Effects Of Surface Conditions On Micro-Resistance Spot Welded Joint Properties Of Superelasticity QBe₂ Sheet

In this paper, 0.2-mm-thick beryllium bronze sheet was welded by small-scale resistance spot method. The effect of heat treatment condition on the mechanical properties and microstructure of the joints and the characteristics of the joint fracture were analyzed, at the same time by adding Nickel layer to improve the quality of the joint, and analyzes the forming process of Ni interlayer effect on the joint. Finally the surface state about properties of micro resistance spot welding was studied by combining with heat treatment state and middle layer.The results show that the size and mechanical properties of the aging joints are higher than the solid solution joints under the same technological parameters. The hardness of the ageing state joint fusion nuclear area was lower than the base metal and the hardness of the solid solution state joint fusion nuclear area was higher than that of the base metal in the welding heat. The aging joint is composed of the axial dendrite in the center zone of fusion core, the axial dendrite and columnar dendrite in the heat affected zone. the nugget grain is perpendicular to the electrode surface at the bottom of the growth direction. The solid solution joints are composed of a thick shaft of dendrite and columnar dendrite crisscross, The grain of the heat affected zone near the fusion zone is small, away from the fusion line close to the base area grain is relatively bulky, and the grain growth direction of the bottom grain is different. There are three kinds of fracture mechanisms in the tensile shear test of beryllium bronze joints, which are along the surface of the joint surface, the central fracture of the joint surface and the button fracture. And the fracture mechanism is affected by the size of the melt core.Nickel interlayer joint microstructure staggered composed of equiaxed dendrite and columnar dendrite, Nickel layer which is not completely melted run through the center of the nugget, non-uniform width Cu-Ni solid solution layer is formed between the Nickel and the Copper. Nickel interlayer changes the mechanism of joint formation, the joint of nickel layer has experienced wet spread, element diffusion and reaction layer grew up in three stages, the molten metal solidified to form a joint at last, the joint is composed of three parts, the copper side melting core, the intermediate reaction layer and the non melted nickel layer, and the joint strength is higher than that of the nickel bearing joint.Pre-welding heat treatment changes the surface hardness of Beryllium bronze sheet, the contact resistance of aging joint is higher than that of solid solution state joint, therefore the aging state joint shows a better performance than the solid solution state joint. After adding the Nickel layer, combined surface hardness decrease reduced the contact resistance, Nickel layer increases the bulk resistance of the joint, the total heat input increased under the same process, at this time, the nugget size increases and part of the nickel layer melting interface connection mechanism changes, leading to the joint strength increased.