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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 aim of this study was to investigate the effects of laser peening (LP) on the friction and wear properties of medical Ti6Al4V alloy. Wear tests were conducted on the samples before and after LP with a ball-on-disk wear tester in Hank's solution. The surface roughness and micro-hardness of the differently treated samples were measured. The friction and wear properties of the samples were evaluated by comparing their coefficient of friction (COF) and wear mass loss. The morphologies of wear scar and the wear mechanism were characterized by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results showed that the surface micro-hardness of the specimen subjected to LP was significantly increased by 25.7% and the corresponding depth of the hardened layer was more than 0.3 mm. The average COF and wear mass loss were significantly reduced to 50% and 29.2%, respectively, as a result of the increasing laser energy and impact times. After LP, the wear mechanism turned from the severe oxidative wear to a slight adhesive wear and abrasive wear. Experimental results proved that LP treatment could effectively improve the friction and wear properties of the medical Ti6Al4V alloy.

Microstructural response of AISI 316L stainless steel to laser peening (LP) was examined by means of optical microscopy (OM) and transmission electron microscopy (TEM) in order to analyze the effects of LP on hydrogen-induced cracking (HIC) resistance. Depth profiles of near-surface microhardness and surface compressive residual stress (CRS) of LP treated specimens were presented respectively. Slow strain rate tensile tests were performed on the hydrogen-charged samples and their corresponding stress-strain curves as well as fracture morphologies were finally investigated in detail. The results demonstrated that LP induced a grain refinement effect on the treated surface while a maximum refining rate of 56.18% was achieved at the laser power density of 10 GW/cm2. The near-surface microhardness also exhibited an attenuation trend with the increasing depth. The surface CRS positively correlated with power density before it reached a threshold value. A special U-shaped dislocation tangle band was observed in the LP treated specimen which served as hydrogen trapping sites. The LP treated samples exhibited better toughness after hydrogen charging from both macro mechanical properties and micro fracture morphologies. LP-induced grain refinement and CRS are believed to be the main contributing factors towards inhibiting the diffusion of hydrogen atoms which ultimately leads to the reduction of the hydrogen embrittlement sensitivity of AISI 316L stainless steel.

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.

Slow strain rate tensile test in a hydrogen-rich environment was performed to study the effects of laser peening (LP) on the tensile properties and martensitic transformation of AISI 316L stainless steel. Underlying physio-chemical processes involved in hydrogen embrittlement of 316L steel in the hydrogen-rich environment has been proposed. Results show that the grain refinement, mechanical twins and high-density dislocations were found in the laser peened specimens. The movement of hydrogen-carrying dislocations and martensite transformation were prevented by complex grain boundaries and high-density dislocations during tensile process. The ultimate tensile strength of the laser peened specimen were increased by 7.73% and 8.45% in air and hydrogen-rich environment respectively compared with that of non-laser peened specimen. What's more, the elongation loss of laser peened sample was far less than that of non-LP sample in a hydrogen-rich environment. LP induced compressive residual stress (CRS), grain refinement, high-density dislocations and a dense surface layer on the treated surface are beneficial for suppressing hydrogen penetration.

Laser peening (LP) treatment with different power densities was used to improve the vibration fatigue resistance of hydrogenated TC4 titanium alloy. Compressive residual stress (CRS), micro-hardness and micro-structure of hydrogenated LPed specimens were firstly measured. Vibration fatigue test was then carried out to compare the fatigue life and fractures of LPed and non-LPed hydrogenated specimens. It was found that LP-induced high level CRS suppressed the permeation of hydrogen atoms through material surface, which may reduce the hydrogen embrittlement sensitivity. The CRS also prohibited the fatigue crack initiation and propagation by increasing the fatigue limit and crack propagation threshold of hydrogenated specimen. Grain morphologies of α phase and β phase were refined after LP. Beneficial crystal defects inside the α phase and β phase tangled more free hydrogen atoms, which was believed to be crucial factors to inhibit the generation of initial hydrogen-induced crack. Fracture morphologies further confirmed that LP changed the location of fatigue crack initiation (FCI) and slowed down the fatigue crack growth (FCG) rate of hydrogenated specimens. Finally, a strengthening mechanism of LP on the vibration fatigue resistance of hydrogenated specimen was proposed.

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.