Research On Microstructure And Mechanical Performance Of Mg-Zn-Ca Cast Magnesium Alloy

In recent years, magnesium alloy has been become a focus topic in the field of metal structure materials due to its unique performance characteristics. As the demand of clean energy is increasing, the development of high performance and lightweight magnesium alloy has caused the wide attention of scholars.In this paper, gravity casting and ultrasonic agitation treatment were used to prepare the Mg-4Zn-xCa(x=0, 0.5, 1, 1.5, 2) alloy and Mg-4Zn-x Ca-yCu(x=0.5, 1.5, y=1, 2) alloys. In order to investigate the effect of as-cast, ultrasonic treatment and heat treatment on the microstructure and mechanical properties of Mg-Zn-Ca alloy, the optical microscope, scanning electron microscope(SEM), energy dispersive spectrometer(EDS), X-ray diffraction(XRD)analysis, universal material testing machine, microhardness test were used to analysis. The results showed that: After adding Ca into the Mg-4Zn alloy, the grain of alloy has been refined,the mechanical properties have been improved. Gray Ca2Mg6Zn3 phase were formed in grain boundary and black Mg2 Ca phase were formed in the distal of the dendrite. The mechanical properties of the Mg-4Zn-1.5Ca alloy is the better than other alloys in this paper, the tensile strength of this alloy is 144 MPa, yield strength is 91 MPa, elongation is 4.2%. The hard phase of Ca2Mg6Zn3 improves the hardness of Mg-4Zn-1.5Ca alloy, the hardness of this alloy is62 HV. After The solution and aging treatment, MgZn2 precipitation was dispersed in the microstructure of the Mg-4Zn-1.5Ca alloy.The aged alloy of Mg-4Zn-1.5Ca alloy reach its peak strengthening at 175℃ and 20 hrs,the tensile strength is 188 MPa, yield strength is 105 MPa. The microstructrue of Mg-4Zn-1.5Ca alloy has different level of refinement with ultrasonic treatment at different times. the as-cast microstructure transform from coarse dendrite to fine uniform round-shaped grain, the tensile strength, yield strength are 154 MPa and 95 MPa, respectively. The microstructure of Mg-4Zn-x Ca(x=0.5, 1.5) alloys are different with adding different amount of Cu, the skeleton-type eutectic microstructrue has transformed to fine needle-shaped phase. The grain boundary phases transformed from discontinous to continuous distribution with the addition of Cu. The second phase in Mg-4Zn-0.5Ca-1Cu alloy is distributed discontinuously in grain boundary, the tensile strength, yield strength and elongation at room temperature of thisalloy are 149 MPa, 102 MPa and 3.5%, respectively. After solution treatment at 420℃ for 8 hrs and aging at 180℃ for 16 hrs, the mechanical properties of Mg-4Zn-0.5Ca-1Cu alloy at room temperature arrive the best level, the hardness, tensile strength, yield strength and elongation are 74 HV, 185 MPa, 119 MPa, 4.8%, respectively. The fracture of aged Mg-4Zn-0.5Ca-1Cu alloy has the different degrees of dimples, shown the quasi cleavage fracture characteristics.