Uniform Residual Stress Research Articles

Mechanisms of laser non-uniform shock peening on the mechanical properties of SUS 304 stainless steel weld joints

The technique of Laser Non-Uniform Shock Peening (LNUSP) is proposed based on the residual stress distribution in welded joints and the shock wave peak pressure of SUS 304 stainless steel. The study investigated the effects of LNUSP on the mechanical properties of laser beam oscillation welded (LWW) joints of SUS 304 stainless steel. The results indicate that LNUSP induced residual compressive stress with an affected layer depth exceeding 0.9 mm, leading to a more uniform residual stress distribution. LNUSP also induced martensitic phase transformation, altered the grain structure orientation, formed parallel dislocation arrays that refined the grains, increased dislocation density leading to dislocation tangles, and induced mechanical twins of 20–70 nm. Furthermore, LNUSP significantly enhanced microhardness and promoted a more uniform hardness distribution, favoring a better balance between strength and ductility. LNUSP notably increased the yield strength (YS) and ultimate tensile strength (UTS) by 49.4% and 5.1%, respectively, compared to the original samples. The LNUSP-treated samples demonstrated superior strength and plasticity, attributed to the uniformity of internal plastic deformation.

Effects of laser non-uniform shock peening on the microstructure and fatigue performance of SUS 304 stainless steel welded joints

Residual tensile stresses and microstructural heterogeneity in welded joints are primary factors that degrade fatigue performance. As an effective post-weld strengthening method, Laser Shock Peening (LSP) can address these issues. Given the personalized demands of different regions, this paper proposes a Laser Non-Uniform Shock Peening (LNUSP) technique. This approach involves applying high pulse energy to the weld zone (WZ) and heat-affected zone (HAZ), and low pulse energy to the base material (BM) to achieve a more uniform residual compressive stress distribution and refined microstructure, thereby improving overall performance and service life. Results indicate that LSP transforms residual tensile stresses on the weld surface into compressive stresses. Compared to original samples, uniformly LSP-treated samples, and locally LSP-treated samples, LNUSP-treated samples exhibit more uniform residual stress and hardness distribution. LSP significantly increases surface hardness, with LNUSP-treated samples achieving a maximum hardness of 308.3 HV. Additionally, LSP induces martensitic transformation in the BM and WZ areas, alters grain orientation, and promotes dislocation movement, resulting in dislocation tangling, dislocation networks, and twin structures, which refine grain size. Consequently, LSP markedly improves joint fatigue performance, delaying fatigue crack propagation. The fatigue life of LNUSP-treated samples is increased by 25.41 % compared to original samples, reaching levels comparable to high-energy large-area impacts. This enhancement is attributed to the uniform residual stress and refined grain structure.

Additive manufacturing Hastelloy X with enhanced properties by optimizing strategies

Optimizing laser additive manufacturing (LAM) scanning strategies is crucial for improving the quality and performance of metallic parts formed by laser powder bed fusion (LPBF), particularly for alloys with high susceptibility to cracking. However, there is limited research on the effects of unidirectional and unidirectional-remelting strategies on the temperature field and forming quality of LPBF-manufactured Hastelloy X (HX), a nickel-based superalloy highly susceptible to cracking. This study focused on the temperature field, surface roughness, crack occurrence, and mechanical and thermal properties of LPBF-manufactured HX, employing four different laser scanning strategies: chessboard, bidirectional, unidirectional, and unidirectional-remelting. Specimens manufactured using the unidirectional-remelting strategy showed the lowest surface roughness (4.12 μm), highest density (99.64 %), superior tensile properties (tensile strength of 1068.29 MPa and elongation of 16.82 %), lowest coefficient of friction (0.65), highest thermal conductivity (9.77 W/m·K), and lowest specific heat capacity (0.38 J/g/K) compared to other strategies. This can be attributed to the unidirectional-remelting strategy resulting in specimens with equiaxed grains, the most uniform elemental and residual stress distributions, as well as the strongest grain boundary and dislocation strengthening. Moreover, it exhibited compressive residual stress in HX specimens. Interestingly, the tensile properties of the unidirectional-formed HX parts ranked second only to the unidirectional-remelting formed parts, owing to the efficacy of texture strengthening. These findings provide insights into the underlying mechanisms of optimizing laser scanning strategies for LPBF fabrication of HX, offering essential references for the LAM of crack-sensitive alloys, especially nickel-based superalloys.

Relief of Residual Stress in Bulk Thermosets in the Glassy State by Local Bond Exchange.

The covalently cross-linked network gives thermosets superior thermal, mechanical, and electrical properties, which, however, squarely makes the large residual stress that is inevitably induced during preparation hardly relieved in the glassy state. In this work, an incredible reduction in residual stress is successfully achieved in bulk thermosets in the glassy state through introducing highly dynamic thiocarbamate bonds by "click" reactions of thiols and isocyanates. Due to the excellent dynamic behaviors of thiocarbamate bonds, local network rearrangement is achieved through thermal stimulation, while the strong 3D cross-linked network is well maintained. Ultimately, a decrease by 44% in residual stress is detected by simply annealing samples at 30°C below glass transition temperature (Tg), during which they could well maintain more than 98.4% of the storage modulus. After the annealing, more uniform residual stress distribution is also observed, showing a 32% decline in sample standard deviation. However, the residual stress of epoxy resin, a typical thermoset as a reference, changes little even after annealing at Tg. The results prove it a feasible strategy to reduce residual stress in bulk thermosets in the glassy state by introducing proper dynamic covalent bonds.

A dynamic installation method to improve the morphological characteristics of the bolt-hole contact interface in CFRP interference bolted structures

Dynamic installation (DI) of composite fasteners can reduce interface damage caused by interference and further improve joint performance. Understanding the low-damage mechanisms during DI based on electromagnetic force is critical. In this paper, a 3-D nonlinear progressive damage model based on continuum damage mechanics (CDM) was developed to predict the geometric dimensional variations of the bolt, the damage behavior and stress distribution of the bolt-hole contact interface after interference-fit connection. Static installation (SI) analysis was also considered as a comparative study. Experimental bolt-hole interface damage behavior validated the FEA results. The results show that DI can significantly reduce the installation resistance by thinning and elongating the bolt, so as to remarkably reduce or avoid severe damage to composite laminates, especially for large interference sizes. The low installation damage of the DI specimens results in more uniform residual stresses and higher amplitudes at the hole edges, which may be more conducive to achieving a more uniform stress distribution in the joints under external cyclic loading, thus helping to reduce the incidence of failure and extend the operating horizons of structures.

Study on the Residual Stress Distribution of Bi-Directional Cold Expansion Process Performed on Open Holes

AbstractTraditional unidirectional cold expansion technology usually generates non-uniform distribution of residual stress in the thickness direction of holes, which is harmful to the improvement of fatigue life of holes. The present work proposed a bi-directional cold expansion procedure to realize the homogenization of residual stress in the thickness direction of the cold expanded hole, thereby further improved antifatigue performance of cold expanded hole. For this aim, a series of finite element (FE) simulations were carried out to investigate the effectiveness of the bi-directional cold expansion procedure and optimize the process parameters. The results showed that the optimized bi-directional cold expansion process generated a more uniform distribution of residual circumferential compressive stress in the thickness direction comparing to the simplified bidirectional cold expansion process using a single mandrel. For the Aluminum alloy 7050-T7451, when the first interference level I1 = 1.8%, the largest and the most uniform residual circumferential compressive stress was achieved, which suggested the best anti-fatigue performance.

Исследование структуры и свойств износостойкого газотермического покрытия с содержанием вольфрама

The paper presents the results of reverse engineering including metallographic, mechanical, and engineering-technical studies of used rods of a compressor produced by the Dresser-Rand company (Siemens, Germany). The study established that the original product is made of AISI 4140 steel with a working coating based on tungsten carbide applied to a depth of 0.2 mm by the HVOF method. The paper contains the results of the development of an import-substituting technological process for producing a wear-resistant powder coating of the Ni–Cr–B–WC system applied by cold gas flame spraying on the surface of a critical unit of compressor equipment in the oil and gas industry. Microanalysis identified that the sprayed spherical WC particles are evenly distributed in the nickel bond without the formation of free cavities at the lamella boundary, retain the size identical to the original powder composition upon the high-speed collision with the substrate, and minimize the level of residual mechanical stresses in the surface layer. The study shows that the sprayed coating has a high microhardness (the bases – 700 HV0.1, WC – up to 2000 HV0.1), which ensures high wear resistance during operation of the rod in a friction pair. A comparative analysis of the tribological properties of the coatings showed that when changing the shape, particle size distribution, and percentage ratio of tungsten carbide from 20 to 70 % in the nickel matrix, the overall wear resistance of the coating equivalently increases. The authors concluded on the possibility of manufacturing an import-substituting product using the gas flame spraying technology with metallurgical powder compositions containing tungsten. The authors developed an industrial technology for applying a wear-resistant coating on the working surface of a rod made of AISI 4140 steel. The paper presents the results of the analysis of the stress state of a material with a coating produced using the developed technology in comparison with the original product. In the product obtained by the experimental technology, in the process of applying the coating and its subsequent mechanical processing, uniform residual mechanical stresses are formed that do not exceed the value of the difference in the principal mechanical stresses. The paper presents the results of the study obtained both on standard samples and on a pilot part.

Fatigue life enhancement mechanism and lifetime prediction of AA6061-T6 open-holed sheet treated by electromagnetic driving dynamic cold expansion

In this study, mechanisms of fatigue life enhancements of electromagnetic driving dynamic cold expansion (DCE) open holed sheets were investigated by testing fatigue life and obtaining residual stress fields experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distributions was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The differences in stress distribution led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Moreover, the predicted fatigue life results were in accordance with experimental data. • Dynamic cold expansion extended larger fatigue life for holed-sheets. • More uniform residual stress was induced by dynamic cold expansion. • Fatigue cracks propagated horizontally in dynamic cold expanded specimens. • Proposed method to predict fatigue life had good agreement with experiments.

Residual stress distribution and fatigue performance investigations of electromagnetic force-based dynamic cold expansion open-holed sheet

In this study, residual stress distributions and fatigue performances of electromagnetic driving dynamic cold expansion (DCE) open-holed sheets were investigated. Fatigue behaviors were tested, and residual stress fields were obtained experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distribution was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The stress distribution differences led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Uniform stress distributions along axial direction in DCE specimens contributed to random initiations and horizontal propagations of fatigue cracks, while the fatigue cracks in SCE specimens usually initiated at the mandrel entrance layer and propagated to the mandrel exit layer. Moreover, the predicted fatigue life results agreed well with experimental data.

Universal Procedure for Correction of Plasticity Effect in Hole-Drilling Uniform Residual Stress Measurement

BackgroundThe hole-drilling method is a well-known and widely used technique for the determination of residual stresses, but is limited to materials with linear elastic behaviour. This can be a problem when high residual stresses are measured, since a local yielding can occur due to stress concentration around the drilled hole.ObjectiveIf the residual stress exceeds about 80% of the material yield stress, the error caused by the plasticity effect becomes significant. In order to correctly evaluate high uniform residual stresses, a universal procedure for a correction of the plasticity effect is introduced.MethodsThe procedure uses a neural network and is capable of correcting any combination of uniform residual stresses with magnitudes up to the material yield stress. It also covers a wide range of material parameters, hole diameters, and strain gauge rosettes and it is independent of the orientation of the strain gauge rosette.ResultsThe correction procedure was tested by more than a million randomly generated stress states that covered the entire range of input parameters and performs remarkably well, since the error of the corrected residual stresses is negligible even for the states with residual stress magnitudes equal to the material yield stress.ConclusionsThe proposed correction extends the application range of the hole-drilling method to high uniform residual stresses and therefore can be very useful for practical measurements.

Beneficial effects of deep cryogenic treatment on mechanical properties of additively manufactured high entropy alloy: cyclic vs single cryogenic cooling

• Cyclic deep cryogenic treatment was utilized to homogenize the residual stress and tune the microstructure of a high entropy alloy fabricated by laser melting deposition, in order to improve its overall mechanical properties. • Cyclic deep cryogenic treatment can induce an increased compressive residual stress and introduce various types of reinforcing defect microstructures into a high entropy alloy, including dense dislocations, intersecting twins, transformed HCP phase and nanograins, thus effectively overcoming the strength-ductility trade-off. • Compared to single-step DCT, cyclic DCT can reduce the residual stress gradients in the LMD-built sample, relieving or removing the tensile residual stress at the top surface. • Transformation to HCP phase can occur during the cyclic DCT process due to the cyclic reverting of the thermal stress to a high level on each cryogenic cooling, which promotes twinning on secondary habit planes and more frequent faulting. Additively manufactured (AM) metallic materials commonly possess substantial tensile surface residual stress, which is detrimental to the load-bearing service behavior. Recently, we demonstrated that deep cryogenic treatment (DCT) is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy (HEA) samples fabricated by laser melting deposition (LMD), by introducing high compressive residual stress and deformation microstructures without destroying the AM shape. However, carrying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process, which are undesirable as the mechanical properties will not be homogeneous within the sample. In this work, we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated CoCrFeMnNi HEA, and improve tensile strength and ductility, compared with single-step DCT of the same cryogenic soaking duration. Under cyclic DCT, the thermal stress is re-elevated to a high value at each cryogenic cooling step, leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation, compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time. The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT. The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.

Surface characteristics and stress corrosion behavior of AA 7075-T6 aluminum alloys after different shot peening processes

Shot peening is often reported as being beneficial to stress corrosion cracking resistance; however, the increased surface roughness induced by shot peening may promote localized corrosion pitting. Here, AA 7075-T6 aluminum alloys were treated by different shot peening processes, and the surface characteristic, including refined microstructure, residual stress and surface roughness, were characterized, then its effect on localized corrosion pitting and stress crack propagation behavior were investigated. Compared with single shot peening, the microstructure refinement was not further enhanced by dual shot peening with adding a subsequent low intensity of micro-shot peening. The surface residual stress distribution become more uniform, and the average surface residual stress value was increased from −158 MPa to −175 MPa. The maximum residual stress was synchronously increased from −302 MPa to −319 MPa. Meanwhile, the surface roughness (Ra) value was decreased from 4.05 μm to 2.05 μm. Compared with single shot peening, the degree of surface localized corrosion pitting was alleviated after dual shot peening due to its low surface roughness and more uniform compressive residual stress distribution. The depth of stress corrosion cracking for dual shot peening was decreased with a minimum value of 33 μm, which may be ascribed to the high maximum compressive residual stress at the subsurface.

Interlaboratory Reproducibility of Contour Method Data in a High Strength Aluminum Alloy

BackgroundThe contour method for residual stress measurement has seen significant development, but an experimental reproducibility study utilizing physical samples has not been published.ObjectiveA double-blind reproducibly study is reported, having scope beginning with EDM cutting and ending with residual stress calculation.MethodsA reinforced I-beam sample geometry is identified for its unique residual stress profile when extracted from residual stress bearing quenched aluminum bar (7050-T74). Contour measurements are prescribed on a midplane of symmetry with dimensions 24.0 mm by 50.0 mm. Fourteen identically prepared samples are fabricated from a single long bar with well characterized and uniform residual stress. Five samples throughout the bar are identified for planning measurements to validate sample uniformity and overall suitability of the residual stress field. The planning measurements employ a range of techniques: contour method, neutron diffraction, and hole-drilling. Eight samples are distributed to an international group of participants to execute their standard measurement practice. A double-blind process is followed to provide anonymity.ResultsResults are provided by eight participants: six being self-similar and two being quite different, the latter set aside as outliers. An average residual stress field is established from non-outlying results and the spatial distribution of reproducibility standard deviation is determined. The average stress field ranges from -60 to 70 MPa and the reproducibility standard deviation averages 8.1 MPa on the measurement plane. The average reproducibility standard deviation is about 3 × larger for points within 1.0 mm of plane boundaries (17.6 MPa) than for the remaining points (6.1 MPa).ConclusionsReproducibility standard deviation (among different labs) for contour method residual stress measurement is found to be very similar to repeatability standard deviation (in a single lab) reported in prior work. The reproducibility observed here, for the entire measurement process, is also similar to that found in a prior reproducibility study limited to contour method data analysis.

A novel method to improve fatigue behaviors of holed structures based on electromagnetic force

In this study, a novel method stress wave strengthening (SWS) process based on electromagnetic force was proposed to improve the fatigue life of holed structures. Corresponding tests were carried out to explore the fatigue performance of SWS. Cold expansion (CE) was also investigated for comparison. The fatigue life of SWS and CE samples were evaluated, moreover, the mechanisms of fatigue failures and life enhancements were also discussed. Results showed that double-side SWS extended fatigue life significantly and reduced stiffness degradation more effectively with respect to CE process. Moreover, fatigue cracks commonly appeared at mid-planes of hole surfaces and horizontally grew in SWS samples, which differed a lot from CE samples. Through the residual stress measurement, it is seen that more uniform residual stress along axial direction can be obtained by SWS compared to CE, which can explain the fatigue life enhancement and failure mechanism of SWS method.

Static and fatigue behavior of hydraulic components produced by different additive manufacturing processes

This study focuses on hydraulic components’ behavior to significant pressure loads both in static and fatigue conditions. This hydraulic block commonly used on large machines (earthmoving, mining, agriculture…) was initially designed to be machined but has been topologically optimized for additive manufacturing methods: laser powder bed fusion (L-PBF), metal binder jetting (MBJ) and lost-wax casting. The material chosen was a martensitic precipitation-hardening stainless steel that provides good corrosion resistance and high mechanical properties. Most of the samples were obtained with laser powder bed fusion and underwent static and fatigue pressure tests, exceeding the technical specifications of the end user. The tested samples have then been screened and went through various fatigue finite element analyses. This approach demonstrated that all the critical fatigue failure locations obtained experimentally were predictable through software simulation, local non uniform roughness and residual stress in failure areas being key factors for good correlation.

Effect of Phase Transformations on Scanning Strategy in WAAM Fabrication.

Due to its high production rates and low cost as compared to other metal additive manufacturing processes, wire arc additive manufacturing (WAAM) has become an emerging technology in the manufacturing industry. However, the residual stress generation and part distortion hinder its widespread adoption because of the complex thermal build-histories of WAAM parts. One of the ways to alleviate this problem is to consider the effects of scan strategies as it directly influences the thermal history of the built part. Since WAAM itself is an evolved welding process and even though it is evident from welding studies that phase transformations directly affect the residual stresses in welded parts, it remains unclear how the consideration of phase transformations for different scan strategies will affect the residual stresses and distortions in the WAAMed parts. A FEM study has been performed to elucidate the effects of phase transformations on residual stresses and the distortion for different deposition patterns. The current findings highlight that for the fabrication of low-carbon martensitic steels: The consideration of phase transformations for line-type discontinuous patterns (alternate and raster) do not significantly affect the residual stresses. Consideration of phase transformations significantly affects residual stresses for continuous patterns (zigzag, in–out and out–in). To accurately simulate complex patterns, phase transformations should be considered because the patterns directly influence the temperature history of the built part and will thus affect the phase transformations, the residual stresses and the warpage. During the fabrication of WAAM parts, whenever possible, discontinuous line scanning patterns should be considered as they provide the part with uniform residual stress and distortion. The alternate line pattern has been found to be the most consistent overall pattern.

Effect of creep annealing on the dimensional stability of dispersion strengthened copper alloy

This study aims to enhance the dimensional stability of alumina dispersion strength copper alloy via creep annealing treatment. The microstructure, residual stress and micro-yield strength of alloys after different treatments were investigated. During creep annealing, the residual stress in the alloy was dramatically reduced due to the decrease of dislocation density. The nano-Al2O3 particles in the alloy did not coarsen or agglomerate during the high temperature treatment and retarded the growth of subgrains effectively, which played important roles in high micro-yield strength of creep annealed alloy. The pores were effectively reduced by the creep deformation during creep annealing, resulting in a more uniform residual stress distribution and an increase in micro-yield strength. Compared with extruded alloy, the residual stress of alloy creep annealed at 950 °C for 12 h was reduced by 88%, while the micro-yield strength increased by 9.7%, leading to a significant enhancement in dimensional stability of the alloy.

Influences of platform heating and post-processing stress relief treatment on the mechanical properties and microstructure of selective-laser-melted AlSi10Mg alloys

This study focused on two essential issues in the fabrication strategy utilized in selective laser melting (SLM) technology: 1) the influence of the hot-build platform and 2) the effects of a post-processing stress relief (SR) treatment on the mechanical properties and microstructure of an AlSi10Mg alloy manufactured by SLM. To examine the mechanical properties, surface hardness measurements and split Hopkinson pressure bar (SHPB) experiments were conducted on samples in the as-fabricated condition and following SR treatment. The samples were extracted from the original SLM product at constant distances from the build platform. Considerable variations in the mechanical properties and damage accumulation resulting from the fabrication process and subsequent post-processing SR treatment were successfully correlated to fundamental characteristics in terms of relative porosity, “on-surface” residual stress, microstructure and texture, solubility, and phase composition. It was found that with increasing distance from the heated build platform, there was a graded increase in the surface hardness and dynamic performance, which are attributed to several competing strengthening mechanisms that were activated owing to the fast cooling rates. Conversely, the reduced thermal gradient and lower solidification rate close to the base led to a higher relative density, as indicated by the smaller size of the keyhole pores. The SR treatment resulted in microstructural changes with uniform and low residual stresses, which led to a significant softening in the mechanical properties, regardless of the building height.

Analysis of stress-strain in the partial heating roll forming process of high strength square hollow steel sections

The demand for roll-formed high strength steel square hollow sections has extensively increased by automotive and construction industries due to their excellent structural performances and attractive appearances. However, due to their low ductility and demand for large forming tool loads, high-strength steels are challenging to be roll-formed at room temperature. Partial heating roll forming is a recently proposed manufacturing technique aiming to overcome these difficulties by applying temperature on bend areas. In this study, the finite element software LS-DYNA was utilized to predict effective plastic strain and stress developments and associated residual stresses induced during the partial heating roll forming method. Furthermore, comparisons were made between cold roll forming and partial heating roll forming methods. The results indicate that the effective stress value during the partial heating roll forming method was lower than the cold forming method. In addition, the product manufactured by the partial heating roll forming method had smaller and uniform residual stress distributions along the section perimeter and across the section thickness than the cold-formed counterpart. Further investigations on corner geometry showed that high strength steel square hollow sections with a minimum bend radius could be successfully manufactured using the partial heating roll forming method.

Numerical and experiment investigation on joining process and failure behaviors of CFRP/Al electromagnetic riveted joint

In this study, electromagnetic riveting (EMR) was proposed to connect CFRP/Al structures due to its little damage on composite. The joining process and failure behaviors of CFRP/Al joints were studied. Specifically, the finite element model was established to analyze the joining process. The riveted joints were tested under various loading velocities to obtain the failure behaviors. The simulation results showed that hole wall of the CFRP and Al sheets had uniform residual stresses, and no damage of the CFRP sheet was observed around the hole after EMR, indicating that the joint had superior performance. The tensile test results showed that the loading speed had a great influence on the performance of the joint. The maximum shear force at lower loading speeds was less than that at higher loading speeds, while the trend of energy absorption was opposite. The main failure position was in CFRP with a mix mode of bearing and shear out. Based on the failure analysis, a simplified analytical model for the maximum shear load was presented.