Experimental Research And Finite Element Simulation Of Laser Shock Processing Of Stainless Steel Weldment

Welding residual tensile stress existing in stainless steel is one of the main factor affecting its service life. For this reason, laser shock processing (LSP) has been developed. As a new surface strengthening technology, LSP uses intense pulse laser to produce shock waves on metal surface, which can improve the distribution of residual stress in stainless steel weldment. Significant compressive residual stresses are mechanically introduced into weldment, which induce strain hardening on the weld surface and prolong the service life. Based on finite element analysis and experimental studies, the influence of LSP on the distribution of residual stress in AISI202 stainless steel welded joints was studied and evaluated.(1) The formation of LSP inducing residual stress field and the propagation mechanism of laser shock wave were discussed. According to the theory of plastic-elasticity dynamics and the theory of shock wave, Fabbro's model and Duan's corrected model in the confined mode were analyzed. The shock wave pressure-time curve was generated, which lay a theoretical foundation for finite element simulation.(2) Experimental researches were made on laser butt welding with AISI202 ,tainless steel sheet and LSP of its weldments, the results of the experiment were analyzed. The results show that LSP can produce significant compressive residual stress on the surface of welding specimen, not only in the impact zone, but also extend to the periphery of processed zone. At the same time, the "surface stress hole", which means the surface stresses at the center of impact zone lower than the edges of the impact, is induced in the process of LSP. Plastic deformation are formed on the surface of metal materials produced by intense laser shock waves. The degree of plastic deformation can reflect that the strengthening effect of multiple LSP is better than single LSP. Laser shock processing induced high density dislocation in the surface layer of stainless steel weldments. The number of austenite nucleation increased and part was transformed into tiny acicular martensite. Under the same parameters, multiple laser shock make microstructure refined, the number of martensite and dislocation density increased. Shock effect has been strengthened by multiple laser shock.In order to verify the effect of LSP on surface stress of welding specimen, the stress corrosion cracking (SCC) accelerated test was carried on in the boiling 42% MgCl₂ solution. The results show that LSP can protecting stainless steel weldments against SCC, owing to the positive contributions of compressive residual stress field induced by LSP. The higher laser overlapping rate, the longer stainless steel weldments produce macroscopic crack in chloride solutions.(3) Based on ANSYS and ANSYS/LS-DYNA software, the finite element analysis model was founded, including stainless steel laser welding and laser shock. Laser welding temperature field, laser welding stress field and laser shock stress field were studied in order. Node temperature getting from laser welding temperature field was used as body load of simulation of laser welding stress field, while laser welding residual stress was used as initial stress of simulation of laser shock stress field. The distribution of residual stress in stainless steel weldment was analyzed after single laser shock and multiple laser shock. The results show that numerical simulation results have a good agreement with the measurement values.