Friction Stir Lap Welding of Magnesium Alloy and Zinc-Coated Steel

Abstract

AZ31 Mg alloy and zinc-coated steel were lap welded using friction stir welding technology. The microstructures and mechanical properties of the joints were examined. The lap shear tensile test results showed that the welding speed had a significant effect on the failure loads of the joints at the rotation speed of 1500 rpm. The maximum failure loads of 3.4 kN, 65% of that of the zinc-coated steel base material, could be obtained when the welding speed was 150 mm/min. Microstructure analysis showed that the intervention of zinc coat promoted the formation of Mg-Zn low-melting-point eutectic structure at the interface. The joining mechanism and the role of Zn coat on friction stir lap welding of Mg alloy and zinc-coated steel were put forward. (doi:10.2320/matertrans.M2009022) Magnesium alloys, the lightest of the structural metals, with a density two-thirds that of aluminum alloy are a promising structural material in vehicle fabrication. Zinc- coated steel with excellent corrosion resistance and good durability is a promising structural material in vehicle applications. In order to achieve combined properties of magnesium alloys and zinc-coated steel, development of reliable joints between magnesium alloy and zinc-coated steel is necessary. As a solid state welding technology, friction stir welding (FSW) process 1) can be used to join Mg alloys 2-6) and steel 7-14) and gets high quality joints than fusion welding technology. Such good merits are also expected for joining dissimilar metal materials. Watanabe et al. 15) studied the weldability of FSW AZ31 magnesium alloy/SS400 steel, and reported that the rotation speed, the position of the pin axis had a significant effect on the strength and the microstructure of the joint. The maximum tensile strength of a butt joint reached about 70% of that of the magnesium base metal and the fracture path was along the joint interface. Liu et al. 16) studied the lap joining of the dissimilar alloys of AZ31B Mg alloy and 304 steel by a laser-GTA hybrid welding technique. A transitional zone formed at the interface of the Mg-Fe during laser-GTA hybrid welding and Mg diffused into the matrix of Fe in the form of oxides and reacted in the transitional zone. During tensile testing, the joints fractured at the interface between the Mg alloy and the steel. The metallic oxides produced at the Mg-Fe interface deteriorated the mechanical properties of the joints. Up to date, current studies only report the weldability of FSW or a laser-GTA hybrid welding of magnesium alloy/ steel and the preliminary results of mechanical properties and microstructure evolution of the joints. 15,16) Investiga- tions on FSW of magnesium alloy and zinc-coated steel have not yet been reported. The object of the present study is to examine microstructure evolution and mechanical properties in FSW magnesium alloy and zinc-coated steel. In this study, AZ31 magnesium alloy and low carbon zinc- coated steel are selected for friction stir lap welding. The magnesium alloy sheet is put on the zinc-coated steel sheet. The inserting depth of the tool, being strictly controlled, is less than the thickness of the magnesium alloy. That is, the probe tip of the tool does not reach the surface of the zinc- coated steel sheet during welding. The emphasis is to study the interface microstructure evolution and clarify the joining mechanism.