Effects Of Ultrasonic Impact Treatment On Weld Microstructure,Residual Stress And Mechanical Properties In Thick Joint

Because of the high temperature and high cooling rate of the weld pool,columnar grains grows easily in the weld metal.welds with large columnar grains usually have poor collaborative deformation capability,which makes propagation of cracks easier during material deformation.In addition,existence of tensileresidual stresses injoints decreases tensileload capacity and fatigue life.Therefore,it is very important to optimize the microstructure,improve the mechanical properties of the weld,and effectively regulate the welding residual stress.In this paper,three 55 mm thick multi-pass arc welding joints were prepared,one was welded with conventional method,another was processed with ultrasonic impact treatment before cap passes,and the third one was welded with layered ultrasonic impact treatment(LUIT)method.The microstructure,residual stress distribution and mechanical property were studied.Counter Method,impact test,hardness test and shear punch test were adopted to perform the investigations,furthermore,the mechanisms of the LUIT on the microstructure and mechanical properties improvement were investigated.Finally,LUIT combined with conventional UIT method was proposed.The investigated results showed that the microstructure of the weld was refined obviously after LUIT,and the coarse column grain in the weld changes to fine equiaxed grains.The UIT-induced oxidation ofmetals can increase the oxygen content in the weld metal and cause the non-metallicoxide inclusion.Thereby,the UIT-processed surface affects the grain growth behavior of the subsequent bead,promoting the non-spontaneous nucleation and resultingin a fine quiaxed-grain microstructure in theweld.The residual stresstest results with two-cut contour method showed that the high tensile longitudinal residual stress is mainly presented in the weld zone,and the peak tensile longitudinal stress appears in the region near the top surface,while the peak tensile transverse stress appears in the region near the bottom surface.The effect of UIT before the cap weld and LUIT on the residual stress distribution was not significant,but the peak value of the longitudinal tensile stress was decreased slightly when apply UIT on the weld.The impact test results demonstrated that the impact absorption energy of the weld metal was increased by 13% with LUIT due to the refinement of the microstructure,while there was no significant change for the joint treated before the cap weld.Shear punch test results showed that due to the improvement of the microstructure,the shear strength of the weld is improvedafter LUIT,especially for the weak weld metal.Fracture photography indicated that the dimples inthe LUIT-process weld were much larger and deeper than those in the conventional weld,and the inclusions were observed obviously.The hardness test results showed that the hardness distributions of three joints were consistent with the shear strength distributions,the hardness of the joint welded with LUIT was obviously enhanced.