下扎深度对回填式搅拌摩擦点焊接头 断裂行为的影响
Effects of Plunge Depth on Fracture Behaviors of Refill Friction Stir Spot Welding

为研究焊接工艺参数对回填式搅拌摩擦点焊(RFSSW)接头断裂行为的影响,以铝合金为例,通过改变焊接过程中套筒的下扎深度对上下板厚度不同的LY12铝合金进行RFSSW试验,从材料流动、原子扩散及接头软化等方面对其接头的断裂机理进行分析,并利用焊点的宏观形貌、显微组织和硬度进行验证。试验及分析结果表明:在RFSSW过程中,当套筒未扎透上板、hook缺陷较平且韧带在整个焊点厚度基本相同时,裂纹主要沿连接强度较弱的搭接面扩展,形成剪切断裂模式;当套筒下扎至下板厚度的15％、hook缺陷向上弯曲且套筒回抽路径产生裂纹缺陷时,裂纹更倾向于沿着原子扩散时间较短的套筒回抽路径扩展,形成剪切塞型断裂模式;随着套筒进一步下扎至下板厚的35％且hook缺陷向下弯曲,裂纹更容易沿着下板中的套筒回抽路径向下扩展;当裂纹扩展到下板的下表面时,形成塞型断裂模式,断口形貌与断裂机理的分析结果相吻合。
Effects of welding parameters on fracture behaviors of refill friction stir spot welding (RFSSW) welds are investigated. RFSSW is used to weld LY12 aluminum alloys with different thicknesses by changing the sleeve plunge depth. The fracture mechanisms of the welds are investigated from the aspects of material flow, atomic diffusion and softening, and are verified by the cross section, microstructure and hardness of the weld. Results show that the crack propagates along the interface of lap weld with lower joining strength and forms a shear fracture mode when the plunge depth is smaller than the thickness of upper plate, the hook defect is flat and thicknesses of bonding ligament in the weld are almost the same, and the crack is more easily to propagate along the withdrawing path with shorter atom diffusion time and results in the shear？？plug fracture. When the sleeve penetrates into the lower plate thickness of 15％, the hook defect bends upwards and the crack defect appears at the sleeve withdrawing path. When the sleeve further plunges into the lower plate thickness of 35％ and the hook defects present along a downward bending direction, the crack grows downwards along the withdrawing path of sleeve. When the crack reaches the bottom of lower plate, the plug fracture of RFSSW weld completed. These results show that the fracture morphologies are in agreement with analytical results of the fracture features