Large Compressive Residual Stress Research Articles

Surface shot peening of cast SiCw/Al composites

ABSTRACT The surface shot peening treatment was carried out on the as-cast SiCw/Al composites, and the distribution of the residual stresses in the surface matrix was measured, the X-ray diffraction method for the yield strength of the surface matrix of the composites was established to explore the strengthening effect of the surface shot peening. The results show that the surface matrix of the composites after shot peening shows a large compressive residual stress state, and the FWHM, that is, the microstructure strengthening effect, is significantly increased., resulting in an increase in the yield strength of the surface matrix of the composites.

Enhanced fretting fatigue strength of TC11 titanium alloy using laser-assisted ultrasonic surface rolling process

In this paper, ultrasonic surface rolling (USRP) and laser heating-assisted USRP (Laser-USRP) techniques are employed to enhance the fretting fatigue strength of TC11 titanium alloy. Furthermore, the surface deformation mechanism and fretting fatigue behavior are investigated. The properties of the reinforced surfaces were observed using transmission electron microscope (TEM), microhardness test, residual stress test and fretting fatigue experiments. The results showed that laser-USRP led to a remarkable enhancement in the fretting fatigue life of TC11 titanium alloy compared to traditional USRP. This was mainly due to the fact that laser heating improved the plasticity of TC11 titanium alloy, contributed to the formation of a smoother surface and increased the degree of surface oxidation, which could improve surface lubrication. Under high temperature, more slip systems were activated and more stable dislocations were formed, which resulted in larger compressive residual stress (CRS) with higher stability. The superposition of the elevated temperature field and the kinetic energy of the ultrasonic vibration produced an annealing effect in localized regions, resulting in the formation of more stable gradient nanograins. This study demonstrated the immense potential of laser-assisted plastic deformation in improving the surface properties of titanium alloys.

Effect of laser shock peening on wear resistance of M50 steel

The effects of laser shock peening (LSP) on the surface state and wear resistance of M50 steel are studied by microstructure observation, hardness, and residual stress detection. Results show that after LSP, the carbides on the surface of steel are broken and micro-pits appear. After removing the surface carbide damage by electrolytic polishing, the number of sub-surface carbides increases, and the maximum content is 26.1%. After LSP, the wear resistance of the steel is improved because LSP causes a large residual compressive stress on the surface. The wear performance of the sample after removing the surface damage becomes better, which is related to the elimination of many crack sources and the larger residual compressive stress.

Magnetron sputter deposition of boron carbide in Ne and Ar plasmas

Conventional magnetron sputter deposition of B4C uses Ar as the working gas. Here, we explore the magnetron sputter deposition of B4C with a Ne plasma, which is expected to exhibit larger sputtering yields than Ar. We study properties of films deposited with different substrate tilt angles with the magnetron source operated in either direct-current (DC) or radio-frequency (RF) mode in an Ar or Ne plasma. Results show that the B4C film properties are determined by a combination of sputtering ballistics and effects of the working gas on the plasma discharge and gas phase scattering of depositing species flux. At constant discharge power, deposition rates for Ar and Ne plasmas are similar, which is attributed to balancing effects of a higher ballistic sputtering yield of Ne and lower ion flux to the target. Both depositing B and C neutral species and bombarding ions have higher energies for the case of Ne plasmas. Films deposited with the RF-driven Ne plasma exhibit a uniform non-columnar structure, lowest oxygen impurity content, and highest mass density and mechanical properties at a cost of Ne incorporation and larger compressive residual stress.

Effect of contact stress and slip amplitude on fretting fatigue behaviour of ultrasonic surface nanocrystallized TC11 titanium alloy

In this paper, combined experimental and numerical methods are applied to investigate the effect of different contact stress and slip amplitude on the fretting fatigue (FF) behaviour of TC11 titanium alloy with gradient nanostructure introduced by ultrasonic surface rolling process (USRP). The properties of gradient nanostructure are characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray residual stress tester, microhardness tester. As evidenced by the FF experimental results, USRP can significantly improve the FF life of TC11 by introducing large compressive residual stress and work hardening layer. The dominant parameters in improving the FF resistance under different loading conditions are investigated. In addition, the simulated results from Abaqus show that contact stress plays a more important role in influencing FF life compared to fretting bridge span, which is attributed to the change in slip amplitude and stress concentration. Fretting fatigue life initially increases and then decreases with contact stress and there is a contact stress threshold that leads to minimum lifetime. This threshold of USRP specimens is greater than that of untreated specimens. The transition of fretting mode along with contact stress is observed and discussed.

Research on the Residual Stress Field of a Compression Bushing-Lug Plate in Cold Expansion Strengthening

The accurate acquisition of the residual stress field is the key to clarifying the cold expansion strengthening mechanism of compression bushings, optimizing the extrusion process parameters, and improving the structural fatigue life. In the actual cold expansion strengthening process, the resultant distribution of residual stresses is influenced by the relative extrusion amount, mandrel structure, material properties, and extrusion speed. In this article, the distribution patterns of residual stress after cold extrusion are investigated through a combination of finite element simulation and experimental measurements using a micro-region stress tester. To examine the redistribution law of the stress field of cold expansion reinforcement under external load, the compression bushing-lug-plate-reinforced structure is loaded and unloaded. The results show that large circumferential residual compressive stresses are distributed in the hole wall of the compression bushing after cold expansion. Radial residual stresses are also compressive stresses, although the values are small. In addition, the reinforced structure after cold extrusion presents a large difference in the stress redistribution rules compared with cold extrusion after the load is applied and removed.

Study on Microstructure Evolution Mechanism of Gradient Structure Surface of AA7075 Aluminum Alloy by Ultrasonic Surface Rolling Treatment.

The materials with grain size gradient variation on the surface, which were prepared with mechanical-induced severe plastic deformation, always show high resistance to high and low cycle fatigue and frictional wear because of their good strength-ductility synergy. The ultrasonic surface rolling treatment (USRT) has the advantages of high processing efficiency, good surface quality, and large residual compressive stress introduced to the surface after treatment. The USRT was used to prepare aluminum alloy (AA7075) samples with a surface gradient structure; meanwhile, the microstructural evolution mechanism of the deformation layers on the gradient structure was studied with XRD, SEM, and TEM. The microstructure with gradient distribution of grain size and dislocation density formed on the surface of AA7075 aluminum alloy after USRT. The surface layer consists of nanocrystals with random orientation distribution, and high-density dislocation cells and subgrains formed in some grains in the subsurface layer, while the center of the material is an undeformed coarse-grained matrix. The results show that the dislocation slip dominates the grain refinement process, following the continuous cutting and refinement of dislocation cells, subgrains, and fragmentation of the second precipitates. This study systematically clarified the mechanism of grain refinement and nanocrystallization on the surface of high-strength aluminum alloys and laid a theoretical foundation for further research on mechanical behavior and surface friction and wear properties of high-strength non-ferrous materials with gradient structure.

Anisotropic response of additively manufactured 316 L stainless steel towards low-temperature gaseous carburization

In order to modify the surface properties of austenitic stainless steel fabricated by Laser Powder Bed Fusion (L-PBF), low-temperature gaseous carburization (LTGC) was employed. During LTGC, a carburized case of expanded austenite forms at the surface, which contains a high surface carbon content (∼2.5 wt%), high nano-hardness (∼12.6 GPa), large compressive residual stresses (from −2.4 GPa to −3.2 GPa), and is free of carbide precipitates. In the expanded austenite zones, different residual stresses were measured with X-ray diffractometry (XRD) for the top plane, i.e. in the plane perpendicular to the build direction, and for the side plane. This anisotropy is suggested to be caused by grain-shape anisotropy for the different specimen planes and inherent to the hierarchical microstructures resulting from L-PBF.

Mechanical Behavior and Low-Cycle Fatigue Performance of a Carburized Steel for GTF Engines

Using nanoindentation technology to analyze the hardness and elastic modulus distributions of the local microzones within materials, it can be determined that the case-carburized specimen is a composite of the carburized case and the pseudo-carburized material in the core. The overall mechanical behavior of the case-carburized material is much closer to that of the completely carburized material, indicating that the carburized case dominates the case-carburized material. Stress fatigue tests conducted on carburized tubular specimens, pseudo-carburized solid specimens, and case-carburized solid specimens showed that the fatigue performance of the completely carburized material is slightly lower than that of the pseudo-carburized specimens due to lower plasticity. However, the fatigue performance of the case-carburized specimens is significantly better than that of the two homogeneous materials. This could be attributed to the graded material behavior and the larger compressive residual stress in the carburized case, which are the primary positive factors for improving the fatigue life of case-carburized materials. SEM fractographs revealed that the fatigue nucleation in the case-carburized specimen initiates from the transition zone rather than from the surface of the specimens as observed in the homogeneous materials. Low-cycle fatigue evaluation of ultra-high-power gear transmission systems should focus on the influences of the carburized case.

Effect of quenching residual stress on precipitation behaviour of 7085 aluminium alloy

The quenching-induced residual stress in ultrahigh strength Al-Zn-Mg-Cu alloys has a great influence on their precipitation behaviour in subsequent heat treatment and the properties of the final products. In the present work, the cladding quenching method was employed for 7085 Al alloy slabs to tailor different quenching residual stresses. Then the effects of quenching residual stress on the precipitation behaviour of the 7085 Al alloy were investigated through finite element analysis, residual stress and hardness distribution measurements and microstructural examinations. The results indicate that the reduction in residual stress increases with increasing cladding layer thickness. For a cladding layer thickness in the range of 1-1.4 mm, the residual stress is almost eliminated. The hardness at the surface of the aged sample with a large compressive residual stress is decreased, while the hardness at the centre of the sample with a large tensile residual stress is increased. The hardness is approximately identical between the aged samples with little or no residual stress. The elastic lattice distortion caused by residual stress is found to promote the nucleation of precipitates during the artificial ageing process. The in-depth mechanisms for the effect of quenching residual stress on the precipitation of 7085 Al alloy are also discussed.

Development of 16Cr8Ni low transformation temperature welding material for optimal characteristics under various dilutions due to all repair welding positions

A new low transformation temperature (LTT) welding material 16Cr8Ni was developed to satisfy the optimal characteristics in diluted welds due to all repair welding positions. Its good weldability for all welding positions was tested. The measured Ms temperature of weld metals with different dilutions was between 150°C and 250°C which is the optimal range for compressive residual stress generation and fatigue life extension. The in-situ observation of phase evolution by the synchrotron-based X-ray diffraction technique validated the solidification mode of LTT welds for the prevention of solidification cracks. Moreover, the mechanical properties in the diluted LTT welds were measured by miniature uniaxial tensile tests. Large compressive residual stresses at the welded joints due to all welding positions were confirmed numerical analyses.

Ironing effect on surface integrity and fatigue behavior during ultrasonic peening drilling of Ti-6Al-4V

Imposing compressive residual stress field around a fastening hole serves as a universal method to enhance the hole fatigue strength in the aircraft assembly filed. Ultrasonic Peening Drilling (UPD) is a recently proposed hybrid hole making process, which can achieve an integration of strengthening and precision-machining with a one-shot-drilling operation. Due to the ironing effect of tool flank surface, UPD introduces large compressive residual stress filed in hole subsurface. In order to reveal the strengthening mechanism of UPD, the influence of ultrasonic vibration and tool dynamic relief angle on ironing coverage rate and its corresponding effect on surface integrity in UPD were analyzed. The experiments were conducted to verify the influence of ironing effect on surface integrity and fatigue behavior of Ti-6Al-4V hole in UPD. The results indicate that the specimen features smaller surface roughness, higher micro-hardness, plastic deformation degree and circumferential compress residual stress under higher ironing coverage rate. The fatigue life increases with the raise of ironing coverage rate, and the fatigue source site in UPD shifts from surface to subsurface comparing with that without vibration assistance. The results demonstrates that a better strengthening effect can be obtained by reasonably controlling the ironing coverage rate in UPD.

On Variable Scale Evolution of Stress and Strain of TA2 Titanium Plate in Combined Hammering

Combined peening composed of multiple peening processes or peening media is a surface treatment method for comprehensive control of the macro shape and performance of the part. Compared to combined peening, the impact kinetic energy of the combined hammering can be easier to control over a wide range, and the hammer tool head size is larger than the shot. This paper focused on investigating the effect of combined hammering treatment, 6 mm and 14 mm tool heads with peening density 3.7 to 4.2/mm2, on the variable scale evolution of titanium TA2. Three types of contact relation between the tool head and existing dimple were proposed for impacting at the same position. The size of the dimple of combined hammering varies in width or depth direction, resulting in nest morphology composed of different size dimples. The cross-section microstructure of the test plate was observed, and the gradient changes of dislocation, slip, and grain size are smoothed by combined hammering. The change in hammer tool head size makes the target plastic deform at different depths. The hammering sequence has a significant influence on the evolution of stress and strain fields. When the tool head is first large and then small, a large compressive residual stress near the surface is introduced, about 1000 Mpa; on the contrary, the compressive residual stress distributes uniformly in the depth direction, with an affected layer depth of about 4.4 mm. The measured dimple size and residual stress verified the reliability of the simulation results.

High-temperature fatigue crack growth characteristics of IN718 Ni-based alloy treated by laser peening

The characteristics of fatigue crack growth (FCG) of laser peened IN718 Ni-based alloy at high temperature with different laser power densities were studied. The residual stress relaxation and micro-structure evolution of the samples at high temperature after laser peening (LP) were analyzed by X-ray diffraction technique and scanning electron microscopy (SEM), respectively. Meanwhile, SEM was used to characterize the fracture morphologies of the samples. The strengthening mechanism of LP on the high-temperature FCG resistance of IN718 Ni-based alloy was discussed. The results showed that large compressive residual stress, fine grains and complex grain boundaries induced by LP could significantly enhance the resistance of FCG at 700 °C. As a result, the FCG rate was decreased and fatigue life of the sample was increased. In addition, the laser power density was found to be inversely proportional to the average spacing of fatigue striations and directly proportional to the size and depth of dimples, which indicated that the increase in laser power density can improve the high-temperature FCG resistance of IN718 Ni-based alloy.

Determination of optimal ultrasonic surface rolling parameters to enhance the fatigue strength of railway axle EA4T steel

The increasing complexity of railway services is placing higher demands on ensuring the fatigue resistance of critical components of high-speed trains. The ultrasonic surface rolling process (USRP) was employed to enhance the fatigue resistance of the EA4T axle steel used for high-speed trains in China. The optimal USRP parameters were first determined by orthogonal testing, with the rotational bending fatigue life as the evaluation index. Multidimensional experimental characterization and finite element simulation results demonstrated that the surface layer of the treated axle steel underwent violent plastic deformation, in which the grain size, hardness and geometrically necessary dislocation density showed distinct variation with depth from the surface. The surface roughness of the USRP-treated specimens was significantly less than for the untreated specimens, and the average compressive residual stress increased by a factor of 4.3. The 46.9 % increase in rotational bending fatigue strength (from 320 to 470 MPa) was attributed to the combined action of the large compressive residual stresses, the work-hardened surface layer, and less surface roughness. These results demonstrate that ultrasonic surface rolling is an effective surface hardening technique to improve the fatigue resistance of railway axles.

Improvement of plasma carbonitriding modified layer on TA15 surface by RASP-assisted DGPSA treatment

In this paper, a dual process combining rotationally accelerated shot peening (RASP) and double glow plasma surface alloying (DGPSA) was designed to prepare Ti(C,N) modified-layer on titanium alloys. The improvement effect of RASP pretreatment on DGPSA technology was analyzed by comparing the surface phase composition, wear resistance and fractography of plasma-carbonitriding (PCN) layers with and without RASP pretreatment. It was found that RASP pretreatment promoted the diffusion and deposition of carbon and nitrogen atoms on the surface of titanium alloy, forming a thick and dense modified layer with a gradient variation in hardness. There is a large residual compressive stress on the surface of the modified layer after RASP-assisted DGPSA treatment. The near-surface fracture morphology indicates that RASP pretreatment can alleviate the adverse effects of surface brittleness and severe grain coarsening caused by heat treatment and improve the surface mechanical properties. The gradient reinforced layer formed by the dual treatment on the subsurface effectively supports the dense compound layer on the surface, resulting in excellent wear resistance.

Microstructure and mechanical properties of composite strengthened high-chromium cast iron by laser quenching and laser shock peening

The study investigated the microstructure and mechanical properties of composite strengthened high-chromium cast iron by laser quenching and laser shock peening. Observation of microstructure, measurement of microhardness, residual stress and FWHM (full width at half maximum), impact toughness and wear tests were carried out on untreated, laser quenched, laser quench-laser shock peened high-chromium cast iron specimens. The research results showed that the laser shock peening will not produce new phases in high-chromium cast iron, and can further promote the transformation of retained austenite into martensite. The grains inside the laser quench-laser shock peened sample were obviously refined, and the carbides were uniformly dispersed. The laser quench-laser shock peened specimen produced large residual compressive stress and FWHM value on the surface, reaching −432.49 MPa and 2.124°, respectively. The problem of residual stress difference between the hardened zone and the transition zone caused by laser quenching was eliminated, and the tensile stress was all converted into compressive stress, which further increases the dislocation density and improves the micro-hardness. The impact toughness value of laser quench-laser shock peened specimen was 4.43 J/cm2, which was 16.27% higher than that of the untreated sample. The friction coefficient and wear rate were significantly reduced, and the wear scar was shallow and narrow, showing weak abrasive wear and oxidative wear, the impact toughness and wear resistance of high-chromium cast iron were improved. The results obtained from this study could be used as reference in future research and applications of laser strengthened high-chromium cast iron.

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.

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.

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.