Generation Of Residual Stresses Research Articles

A simple yet consistent constitutive law and mortar-based layer coupling schemes for thermomechanical macroscale simulations of metal additive manufacturing processes

This article proposes a coupled thermomechanical finite element model tailored to the macroscale simulation of metal additive manufacturing processes such as selective laser melting. A first focus lies on the derivation of a consistent constitutive law on basis of a Voigt-type spatial homogenization procedure across the relevant phases, powder, melt and solid. The proposed constitutive law accounts for the irreversibility of phase change and consistently represents thermally induced residual stresses. In particular, the incorporation of a reference strain term, formulated in rate form, allows to consistently enforce a stress-free configuration for newly solidifying material at melt temperature. Application to elementary test cases demonstrates the validity of the proposed constitutive law and allows for a comparison with analytical and reference solutions. Moreover, these elementary solidification scenarios give detailed insights and foster understanding of basic mechanisms of residual stress generation in melting and solidification problems with localized, moving heat sources. As a second methodological aspect, dual mortar mesh tying strategies are proposed for the coupling of successively applied powder layers. This approach allows for very flexible mesh generation for complex geometries. As compared to collocation-type coupling schemes, e.g., based on hanging nodes, these mortar methods enforce the coupling conditions between non-matching meshes in an L^2-optimal manner. The combination of the proposed constitutive law and mortar mesh tying approach is validated on realistic three-dimensional examples, representing a first step towards part-scale predictions.

Understanding the Relations between Surface Stress State and Microstructure Feature for Enhancing the Fatigue Performance of TC6 Titanium Alloy

Fatigue performance has always been an important factor affecting the application of titanium alloy. The service life of TC6 titanium alloy is easily reduced under a continuously alternating load. Therefore, there is an urgent need for a new method to improve fatigue performance. Laser shock peening (LSP) is a widely proposed method to enhance the fatigue performance. Here, through experiments and finite element simulations, it was found that LSP can prolong the fatigue life of TC6 by improving the surface stress state. In strengthening processes, the generation of residual stress was mainly attributed to the change of microstructure, which could be reflected by the statistical results of grain sizes. The content of grains with a size under 0.8 μm reached 78%, and the microhardness value of treated TC6 was 18.7% higher than that of an untreated sample. In addition, the surface residual compressive stress was increased to −600 MPa at the depth of 1500 μm from the surface. On this basis, the fatigue life was prolonged to 135%, and the ultimate fracture macroscopic was also changed. With the treatment of LSP, the fatigue performance of TC6 is highly promoted. The strengthening mechanism of LSP was established with the aim of revealing the relationship between microstructure and stress state for enhancing the fatigue performance in whatever shapes.

Comparison Between Surface and Near-Surface Residual Stress Measurement Techniques Using a Standard Four-Point-Bend Specimen

BackgroundDetermination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties.ObjectiveThis study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI).MethodsThese measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties.ResultsThe XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation.ConclusionThe ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method.

Analysis of abrasive belt wear effect on residual stress distribution on a grinding surface

Owing to elastic grinding and other favorable processing characteristics, belt grinding has been widely used in many industrial applications, including machining titanium alloys of difficult-to-cut metals. Abrasive belt wear inevitably occurs in the grinding process, but its mechanism and effects have not been fully understood, especially its effect on the residual stress (RS). To overcome this shortcoming, this study experimentally investigates the abrasive wear influence on the surface RS distribution of titanium plate belt grinding through a synthetic cause analysis. First, abrasive belts with different wear stages are prepared according to the wear law throughout the abrasive belt life cycle, and their wear degrees are quantitatively evaluated. Then, comparative grinding experiments of the TC4 flat plate are conducted using the prepared abrasive belts. Finally, the surface RS distribution of the grinding traces and stress differences induced by the abrasive belt wear are analyzed and discussed comprehensively. The results indicate that the more severe the abrasive belt wear is, the greater the accumulation of grinding heat and the greater the ratio of the grinding energy will be, which will further lead to residual tensile stress (RTS) generation, especially in the grinding direction. In addition, owing to the fast heat dissipation on both sides of the contact wheel, the surface RS along the grinding traces' vertical direction obeys an approximately symmetrical distribution, having low values on the sides and a peak in the middle. The strong accumulation of grinding heat caused by severe wear makes the difference between the stress distribution extremums increase with the abrasive belt wear.

Changes in the microstructure and mechanical properties of Al2O3/YSZ directionally solidified eutectic ceramics during long-time heat treatment

Al2O3/YSZ directionally solidified eutectic ceramics (DSECs) were prepared using the induction heating zone melting (IHZM) method. The microstructures of the DSECs formed with various crucible pulling rates (2.5–300 mm/h) changed from the discontinuous lamellar into the colony structure, finally into the cell structure. The intracolony had better microstructural stability than the intercolony during heat treatment due to two factors: the size effect and the t-m transformation effect. The hardness changes were mainly related to the release and generation of residual stress caused by repeated heating and cooling. The quantity of cracks in DSECs increased with the heat treatment time, leading to a gradual decrease in the fracture toughness, 300 mm/h DSEC had the lowest reduction in fracture toughness after 500 h heat treatment due to fine microstructure, the fracture toughness was 7.15 MPa·m1/2, which was 76 % of that before the heat treatment (9.39 MPa·m1/2).

Influence of 3D printing stress wave on residual stress

As a popular rapid manufacturing technology, three-dimensional printing (3D printing) has been widely adopted in medical, automotive, aerospace, construction and other industrial fields. The stereo lithography apparatus (SLA) is an effective 3D printing method for ceramic printing. However, shrinkage from monomer to polymer and laser stress wave cause residual stress in the formed parts during SLA. Serious quality defects including cracks, warpage and deformation caused by residual stress have remained to be a problem. Basically, the laser stress wave plays an important role in the generation of 3D printing residual stress. In this work, to investigate the propagation mechanism of the laser stress wave, the finite element method was adopted to simulate the SLA process of zirconia. The influence of 3D printing factors on the residual stress was obtained, and we found that the wave velocity of the stress wave obtained by the simulation model was highly consistent with the theoretical wave velocity. Meanwhile, the attenuation formula of the stress wave in the 3D printing process was obtained by fitting to investigate its attenuation law. Based on the above results, the attenuation law of the 3D printing stress wave has a direct influence on the development and variation trend of its residual stress.

Analysis of residual stress relaxation of aluminum alloys EN AW 6061/‐82 T6 under cyclic loading

AbstractStress relaxation describes the reduction of stress under static or cyclic loading at a constant strain level. Several processes induce intentionally residual stresses, for example, autofrettage of thick‐walled pressurized tubes to improve their fatigue life. This well‐known process induces residual compressive stresses at the critical inner surface by using a single static but controlled overloading internal pressure. Relaxation of residual stresses due to cyclic loading in service would endanger the effectiveness of autofrettage and could finally lead to unexpected fatigue failure. In this study, strain‐controlled experiments up to 500,000 load cycles and amending nonlinear finite element simulations were done for the aluminum alloys EN AW 6061 T6 and EN AW 6082 T6 to study potential cyclic stress relaxation in four‐point bending tests after controlled single static plasticization for residual stress generation. This analysis identifies almost stable residual stresses for both materials under different cyclic strain‐controlled load levels.

Study of the Contact Thermal Compression Behavior of Copper using Scanning Contact Potentiometry Method and Finite Element Analysis

The issue of tracking residual stresses initiation in welding copper under various affected conditions is essential in increasing safety through improving the welding quality in particular, in the nuclear industry. This study investigated the behavior of welded copper numerically and experimentally under contact-heating compression test with constant clamping force. The scanning contact potentiometry (SCP) method was used to track the initiation and development of residual stresses within the weld zone. Furthermore, the finite element analysis (FEA) method was used to simulate and study the effect of thermal variations, with a constant compressive force, on mechanical factors that contribute to residual stresses formation within the weld zone. Additionally, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to get further information about the topography and composition of the specimen's surface. SCP results show that residual stress initiated from within the volume after 200 °C, and with further oxidation, its formation began on the surface after 250 °C. Using the relationship between maximum values of linearized von Mises stresses and maximum values of ADS at high SLS = 4.523, it was found that residual stresses generation began after 150 °C within the weld zone, and thermal stresses linearly increase with temperature due to further thermal expansion, which is associated with variation in linearized von Mises stresses and ADS maximum values. Comparison between potentiograms after 300 °C and FEA results have shown that the distribution of von Mises stress, normal stress, and total deformation are matching those in the distribution of ADS and are localized within the weld zone.
 HIGHLIGHTS
 
 The behavior of welded copper under compression at medium-low temperature range
 Residual stresses detection at an early stage localized within weldment zone
 Residual stresses increase with increased oxidation at copper surface
 Matched the SCP Experimental and FEA simulation results
 
 GRAPHICAL ABSTRACT

Compressive residual stress generation at crack tip in bulk metal using acoustic cavitation

We propose to generate compressive residual stress at the crack tip in the bulk metal by introducing acoustic cavitation to the crack tip. We conducted experiments using pre-cracked specimens to measure compressive residual stress at the crack tip achieved by the acoustic cavitation. In the experiments, the pre-cracked metal specimens were subject to an acoustic load with a frequency of 20 kHz to introduce acoustic cavitation to the crack tip. We found that the acoustic cavitation generates a significant amount of compressive residual stress at the crack tip and the generated compressive residual stress effectively retarded the crack growth.

Modeling the superposition of residual stresses induced by cutting force and heat during the milling of thin-walled parts

Previous studies on machining-induced residual stress mainly concentrated on qualitative analyses of mechanical load and thermal load. Nevertheless, it remains a major challenge to quantify the influence of cutting force and heat on residual stress generation due to the lack of superposition mechanism. A newly empirical model of milling residual stress superposition was constructed to distinguish the effect of milling force from the effect of milling heat. The influence proportions of milling force and heat on the residual stress generation have been determined. Firstly, the residual stresses induced by the milling force, heat and force-heat coupling load were simulated independently, and the simulation models were verified by experiments. The simulation results presented that the milling force caused both residual compressive stress and residual tensile stress on the machined surface, while milling heat induced mainly residual tensile stress. Then, a mathematical model was put forward to quantify the superposition relationship between the force-induced residual stress and the heat-induced residual stress presented in the simulations. It has been found that the influence of milling force accounts for 53% to 78% in the generation of the surface residual stress when the force-to-heat ratio (RFT) varies from 0.09 to 0.39. Finally, with a constant material removal rate, the conclusion can be drawn that the parameter combination with a higher force-to-heat ratio is more beneficial for the generation of residual compressive stresses on the machined surface, which has been confirmed by the case experiments. Therefore, quantitative analysis of residual stress generation and superposition turns to be possible by using the proposed modeling.

Generation mechanism of residual stress at press-blanked and laser-blanking edges of 1.5 GPa ultra-high strength steel sheet

The generation mechanism of the residual stress at press-blanked and laser-blanked edges of a 1.5 GPa ultra-high strength martensite steel sheet was investigated. The residual stress at the press-blanked edges of the 1.5 GPa sheet is highly tensile, whereas the residual stress at the laser-blanked edges is compressive. The high tensile residual stress causes the occurrence of hydrogen-induced delayed cracking and the reduction in fatigue strength. In press blanking of the 1.5 GPa sheet, considerably high compressive stress was generated under the punch corner just before the separation due to a small critical punch stroke, and high tensile residual stress on the fracture surface of the blanked edge was induced by releasing this high compressive stress for the separation. Not only the high strength of the 1.5 GPa sheet but also the low ductility influences the generation of the high tensile residual stress on the fracture surface because of the small critical stroke. On the other hand, the compressive residual stress at the laser-blanked edges of the 1.5 GPa sheet was generated by the volume increase of the martensite transformation at a comparatively low temperature. In addition, an approach for determining the allowable amount of hydrogen for the occurrence of delayed cracking at the edges was proposed, and the relationship between the critical residual stress and the amount of hydrogen for the 1.5 GPa sheet was obtained.

Thermo-mechanical analysis of laser welding of Grade 91 steel plates

Generation of residual stresses and distortion in weld component is a major concern in Grade 91 steel thin plates. In the present study, FEM based Laser welding analysis of Grade 91 steel is carried out for 3 thick plates using SYSWELD software. Heat input optimization has been carried out by bead-on-plate welding at different heat input. Optimized heat input with full depth of penetration is used for a square butt joint of Grade 91 steel laser welding. Experimentally measured residual stresses using X-ray diffraction technique are compared with predicted results. Residual stresses comparison shows that due to phase transformation effect the peak tensile stress values are observed next to heat affected zone. The predicted distortion in longitudinal and transverse directions is validated with experimental results.

Comparison of Process Signatures for thermally dominated processes

Four machining processes are analyzed with respect to the internal material loads and modifications in the surface layer. The processes considered are grind hardening, induction hardening, laser hardening as well as Electrical Discharge Machining (EDM), which all impose primarily heat loads on the workpiece. The concept of Process Signatures is assumed, in which suitable descriptive quantities of the internal material loads are correlated to the resulting modifications in the surface layer. The surface layer modifications chosen here are the residual stresses, phase compositions and the thickness of the transformation zone. Validated numerical simulation methods are used to determine the material modifications for grind hardening and induction hardening. For laser hardening probe measurements are used for the residual stresses and the thickness of the transformation zone. For the EDM the residual stresses were also measured. The thickness of the white layer and the transformation zone were derived from simulations. In all cases, numerical simulations are conducted to determine the temperature field during the machining process. The analysis of the involved mechanisms such as martensite formation suggests that the maximum temperature gradient is a suitable descriptive quantity for the generation of residual stresses. The results obtained for the four machining processes confirm this behavior. The depth of the transformed zone is shown to correlate with the square of the maximum temperature multiplied by the square root of the contact time. Although the different processes investigated show several orders of magnitude different heating and cooling rates, the results achieved are promising and confirm the applicability of the Process Signature concept.

Influence of process parameters on the residual stress state and properties in disc springs made by incremental sheet forming (ISF)

Disc springs are machine elements that are used when high forces need to be supplied and in limited installation space. They need to fulfil high demands on the stability of the spring characteristics, reliability and lifetime. In corrosive environments, metastable austenitic stainless steels (MASS) disc springs are often used. Tensile stresses that occur during service limit the lifetime of disc springs. Usually, their durability is enhanced by generating favorable compressive residual stresses using shot peening operations. Such operations lead to extra efforts and additional production costs. In this study, the adaptive and targeted generation of residual stresses via incremental sheet forming (ISF) is investigated as alternative to shot peening focusing on EN 1.4310 and EN 1.4401 stainless steel. Previous work has shown that ISF is capable of controlling the radial and tangential stresses in the springs. However, no analysis of the influence of the residual stress state in the rolled sheet strips and the ISF process parameters was performed. The goal of the current work is to analyze the evolution of residual stress during rolling and subsequent incremental forming of disc springs. In order to examine the role of dissipation and temperature increases in the rolling process, sheet blanks rolled at room and elevated temperature are analyzed. The characteristics of the compressive residual stresses induced by ISF are studied for different process parameters. X‑ray diffraction is used to investigate the buildup of these stresses. Using ISF, the generation of compressive residual stresses can be integrated into the forming process of disc springs, and further post-treatment may be skipped. The results show that the residual stress state in the rolled material is crucial, which requires tight control of the rolling temperature. Another result is that ISF is able to yield high compressive residual stresses and improved spring characteristics when small tool diameters and step-down values are used.

Thermal Effects on Surface and Subsurface Modifications in Laser-Combined Deep Rolling

Knowledge of the relationships between thermomechanical process loads and the resulting modifications in the surface layer enables targeted adjustments of the required surface integrity independent of the manufacturing process. In various processes with thermomechanical impact, thermal and mechanical loads act simultaneously and affect each other. Thus, the effects on the modifications are interdependent. To gain a better understanding of the interactions of the two loads, it is necessary to vary thermal and mechanical loads independently. A new process of laser-combined deep rolling can fulfil exactly this requirement. The presented findings demonstrate that thermal loads can support the generation of residual compressive stresses to a certain extent. If the thermal loads are increased further, this has a negative effect on the surface layer and the residual stresses are shifted in the direction of tension. The results show the optimum range of thermal loads to further increase the compressive residual stresses in the surface layer and allow to gain a better understanding of the interactions between thermal and mechanical loads.

Analysis of Residual Stress of Injected Plastic Parts: A Multivariable Approach

The analysis of residual stresses generated during the injection molding process is crucial for the part quality assessment. The present study evaluates the generation of residual stress considering different variables that affect them. For that purpose, diverse part geometries were evaluated with different polymers (polycarbonate, ABS and polypropylene) and some Design of Experiments (DOE) were implemented in a simulation software of injection molding process (Moldex 3D). The results show higher residual stresses in thick wall parts due to unbalanced cooling through-thickness. For thin-wall parts, residual stresses were lower because of better cooling through its thickness. The polycarbonate was the more sensitive polymer to residual stresses upon its processing conditions, being the mold temperature and the packing stage the most critical variables to increase or reduce residual stresses.

Targeted residual stress generation in single and two point incremental sheet forming (ISF)

The mechanical properties and the operating life of a formed component are highly dependent on the residual stress state. There is always a high magnitude of residual stresses in the components formed by incremental sheet forming (ISF) due to the localized deformation mechanism. Hence, a thorough understanding of the generation of the residual stresses by ISF is necessary. This study investigates the residual stress generation mechanism for two process variants of ISF, i.e., Single Point Incremental Forming (SPIF) and Two Point Incremental Forming (TPIF). This understanding is used to control and targetedly generate the residual stresses to improve the part performance. In this regard, the residual stress state in a truncated cone geometry manufactured using SPIF and disc springs manufactured using TPIF was experimentally analyzed. Validated numerical models for both process variants were developed to study the residual stresses in detail. The residual stress state in SPIF is such that the tool contact side develops tensile residual stresses and the non-contact side undergoes compressive residual stresses. The tool step-down variation was used to control residual stresses and improve the fatigue strength of truncated cones manufactured using SPIF. For TPIF, two different forming strategies were used to analyze the residual stress generation mechanism and the role of major process parameters. The residual stresses for TPIF are pre-dominantly compressive in both directions of forming tool motion. For both process variants of the ISF process, it is shown that the residual stresses can be beneficially utilized to improve mechanical properties of the components.

Recent Developments and Novel Applications of Laser Shock Peening: A Review

By inducing work hardening and beneficial compressive residual stresses in near‐surface regions, laser shock peening (LSP) improves the fatigue performance of many metallic components. In recent years, LSP has found many new applications in emerging fields: additive manufacturing, ceramics, and metallic glasses. In addition, innovative process development based on LSP has been reported, which includes warm LSP, cryogenic LSP, electropulsing‐assisted LSP, laser peening without coating, femotosecond LSP, and laser peen forming. Herein, a comprehensive review of the LSP process with a focus on the novel applications and innovative process development is aimed for. The history of LSP with key events is briefly reviewed. The fundamental mechanisms of LSP, including the generation of shockwaves by a high‐energy pulsed laser, the generation of compressive residual stresses by the shockwave, and its effect on crack propagation, and how LSP induces grain refinement are also discussed. The effects of compressive residual stresses and grain refinement on the mechanical properties of metallic materials are reviewed. Recent developments in LSP, such as innovative process development based on LSP and its novel applications, are discussed. Finally, the current challenges the LSP technology faces and its future directions are also discussed.

Reduction of dislocations in α-Ga2O3 epilayers grown by halide vapor-phase epitaxy on a conical frustum-patterned sapphire substrate.

The compound α-Ga2O3 is an ultra-wide-bandgap semiconductor and possesses outstanding properties such as a high breakdown voltage and symmetry compared with other phases. It has been studied for applications in high-performance power devices. However, it is difficult to obtain a high-quality thin films because α-Ga2O3 can only grow heteroepitaxially, which results in residual stress generation owing to lattice mismatch and thermal expansion between the substrate and α-Ga2O3. To overcome this, α-Ga2O3 was grown on a conical frustum-patterned sapphire substrate by halide vapor-phase epitaxy. The surface morphology was crack-free and flat. The α-Ga2O3 grown on a frustum-patterned substrate and a conventional sapphire substrate at 500°C exhibited full-width at half-maxima of 961 and 1539 arcsec, respectively, for 10-12 diffraction. For the former substrate, lateral growth on the pattern and threading dislocation bending towards the pattern suppressed the propagation of threading dislocations generated at the interface, which reduced the threading dislocation propagation to the surface by half compared with that on the latter conventional substrate. The results suggest that conical frustum-patterned sapphire substrates have the potential to produce high-quality α-Ga2O3 epilayers.

Numerical modelling of residual stresses during orthogonal cutting of Ti6Al4V using internally cooled cutting inserts

Residual stress has an adverse effect on the fatigue strength of machined components. The parts used in high end industries like aerospace and automobile sector needs to be carefully examined for the residual stress development during machining process. In this paper, both thermal and mechanical effects have been taken into account to predict the residual stress in workpiece while machining using internal cooled cutting inserts. The internal cooling system has now a days being used as an advanced cooling system for Ti and Ni based materials. In the present paper, Oxley’s model for slip line theory and contact theory using McDowell’s hybrid method have been used to predict the forces and residual stress respectively. The effect of change in the flow of lubricant from laminar to turbulent in internal channel of cutting insert has also been analysed. It has been found analytically that the cutting force of 761 N during dry turning is decreased to 511 N and 494 N in case of laminar and turbulent flow respectively. Besides this, the generation of compressive residual stress is found at the surface layer of machined specimen for internally cooled cutting inserts. The maximum compressive stress of 150 MPa at 0.03 mm depth of work piece has been found in case of laminar flow and 100 MPa for the turbulent flow.