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The mechanical properties of laser peening (LP) treated TC4 titanium alloy before and after hydrogen charging were compared by means of slow-rate tensile tests. Fracture morphologies of the specimens were observed by SEM to identify the fracture mechanism under the interaction of LP-induced compressive residual stress, refined microstructures and hydrogen permeation. Cross-sectional TEM observation was also conducted to investigate the effects of LP and hydrogen charging on the microstructural evolution of the alloy. Under hydrogenated and unhydrogenated conditions, the LPed specimen presented better UTS results. Furthermore, increasing laser power density decreases the rate of hydrogen-induced elongation loss, as well as reducing the hydrogen-induced plasticity loss. LP induced microstructures, like high tangled dislocations, mechanical twins, and multi-directional slip bands are believed to be the potential factors of trapping hydrogen atoms movement, which ultimately reduced the hydrogen embrittlement (HE) of the alloy.

The effects of laser peening (LP) on the surface micro-structure and hydrogen embrittlement behavior of 316L stainless steel specimens were investigated. 316L stainless steel samples were treated by LP first, and then hydrogen charged electrochemically in 0.5 mol/L dilute sulfuric acid solution in the presence of 1.25 g/L sodium phosphate. Surface residual stress and microhardness of the specimens were recorded before and after hydrogen charging, while the surface microstructures of the specimens were also observed by means of optical microscope (OM) and scanning electron microscope (SEM). Furthermore, a slow rate tensile test was conducted by an electronic universal testing machine to study the toughness loss during the hydrogen charging process. The results showed that the LPed areas were less influenced by hydrogen atoms than the base metal. Also, susceptibility to hydrogen embrittlement was decreased after LP, and the strengthening mechanism of LP on hydrogen embrittlement resistance was subsequently revealed and analyzed on the basis of compressive residual stress, grain refinement and dislocation changes.

The main purpose of this study is to investigate the mechanical properties and micro-structures of IN718 super-alloy subjected to Cryogenic Treatment (CT). Tensile tests were performed and their engineering stress-strain curves were examined. Fracture morphologies of samples were observed with the help of Scanning Electron Microscopy (SEM). The micro-structural evolution of IN718 super-alloy before and after CT was investigated by both Optic Microscope (OM) and SEM. The composition of the precipitated phase was detected by Energy Dispersive Spectrometer (EDS). In order to better analyze the evolution of micro-structures, Transmission Electron Microscopy (TEM) observation was also carried out. Additionally, XRD technique was used to analyze the surface residual stress and microstructural phase. The results show that after two cycles of CT (CT-2), the tensile properties of IN718 super-alloy were found to be substantially improved at room temperature. Also, in comparison to the ductile fracture mode with very small dimples and some quasi-cleavage planes of untreated specimen, CT-2 specimen exhibited a more ductile fracture mode. The grains of CT-2 sample were significantly refined and the size became much more uniform. Compared to untreated specimen, the quantity of precipitated phases at grain boundaries was increased while the size of the precipitates was smaller and the distribution was more uniform after CT-2. High density dislocations were found to pile up at the grain boundaries. The occurrence of this phenomenon is expected to be explained as a result of micro plastic deformation and the increase of internal stress induced by volume shrinkage during the CT process.

The hydrogen permeation resistance of Ti-6Al-4V alloy treated by laser peening (LP) was investigated. Electrochemical hydrogen charging, hydrogen permeation experiments were carried out respectively. The residual stress, micro-hardness of hydrogenated and non-hydrogenated specimens were analyzed. The apparent hydrogen diffusion coefficient, lattice hydrogen concentration and hydrogen trap density were compared. LP process and hydrogen diffusion process were numerically simulated by ABAQUS software. The results show that LP can effectively increase the compressive residual stress (CRS) and micro-hardness on the surface of Ti-6Al-4V alloy. LP also promoted the closure of the original micro-cracks by forming a dense strengthening layer on the surface of the material, which reduced the hydrogen concentration and the diffusion coefficient in the crystal lattice. The laser power was inversely related to the hydrogen diffusion coefficient. The numerical simulation results reveal that the hydrogen concentration in the depth direction of LPed specimen decreases significantly, and the maximum drop of 50% for hydrogen concentration was appeared at the depth of 0.6 mm from the top surface. Moreover, the distribution trend of hydrogen concentration in the LP area is consistent with that of CRS, which indicates that CRS plays a positive role in the inhibition of hydrogen diffusion.

The aim of this paper is to study the effects of laser peening (LP) on the residual stress distribution and microstructure evolution of AA 6061-T6 under different temperatures. A laser peening experiment was conducted on the square-shape samples by using single spot and 50% overlap shock. Three-dimensional surface morphologies of treated samples were observed. The influence of peening temperature on the distribution of compressive residual stress was analyzed. An optical microscope (OM) and a transmission electron microscope (TEM) were employed to observe the microstructure evolution of the samples before and after LP. The results indicate that, as the peening temperature increases, the micro-hardness increases first and then decreases. The LP process induces high-amplitude compressive residual stress on the surface at different temperatures even if the compressive residual stress slightly reduces with increases in temperature. The maximum compressive residual stress affected layer depth is about 0.67 mm, appearing at a temperature of 160 °C. The OM test revealed that the grain size was significantly decreased after warm laser peening (WLP) and that the average value of grain size was reduced by 50%. The TEM test shows that more dislocation tangles were produced in AA 6061-T6 after WLP; compared to the LP process, the precipitate-dislocation interaction can benefit both strength and ductility for AA 6061-T6, thus enhancing the mechanical properties of the material.

The aim of this study was to investigate the effects of the cryogenic treatment (CT) using liquid nitrogen on tensile properties and microstructures of the 2024-T351 aluminum alloy. Tensile tests were carried out, and tensile fractures were observed using a scanning electron microscope (SEM). The microstructure evolution of 2024-T351 subjected to CT was also studied using both an optic microscope (OM) and a SEM. The components of the second phase were tested with an energy dispersive spectrometer (EDS). The results showed that both the ultimate strength and the yield strength of the 2024-T351 aluminum alloy could be improved through CT without the sacrifice of elongation. In addition, tensile fractures showed that the plasticity of 2024-T351 aluminum might also be improved, as the dimples in the fracture of the CTed specimens were markedly more uniform compared with the untreated specimen. The phenomenon of grains refinement (GR) was found through microstructure observation. It was also found that the second phases were distributed more uniformly after CT. A conceivable mechanism concerning the shrinking effect and crystal grain movement was raised to explain the experimental phenomena. The effects of CT on residual stress in the 2024-T351 aluminum alloy are discussed herein. Measurements showed that tensile residual stress in 2024-T351 was removed, and slight compressive residual stress was generated after CT. This may also contribute to the improvement of the tensile properties of the alloy.