Breaking the trade off between corrosion resistance and fatigue lifetime of the coated Mg alloy through cold spraying submicron-grain Al alloy coatings

Although magnesium (Mg) alloys are the lightest among structural metals, their inadequate corrosion resistance makes them difficult to be used in energy-saving lightweight structures. Moreover, the improvement in corrosion resistance by the conventional surface treatments is always achieved at the expense of sacrificing the fatigue lifetime. In this study, high purity aluminum (Al) and AlMgSi alloy coatings were deposited on Mg alloys via an in-situ micro-forging (MF) assisted cold spray (MFCS) process for simultaneous higher corrosion resistance and longer fatigue lifetime. Besides contributing to a highly dense microstructure, the in-situ MF also greatly refines the grain of the deposited Al alloy coating to the sub-micrometer range due to the enhanced dynamic recrystallization and also generates notable compressive residual stress up to 210 MPa within the AlMgSi coating. The absence of secondary phases in the AlMgSi alloy coatings enable the coated Mg alloy with corrosion resistance, which is even better than its bulk AlMgSi counterparts. The unique combination of refined microstructure and the prominent compressive residual stress within the AlMgSi coatings, effectively delayed the crack initiation upon repeated dynamic loading, thereby leading to ~10 times increase in the fatigue lifetime of the Mg Alloy. However, although residual stress is also generated in the submmicro-sized grained pure Al coating, the low intrinsic strength of the coating layer leads to a lower fatigue lifetime than the uncoated Mg alloy substrate. The present work is aimed to provide a facile approach to break the trade-off between corrosion resistance improvement and fatigue lifetime of the coated Mg alloys.