Research On The Numerical Simulation And Impact Effects Of Laser Shock Processing

Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue, corrosion and wearing resistance of metals. More recently, lower power lasers operated at higher frequencies, have been explored for laser shock processing. This thesis presented a systematic research on the numerical simulation and impact effects of laser shock processing with a small energy Nd:YAG laser. Main research and originality innovations are as follows:An experimental technique for successive small energy impacts was developed. Great efforts have been devoted to attain the process of small energy impacts based on the mechanism of laser shock processing. A beam path facility and a sample holder were developed to work with a small energy Nd:YAG laser head. Selections of the transparent overlay and absorbent coating were intensively investigated by experiments. The results showed that flowing water used as transparent overlay, and a black tape used as absorbent coating could ensure an effective shock wave induced by laser shock. The developed experimental setup offered a reliable equipment to perform the investigation on the small energy laser shock processing.Modeling of entire treatment process of laser shock processing was accomplished. Complete numerical simulation was performed by a sequential application of two sub-models: a confined plasma development model to predict the time history of shock pressure firstly and then a FEM model to investigate the mechanical behavior of the specimen. FEM analysis procedure was composed of dynamic analysis and static analysis. The methods for determining the time step and solution time for dynamic analysis were both proposed. The experimental validations showed that the numerical model could accurately predict the surface topography and residual stress field of the treated material.The characteristic of the effects induced by single and multiple impacts was investigated. The research on the single impact showed that increasing the laser intensity could enhance the compressive residual stress field, but may induce"residual stress hole" on the impact center. Surface deformation induced by laser shock was firstly investigated. A shallow pocket about 1μm in depth was induced on the top surface of material by multiple impacts. Also, residual stress and surface deformation were increased and gradually reached the saturated state with the increase of impacts.The mechanism of"residual stress hole"was put forward. Propagation and interaction of stress waves in the treated material was analyzed based on the stress wave theory and numerical simulation. The stress wave structure, propagating velocity, high strain-rate effect and attenuation of stress wave were intensively investigated. And the mechanism of"residual stress hole"was put forward: when the shock pressure is high enough, the release waves produced at the boundary of the impact region propagate to the impact center on the top surface, which will induce reverse lateral deformation to decrease the compressive residual stress on the impact center.A simplified method was given to attain the simulation of overlapping laser shock processing. A finite element model using a newly developed and representative symmetry cell was adopted to simulate the process of overlapping laser shock processing. The dimension of symmetry cell was determined according to the symmetric properties of spots distribution and the active region of single impact. The good agreements have been shown with the experimental results under different overlapping rates, proving the validity and efficiency of the developed model.The effects of overlapping laser shock processing were investigated. The treatment of AISI 1045 steel was attained by the overlap of small laser spots. The changes of mechanical properties of the specimen treated by different overlapping rates were investigated by both experiments and numerical simulation. Experimental results showed that surface quality was essentially unaffected after treatment. Plastically affected depth was much shallower than that obtained with larger spot sizes, due to rapid attenuation of shock waves. Induced residual stress field was uniform on the top surface and was enhanced with the increase of overlapping rate. Surface micro-hardness reached a larger value than that reached on the untreated region and was also improved with the increase of overlapping rate. A plastically deformed martensite transformation zone was found in an extremely thin layer near the top surface due to the heat effect.