Mechanism Research Of Nano-second Laser Induced Shock Wave On The Surface Topography And Performance Of Metals

Shock wave induced by high energy nanosecond plused laser under confined regime has attracted much attention nowadays. It is applicated in many fields such as laser shock processing (LSP), laser shock forming, laser-induced spalling, laser shock wave implanting. Among them, LSP is the most attractive technology. LSP induces high compressive residual with great depth on the surface layer of metals and alloys, which can greatly improve the fatigue life of treated materials. LSP has the advantage of non-contact, strong controllability and local treatment, which makes it having unparalleled advantage in improving the fatigue life of aircraft engine blade and vane-integrated disk. The most important factor of influencing fatigue life is surface integrity of mechanical parts, which includes the surface roughness, work-hardening effect and residul stress distribution. Residual stress distribution is widely studied by many researchers about LSP. However, the surface roughness has not attracted enough attentions. Moreover, extending the application scope of laser shock waves is also a meaningful work. Shock waves (with peak pressure of several GPa) induced by nanosecond laser systems are used in this paper. Surface integrity expecially surface roughness is especially concerned after LSP in this paper. A full investigation about the factors which will influence surface roughness is conducted in LSP. The dimension of laser spot and pressure pulse induced by LSP are optimized with simulation and experiment. Moreover, two interesting technologies based on the effect of laser shock wave are invented, which is net mask laser shock processing (NMLSP) and laser shock precessing mocro indention (LSPMI). The detailed contents are as belows.(1) The theoretical caculation of surface roughness with different overlapping styles after LSP.Effects of the usually used overlapping styles to the surface roughness with different overlapping rates are studied, and the overlapping styles are isosceles-righttriangle-style and equilateral-triangle-style. Surface roughness includes Ra, Rq and Rz is theoretically caculated with different overlapping rates. The optimized overlapping style and overlapping rates are successfully attained.(2) Full investigations about surface integrity of LY2aluminum alloy on the scope of laser spot are conducted after LSP.Surface integrity of LY2aluminum alloy on the scope of laser spot are studied after LSP, and the defects of surface are carefully studeid and the formation mechanism of defects are analysised. The surface roughness, microhardness and residual stress distribution of LY2aluminum alloy with different initial surface topography are investigated under different LSP parameters. It is found that there is an ultimate value about surface roughness at the condition of soft-contact no matter how the initial surface topography is. While the surface roughness of rigid-contact condition depends on the initial surface topography of rigid foil. The theoretical foundation and total solution on the improvement of surface roughness in LSP are provided. Compressor blades made of LY2aluminum alloy are treated by LSP with different overlapping rates and styles. Surface roughness and line roughness measurement are conducted after LSP. The optimized overlapping style and overlapping rates are experimentally verified.(3) The effects of laser spot dimension and spatial distribution of pressure pulse induced by LSP on the residual stress distribution and plastically affected depth are simulated and experimentally validated.The ABAQUS software is used to simulate the effects of laser spot diameter on the plastically affected depth and residual stress distribution of pure Al treated by LSP. The simulated results are validated by experiments. The results show that there are some defects in the model demeloped by Ballard. The relations between the laser spot diameter and plastically affected depth are found. Moreover, effect of spatial distribution of pressure pulse induced by LSP on the residual stress distribution is simulated, and the optimized spatial distribution of pressure pulse is attained.(4) Fabrication of micro-structure based on laser shock wave.Based on the laser shock wave effects, a new technology call net mask laser shock processing (NMLSP) is invented. Micro-dent arrays are successfully fabricated by this technology on the surface of LY2aluminum alloy. Although the shape of net mask was square, the as-fabricated micro-dents were close to be circular which was just due to the edge effects and lateral wave propagation. Moreover, it was found that the interference of adjacent surface shock waves would affect the topography of micro-dents. A FEM model was performed to simulate the formation process of micro-dents by ABAQUS. The simulation results indicated that the existence of absorbing layer (Al foil) influenced the contact pressure contours on the surface of the LY2Al alloy sample, which influenced the depth profile of micro-dents fabricated by NMLSP. Moreover, the tribological behavior of A304stainless steel treated by NMLSP is investigated. The results show that tribological and wear properties are improved when treated by NMLSP. Another new technology call laser shock processing micro indention (LSPMI) is also invented. The topography of micro indention fabricated on LY2Al alloy samples by this technology is investigated and the formation process of micro indention is analysed.