X.D. Ren

Study of the Effect of Coatings on Mechanical Properties of TC4 Titanium Alloy during Laser Shock Processing

During the process of laser shock processing (LSP), we use high power Nd:Glass laser systems which result in significantly improve fatigue properties and stress corrosion cracking in titanium components. An ablative coating such as black painting and aluminum foil are used to protect the titanium component from surface melting by laser pulse, which adversely affects sample fatigue life. Three different shock-processing configurations are considered: non-ablative exposure, aluminum foil and confined ablation with black painting. We analyzed effect of the coatings on the shock wave propagation into the titanium specimen and the resulting change in mechanical properties versus depth. Near the surface, yield strength and hardness are found to be increased by laser shock, the titanium surface for all coatings increased in yield strength by 16% to 37%. The result have demonstrated that surface hardness can be improved by 31 percent over a machine worked surface with black paintings and be improved by 25 percent with aluminum foil .The experiment aim is to report the effect of the ablative, sacrificial coatings on the laser pulse generation of shock waves and their propagation into the titanium alloys and the resulting change in mechanical properties below the surface versus depth.

Residual stress induced convex bending in laser peen formed aluminum alloy

Laser peen forming is a purely mechanical forming method developed to accurately bend, shape, precision align, or repair components using nanosecond-pulsed laser induced shock waves. This paper aims to clarify the residual stress induced bending process in laser peen formed aluminum alloy plates. Finite element simulations were performed to investigate the bending angle and residual stress redistribution before and after bending. A reasonable agreement between simulation and experiment results was achieved. The results showed that larger bending angle was obtained by increasing the number of laser impacts and laser peened area. Besides, the bending angle in 3 mm plates was larger than that in 2 mm plates under the same processing condition. The laser peening induced bending angle is controlled by a residual stress gradient in the thickness direction.Laser peen forming is a purely mechanical forming method developed to accurately bend, shape, precision align, or repair components using nanosecond-pulsed laser induced shock waves. This paper aims to clarify the residual stress induced bending process in laser peen formed aluminum alloy plates. Finite element simulations were performed to investigate the bending angle and residual stress redistribution before and after bending. A reasonable agreement between simulation and experiment results was achieved. The results showed that larger bending angle was obtained by increasing the number of laser impacts and laser peened area. Besides, the bending angle in 3 mm plates was larger than that in 2 mm plates under the same processing condition. The laser peening induced bending angle is controlled by a residual stress gradient in the thickness direction.

Coating Influence on Residual Stress in Laser Shock Processing

Laser shock processing is an important surface treatment that induces compressive residual stress to components, where the coating plays an important role. This paper deduce a general formula of the optimum thickness of coating according to the law of energy conservation and analysis the influence of coating on residual stress of the titanium alloy in laser shock processing. Titanium alloy with black paint, silica acid black paint and without coating were shocked by laser system respectively. It was found that coating could increase shock pressure amplitude and laser density absorption. Compressive residual stresses at the surface of the sample with the black paint and silica acid black paint are about -212.2MPa and -264.2MPa respectively, while the surface stress on the uncoated specimen is very high tensile stress. The bare surface due to melting and vaporization, leads to a very rough surface. The depth of induced compressive stress could reduce stress corrosion cracking in titanium alloy and improve fatigue lifetime.

Study on Residual Stress of 3A21 Metal Sheet by Laser Shock in Oblique Angle

3A21 metal sheet was shocked by Nd-glass laser in oblique angle under the function of pulse energy 42 J, pulse width 23 ns, pulse power 1.2×109 W, the angle between input beam and normal of sample is 30°. The center of the deformation of the sheet is 3 mm away from the geometric center. Residual stresses of crossing center side in length direction and the distribution of space in the positive and negative sides are measured by XRD, which are compressive stress 100 MPa. The diagonal length of the sheet is longer than the one of the crossing center side in length direction, the force that needs to form in the diagonal direction is bigger, so compressive stress is much bigger. The capstone of the square sheet is squeezed, the procession seriously increase, so compressive stress is the biggest one.

Study on Deformation Behaviors of LD31 (6063) Aluminum Alloy Sheet by Oblique Angle Laser Shock

Laser shock forming of metal sheet usually effects on the surface of work-piece in normal direction by laser beam, and produces shock wave force which is vertical to the surface of work-piece. But it is difficult to shock in normal direction during processing on the surface of forming camber, so it needs to study on oblique angle laser shock processing. LD31 forging aluminum alloy sheet is shocked by Nd-glass laser in the paper, model and mode of oblique angle laser shock processing is found, and transmitting characteristics of plasma stress wave is discussed. The experiment results show that the angle between input beam and sample normal is bigger, component of forces that parallel with sample surface is bigger, the deformation eccentricity is bigger and bigger. When the angle is bigger, the area of laser beam spot becomes bigger too, energy density becomes smaller, deformation becomes shallower.

Laser High-Speed Impact Composite Forming of Aluminum/Aluminum

Based on the laser-driven flyer micro forming and laser high-speed impact welding, this paper put forward the laser high-speed impact synchronous welding and forming new process, and builds the compound welding experiment platform. The three-dimensional deep field digital microscope of KEYENCE VHX-1000C was used to measure the surface morphology and the maximum deformation depth of the welding and formingsamples. By observing the surface morphology of the sample, it was found that strong plastic deformation occurred on the surface of the materials and well reproduced the shape of the mold. When the laser energy was below 4.5J, the maximum deformation depth of the samples increased with the laser energy. However, the maximum deformation depth decreased due to the spring back phenomenon when the laser energy was larger than 4.5J. The Axio CSM 700 confocal microscope was used to measure the morphology of the welding interface. The cross profile of the welding interface showed that most regions had been welded and the welding interface was nearly flat.

Fatigue Crack Propagation Experiment and Simulation on 7050 Aluminum Alloy

In this paper, an experiment of fatigue crack propagation in 7050 aluminum alloy was presented. Laser shock processing (LSP) is used to shock the crack surface. Compared with the specimen without LSP, the fatigue life after LSP increased greatly. The simulation of the fatigue crack growth in 7050 aluminum alloy is implemented in FRANC2D. Simulating result is in accordance with the result of the experiment well. Laser shock processing increases the fatigue life and reduce fatigue crack growth rate, it has good prospect on the study of crack arrestment.

Effects of Laser Multiple Processing on Properties of Heat-Resistant Steel

The heat-resistant steel after aluminized was treated by laser shock processing (LSP) with high power Nd:YAG laser, and then was tensile tested at 400°C. The effects of the high-temperature behavior after LSP were analyzed from residual stress and fracture organization. The results showed that the yield strength and tensile strength of heat-resistant steel after aluminized were improved obviously during the tensile testing at high temperature, and the High-temperature fatigue life of 00Cr12 with composite processing was enhanced vastly. Compared with the LSP, the High-temperature fatigue life of 00Cr12 heat-resistant steel by aluminizing and LSP had a larger increase.

Effects of Laser Shock Processing on Fatigue Lives of Heat-Resistant Steel and Estimation of Fatigue Safe Lives

Fatigue tensile treatment was performed on 00Cr12 heat-resistant steel specimens which had been treated by laser shock processing (LSP). The unilateral tolerance factor statistical analysis method and the two-dimensional Weibull distribution method were used to predict the values of fatigue safe lives of the specimens respectively. The results indicate that the fatigue lives of 00Cr12 specimens were enhanced greatly after LSP, and improved by 62% compared with the specimens which were not treated by LSP. The unilateral tolerance factor method obtained an exact estimation value of the fatigue safe life; while the two-dimensional Weibull distribution method can get a range of values of fatigue safe lives, the fatigue safe lives of the specimens which after LSP were a range from 116570 to 150230, the average value was 132330. The two-dimensional Weibull distribution method has more engineering applicability and can be used to estimate the fatigue safe lives with fewer experiments.

Influence of compound laser life-extension treatment on mechanical properties of key parts of pellet traveling grate

An new composite strengthening and prolongation technique was discussed on the key components of pellet traveling grate, which are prone to thermal vibration fatigue, contact fatigue and wear resistance under the alternating high/low temperature. CO2 laser remelting was used to subject the key components of pellet traveling grate, improving the materials surface hardness, and the advanced surface treatment technique by utilizing intense laser shockwave was adopt. The impact of residual compressive stress generated by laser shock processing on fatigue life of surface cracks was discussed, and laser shock could effectively alleviate the residual stress at the surface and significantly improve the fatigue life and anti-stress corrosion performance. The result shows that composite laser treatment could effectively close up and repair cracks.