Interface Structure And Mechanical Performance Improvement Of Disimilar Friction-stir-welded Joint

In this paper, we chose industrial pure aluminum(Al), pure copper(Cu) and AZ31 magnesium alloy as model metals to study dissimilar metals of Friction Stir Welding(FSW) research and for the purpose of further improving the mechanical properties of the dissimilar joints and exploring the interface behavior. Optimizing the microstructure and improving the mechanical properties of the dissimilar joints and discussing their relationship by the means of FSW, cold-rolling and annealing treatments.We obtained the non-defects of Al-Cu and Al-Mg dissimilar joints by using the large shoulder tool, tool offset, medium tool rotation speed and low travel speed process. Subsequently, we performed a treatment of cold-rolling and annealing to the as-welded joints to further improving of the mechanical properties; the as-welded Al-Mg dissimilar joints were annealed for the purpose of further exploring the effects of intermetallic compounds(IMC) on mechanical properties of the disimillar joint.We fabricated high strength with superior plastic Al-Cu heterogeneous butt plates by post-weld cold-rolling and annealing treatments. The strength of the dissimilar Al-Cu butt joints increased from 79 MPa to 231 MPa for the as-welded state and the post-weld cold-rolled and annealed treated state, respectively;Meanwhile the elongation increased from 1.0% to 17.5%. Moreover the tensile fractures occurring along the Al-Cu interface of the as-welded joint were effectively inhibited by post-weld cold-rolling treatment. The fracture of the post-weld cold-rolled Al-Cu joint changed from brittle to ductile after annealing. The improved mechanical properties of the Al-Cu dissimilar joint can be mainly attributed to optimized distribution of the Al-Cu interface due to the post-weld cold-rolling. In addition, the reduced property gradients and the internal restrain effects resulting from the post-weld annealing treatment have important contributions for the improvement of the tensile properties of the Al-Cu dissimilar joints. The strength of the post-weld cold-rolled joint significantly decreased with the increase of annealing temperature or time, the elongation decreased after an initial increasing. The fracture mode changed from brittle to ductile, and then changed from ductile into a brittle. The Al-Cu IMC layer had a significantly grown up during annealing process, which the mechanism is volume diffusion.The tensile properties of the Al-Mg dissimilar FSW joint are equivalent to the base material(BM) of pure Al and the fracture occurred at the nugget zone(NZ) of Al side with ductile feature. The strength of the joint significantly decreased with the increasing of annealing time; meanwhile the elongation has a significant decreasing for the post-weld annealed joints and the tensile fractures occurred at the Al-Mg interface with brittle fracture. During annealing process, the IMC layer at the Al-Mg interface has a significantly grown up; furthermore the Al12Mg17 and Al3Mg2 of IMC were detected by XRD and DSC in the as-welded and post-weld annealed joints.