Study On Microstructure And Mechanical Properties Of ZA105 High Zinc Magnesium Alloy With Cu And Sb Additions

Mg-Zn-Al alloy has been considered as a promising high-temperature magnesium alloy because of its high heat-resistance, lower cost and sound castiability. Generally, the main heat-resistant phases areÏ„-Mg32(Al,Zn)49 phase and a small amount ofφ-Al2Mg₅Zn₂ phase in ZA alloy, in which Zn content is above 5wt.%. However, the casting properties of this alloy are unsatisfactory and there are still some problems in mechanical properties both at elevated temperature and at room temperature. Therefore, in order to solve these problems, the new typical and low-cost high zinc magnesium alloy was developed by optimizing alloy composition of Mg-10Zn-5Al alloy in which Sb was used as modifier and Cu as alloying element.Effect of Sb,Cu and heat treatment on microstructure and mechanical properties of Mg-10Zn-5Al magnesium alloy were investigated by using modern analytical technology of optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), micro-hardness test, impact-toughness test and tensile-strength test, hoping to provide theoretical foundation for the improvement of mechanical properties of high-zinc magnesium alloy.Results of Sb-added alloys indicate that : the microstructure of the Mg-10Zn-5Al-2Cu alloy are composed ofα-Mg,Ï„-Mg32(Al,Zn)49 phase,φ-Al2Mg₅Zn₂ phase and Mg₂Cu phase. Compared with the unmodified Mg-10Zn-5Al-2Cu alloy, the new dystectic Mg₃Sb₂ black particles with hexagonal D52 structure appear in the matrix. When the content of Sb is 0.1wt.%, the microstructures of the alloy is refined dramatically. The hardness of Mg-10Zn-5Al-2Cu alloy shows a gradually rising tendency with Sb increasing. When the addition amount of Sb is 0.1wt.%, the impact-toughness rises to the maximum 6J/cm2, which is 50% higher than that of Sb-free alloy, and the tensile strength is up to 190MPa. In particular, when the content of Sb is 0.2wt.%, the tensile-strength reaches the maximum 195 MPa, which increases by 5.4%, while the impact-toughness drops sharply. As the addition of Sb exceeds 0.2wt.%, the tensile-strength begins to decline either. So the appropriate Sb addition is 0.1wt.% in the experiment. The microstructure of Mg-10Zn-5Al-2Cu magnesium alloy is the most refined and better comprehensive mechanical properties are achieved.The analysis results of Cu strengthening indicate that: Cu addition into the Mg-10Zn-5Al-0.1Sb high-zinc magnesium alloys results in formation of the fishbone-like Mg₂Cu eutectic structure on theÏ„-Mg32(Al, Zn)49 second phase in the alloy. When the Cu addition is less than 2.0wt.%, the hardness and the tensile-strength arise gradually with the Cu increasing, while the impact-toughness shows downward trend. Especially when the content of Cu is 2.0wt.%, the hardness reaches the maximum, 79.35HB, 9.65% higher than that of mother alloy. The room temperature and elevated temperature (200℃) tensile strengths reach 190MPa and 160MPa, which increase by 21.1 % and 14.3%, while the impact-toughness is up to 6 J/cm2. Further Cu addition leads to the coarseness of microstructures and rapid decrease of hardness, tensile-strength, and impact-toughness. In a word, the microstructure of Mg-10Zn-5Al-0.1Sb magnesium alloy is the most strengthened and better comprehensive properties are obtained with appropriate Cu addition of 2.0wt.%.The experiment results of heat treatment indicate that:(1) After solution treatment, the eutecticα-Mg phase around in vicinity of grain boundaries and theφ-Al2Mg₅Zn₂ phase in grains of the Mg-10Zn-5Al-0.1Sb-2.0Cu alloy dissolve into to the matrix microstructure. And micro-morphology and distribution of theÏ„-Mg32(Al,Zn)49 phase change obviously. But theÏ„-Mg32(Al,Zn)49 phase does not completely dissolve into the matrix in the solid solution treatment. Dystectic heat-stabilityÏ„-Mg32(Al,Zn)49 phase and Mg₂Cu phase still exist at the grain boundaries, the micro-hardness of the matrix in the Mg-10Zn-5Al-0.1Sb-2.0Cu alloy is up to 85 Hv, which is 16.6% higher compared with as-cast alloys after 24h solution treatment.(2) The massive ternary phases are precipitated at or around grain boundary in Mg-10Zn-5Al-0.1Sb-2.0Cu alloy aging for 36h. The precipitated ternary strengthening phases are identified asÏ„-Mg32(Al,Zn)49 phase andφ-Al2Mg₅Zn₂ phase. With aging time elongating to 48h, the ternary precipitations enrich and grow. The micro-hardness increase first and then decrease with the elongating of aging time. When the aging time is 36h, the micro-hardness is up to the maximum, 105.9Hv, 24.6% higher than that of alloys in solid solution state. Tensile strengths at room temperature and high temperature by solution and ageing treatment are much higher than that of as-cast alloys and variation tendency of tensile strengths is the same as that of as-cast alloys. When the addition of Cu is 2.0wt.%, the tensile-strengths of solution and ageing treated alloys at room temperature and high temperature (200℃) are 228MPa and 176MPa, which are 20% and 10% higher than that of as-cast alloys respectively. Therefore, the optimal aging time in the experiment is 36h.