect of surface nanocrystallization produced by laser shock processing (LSP) on the corrosion fatigue behavior of 300M steel was systematically investigated. The surface integrity, microstructure evolution and residual stresses before and after corrosion fatigue were characterized. The results showed that afte LSP the surface grain size of 300M steel was sufficiently refined down to nanoscale after LSP, and a large residual compressive stress formed. Under the same loading stress level, the corrosion fatigue life of LSP-treated 300M steel was improved significantly. This improvement increased with increase of LSP pulse energy. After corrosion fatigue, the surface grain of LSP-treated 300M steel remained at the nanoscale, showing a good stability. The dislocation density and the number of deformation twins in the subsurface layer became higher with increase of LSP pulse energy to accommodate deformation. Surface residual compressive stress induced by LSP has a relaxation during corrosion fatigue, depending on the laser pulse energy and fatigue loading force.

Microstructure and mechanical performance of dissimilar material joints of 2024Al and SiO2 glass by ultrasonic assisted soldering with Cu interlayer

Ultrasonic assisted soldering was used to join SiO2 glass and 2024Al with Sn solder. The soldering temperature and pressure were set at 300 °C and 0.2 MPa respectively. The Sn solder had good wettability on the SiO2 glass surface by ultrasonic cavitation. At the same time, ultrasonic accelerated the dissolution of 2024Al in molten Sn solder. With the extension of ultrasonic time, Sn solder had better wettability on the surface of base material. The shear strength of the SiO2/Sn/2024Al joint was stabilized at 14–17 MPa by ultrasonic wetting for 5 min before soldering. Due to the large residual stress in the joint, the brittle fracture of the glass occurred during the shear process. To improve the load capacity of the joint, Cu interlayer was added to the joint to relieve the thermal stress. Cu foil effectively relieved the residual stress of the joint during the cooling process. Molten Sn solder reacted with Cu foil and formed intermetallic compounds (IMCs). IMCs effectively block the deformation of filler metal and reduce the generation of stress. When the ultrasonic assisted soldering time was 5 s, the SiO2/Sn/Cu/Sn/2024Al joint reached the highest shear strength about 28 MPa.

A new EPS beads strengthening technology and its influences on axial compressive properties of concrete

Abstract In this article, a new technology of surface wrapping and strengthening of expandable polystyrene (EPS) beads is proposed. Three wrapping materials including magnesium phosphate cement (MPC), ultra-high performance concrete (UHPC), and water-borne polyurethane (WPU) were evaluated. Four groups of uniaxial compression tests of EPS concrete specimens were conducted: the failure process and the characteristics of the specimens were analyzed; and the complete stress–strain equation is proposed based on these analyses. The research shows that the EPS beads were strengthened by wrapping, and the axial compressive strength of concrete is increased by 50–75%; the concrete with EPS beads wrapped by MPC and WPU has obvious brittleness; the concrete with EPS beads wrapped by UHPC has axial compressive strength close to that of EPS concrete wrapped with MPC and WPU but has a larger ductility coefficient (150%), a larger toughness coefficient (50–90% larger), a higher energy dissipation coefficient (40% larger), and larger residual stress. The proposed stress–strain model is in good agreement with the experimental results, which can accurately describe the stress and deformation characteristics of EPS concrete under uniaxial compression and has good applicability. The discovery of this study provides background knowledge for the structural analysis and design of this kind of concrete; it provides a basis for selecting EPS concrete as structural materials.

Effect of shot peening on static and fatigue properties of self-piercing riveting joints

The joint strength of self-piercing riveting is a key factor for evaluating riveting quality. Shot peening is a useful process to strengthen sheet metal. To improve the performance of self-piercing riveting joints, the shot peening process and self-piercing riveting process are combined in this work. The shot peening of AA5052-H32 aluminum alloy sheets, static and fatigue experiments of the self-piercing riveting joints are executed, the effects of shot peening on the mechanical performance of the joints are performed. The results show that: The shot peening process can induce residual compressive stress in the surface layer of aluminum sheets, which can improve the performance of the joints; In the shot peening process, the largest residual stress is around −187.3 ± 5.1 MPa; The shot peening process can improve the static strength of the joints by about 9.5%, and has little effects on the tensile failure forms of the joints; The fatigue strength of the shot-peened self-piercing riveting joints is increased by about 9.2%. The main fatigue failure type is the damage of the lower sheets at the contact zones with the rivet leg. The contact surfaces for the shot-peened self-piercing riveting joints are suffered less from the fretting damage than that of non-shot peened joints. The static strength, fatigue strength are increased, and the fatigue crack growth rate is decreased for shot-peened joints. The shot peening process can be innovatively combined with the self-piercing riveting process to improve self-piercing riveting joint performance. This work provides new ideas to improve the self-piercing riveting quality.

Residual stress and microstructure properties by trailing cooling of argon gas of wire and arc additive manufacturing

To reduce the large residual stress and refine the coarse grains of the deposited layers caused by excessive heat input in the wire and arc additive manufacturing (WAAM), a trailing cooling of argon gas-based WAAM method was proposed. A finite element model of stress field in WAAM with and without cooling source is established. The influence of the cooled gas on the stress and deformation of the additively manufactured parts is analyzed in detail. The results show that, compared with additive manufacturing without cold source, the application of cold source can effectively reduce the longitudinal stress level in the deposited layer area and improve the dimensional accuracy of the deposited layers under the condition of cold source additive manufacturing, the maximum joint displacement is reduced by 24%. Analysis of the microstructure and properties of the deposited layers shows that the cooled trailing gas can refine the grain size of the deposited metal and improve the tensile strength and elongation of the deposited metal, which may be increased by 10% and 8%, respectively.

Unveiling the origins of work-hardening enhancement and mechanical instability in laser shock peened titanium

Laser shock-peening (LSP) produces complex gradients in microstructures and residual stresses, which enables fabrication of metallic engineering components with superior mechanical properties. Here we use non-destructive and spatially resolved high-energy X-ray diffraction (HE-XRD) to investigate the influences of LSP on microstructure, surface topography, and residual stress for high purity titanium plates. LSP is found to produce large compressive in-plane residual stresses near the peened surface which monotonically decay to zero about 2.5 mm below the surface. These properties were also tracked during in-situ tensile loading, allowing stress partitioning and work-hardening rates to be measured as a function of depth. The surface region is found to have the highest work-hardening rate and remain mechanically stable up to sample failure. In addition, a crystal rigid rotation along a transversal axis of about 27° was observed at large applied strain near the surface region, which is attributed to the formation of denser dispersed shear bands. Furthermore, a sudden drop in tensile stress was observed at a depth of ∼160 µm from the LSP surface, indicating the formation of a localized mechanical instability zone (LMIZ). The origin of this LMIZ could be attributed to the increase in shear stress induced by a load transfer from LSP surface due to the formation of denser dispersed shear bands. These observations indicate a localized transition in deformation behavior has occurred from dispersed shear banding to LMIZ and homogeneous deformation.