Search Results (14)

A surface modification through multiple microshot peening (MSP) was performed on Mg–steel weldment. Application of MSP was found beneficial to the elimination of surface microdefects owing to severe plastic deformation induced by MSP. Moreover, MSP treatment transformed the residual tensile stress of the weld surface into residual compressive stress, which was beneficial to inhibit the initiation and propagation of surface microdefects. Strain strengthening and grain refining were introduced into the shot peened joint, resulting in the notable increase in surface hardness and tensile strength. Compared with an untreated joint, the tensile strength of optimized Mg/steel weldment was markedly enhanced and raised 28% to 244 MPa, and fracture ultimately occurred in the Mg alloy base material. Moreover, the refinement of weld grain induced by MSP treatment was beneficial to strengthen the stress corrosion sensitivity of Mg/steel joints, while also promoting the formation of a denser Mg(OH)₂ passivation film on the weld surface and enhancing the corrosion resistance of the joints. Full article

Titanium alloys are acclaimed for their remarkable biocompatibility, high specific strength, excellent corrosion resistance, and stable performance in high and low temperatures. These characteristics render them invaluable in a multitude of sectors, including biomedicine, shipbuilding, aerospace, and daily life. According to the different phases, the alloys can be broadly categorized into α-titanium and β-titanium, and these alloys demonstrate unique properties shaped by their respective phases. The hexagonal close-packed structure of α-titanium alloys is notably associated with superior high-temperature creep resistance but limited plasticity. Conversely, the body-centered cubic structure of β-titanium alloys contributes to enhanced slip and greater plasticity. To optimize these alloys for specific industrial applications, alloy strengthening is often necessary to meet diverse environmental and operational demands. The impact of various processing techniques on the microstructure and metal characteristics of titanium alloys is reviewed and discussed in this research. This article systematically analyzes the effects of machining, shot peening, and surface heat treatment methods, including surface quenching, carburizing, and nitriding, on the structure and characteristics of titanium alloys. This research is arranged and categorized into three categories based on the methods of processing and treatment: general heat treatment, thermochemical treatment, and machining. The results of a large number of studies show that surface treatment can significantly improve the hardness and friction mechanical properties of titanium alloys. At present, a single treatment method is often insufficient. Therefore, composite treatment methods combining multiple treatment techniques are expected to be more widely used in the future. The authors provide an overview of titanium alloy modification methods in recent years with the aim of assisting and promoting further research in the very important and promising direction of multi-technology composite treatment. Full article

As a surface peening technique, shot peening introduces residual compressive stresses to the surface of the part, which effectively increases the fatigue life of the structure and material. However, when structures are subjected to alternating loads, this can lead to stress relaxation on the material surface, weakening the effectiveness of the shot peening process. In addition, reasonable shot peening parameters are essential. In this paper, the effects of shot peening pressure and shot coverage on the fatigue life of materials during shot peening were investigated, followed by fatigue tests on 20CrMnTi specimens using a high-frequency fatigue testing machine to study the effects of shot peening and re-shot peening on the fatigue life of shot-peened materials after different load cycles. The results show that a reasonable shot peening pressure and coverage rate can significantly improve the fatigue life of the material, while a shot peening pressure higher than 0.4 MPa will reduce the fatigue life of the material 20CrMnTi. Coverage rates of 100% and 200% can both improve the fatigue life of the material, while a 200% coverage rate has a better strengthening effect. Re-shot peening removes the residual compressive stress relaxation on the surface of the material caused by cyclic loading and improves the fatigue life of the material. The maximum value of the residual compressive stress on the surface of the test material after shot peening is 443 MPa, and after a certain number of fatigue loads, the residual compressive stress on the surface is reduced to 203 MPa, which is subjected to secondary shot peening, and the residual compressive stress is restored to 415 MPa, and the fatigue life is significantly increased. When the second shot peening time is taken as 25% of the fatigue life of the initial shot blasting of the material, the shot peening effect is better. Full article

Combined peening composed of multiple peening processes or peening media is a surface treatment method for comprehensive control of the macro shape and performance of the part. Compared to combined peening, the impact kinetic energy of the combined hammering can be easier to control over a wide range, and the hammer tool head size is larger than the shot. This paper focused on investigating the effect of combined hammering treatment, 6 mm and 14 mm tool heads with peening density 3.7 to 4.2/mm${}^2$, on the variable scale evolution of titanium TA2. Three types of contact relation between the tool head and existing dimple were proposed for impacting at the same position. The size of the dimple of combined hammering varies in width or depth direction, resulting in nest morphology composed of different size dimples. The cross-section microstructure of the test plate was observed, and the gradient changes of dislocation, slip, and grain size are smoothed by combined hammering. The change in hammer tool head size makes the target plastic deform at different depths. The hammering sequence has a significant influence on the evolution of stress and strain fields. When the tool head is first large and then small, a large compressive residual stress near the surface is introduced, about 1000 Mpa; on the contrary, the compressive residual stress distributes uniformly in the depth direction, with an affected layer depth of about 4.4 mm. The measured dimple size and residual stress verified the reliability of the simulation results. Full article

Shot peening technology is used to improve the fatigue strength of materials and parts, and is one of the most effective surface engineering techniques to prolong fatigue life. In this paper, according to the finite element simulation analysis of shot peening, a randomly distributed multiple-shot finite element model was established. The superimposed effects of multiple projectile impact craters in shot peening are fully considered. The effects of shot velocity, shot peening angle and shot coverage on the residual stress field and surface roughness were studied. The alloy steel 20MnTiB, widely used in the automotive industry, was used as the raw material to process the specimens. The shot peening tests of different process parameters were carried out. The test results verified the correctness and accuracy of the random distribution model of multiple-shot. The shot-peening simulation model proposed in this paper allows a more accurate analysis of the effect of shot-peening parameters on the surface residual stress field and helps to quickly set the correct shot-peening process parameters. This paper further investigates the effect of shot peening parameters on fatigue life, providing a basis for the rational development of shot peening solutions. Full article

Surface nanocrystallization of metals and alloys via high-frequency ultrasonic shot peening (USP) can significantly increase the mechanical properties of the materials. However, the relationship between the external process parameters and the internal microstructure of the materials is still unclear and an accurate numerical model to simulate the USP process is urgently required for better control of the grain refinement process. In this study, we successfully realized surface nanocrystallization of 316L stainless steel using USP with an ultrasonic frequency and amplitude of 20 kHz and 50 μm, respectively. The microstructure evaluation of 316L stainless steel during USP was revealed. We established a finite element numerical model to simulate the high-frequency USP process and calculated the plastic strain and stress distribution of 316L stainless steel during the grain refinement process. We investigated the effects of the ultrasonic frequency, working distance, and ultrasonic amplitude on the plastic strain and stress distribution on the materials using the finite element simulation method. The dynamic behavior of the shot during the USP process was studied using a high-speed camera, and the FE simulation results agreed well with the experimental results. We also investigated the impact of multiple shots during the USP process by the high-speed camera observation and FE simulation. Research results indicate that high-frequency USP is an effective method to obtain large-scale bulk nanocrystalline materials and the finite element simulation can help materials scientists and engineers to better understand the relationship between the process parameters and microstructure evaluation of 316L stainless steel. Full article

In this study, multiple shot peening was performed on a carbon nanotubes−reinforced aluminum matrix composite, of which residual stress fields and tissue structure evolution were investigated. It is shown that the multiple shot peening could significantly increase the magnitude of compressive residual stress field, modify surface morphology of the specimens, and further refine the grain sizes of the near surface layer. Dislocation density in the near−surface layers were also elevated by multiple shot peening. Moreover, enhanced microhardness with more even distribution were obtained in the modified peened layers ascribed to the raised compressive residual stress field and microstructure which could give rise to the strain−hardening effects. Full article

A known problem of additive manufactured parts is their poor surface quality, which influences product performance. There are different surface treatments to improve surface quality: blasting is commonly employed to improve mechanical properties and reduce surface roughness, and electropolishing to clean shot peened surfaces and improve the surface roughness. However, the final surface roughness is conditioned by multiple parameters related to these techniques. This paper presents a prediction model of surface roughness (Ra) using an Artificial Neural Network considering two parameters of the SLM manufacturing process and seven blasting and electropolishing processes. This model is proven to be in agreement with 429 experimental results. Moreover, this model is then used to find the optimal conditions to be applied during the blasting and the electropolishing in order to improve the surface roughness by roughly 60%. Full article

The work aims to study the change rule of the surface properties of aluminum (Al) alloy after shot peening (SP), and obtain the corresponding relationship between the surface material characteristics and SP parameters. Firstly, the Finite Element Model (FEM) of multi-projectile impact Al alloy specimen was established by ANSYS/LS-DYNA (Version: Ansys15.0). Box-Benhnken design method (BBD) was used to design 3-level and 3-factor shot peening experiment with shot peening pressure. Projectile size, jet distance, the surface residual stress, as well as deformation were taken as responses. The surface stress and the deformation at the crater were obtained according to the experiments. Then, Design-Expert software was adopted to fit the values to attain the multiple regression quadratic equations, and the response surface methodology (RSM) was applied to analyse the interaction between the various factors. The degree of model-fitting was simultaneously identified in accordance with an analysis of variance of the function models. Finally, the shot peening test was carried out using 7075-T651 Al alloy as the specimen. Combined with the X-ray diffraction (XRD) stress test and the optical microscopic observation of the crater section, the value of stress and deformation value could be determined to verify the accuracy of the model. The adjusted R² of the stress function model and the deformation function model were 94.85% and 95.08%, respectively. The deviation between calculated stress value and the experimental value was less than 5.5%. The deformation of the section showed that the deformed layer of the specimen was approximately the same as calculation value. The result indicated the function model had high accuracy. The analysis suggested that the function model could quickly and precisely deduce the parameter combination of the SP from the surface stress or deformation of the material, which provided a diversity reference for the surface stress and hardness strengthening of SP. Full article

Abrasive waterjet peening (AWJP) as an important surface strengthening method can effectively improve surface properties. In this study, after multiple AWJP, the distribution of compressive residual stress and roughness on the surface of 18CrNiMo7-6 steel has been evaluated by an X-ray diffraction (XRD) method and a 3D surface topography system, respectively. Compared with the single AWJP, multiple AWJP can obviously increase the surface residual stresses (−1024 MPa to −1455 MPa) and the depth of maximum compressive residual stress (100 μm to 120 μm), as well as make the stress distribution more uniform. In terms of the surface roughness, multiple AWJP influences its uniform distribution and reduces the surface roughness (S_a = 0.69 μm), compared with a single AWJP (S_a = 2.96 μm), due to the smaller shot balls and a uniform deformation during multiple AWJP. In addition, we have studied the effects of multiple AWJP on the hardness of the surface layer. The results show that multiple AWJP increases the hardness by up to 15.9%, compared to the single AWJP. These studies provide useful insight into improving the surface properties of 18CrNiMo7-6 steel by multiple AWJP. Full article

Bolted joints are highly loaded components and serious issues may arise from improper fastening and in particular from too high or too low preload. Friction at the underhead plays an important role, as it significantly affects the achievable preload for fixed and controlled tightening torque. In addition, multiple tightening is usually performed on connecting rod screws, which may be a further source of friction increment. This study investigates the effect of two surface treatments, shot-peening and deep-rolling, on the tribological properties upon bolt fastening. This topic was tackled experimentally and the campaign involved MJ9 X 1 4 g grade 13.9 36 NiCrMo connecting rod screws, in both lubricated and dry conditions. The results, processed by statistical tools, indicate that deep-rolling does not affect friction, whereas shot-peening yields significant benefits. As an effect of the generation of dimples and multiple contacts, it is able to lower (up to 25%) the bearing frictional coefficient in lubricated conditions, also making the friction level independent of the number of re-tightenings. For a dry surface, an even higher friction decrease (up to 30%) is achieved. Without lubrication, the friction coefficient keeps increasing for the incremented number of tightenings, but the increase rate is lowered with respect to the untreated surface. Full article

Laser-based powder bed fusion (L-PBF) is an additive manufacturing (AM) technique that uses a computer-controlled laser beam as the energy source to consolidate a metal powder according to a layer-upon-layer strategy in order to manufacture a three dimensional part. This opens the way for an unprecedented freedom in geometry, but the layer-wise build-up strategy typically results in a very poor surface finish, which is affected by the staircase effect and by the presence of partially molten particles. Surface finishing treatments are therefore necessary to obtain an adequate surface finish, to improve the fatigue behavior and to meet mechanical and aesthetic needs. The present contribution systematically compares numerous surface finishing techniques, including laser shock processing, plastic media blasting, sand blasting, ceramic shot peening and metal shot peening with steel particles of different sizes (ϕ = 0.2 mm and ϕ = 0.4 mm). The results show that all the proposed methods improve the surface quality and the fatigue life of A357.0 L-PBF parts. However, the achievement of the lowest surface roughness does not necessarily correspond to the best fatigue performance, thus suggesting that multiple mechanisms may be active and that besides surface roughness also residual stresses contribute to increase the fatigue strength. Full article

Shot peening (SP) is a controlled and systematic process of surface treatment that has a large number of controllable process parameters that make its application highly challenging. It involves the shooting of small and hard metallic balls at a targeted surface, with the aim of enhancing the fatigue strength of the workpiece under unfavorable service conditions. The compressive residual stress (CRS) induced by this application is expensive to evaluate experimentally. This paper presents a numerical model of the impact of a single-shot on a metallic surface, with the aim to set the stage for a realistic multiple shots peening simulation. The approach proposed herein is a sequential Discrete Element-Finite Element (DE-FE) coupled simulation, based on the use of different types of coefficients of restitution (CoRs) with emphasis on the energetic CoR. The energetic CoR relates the shot/target contact forces to the fractional strain energy needed for localized plastic deformation of the near-surface layer in the workpiece. The generated results of the induced compressive residual stresses (CRS) and equivalent plastic strain (PEEQ) from single-shot simulations are validated with similar results from the literature. Our study clarifies the strain energy aspects of a single-shot impact responsible for the desired effects of CRS and PEEQ, thereby laying the groundwork for accurate and realistic modeling of the SP process via the DEM-FEM approach. Full article

Shot peening is one of the most famous mechanical surface treatments to improve fatigue performance of metallic components, which is attributed to high amplitude compressive residual stresses. A numerical approach is developed to analyze the residual stresses in 301LN metastable austenitic stainless steel by shot peening. The material behavior is described by a proposed constitutive model in which strain-induced martensitic transformation, isotropic hardening and kinematic hardening effects are taken into account properly. Both single shot and random multiple shots peening were simulated and analyzed. A numerical method is presented with the Python programming language to make the multiple shots follow a random probability distribution. Results demonstrate that the simulated equivalent plastic strains and martensitic volume fractions agree well with the experimental ones, which verify the validity of the constitutive model. Besides, the numerical method is effective at achieving a realistic surface coverage. The maximum compressive residual stress by the Johnson–Cook model is 12% higher than that of the proposed model. Additionally, each hardening effect has an effect on the simulated residual stress. The developed numerical approach can provide a feasible simulation of the shot-peening process and makes an accurate prediction of the residual stress field in 301LN steel. Full article