Effect Of Intermediate Layer On Microstructure And Mechanical Properties Of Friction Stir Welded Joint Of Ti/Al Dissimilar Metals

The dissimilar metal structure of Ti/Al shows wide application perspective in aeronautical, aerospace?high speed railway and other fields. But it is hard to joining titanium to aluminum for its big difference of physical and chemical properties. In this paper, 3mm thick of TC4 titanium and 2A14 aluminum dissimilar metals were joined by friction stir welding method through add intermediate layer. The influence of process parameters on weld formation and tensile strength were investigated, moreover, the impaction of intermediate layer materials on micro-structure and performance of the joint were studied by the scanning electron microscope(SEM)?the energy disperse spectroscopy(EDS) and the X-ray diffraction(XRD). The results show that:For Al/Zn dissimilar metal FSW, the well-formed and “double-hook-like” butt joint can fabricated by friction stir welding with the parameters that welding velocity is 70mm/min, rotation speed is 500 rpm, which the tensile strength exceed to 152.7MPa, nearly 80.4% of the zinc. Aluminum alloy and zinc mixed sufficiently in hook-like region where appeared local onion rings?alluvial plain morphology, and different solid solutions that transition from zinc-based to aluminum-based from zinc side to aluminum side generated in the region. As for Ti/Zn dissimilar metal FSW, joint was obtained by way of mechanical-interlocking, the process parameters range was narrowly due to big difference of physical and chemical properties between Ti and Zn. The tensile strength 75.9MPa was achieved when rotational speed was 500rpm?welding speed was 70mm/min, which only 39.5% of the zinc.Micro-hardness of the joint was decreased after add intermediate layer material Zn. When the welding velocity, rotation speed are 75mm/min and 375 rpm, respectively, the maximum tensile strength exceed to 237.3MPa, nearly 56.7% of the aluminum alloy. To a certain extent, intermediate layer material Zn can reduce the formation of Ti-Al inter-metallic compounds in joint and promote fracture mechanism of joint transition from brittle fracture to brittle and ductile inter-mix fracture. However, intermediate layer material Zn was easy to melt for its low melting point, so it often cause defect at the interface what reduce the mechanical properties of the joint.When the welding velocity, rotation speed was 75mm/min and 375 rpm, respectively, the maximum tensile strength exceed to 285.3MPa, nearly 68% of the aluminum alloy. At the bottom of joint, only large degree of deformation was occurred of the intermediate layer material Ni, what has not been stirred into a granular. The gap caused by incomplete integrate at the bottom of joint between Ti and Al become weak parts. Brittle and ductile inter-mix fracture mechanism was found according to study fracture of Ti/Ni/Al. The tensile strength was increased after add intermediate layer material Ni since it can reduce brittle phase of joint, and impel Ti-Al inter-metallic compound transition from Ti3 Al to TiAl what has some toughness.