The Effect Of High Energy Shot Peening On The Microstructures And Properties Of TIG Welded Joints Of The Same And Dissimilar Metallic Materials

In this paper,AZ31 B magnesium alloy,304 stainless steel and TC4 titanium alloy were used as the research object,AZ31 B magnesium alloy welding wire was used as filler metal,and TIG welding brazing technology was used to weld AZ31B/AZ31 B,AZ31B/304 and AZ31B/TC4 joints.Through the analysis of mechanical properties,surface morphology,surface residual stress,surface grain size and dislocation density of welded joints,the changes in microstructure and properties of AZ31B/AZ31 B welded joints before and after shot peening were explored.The effect of different shot peening intensity on the microstructures and properties of different welded joints also were studied during the high energy shot penning processes.For AZ31B/AZ31 B welded joints,the optimal welding current parameter was 90 A,the maximum tensile strength of the joint after high energy shot peening reached 262 MPa,which was 13.4% higher than that of the joint without high energy shot peening(208 MPa),almost equivalent to the tensile mechanical properties(272 MPa)of the magnesium alloy substrate,the fracture position occurred near the Mg alloy substrate rather than the weld seam.After high energy shot peening,the defects such as micro-cracks and voids on the surface of AZ31B/AZ31 B welded joints were eliminated and the surface was strain-strengthened.When the shot peening strength reached to 0.20 mm N,the thickness of the hardened layer was approximately 120 ?m,and the largest surface hardness value reached to 63.9 HV,and with the distance from the joint surface increasing,the hardness drops sharply and tends to a constant value of 47.5 HV.When the shot peening strength was 0.16 mm N,the compressive residual stress of the surface reaches a maximum value of 54 MPa;at the same time,the surface grains were refined.For AZ31B/304 welded joints,the optimal welding current parameter is 80 A.The maximum tensile strength of the joint after high energy shot peening reached 251 MPa,which was 20.7% higher than that of the joint without high energy shot peening(208 MPa).The fracture position occurred in the heat-affected zone near the fusion zone.After high energy shot peening,the micro-cracks and voids on the surface of AZ31B/304 welded joints were eliminated and the surface was strain-strengthened.When the shot-peening strength reached to 0.20 mm N,the maximum surface hardness value reached 64.3 HV.When the shot peening strength is 0.16 mm N,the compressive residual stress of the surface reaches a maximum of 52 MPa;at the same time,the surface grains were refined.For AZ31B/TC4 welded joints,the optimal welding current is 60 A.The maximum tensile strength of the joint after high energy shot peening reached 241 MPa,which was 24.2% higher than the joint strength(194 MPa)without high energy shot peening.The fracture location even occurs on magnesium alloy substrates.After high-energy shot peening,the micro-cracks and voids on the surface of AZ31B/TC4 welded joints were eliminated,and the surface was strain-strengthened.When the shot-peening strength reached to 0.24 mm N,the maximum surface hardness value reached 64.5 HV.When the shot peening strength is 0.16 mm N,the compressive residual stress of the surface reached a maximum of 105 MPa;at the same time,the surface grains are refined,which was deemed to improving the mechanical properties of the welded joint.However,when the shot peening strength reached to 0.24 mm N,serious internal cracks occurred in the weld seam,which resulted in a decrease in the bond strength of the joint.In short,during the high energy shot peening process,the improvement of the mechanical properties of TIG welded joints can be attributed to the combination effects of the elimination of joint surface defects,the generation of compressive residual stress,strain strengthening,and fine grain strengthening.