High Compressive Residual Stress Research Articles

Analysis and design optimization of double-sided deep cold rolling process of a Ti-6Al-4V blade

Deep cold rolling (DCR) is a promising mechanical surface treatment which can be effectively used to improve the fatigue life of components by introducing deep and high beneficial compressive residual stresses on the surface and sub-surface layers. However, the application of conventional DCR on thin-walled geometries such as compressor blades can be very challenging as the applied load can damage the component. Double-sided deep rolling on thin-walled components has been proven to be a viable alternative solution as both sides of the component are treated simultaneously which thus decreases the risk of component distortion. In this study, a high-fidelity non-linear finite element model has been developed to simulate the double-sided DCR process on thin Ti-6Al-4V plate and to predict the residual stress profile introduced by the process and after thermal relaxation due to subsequent exposure to high temperature. The accuracy of the developed finite element model is validated by comparison with the experimental measurement available in the literature. Response surface method (RSM) has then been carried out on results obtained by the high-fidelity FE model to develop predictive analytical models to approximate residual stress profiles induced by the process. The developed analytical models can efficiently replace FE models to perform sensitivity analysis and design optimization of process parameters. Load distribution at high stress areas of a generic compressor blade is considered to formulate a design optimization problem of double-sided DCR process in order to achieve optimal residual stress distributions at room temperature and after thermal relaxation at elevated temperature of 450 °C.

3D DIC-assisted residual stress measurement in 316 LVM steel processed by HE and HPT

A method has been developed for determining residual stress based on displacement fields near drilled holes analyzed using 3D digital image correlation. Finite element modeling was used to determine corrections for analytical equations describing displacement fields near the blind holes, which made it possible to determine the residual stress distribution over a wide range of hole depth-to-hole diameter ratios and various areas of displacement field measurements using inverse method iterative calculations. The proposed method eliminates many drawbacks of traditional procedure based on strain gauges as hole eccentricity sensitivity and requirement of the relatively large span between holes. The method and testing setup, build-up of generally available components, were used to determine the residual stress distribution for 316 LVM samples processed by two methods from the large deformation group: hydrostatic extrusion (HE) and high-pressure torsion (HPT), by drilling 1.75 and 0.58-mm-diameter blind holes, respectively. In the case of the measurements performed on the surface of a HE-processed 16 mm bar cut along its diameter, a gradual change was revealed—from a compressive to a tensile residual stress distribution (from ~ − 300 MPa in the center to 400 MPa in 4 mm distance from the edge) in the longitudinal direction, with near-zero values in the radial direction. Moreover, the method was also adapted to perform measurements on the outside surface of the bar, which gave results consistent with those taken along the radius profile (~ 600 MPa longitudinal stress). Measurements on the top surface of a cylinder 10 mm in diameter and 1 mm high processed by HPT showed a high compressive residual stress in the center and a dominant shear component for the holes drilled at different distances from the center.

Effect of residual stresses on the mechanical properties of Ti-TiAl laminate composites fabricated by hot-pack rolling

The Ti-TiAl laminate composites with different titanium alloy volume fraction were successfully designed and fabricated by hot-pack rolling. Due to the mismatch of coefficient of thermal expansion (CTE), the composites show pronounced residual stresses. The TiAl alloy possesses residual tensile stress while the titanium alloy possesses residual compressive stress. With the increase of titanium alloy volume fraction, the residual compressive stress is decreased but the residual tensile stress is opposite. The residual compressive stress is beneficial for improving the mechanical properties of Ti-TiAl laminate composites, which is because it enhances the strength and postpones the rupture of composites during tensile test. The higher residual compressive stress also improves the fracture toughness of composites by extending crack propagation. The composite with 42 vol% titanium alloy shows the optimal comprehensive mechanical properties, which is higher than most of other Ti-Al based metal-intermetallic laminate composites.

Microstructure and mechanical properties of laser shock peened 38CrSi steel

Laser shock peening (LSP) induces severe plastic deformation and high compressive residual stress in the metal surface, which can effectively enhance mechanical properties. In this work, the effects of LSP on the microstructure, microhardness, residual stress, tensile and fatigue properties of 38CrSi steel were investigated. After LSP, high-density dislocations such as dislocation walls, dislocation cells, and deformed cementite lamellae structures were observed in the surface layer. The dislocation density was roughly calculated by TEM images. Surface microhardness strengthened by dislocations increased by 24%. The maximum value of compressive residual stress reached −404 MPa. The depth of residual stress affected layer was 750 μm. The peening area had a great influence on the fatigue property. Compared with the base metal (BM), the fatigue life of specimens peened on Area I was improved by 76%, while the fatigue life of specimens peened on Area II was improved by 125%. Finite element analysis revealed that the compressive residual stress induced by LSP in the whole edge of specimen peened on Area II was the main factor for further improving fatigue property. The relationship between compressive residual stress field and crack path was investigated to clarify the strengthening mechanism in different peening areas.

Study of Impact Velocity on Residual Stress and Surface Hardness of SKD11 in Shot Peening Process

Shot peening process could create compressive residual stress and increase surface hardness and hence also used to improve material surface properties in case thermal effect is to be avoided. The shot peening process parameters such as pressure which result in different shot impact velocity could affect the compressive residual stress distribution which results in different post-process material properties. The ability to understand and predict the effect of process parameters on stress distribution could be very useful to control and obtain material properties as required. In this work, a shot peening process commercially available locally was investigated. The residual stress distribution after shot peening of SKD11 was studied using the finite element (FE) technique. A single shot impact was simulated. A maximum velocity with a miximum impact angle was assumed. The computational predictions showed higher compressive residual stress developed with increasing shot velocity as expected due to higher impact energy. However, experimental results suggested that the process arrangement and machine control highly affect the properties of the material after process. The compressive residual stress and surface hardness obtained experimentally was almost unchanged with an increase in pressure from 0.35MPa to 0.6MPa. It was found that, due to machine arrangement, an increase in impact velocity at higher pressure was relatively small and did not observed in all effected area due to fixed arrangement of nozzle and samples. Hence, research results suggested that a detail computational methodology including the effect of unevent impact velocity and impact angle should be employed to increase the predictive ability of the FE model. The current work could be extended to include such effects with no major difficulty to develop useful information for the design of shot peening process for any specific machine and arrangement.

Residual Stress Control on Bearing Steel by Surface Cooled Induction Heating Fast Tempering

In order to reduce the amount of CO2 emission during carburizing while keeping the advantages of case hardening, a surface cooled induction heating fast tempering process has been developed for the production of bearing elements. This process is applied to SUJ2 bearing steel, followed by conventional hardening. By surface cooled induction heating fast tempering, high temperature at the center of the test piece and relatively low temperature at the surface cause a difference in tempering temperature, leading to compressive residual stresses at the surface because of the difference in volume change during tempering. As a result, the bearing made by this process has the same rolling contact fatigue life as a carburized bearing due to high compressive residual stress. Process simulation of surface cooled induction heating fast tempering shows that residual stress is mainly caused by the difference in volume change of martensite during this process between the surface and the center. However, the decomposition of retained austenite appears to shift residual stress toward tensile stress.

Analytical modeling and experimental verification of surface roughness in the ultrasonic-assisted ball burnishing of shaft targets

Ultrasonic-assisted ball burnishing (UABB) is a promising surface strengthening technology for improving the properties of components. This method superimposes ultrasonic vibration to the traditional deep rolling; it can improve the surface roughness of a material, enhance its surface hardness, and introduce a high compressive residual stress, thereby improving the fatigue performance of components. A mathematical model should be established to predict and analyze the relationship between the experimental process parameters and results and to predict the effect of UABB on surface roughness. In the present work, a mathematical model of surface roughness after UABB was established on the basis of Hertz contact theory, comprehensively considering the process parameters, dynamic vibration forces, and elastic rebound of the targets. UABB experiments were carried out to verify the correctness of the mathematical model. Results showed that UABB can significantly reduce the surface roughness of the material, and the values calculated from the derived mathematical model showed good agreement with the experimental findings. The surface roughness decreased with the burnishing force (50–250 N) and amplitude (4–10 μm) increased, whereas the feed rate decreased. The increases of radii of component and ball were detrimental to the improvement of the surface roughness. And elastic rebound was not conducive to roughness reduction and its influence was negligible. Finally, the derived surface roughness model can be used to predict the component values after UABB and understand the effects of machining parameters on the shaft targets during processing.

Effect of phase in surface layers on rotating-bending fatigue strength of SCM415 steel after austenitic nitriding

The rotating bending fatigue strength of JIS-SCM415 steels after austenitic nitriding was evaluated to find the surface phase that gave the highest strength. Nitriding was performed at 903 K under conditions of KN = 0.15, 0.25 and 0.35 atm-1/2. In addition, specimens which were tempered at 523 K for 7.2 ks after nitridings with KN = 0.15 and 0.25 atm-1/2 were prepared. A specimen prepared by nitrocarburizing at 853 K was also evaluated as a comparison. Specimens nitrided at 903 K at KN = 0.15 atm-1/2 showed the highest fatigue limit because the 10 μm thick surface layer was a dual phase of austenite and martensite that might have a fine microstructure which inhibited initial crack generation and growth. Specimens nitrided at 903 K at KN = 0.35 atm-1/2 showed the second highest fatigue limit because the ε compound layer that was 20 μm thick had a high compressive residual stress. Specimens nitrides at 903 K at KN = 0.25 atm-1/2 showed the third highest fatigue limit because of the formation of a 6 μm thick compound layer which was a dual phase of ε and γ’ that had few voids and a high toughness (low hardness) in addition to its compressive residual stress. Tempered specimens showed a lower fatigue limit than that of non-tempered specimens. This was because the surface layer became brittle in tempered specimens. Specimens other than the tempered specimen in KN = 0.25 atm-1/2showed a fatigue strength higher than that of a sample nitrocarburized at 853 K.

Fatigue behaviors of foreign object damaged Ti-6Al-4V alloys under laser shock peening

Two-sided and simultaneous laser shock peening (TSLSP) was used to strengthen the Ti-6Al-4V titanium alloy. The fatigue crack growth rate of the specimens was investigated by fatigue test. The results show that both direct TSLSP (TSLSP-D) and indirect TSLSP (TSLSP-I) can reduce the fatigue crack growth rate by producing high magnitude compressive residual stress. The maximum fatigue life increases by 94% (TSLSP-D) and 169% (TSLSP-I) compared with the original specimens. Moreover, the TSLSP-D results show decreased resistance to foreign object damage because of decreased plasticity while the TSLSP-I simultaneously achieves superior foreign object damage resistance and fatigue performance.

Improving fatigue resistance of high strength bolt under cyclic transverse loading by fine particle peening

In order to avoid the fatigue failure of high strength bolt under cyclic transverse loading, fine particle peening (FPP) treatment was conducted to the bolt using steel beads. The surface roughness, residual stress and surface hardness of the treated bolts were examined. Fatigue tests were performed for bolts with or without FPP treatment under transverse loading. The test results show that FPP treatment effectively improves the fatigue resistance of high strength bolt through reducing the surface roughness, increasing the surface hardness and introducing high compressive residual stress in the surface layer. For the FPP treated bolts, failure location changes from the bolt head fillet to thread root as loading displacement decreases to 0.3 mm. A finite element analysis was performed to analyze the failure behavior before and after FPP treatment.

Synchronous Shot Peening Applied on HVOF for Improvement on Wear Resistance of Fe-based Amorphous Coating

Shot peening was used synchronously to improve Fe-based amorphous coating performance by delivering ZrO2 ceramic particles into a low-temperature region of a flame during the high velocity oxygen flame (HVOF) spray process. The coating became denser, and its hardness became higher via the new process. Moreover, the compressive residual stress was induced by shot peening. The results from the dry friction test indicated that the coating’s wear resistance was enhanced obviously. The wear mechanism of coatings with and without shot peening is an abrasive wear combined with an oxidation wear at wear test conditions of a low load and a low frequency. The coating with the best wear resistance did not have the strongest microhardness but had the highest compressive residual stress. The compressive residual stress had a significant positive influence on the wear resistance at a low frequency, while its effect is weakened at a high frequency.

Correlation of Microstructure and Properties of Cold Gas Sprayed INCONEL 718 Coatings

In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness.

Nanoscale evolution of stress concentrations and crack morphology in multilayered CrN coating during indentation: Experiment and simulation

The layered architecture approach allows designing mechanical and fracture properties of hard coatings. The current study investigates the performance of a multilayered CrN coating, consisted of 5 μm CrN sublayers of very similar mechanical properties and microstructure but different residual stress states, during in-situ wedge indentation. A finite element model of the indentation was developed and validated against measurements of the local multiaxial stress fields during indentation, characterized by means of X-ray nanodiffraction analysis with a spatial resolution of 500 nm. By means of numerical fracture mechanics the effect of the multilayered structure on the formation and morphology of mode II cracks is analyzed. The configurational force concept was applied to investigate the crack driving forces and crack extension angles of static cracks in different geometrical arrangements. The simulation results agree well with the experimental findings and reveal a shielding effect preventing an interface-near crack from entering the CrN layer with the higher compressive residual stresses. Furthermore, the possibility to match the numerical results with the locally resolved experiments allowed determining validated material parameters for the deformation and fracture behavior. The work revealed e.g. that a KIIC of around 1 MPa∙m1/2 is an appropriate choice for the investigated CrN coating.

Evaluation of Mechanical Behavior and Surface Morphology of Shot-Peened Ti-6Al-4V Alloy

The effects of shot peening (SP) on mechanical behavior and surface morphology of Ti-6Al-4V were investigated in detail. After SP, the uniformity of compressive residual stress (CRS) was analyzed. The CRS decreased as SP intensity increased because of the severe impact of shots. The CRS was enhanced from − 762.7 to − 795.5 MPa with an increase in SP intensity. The fatigue life was increased from 0.46 × 106 cycles to 8.10 × 106 cycles after SP when the loading amplitude was 610 MPa. This increase was mainly attributed to the enhancement of the crack initiation hindering and propagation by the high CRS and refined domain sizes. The results of surface morphology showed that the convexity and concave regions were generated after SP and the improvement in SP intensity aggravated the increase in roughness. Although the roughness increased after SP, it did not hinder the enhancement of fatigue lives, indicating that CRS played a leading role in the improvement in fatigue lives. Thus, the analysis and discussion revealed that SP was beneficial for the improvement of the surface mechanical behavior of titanium alloys.

Nitriding behavior and mechanical properties of carburizing and nitriding duplex treated M50NiL steel

In this study, carburizing and nitriding duplex treatments were carried out for 10 h at various nitriding temperatures in the range of 460–540 °C on M50NiL steel. The results show that an increase in nitriding temperature greatly increases the thickness of the nitrided layer, but a further increase in the nitriding temperature to 540 °C does not obtain a deeper nitrided layer. Moreover, an increase in nitriding temperature promotes the dissolution of grainy carbides and leads to a precipitation of intergranular precipitates. The pre-existing carbides are easier to be transformed into M(N,C) at a low nitriding temperature. The compound layer is a mixed phase of γ′ (Fe4N) and ε (Fe23N). With increasing the nitriding temperature, the magnitude of the residual compressive stresses decreases, the surface roughness increases, and the proportion of γ′ phase increases. The results of the wear tests carried out at different loads demonstrate that the wear mechanism controlling the wear rate transfers from oxidative wear to abrasive wear when the load increases. The sample nitrided at 500 °C shows the best wear resistance to abrasive wear under a high load due to its thicker compound layer, high ε proportion and high surface residual compressive stress.

Effect of liquid nitrogen cooling on surface integrity in cryogenic milling of Ti-6Al-4 V titanium alloy

Owing to poor thermal conductivity, heat dissipation, and high chemical reactivity toward most of the tool materials, temperature elevation in the machining of titanium alloy leads to poor surface quality. Based on analyzing the variation laws of the milling forces under cryogenic cooling, the present investigation concerns the surface integrity (surface roughness, micro-hardness, microstructures, and residual stresses) in cryogenic milling of Ti-6Al-4 V alloy under the application of liquid nitrogen (LN2) as a cooling mode. Findings have indicated a dramatic increase in milling forces, and decreasing surface roughness was observed under variation of jet temperature (20~−196 °C). Besides an increase in cutting speed from 60 to 120 m/min, a linear increase in cutting forces, surface roughness, micro-hardness, and residual compressive stress was observed. The minimum micro-hardness decreased at cutting speed of 90 m/min and up to 30 μm in depth. A holistic comparison between obtained results under cryogenic milling and previously studied results under dry milling at same cutting conditions depicted higher micro-hardness and higher compressive residual stress under cryogenic LN2 on the machined surface. However, the residual stress under LN2 cooling conditions tends to decrease relatively slower compared to dry milling. Also, there are no significant differences in grain refinement and twisting under dry and cryogenic LN2 machining. The research work proves the effectiveness of cryogenic milling in improving the surface integrity of the Ti-6Al-4 V alloy.

Investigation of subsurface fatigue crack growth behavior of D2 tool steel (JIS SKD11) based on a novel measurement method

In our preliminary study of the fatigue properties of D2 tool steel (JIS SKD11), it was found that owing to grinding in the specimen preparation process, high compressive residual stresses were introduced in the surface layers of the fatigue specimens, and as a result, the fatigue crack growth of the specimen was entirely subsurface. As relatively little is known about the subsurface growth of fatigue cracks, this finding gave us the opportunity to learn more about the subsurface fatigue crack growth process. An S–N curve covering fatigue lifetimes from 103 to 108 cycles was established for R = −1 loading condition. The velocity of fatigue crack growth as a function of the maximum stress intensity factor Kmax was determined to be in the range of 10−11–10−7 m/cycle. The Murakami √area method and a new experimental procedure were used to estimate the stress intensity factor and crack growth velocity under the specimen surface, respectively. An analytical equation for the velocity of crack growth as a function of the maximum stress intensity factor Kmax was developed, which agrees with the experimental results. The integration of this equation led to estimates of the fatigue lifetime for given stress ranges. These estimates were also in agreement with the experimental results.

The effect of ultrasonic surface rolling process on the fretting fatigue property of GH4169 superalloy

To improve the fretting fatigue (FF) resistance of GH4169 superalloy, ultrasonic surface rolling process (USRP) is carried once and three times on material surface and the effect of surface integrity on FF is investigated. The results show that USRP significantly reduces the surface roughness of GH4169 superalloy and improves surface micro-hardness as well as induces a high intensity compressive residual stress at deep thickness. The dislocation density beneath the top surface is increased and the grains of the material surface are refined. A gradient nanostructured layer is observed beneath the surface and the equiaxed nanograins are generated with the size of around 37.6 nm at the top surface of USRP-3 sample. The FF test indicates the FF life of GH4169 superalloy increase by 3.6 times and 11 times, by one and three USRP treatments, respectively. In addition, the factor separation test indicates that the compressive residual stress plays an important role in improving the FF life of GH4169 superalloy.

Residual stress in laser cladded heavy-haul rails investigated by neutron diffraction

Abstract Residual stress is one of the critical parameters affecting the fatigue behaviour of tribological components, which can be introduced by a thermo-mechanical process such as laser cladding. In this study, the residual stress distribution of laser cladded rails was evaluated using a neutron diffraction technique. The substrate rail for the laser cladding was hypereutectoid rail steel used in Australian heavy-haul railway track, and the cladding materials were 410L (a low carbon content stainless steel alloy) and Stellite 6 (a Co-based alloy). The cladding materials were selected based on their high wear, corrosion and fatigue resistance properties. This study measured the residual stress in full-scale laser cladded rails where the residual stresses were measured in the cladding layer, heat affected zone (HAZ) and substrate zone of the railhead. A new sample preparation strategy was developed to quantify the residual stresses in the full-scale rails with high spatial resolution. Higher compressive residual stress was found in the cladding layer, which may have resulted from the martensitic transformation occurred in that region. Tensile stresses occurred in the HAZ to a depth of 4 mm, which might be mainly caused by thermal contraction and volumetric change in the microstructure. The addition of a second cladding layer did not significantly affect the magnitude of the residual stresses, but the peak tensile residual stress shifted to a deeper location from the surface, which is beneficial in resisting wear. Post-cladding heat treatment significantly reduced the undesirable high residual stress from the cladding layer and HAZ.

Optimization of Inconel 718 thick deposits by cold spray processing and annealing

Cold Spraying of high strength materials, i.e., Inconel 718 is still challenging due to the limited deformability of the material restricting the quality of deposits. Thus, process parameters must be tuned for reaching higher particle impact velocities and temperatures to allow for maximum amounts of well-bonded particle-substrate and particle-particle interfaces. In the present study, Inconel 718 powder was cold sprayed under varied process gas temperatures for a systematic study of the influence on the quality of thick deposits. In addition, one set of samples of each batch was exposed to post heat treatment procedures by hot isostatic pressing, thermal soft annealing, and aging for attaining hard bulk material properties. Deposits microstructure, porosity, electrical conductivity, hardness, and residual stress were analyzed in as-sprayed and as-heat treated conditions. Results are discussed in terms of the “coating quality parameter”, defined as the ratio between particle impact velocity and critical velocity. As-sprayed deposits exhibit microstructures with highly deformed particles and well bonded internal interfaces. X-ray diffraction reveals that powder and deposits present a γ-solid-solution phase, allowing to assume conventional softening behavior for estimating critical conditions for bonding. Increasing the process gas temperature leads to lower coating porosity and higher electrical conductivity. Deposits showed similarly high microhardness and compressive residual stresses, both caused by work hardening during cold spraying. Subsequent heat treatments improved the quality of internal interfaces, mostly for deposits with high values of “coating quality parameters”. By distinguishing influences on several coating properties, these results contribute to gain basic knowledge for successful manufacturing of Inconel 718 thick deposits by cold spraying, particularly concerning needed coating quality parameters for adjusting desired properties.