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In order to investigate the effect of Mg2Si formation on the microstructure and properties of an Al−Si alloy, the critical point of a hypereutectic Al−17Si−4Cu−Mg alloy was calculated by Pandat software. The calculation results of the equilibrium phase diagram show that the critical point for Mg2Si phase formation for the alloy was obtained when the Mg content was 2.2%. The contents of 0.5 wt.% Mg and 2.5 wt.% Mg were selected as the research object. The content of Mg increased from 0.5 wt.% to 2.5 wt.%, the eutectic Si in the matrix was reduced, and the Chinese character-like Mg2Si phase appeared in the microstructure. In the peak ageing state, in addition to θ″ and Q′ phases that were mainly precipitated, there was also needle-like β″ precipitation in the 2.5 wt.% Mg content alloy. Larger precipitates were found in 2.5 wt.% content alloys, mainly due to the promotion of the solid solution having the aggregation and segregation of more solute elements in the matrix. The tensile strength, elongation, and hardness of hypereutectic Al−17Si−4Cu−0.5Mg alloy under peak ageing were 331 MPa, 3.11%, and 152.1 HB, respectively. The tensile strength and the elongation decreased while the hardness increased with the 2.5 wt.% Mg content, which is due to the formation of hard and brittle Mg2Si and Al8FeMg3Si, which has a splitting effect on the matrix.

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In order to provide an effective anticorrosive Cu-based coating for magnesium alloy, cold spray and ultrasonic shot peening (USSP) were used. A series of Cu/Ni composite coatings with different content of Ni were deposited on the surface of the Mg alloy. The obtained coatings were strengthened by USSP. The results revealed that the structural compactness and protecting effect of the cold-sprayed pure Cu coating was enhanced by USSP. The micro-hardness values of the Cu/ Ni composite coatings were increased with the Ni content. The 16 vol.% Ni powders improved the corrosion resistance of the cold-sprayed pure Cu coating. Thus, the cold-sprayed Cu/Ni composite coating with excellent mechanical properties and corrosion resistance can be applied to magnesium alloy surface.