Depth Of Compressive Residual Stress Research Articles

Femtosecond laser shock peening Cu-Zn welds: Experiments and multi-scale modeling investigation

Laser shock peening of the weld surface is a cutting-edge treatment technology following welding. However, the conventional nanosecond pulsed lasers used for this process can unintentionally damage the weld surface quality due to secondary melting, which can reduce the comprehensive mechanical properties of the material. Here, we applied femtosecond laser shock peening (FLSP) to Cu-Zn welds as a paradigm. Experimental results show that FLSP cause the finer grain size on the surface of welds, with the maximum increases of approximately 17.6 % in surface hardness of the welds. The residual stress on the surface was transformed from tensile residual stress to compressive residual stress. Crucially, to provide a more precise depiction of the FLSP process, we developed a comprehensive multi-scale modeling approach that involves integrating the inputs and outputs among models with different scales. This multi-scale modeling approach provides an enhanced understanding of the FLSP process compared to traditional single scale models. The high-density electron pressure evolution process, the shock compression fragmentation process of the weld dislocation network during FLSP and the depth of compressive residual stress could be investigated by this model. These findings provide electronic-level and atomic-level insights and imply promising applications of FLSP on various welds surface.

Effect of Laser Scanning Speed and Fine Shot Peening on Pore Characteristics, Hardness, and Residual Stress of Ti-6Al-4V Fabricated by Laser Powder Bed Fusion

There is a concern regarding sub-surface pores within laser powder bed fusion of Ti-6Al-4V, which can initiate cracks and reduce mechanical properties, especially after machining for surface finishing. This study investigated the effect of laser scanning speed and fine shot peening on the pore characteristics, hardness, and residual stress of Ti-6Al-4V fabricated by laser powder bed fusion using scanning electron microscopy, X-ray micro-computed tomography, Vickers hardness, and X-ray diffraction. As the laser scanning speed increased, the number of pores and pore size increased, which reduced the hardness of Ti-6Al-4V. Most pores were less than 20 µm in size and randomly distributed. The fine shot peening generated plastic deformation and compressive residual stress on the surface, leading to higher hardness, with similar surface properties at all scanning speeds. The depth of compressive residual stress by fine shot peening varied corresponding to the scanning speeds. Increasing the scanning speed accelerated the rate of conversion between the compressive and tensile residual stresses, and decreased the depth of the maximum hardness by the fine shot peening from initial tensile residual stress within Ti-6Al-4V fabricated by laser powder bed fusion, thus reducing the enhancement achieved by the fine shot peening.

Enhancement of surface characteristics of additively manufactured γ-TiAl and IN939 alloys after laser shock processing

The enhancement in laser technology opened up new methods such as additive manufacturing (AM) of metals. While AM offers high design flexibility and reduced material usage, it can also produce problematic parts due to poor surface quality. A variety of post-processing techniques are available to address the drawbacks associated with the surface properties of AM. Laser Shock Processing (LSP) is one of the unique methods that induces compressive residual stress (CRS) on the surface and subsurface by creating severe plastic deformation. In this study, two critical aerospace alloys (γ-TiAl and IN939) were manufactured through two different methods called Powder Bed Fusion with Electron Beam (PBF-EB) and Powder Bed Fusion with Laser Beam (PBF-LB). Subsequently, AM samples were investigated to observe single layer LSP effects on the surface. The results of the laser peening process were determined by residual stress, microhardness and surface roughness measurements. The residual stress profiles showed that LSP significantly induces CRS on the surfaces of AM alloys. For the γ-TiAl and IN939 samples, the maximum values of CRS and depth of CRS were −460 MPa/1000 µm and −516 MPa/700 µm, respectively. Similarly, the microhardness of the materials was increased by 44.4 % for γ-TiAl and 18.2 % for IN939 by laser post-processing. In addition, a comparison of the roughness of the unground and ground AM samples was carried out. Depending on the surface condition of the AM samples, LSP had different outcomes. For example, the extreme roughness of the AM samples was partially reduced by the thermal effects of the high-energy laser shots. When comparing the roughness values in terms of Ra and Rz, there were decreases in the range of 14.7–21.3 % and 3.34–39.3 % for unground IN939 samples. However, for unground γ-TiAl samples, depending on the direction of measurement, roughness variations were observed as a 0.99 % decrease − 5.3 % increase for Ra and a 2.2 % decrease − 14.2 % increase for Rz. The roughness values for ground samples of both alloys were drastically increased, varying between 42.1 % and 7x increase. Sa values were recorded on unground AM samples. For IN939 and γ-TiAl samples these values decreased by 8.41 % and 15.8 % respectively.

Surface Conditions after LASER Shock Peening of Steel and Aluminum Alloys Using Ultrafast Laser Pulses.

Laser shock peening (LSP) is a mechanical surface treatment process to modify near-surface material properties. Compared to conventional shot peening (SP) the process parameters can be finely adjusted with greater precision and a higher penetration depth of compressive residual stresses could be reached. However, high process times of LSP leads to high production costs. In this study, ultrafast LSP (U-LSP) with an ultrafast laser source (pulse time in the picosecond range) was applied on specimens made of X5CrNiCu15-5 and AlZnMgCu1.5. The surface characteristics (surface roughness) and surface-near properties (microstructure, residual stresses, and phase composition) were compared to the as-delivered condition, to conventional laser shock peening (C-LSP), and to SP, whereas metallographic analyses and X-ray and synchrotron radiation techniques were used. The process time was significantly lower via U-LSP compared to C-LSP. For X5CrNiCu15-5, no significant compressive residual stresses were induced via U-LSP. However, for AlZnMgCu1.5, similar compressive residual stresses were reached via C-LSP and U-LSP; however, with a lower penetration depth. A change in the phase portions in the surface layer of X5CrNiCu15-5 after C-LSP compared to SP were determined.

Effect of laser shock peening on austempered ductile iron

In this study, laser shock peening (LSP) has been applied on austempered ductile iron (ADI) to determine the optimum laser power and spot overlapping rate required for automotive gear applications. The surface roughness and phases present were studied to understand the effect of the laser shock wave application on the material. An increase in the mean surface roughness was achieved when the laser spot overlap increased from 20% to 60%. A significant increase in the microhardness was obtained in the surface and sub-surface, indicating an increased dislocation density of the material. Compressive residual stress depth and magnitude characterized by X-ray Diffraction (XRD) and Incremental Hole Drilling (IHD) were found to increase with laser power density. This work significantly identifies the pathway toward the engineering manufacture of austempered ductile iron and the application of LSP for the automotive drive-train industries.

Influence of Slide Burnishing Parameters on the Surface Layer Properties of Stainless Steel and Mean Positron Lifetime.

This paper presents the results of an experimental study on the impact of slide burnishing on surface roughness parameters (Sa, Sz, Sp, Sv, Ssk, and Sku), topography, surface layer microhardness, residual stress, and mean positron lifetime (τmean). In the study, specimens of X6CrNiTi18 stainless steel were subjected to slide burnishing. The experimental variables were feed and slide burnishing force. The slide burnishing process led to changes in the surface structure and residual stress distribution and increased the surface layer microhardness. After slide burnishing, the analyzed roughness parameters decreased compared with their pre-treatment (grinding) values. The slide burnishing of X6CrNiTi18 steel specimens increased their degree of strengthening e from 8.77% to 42.74%, while the hardened layer thickness gh increased after the treatment from about 10 µm to 100 µm. The maximum compressive residual stress was about 450 MPa, and the maximum depth of compressive residual stresses was gσ = 1.1 mm. The positron mean lifetime τmean slightly yet systematically increased with the increase in burnishing force F, while an increase in feed led to changes of a different nature.

A Review of Laser Peening Methods for Single Crystal Ni-Based Superalloys

Single crystal Ni-based superalloys are often used to create gas turbine engine blades for their high strength under intense thermo-mechanical loading. Though they are remarkably capable under these conditions, a particular class of premature failure mechanisms known as surface-initiated damage mechanisms can lead to the early fracture of an otherwise healthy blade. This review paper discusses the current progress of post-processing techniques that can greatly mitigate the potency of surface-initiated damage mechanisms. In particular, laser peening (LP) is of significant interest due to the relatively low amount of cold work it induces, greater depth of compressive residual stresses than other cold working methods, ability to accommodate complex part geometries, and the minuscule effect it has on surface roughness. The residual stresses imparted by LP can greatly hinder crack growth and consequently allow for enhanced fatigue life. Given that turbine blades (constructed with single crystal Ni-based superalloys) are prone to fail by these mechanisms, LP could be a worthy choice for increasing their service lives. For this reason, initiative has been taken to better understand the mechanical and microstructural modifications imparted by LP on single crystal Ni-based superalloys and a summary of these investigations are presented in this review. Results from several works show that this class of alloy responds well to LP treatment with improvements such as ~30–50% increase in microhardness, 72% increase in low cycle fatigue life, and elevated resistance to hot corrosion. The primary objective of this review is to provide insight into current state-of-the-art LP techniques and summarize the findings of numerous works which have utilized LP for increasing the service lives of single crystal Ni-based superalloy components.

Investigations for improving fatigue strength by low energy laser peening via bending fatigue test

Laser peening can introduce compressive residual stresses on the surface of various materials, thereby effectively prolonging their fatigue lives. In this study, the effects of laser peening with pulse energies of 20 and 70 mJ on residual stress and fatigue life were investigated on two types of HT780 box-welded specimens of different sizes, considering the realization of a portable laser peening system equipped with a small laser device with low pulse energy. As a result of such low pulse energy laser peening, it was found that the depth of compressive residual stress becomes shallower compared to the current laser peening with a pulse energy of 200 mJ, while the residual stress on the surface remains at the same level. Bending fatigue tests were conducted with the stress ranges at 100 and 150 MPa, which revealed that the fatigue life of the specimens with low pulse energy laser peening was at least 50 times and eight times longer than that of the as-welded specimens, respectively. Comparing the fatigue test results of different size specimens, it is clear that laser peening with low pulse energy is effective in extending the fatigue life of HT780 box-welded specimen regardless of its size.

High temperature behaviour of a mechanically surface hardened single crystal nickel-based superalloy

Surface hardening procedures such as mechanical shot peening and deep cold rolling introduce extensive surface cold work and compressive residual stress. While both methods are designed to increase material fatigue strength, single crystal nickel-based superalloys used for turbine blade applications are routinely exposed to temperatures above 900 °C. The thermal stability of the cold work and the residual stress induced in the sub-surface of the material is therefore a key design requirement. In this study, mechanically shot peened and deep cold rolled samples of the superalloy CMSX-4 were annealed in vacuum at either 900 °C or 1100 °C for between 100 and 1000 h. Using scanning electron microscopy, electron backscattered diffraction and centre hole drilling, the cold work, the residual stress and the material response to annealing heat treatments were all studied. Significant recrystallisation of the cold worked material was observed, typically after 500 h at 900 °C and after 5 h at 1100 °C. Undesirable topologically close-packed precipitates began forming on slip bands generated by both surface hardening techniques after only 10 h at 900 °C, whereas at 1100 °C their formation was hindered by the recovery of slip bands. Rafting of the γ/γ' microstructure took place at 1100 °C, with the extent of the rafting found to be limited by the depth of compressive residual stress. Samples treated by deep cold rolling generally displayed smaller recrystallisation depths, lower rates of topologically close-packed precipitate formation and deeper rafting than those treated by mechanical shot peening.

Investigation on effects of single- and multiple-pass strategies on residual stress in machining Ti-6Al-4V alloy

Residual stresses in the surface and subsurface of the machined part are important factors for surface integrity, which can significantly influence its performance and service life. Generally, the final machined surface could be finished by single- or multiple-pass cutting operation and the formation of residual stresses in each process have been widely studied, however few studies pay attention to the differences of residual stress generated by single- and multiple-pass strategy on the final machined surface. In single-pass cutting, the original unprocessed surface is often employed as initial state. While in multiple-pass, the previous cut often cause accumulated strain/stress and temperature on the machined surface/subsurface, which will affect the cutting forces, thermal distributions, plastic deformation in the subsequent cuts and eventually influence the final residual stresses. Thus this paper investigates the cutting forces, temperatures and residual stresses distributions in machining Ti-6Al-4V with the special emphasis on the differences between the single- and multiple-pass strategies. Coupled Eulerian-Lagrangian (CEL) method is used in FE modelling and experiments are also implemented. Good agreements were found between the experimental and numerical results, then further discussions based on the FE results can draw the conclusions that, the first cut in multiple-pass cutting strategy can lead to a decrease in cutting force and surface tensile residual stress in the second cut. The multiple-pass cutting strategy can result in a lower magnitude and depth of compressive residual stress in subsurface, and this is more obvious when a smaller ap is implemented in the sequential cut. This work can provide practical guidance for optimizing the cutting parameters to get the desired residual stress.

Residual stress and microhardness evolution induced by conventional and ultrasonic shot peening

Very limited comparative research has been conducted on microstructure evolution induced by different peening methods. Herein, the 2024T351 aluminium alloy was subjected to conventional shot peening (CSP) and ultrasonic shot peening (USP) under the identical Almen height to investigate the differences in surface roughness, residual stress and microhardness. The results demonstrated that under the same Almen height, the CSP treatment resulted in a rougher surface with higher surface roughness and more surface defects, and higher surface and maximum compressive residual stress (CRS) than the USP treatment. In contrast, the CRS depth was relatively lower for the CSP treated samples. In addition, the USP treatment produced more significant microhardness and hardened layer than the CSP treatment.

Effect of Laser Shock Peening on micro-structure and mechanical properties of Friction Stir Welded CuCrZr sheets

Friction stir welding (FSW) of CuCrZr sheets was performed and Laser Shock Peening (LSP) was carried out as a post-weld surface treatment. Effect of LSP on hardness, tensile and fatigue strength was investigated. Depth of compressive residual stress in base material increased from 550 μm to 700 μm when the number of LSP shots was increased from one to five. However, noticeable change in surface residual stress could not be observed which is attributed to the saturation of dislocation density or plastic deformation at the surface. An increase in yield strength up to ∼35% as well as an improvement in the fatigue life up to ∼70% after LSP could be achieved for base and FSW specimens. Increase in low angle grain boundaries after LSP indicates the increase in substructure induced by plastic deformation. A quantitative analysis using grain boundary misorientation and kernel average misorientation was used to understand microstructural changes due to the LSP and correlate it with the mechanical properties.

A parametric neutron Bragg edge imaging study of additively manufactured samples treated by laser shock peening

Laser powder bed fusion is an additive manufacturing technique extensively used for the production of metallic components. Despite this process has reached a status at which parts are produced with mechanical properties comparable to those from conventional production, it is still prone to introduce detrimental tensile residual stresses towards the surfaces along the building direction, implying negative consequences on fatigue life and resistance to crack formations. Laser shock peening (LSP) is a promising method adopted to compensate tensile residual stresses and to introduce beneficial compressive residual stress on the treated surfaces. Using neutron Bragg edge imaging, we perform a parametric study of LSP applied to 316L steel samples produced by laser powder bed fusion additive manufacturing. We include in the study the novel 3D-LSP technique, where samples are LSP treated also during the building process, at intermediate build layers. The LSP energy and spot overlap were set to either 1.0 or 1.5 J and 40% or 80% respectively. The results support the use of 3D-LSP treatment with the higher LSP laser energy and overlap applied, which showed a relative increase of surface compressive residual stress (CRS) and CRS depth by 54% and 104% respectively, compared to the conventional LSP treatment.

Small–Scale Experimental Investigation of Fatigue Performance Improvement of Ship Hatch Corner with Shot Peening Treatments by Considering Residual Stress Relaxation

Ship hatch corner is a common structure in a ship and its fatigue problem has always been one of the focuses in ship engineering due to the long–term high–stress concentration state during the ship’s life. For investigating the fatigue life improvement of the ship hatch corner under different shot peening (SP) treatments, a series of fatigue tests, residual stress and surface topography measurements were conducted for SP specimens. Furthermore, the distributions of the surface residual stress are measured with varying numbers of cyclic loads, investigating the residual stress relaxation during cyclic loading. The results show that no matter which SP process parameters are used, the fatigue lives of the shot–peened ship hatch corner specimens are longer than those at unpeened specimens. The relaxation rate of the residual stress mainly depends on the maximum compressive residual stress (σRSmax) and the depth of the maximum compressive residual stress (δmax). The larger the values of σRSmax and δmax, the slower the relaxation rates of the residual stress field. The results imply that the effect of residual stress field and surface roughness should be considered comprehensively to improve the fatigue life of the ship hatch corner with SP treatment. The increase in peening intensity (PI) within a certain range can increase the depth of the compressive residual stress field (CRSF), so the fatigue performance of the ship hatch corner is improved. Once the PI exceeds a certain value, the surface damage caused by the increase in surface roughness will not be offset by the CRSF and the fatigue life cannot be improved optimally. This research provides an approach of fatigue performance enhancement for ship hatch corners in engineering application.

Analysis on the Fatigue Properties of Shot-Peened Al-Si-Mg Alloy and Its Fatigue Life Prediction

Ceramic micro-shot peening (CMSP) and steel micro-shot peening (SMSP) were utilized to investigate the effect of micro-shot peening (MSP) on the high-cycle fatigue properties of Al-7Si-0.3Mg casting aluminum alloy in a previous study. However, the improvement effects of CMSP and SMSP on the fatigue strength (at 5 × 107 cycles) were only 33% because the depth of harden layers was only 20 and 55 μm while the depth of compressive residual stress affected layers was only 37 and 68 μm. In this study, conventional shot peening (CSP) was utilized, and the results were compared with those of MSP, with the expectation that CSP would provide a greater improvement in the fatigue strength. The affected surface layers of the shot-peened specimens were characterized using surface morphology, microhardness, and residual stress analyses. In addition, the effect of CSP on the fatigue strength at 5 × 107 cycles was investigated using a rotating bending fatigue test (R = − 1). An investigation of the extensive surface compressive residual stress relaxation process for the three different shot-peened specimens during cyclic loading was conducted using x-ray diffraction. In addition, the initiation sites for fatigue cracks on the fracture surface were observed using scanning electron microscopy. Furthermore, the fatigue life of the samples with the internal casting defect failure mode was predicted using linear elastic fracture mechanics, while that for samples with the surface crack initiation failure mode was predicted using the modified Morrow model considering the residual stress.

Development of design methodology of technological process of ball-rod hardening with account for formation of compressive residual stresses

Introduction . The study results on the multicontact shock-vibrating machining using a ball-rod hardener are presented. Methods of surface plastic deformation processing, their advantages are described. The tool circuit diagram is given. Features, technological advantages, and the application area of the ball-rod hardening are specified. Materials and Methods . When conducting theoretical studies on the processing, factors, which affect the quality of the surface layer of the machined parts, were established. Dependences are given for calculating the surface roughness, the hardened layer depth, and the deformation ratio under ball-rod hardening. While studying the generation of residual stresses, the dependence for calculating the residual stresses generated in the surface layer of the machined part was specified. Results . The experimental findings of the processing requisite for verification of the adequacy of the theoretical models, as well as the routine of the experiments, are presented. A table and graphs clearly confirming good convergence of the theoretical and experimental data are given (the difference does not exceed 20%). Residual stresses in the surface layer are compressive which enables to predict high performance properties of the machined parts. The value of residual stresses on the workpiece surface is in the range of 130÷200 MPa. The depth of compressive residual stresses is in the range of 0.9-1 mm. The fatigue characteristic variation, the ultimate stresses of the cycle in depth, which affects the endurance limit, is calculated. It has been established that the processing of workpieces by a ball-rod hardener provides increasing the ultimate cycle stress under repeated loading by 27-35%. Discussions and Conclusions . The design methodology of technological process of ball-rod hardening can be used under the development of production at the machine-building enterprises. In accordance with the recommendations, the limits of the required quality parameters of the workpiece surface layer are set; the parameters of the ball-rod hardener, the interference fit and the radius of rod sharpening are selected. Quality parameters of the surface layer are calculated. Correction of the selected modes and re-calculation of the parameters of the machined surface are carried out until all the specified characteristics are located within the required limits.

Fatigue Limit of Custom 465 with Surface Strengthening Treatment.

In order to study the effect of nitriding or shot peening on the surface modification and fatigue properties of martensitic stainless-steel Custom 465, the residual stress and micro-hardness of the strengthened layer are determined by X-ray and micro-hardness tester, respectively. The up-and-down method is used to measure the rotational bending fatigue strength at 1 × 107 cycles, and the fatigue fracture characteristic is observed by scanning electron microscopy. The relationship between surface residual stress and internal fatigue limit of surface strengthening treatment is discussed. Results show that nitriding or shot peening surface strengthening layer forms a certain depth of compressive residual stress, where in the surface compressive residual stress of the nitrided specimens is greater than the shot peened specimens. The micro-hardness of the nitrided or shot peened surface strengthening layer is significantly improved, where in the surface micro-hardness of nitriding specimens are higher than shot peening specimens. The nitriding or shot peening surface strengthening can significantly improve the fatigue limit of Custom 465, wherein the fatigue limits of nitrided and shot peened surface strengthened specimens are 50.09% and 50.66% higher than that of the un-surface strengthened specimens, respectively. That is, the effect of the two strengthening methods on fatigue limit is not very different. The fracture characteristics show that the fatigue crack of the un-surface strengthened specimens originates from the surface, while the fatigue crack of surface strengthened specimens originates from the subsurface layer under the strengthened layer. The relationship between the internal fatigue limit and the surface residual stress of the surface strengthened specimen can be used as a method for predicting the fatigue limit of the surface strengthened specimens.

Effect of prebending curvature on bending strain and surface properties of stiffened plate during prestressed ultrasonic peening

Stiffened plates are widely used in components of airplane for its light weight and high stiffness and ultrasonic peening (UP) is the key forming technology. However, insufficient bending deformation and poor mechanical properties usually appear during free UP, and elastic prestressed UP is adopted. The effect of prebending curvature radii on bending deformation and surface properties is experimentally conducted. The results show that chordwise peening path contributes to enhancement of chordwise bending strain and smaller spherical deformation. Compared with free UP, more sufficient bending deformation, larger surface hardness with smaller surface roughness, and larger and deeper compressive residual stress are generated in prestressed UP. Spherical deformation coefficient is adopted to quantitatively describe the spherical deformation. With decrease of prebending curvature radius, bending deformation, surface hardness, and absolute value of maximum compressive residual stress are significantly increased; meanwhile, the spherical deformation coefficient is obviously decreased. More uniform surface morphology and larger total depth of compressive residual stress are acquired by applying a prebending moment before UP.

Multiple laser shock peening effects on residual stress distribution and fatigue crack growth behaviour of 316L stainless steel

Laser shock peening (LSP) is one of the life enhancement processes by inducing compressive residual stresses into the material. The effects of repeating LSP pattern on the residual stress is investigated numerically in this study. A 3-D finite element model was developed to obtain residual stress field in the material. FEM results have shown that increasing the number of peening impacts increase the magnitude and depth of compressive residual stress. Also the average surface residual stress in the impact region increased from −291 after 1 impact to −326, −343 and −356 MPa after 2, 3 and 4 impacts respectively. Moreover, the effects of repeating LSP pattern on fatigue crack growth properties were investigated experimentally by performing tests on C(T) specimens made of 316L stainless steel. The test results have shown that the fatigue crack growth life is very sensitive to the number of peening impacts and increased from 84,380 cycles in untreated sample to 104,130 cycles in sample with 1 time LSP and 170,500 cycles to sample with 2 times LSP. Also the fatigue crack growth rates decreased by repeating LSP patterns on the test specimens.

Multi-Objective Optimization of Residual Stress and Cost in Laser Shock Peening Process Using Finite Element Analysis and PSO Algorithm

Laser shock peening (LSP) is an effective process utilized for surface enhancement of metal parts so that generating compressive residual stresses (RS) on the surface improves fatigue life of the material. The main affecting parameters on surface negative residual stress are laser power, laser beam size and shape, peening pitch and pattern. Varying these parameters alters the magnitude and depth of RS as well as the cost of LSP. An integrated method for simulation of optimum LSP process is presented in this paper, in which Particle Swarm Optimization (PSO) technique was employed utilizing Python coding in ABAQUS finite element environment to maximize the uniformity of compressive RS and minimize LSP cost on an Inconel 718 super-alloy specimen. The mentioned affecting parameters were selected as optimization parameters, and minimum acceptable amounts and depth of compressive RSs were two main design constraints. Simulation results were compared with previously published experimental ones, and optimum LSP variables were finally determined and presented for certain amount of design constraints. It was revealed that, relatively small circular laser beam, shot by square scanning pattern, leads to generate the most uniform RS with minimum LSP cost.