Effect of Pre-Heat Treatment on Microstructure and Properties of As-Extruded AZ91-CaO Alloy

The effects of different pre-heat treatments on the microstructure, mechanical properties and corrosion resistance of extruded AZ91 and AZ91-CaO alloys were investigated. The results showed that the microstructure of AZ91 is clearly refined after adding CaO modifier. After solution heat treatment (T4), most of the second phase was dissolved into the matrix, while after aging heat treatment (T6), new fine and discontinuous Mg17Al12 were precipitated in both AZ91 and AZ91-CaO alloys. The average grain size of the extruded AZ91-CaO-T6 alloy was 0.9 mu m, and the second phase has the smallest size and more uniform dispersion. Therefore, the mechanical properties of AZ91-CaO-T6-EX are optimal, and its ultimate tensile strength, tensile yield strength and elongation reach 367.6 MPa, 320.2 MPa, and 9.5%, respectively. Meanwhile, the electrochemical and salt spray corrosion experiments also showed that the AZ91-CaO-T6-EX alloy had the best corrosion resistance; the self-corrosion potential was -1.08 mV, the corrosion current density was 5.65 mu A/cm(2), and the annual corrosion rate was 0.36 mm/y after 72 h salt spray treatment. This high corrosion performance was mainly attributed to the dispersed second phase with small size and fine grain size.