Coating Design for Controlling  Phase IMC Formation in Dissimilar Al-Mg Metal Welding

The brittle intermetallic compound (IMC), -Al3Mg2, grows very rapidly at the interface during dissimilar welding of Al to Mg. This currently prevents acceptable joints being produced between these two important light alloys. In the present study, the use of Al-alloy inter-layer coatings has been investigated to address this problem, for application to friction welding processes. Thermodynamic principles were employed to aid design of the coatings. Two types of coatings were selected: i) a Si-rich coating, which promoted the slower growing and more stable Mg2Si phase; ii) an Al-Zn-rich solid solution coating, which replaced the undesired Al3Mg2 phase with a ternary (Al,Zn)49Mg32 intermetallic compound. In statically annealed diffusion couples both coatings were found to inhibit the growth of Al3Mg2. Refill friction stir spot welding (RFSSW) experiments also showed that both coatings were effective in reducing interfacial reaction and can improve the mechanical properties of Al - Mg dissimilar joints.