AA5754铝合金电阻点焊接头的微观组织和力学性能

Microstructure and mechanical property of AA5754 aluminum alloy resistance spot welds

以汽车车身用AA5754铝合金板材制备的电阻点焊接头为研究对象,通过金相观察、显微硬度矩阵测试和拉伸试验对焊接接头的微观组织、工艺缺陷与力学性能进行表征与分析。结果表明:根据组织和性能的差异,接头可分为3个典型区域:焊核区、热机械影响区和母材区。接头焊核心部为等轴枝晶组织,焊核边部为柱状枝晶组织,焊核周边的热机械影响区内的材料发生部分熔化,产生了晶内及晶界枝晶。接头中主要缺陷为喷溅、焊核区气孔、缩孔和沿晶裂纹、液化裂纹。母材区硬度沿厚度方向渐变,边部呈现最大值,心部呈现最小值;焊核硬度与母材硬度相近;焊核两侧的热机械影响区中,在接头的压痕边缘和板材接触面缝隙尖端处存在1~2 mm宽的硬化区。接头的强度与焊点直径呈正相关关系,拉剪试验的失效模式为焊点熔核界面断裂,剥离试验的失效模式为纽扣剥离断裂。

AA5754 aluminum alloy sheets used for automotive body were joined by resistance spot welding. The microstructure, discontinuities and mechanical properties of the welds were characterized and analyzed by optical microscope observation, Vickers microhardness grid measurement and tensile tests. The results show that, according to the differences of microstructure and properties, the weld joint can be divided into three regions, i.e. nugget, thermo-mechanically affected zone（TMAZ） and base metal（BM）. An equiaxed dendritic structure exists in the nugget center while a columnar structure is observed at the nugget perimeter. Minor amounts of inner and inter-grain dendritic structures, resulting from partial melting, is found in TMAZ. Discontinuities in the welds are expulsion, gas bubbles, shrinkage cavity, shrinkage cracks and liquation cracks. The thru-thickness hardness of the BM sheet displays a distinct variation. A 1-2 mm wide hardened zone was observed around the indentation perimeter and crack tip on the faying surface in the TMAZ. The positive correlation is found between the weld strength and weld button diameter. The fracture mode for tensile-shear tests is interfacial fracture while button pull-out fracture for coach-peel tests.