| In this paper,micro-resistance spot welding of 0.2mm thick TC4 and 2A12 thin plates is carried out,and the effect of process parameters on tensile shear force of the joints is studied.The connection characteristics and fracture behavior of spot welded joints were studied by scanning electron microscope and energy spectrometer.The phase composition of the joint was determined by means of X-ray diffractometer.The results show that the welding time has no significant effect on the tensile shear force of the joint.When the welding current is 4.2k A and the electrode pressure is 10 ms,the maximum shear strength is 125.82 N.the joint has two fracture modes.The fracture along the center of the nugget shows the characteristics of brittle fracture,the center of the buttonlike fracture shows the characteristic of ductile fracture,and the heat affected zone shows the characteristic of brittle fracture.Welding current will significantly affect the thermal input of micro-resistance spot welding process.The microstructure of Ti/Al spot welding joints with different welding current is different.For the joints with small heat precipitation,the composition of the joints at the bottom of the nugget is undercooled due to the different concentration of solute.The results show that the optimum growth direction varies from region to region.The central region of the nugget is a coarse and continuous columnar crystal.For the joints with large precipitated heat,The "net basket" ? structure can be observed inside the nugget,and the width of the heat-affected zone is larger than that of the heat-affected zone with small precipitated heat joint.The grains at the bottom of the nugget grow perpendicular to the edge of the nugget.Near the edge of the nugget,the grain size is smaller because of the fast heat dissipation near the electrode head.The central region of the nugget is a coarse columnar crystal and "basket" ? tissue.The heat affected zone of spot welding joints with large heat precipitation is larger.The main products of metallurgical reaction between nugget and Al side are Al3 Ti,and the products of metallurgical reaction with Ti side are mainly Al Ti3 and Al Ti2.The Al3 Ti intermetallic compound is the main component in the inner region of the nugget,while the Ti Al intermetallic compound is found in the inner defect of the nugget.The joint formation process can be divided into four stages: "solid phasebonding","brazing bonding" and "melting bonding" "formation and growth of the nugget". |
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