Microstructure, Tensile Properties and Fracture Behavior of Squeeze-Cast Mg Alloy AZ91 with Thick Cross Section

As the most popular magnesium alloy, AZ91 is desired in the automotive-related manufacturing industry because of its high specific strength and good corrosion resistance. AZ91-based automotive components are made by conventional high-pressure die casting (C-HPDC) and are unable to provide adequate mechanical properties to components with heavy cross sections because of the high level of gas and shrinkage porosity in C-HPDC parts. To expand the usage of AZ91, a replacement for the C-HPDC to minimize the porosity content is needed. In this research, Mg alloy AZ91 with a cross-sectional thickness of 10 mm was squeeze cast (SC) under an applied pressure of 90 MPa. A counterpart was made by C-HPDC for comparison with SC AZ91. The porosity of the SC and C-HPDC AZ91 specimens was evaluated by optical microscopy and density measurements. The SC AZ91 was almost pore free, whereas the counterpart had a porosity of 3.57%. Microstructural analyses by scanning electron microscopy showed that the SC and C-HPDC AZ91 contained phases of primary alpha-Mg, beta-Mg(17)Al(12)eutectic and Al-Mn intermetallic. Tensile testing indicated that the ultimate tensile strength and elongation (e(f)) of the SC alloy were 171.7 MPa and 3.0%, which resulted in improvements of 33 and 77% compared with those of the C-HPDC counterpart. The strain-hardening rate and tensile toughness of the SC AZ91 during deformation were higher than those of the HPDC specimen. SEM fractography revealed characteristics of plastic deformation on the flat fractured surface of the SC AZ91, whereas brittle fracture prevailed on the HPDC counterpart.