| DOU Yang,ZHOU Wang-fan,WU Yong-sheng,CHEN Lan,REN Xu-dong.Thermal Effect of Laser Shock Peening without Coating[J],52(5):347-355 |
| Thermal Effect of Laser Shock Peening without Coating |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.05.034 |
| KeyWord:laser shock peening pure titanium thermo-mechanical coupling residual stress microstructure temperature field |
| Author | Institution |
| DOU Yang |
School of Mechanical Engineering, Jiangsu University, Jiangsu Zhenjiang, |
| ZHOU Wang-fan |
School of Mechanical Engineering, Jiangsu University, Jiangsu Zhenjiang, |
| WU Yong-sheng |
School of Mechanical Engineering, Jiangsu University, Jiangsu Zhenjiang, |
| CHEN Lan |
School of Mechanical Engineering, Jiangsu University, Jiangsu Zhenjiang, |
| REN Xu-dong |
School of Mechanical Engineering, Jiangsu University, Jiangsu Zhenjiang, |
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| Abstract: |
| Laser Shock Peening without Coating (LSPwC) technology is suitable for special processing environment while ensuring the strengthening effect and processing efficiency. The improved surface properties of metal materials after LSPwC attribute to the introduction of compressive residual stress and modification of microstructure. However, the thermo-mechanical coupling effect on stress state and microstructure has not been well understood. This paper aims to investigate the formation mechanism of residual stress and microstructure in TA2 pure titanium under laser induced thermal and mechanical effects induced by LSPwC. The investigated material was rolled TA2 pure titanium sheet with a thickness of 3 mm. The laser had a wavelength of 532 nm, pulse width of 10 ns, spot diameter of 0.4 mm and pulse energy of 30 mJ. The overlapping rate of two adjacent laser spots was 50%. Running water with a thickness of 2 mm was used as the constraint layer. The sample size, LSPwC processing area size and laser scanning path are shown in Fig.1. The measurements of residual stresses were performed on an x-350a X-ray stress meter based on sin2Ψ method. The surface microstructure was analyzed by an FEI Talos f200x transmission electron microscope (TEM). The finite element simulation of LSPwC process was completed on ABAQUS software. The two-dimensional axisymmetric model was used to simulate the single-point LSPwC, and the element type was set as CAX4RT. The heat input induced by nanosecond-pulsed laser was equivalent to the Gaussian heat flux while the shock wave was equivalent to the pressure pulse applied on the material surface. The effects of heat conduction, convection and radiation as well as the effects of evaporation, melting and lattice phase transition on the temperature field were considered in this work. The constitutive model of TA2 pure titanium during LSPwC was Johnson-cook plastic model. During LSPwC, the heat affecting time of each layer and heat affected depth increased with laser pulse energy and laser spot diameter. Moreover, with the increase of laser pulse energy and spot diameter, the cooling rate of each layer decreased. After LSPwC, a tensile residual stress layer with a thickness of approximately 50 μm formed, and the maximum compressive residual stress appeared in the sub-surface layer. The depth of compressive residual stress layer increased with the increase of laser energy and laser spot diameter. The shock wave with high amplitude pressure increased the plastic strain and penetration depth of plastic wave. Therefore, the depth of compressive residual stress increased with the increase of laser pulse energy. As the spot diameter increased, the laser-induced shock wave tended to propagate in the form of plane wave. Compared with the spherical wave induced by small laser spot, the attenuation speed of plane wave was slower, so the depth of compressive residual stress layer increased with laser spot diameter. The surface layer underwent rapid cooling process after the plastic deformation was completed during LSPwC. In the LSPwC process, the TA2 surface melted and then solidified, and the cooling rate exceeded 106 ℃/s. Therefore, ultra-fine martensite grains formed on the surface. A small amount of martensites could also be observed at the depth of 20 μm from the topmost surface. The reason was that the temperature of this layer was higher than the lattice transformation temperature of pure titanium, and martensite transformation occurred during rapid cooling. Martensitic transformation lead to lattice distortion, so dislocation tangles appeared near the martensite boundaries. The microstructure at the depth of 100 μm from the topmost surface was characterized as high-density dislocation tangles, which attributed to laser-induced shock wave. The formation mechanism of LSPwC induced residual stress and microstructure can be summarized as follows:the material surface absorbs laser energy and then vaporizes and ionizes to form a shock wave propagating to the interior of the material; the volume of surface layer expands due to melting, and the subsurface is stretched along the direction perpendicular to the laser incidence by the shock wave, resulting in high-density dislocations; the surface layer solidifies and shrinks, forming a large number of ultra-fine martensite grains; the near surface is heated and then expands, which produces further stretching along the direction perpendicular to the laser incidence, and then cools rapidly to form a small amount of martensites; ultra-fine martensite grains and tensile residual stress form on the surface; a large number of dislocation tangles and compressive residual stress generate in the subsurface layer. |
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