选取半径分别为100、75、50和25 mm的球面电极，采用三相次级整流点焊机进行1 mm厚2A12铝合金点焊试验。利用光学显微镜和扫描电镜观察焊后熔核的形貌、裂纹特征及显微组织，利用低倍测量显微镜和拉伸试验机测试熔核的尺寸和拉剪力，研究球面电极半径对2A12铝合金点焊熔核裂纹的分布特征、形成机理以及熔核尺寸和拉剪力的影响。结果表明：球面电极半径越大，熔核内越容易产生裂纹，且熔核直径总体上呈下降趋势。当电极半径分别为75和100 mm时，在熔核中间区域产生贯穿整个熔核的沿晶纵向结晶裂纹，在熔核的热影响区出现液化裂纹，熔核内没有横向裂纹产生。
Selecting 100, 75, 50 and 25 mm as the radius of spherical electrode, spot welding experiment of the aluminum alloy 2A12 sheet with thickness of 1 mm was performed on the three-phase secondary rectification welding machine. Using optical microscopy and scanning electron microscopy, the microstructure and crack characteristics of the nugget were observed. The low-magnification measuring microscope and the tensile test machine were used to test the nugget diameter and the tensile-shear strength of the weld, and then the effect of spherical electrode radius on the crack distribution, formation mechanism, nugget diameter, and the tensile-shear strength of spot welding joint of the aluminum alloy 2A12 were investigated. The results show that as the spherical electrode radius increases, more likely the cracks occur in the nuggets, and nugget diameters present decreasing tendency. When the spherical electrode radii are 75 and 100 mm, respectively, the intergranular longitudinal solidification crack locates in the middle of the nugget, which almost transverse across the whole weld nugget, and liquation crack is visible in the heat-affected zone (HAZ), but there is no transverse crack in the nugget.
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