Influence mechanism of pin length and shape on the heat-flow field of Al/Cu friction stir lap welding

The hydrodynamic models of different pin lengths and shapes were established for Al/Cu heterogeneous friction stir lap welding (Al/Cu-FSLW) with volume of fluid (VOF) method. The models with cylindrical pins with lengths of 2.4 mm, 2.6 mm and 2.8 mm, and conical pin with length of 2.6 mm were built to analyze the heat-flow and heat-mass transfer during Al/Cu-FSLW. The interfacial friction shear stress and heat flow were calculated by the relative sliding state of the tool/workpiece contact interface, and an interfacial dynamic adaptive thermal model changing with the material flow state was established. The results revealed that an increase in the pin length could effectively strengthen the mixing of the Al/Cu materials and increase the plastic deformation in the stirring zone. With increasing pin length, the copper was more violently disturbed at the bottom of the mixing zone, which led to a sharper hook at the joint and higher temp. For the same length of pin, the stirring effect and the material plastic flow under the action of conical pin were weaker than that of the cylindrical pin. According to the heat generation statistics, the conical pin generated the least heat. With increasing pin length, the frictional heat gradually decreased, while the viscous-plastic heat increased. The strengthened migration of Al/Cu materials and temperature promoted the formation of Al/Cu intermetallic compounds. Therefore, it was beneficial to reduce the formation of intermetallic compounds and improve the weld quality by using conical pin or reducing the length of the pin.