Effect of pouring temperature during a novel solid–liquid compound casting process on microstructure and mechanical properties of AZ91D magnesium alloy parts with arc-sprayed aluminum coatings

Abstract

Aluminum (Al)/AZ91D magnesium (Mg) alloy bimetals, i.e., AZ91D magnesium alloy parts with Al coatings, were produced by pouring liquid Mg alloy, at temperatures of 933, 963, 993 and 1023 K, into molds with an arc-sprayed Al coating deposited. It was shown that an interface zone was generated in the Al/AZ91D bimetal produced by this novel solid–liquid compound casting technique, when the pouring temperature wasn't lower than 963 K. A mixing zone, composed of Al, alumina and MgAl2O4, was formed in the bimetal cast at 933 K. When the pouring temperature was increased from 963 to 1023 K, the interface zone mainly comprising the Al–Mg intermetallic compounds increased in thickness, and the γ-Al12Mg17 intermetallic compounds in this zone tended to grow toward the Mg alloy side. The obtained interface zones of the Al/AZ91D bimetals showed greater microhardness than the substrate and the coating metal. At a pouring temperature of 993 K, the Al/AZ91D bimetal had the thinnest interface zone without any untransformed Al pieces and thread-like MgAl2O4, leading to the highest shear strength of 25 MPa. Since the expansion of the interface zone increased the possibility of brittle fracture, an excessive increase in the pouring temperature did not necessarily improve the adhesive strength between the as-cast AZ91D and the arc-sprayed Al coating.