| The semi-solid forming technology has been considered to be one of the most advanced metal forming technology, which is characterized by easy to near-net shape, low molding temperature, and energy conservation. Magnesium alloy is easily oxidized and burned in the usual forming process, but the semi-solid forming technology is effective to solve this problem. At present, the semi-solid forming of magnesium alloys is mainly for the existing alloys of AZ91D, AM60and AM50. However, on the one hand, these alloys can not fully meet the requirements of semi-solid forming controllability, on the other hand, they can not give full play to the strengths advantage of semi-solid forming material by heat treatment because of their bad strengthening effect of heat treatment. Therefore, it is necessary to develop a high-strength magnesium alloy for semi-solid forming. First, to obtain the high strength magnesium alloys which have remarkable effect of heat treatment strengthening, then using semi-solid forming to improve its mechanical properties, and finally through the heat treatment further strengthened. Mg-Zn-Al alloy has higher mechanical properties at room temperature and high temperature, and it has low cost, therefore it widely researched and used, further strengthen it by semi-solid forming is feasible. The microstructure and mechanical properties of Mg-Zn-Al alloy at cast and heat-treated condition were investigated in this study. The main results are found as follows:1. The microstructure and mechanical properties of Mg-(5-20%)Zn-(0-6%)A1alloys were studied. The results indicated that the alloys are mainly composed of a-Mg、MgZn、Mg32(Al,Zn)49and Mg17Al12. With increasing of Zn and Al, the coarse dendrites transite to fine equiaxed grains and the grain size is gradually reduced; While the number of eutectic phase and its size both gradually increased. Mg-Zn-Al alloys have higher tensile strength at room temperature, which is between180-240MPa, the tensile strength of ZA54、ZA56and ZA202reaches to227,228and237MPa, respectively. The elongation of alloys decreased gradually with increasing of Zn content.2. The tensile strength of Mg-Zn-Al alloys has significantly improved by heat treatment, while the tensile strength of ZA74and ZA72alloy is338and337MPa, respectively. Compared to as-cast alloys, it has been improved by68and66%, respectively. The tensile strength of ZA54and ZA56alloy could get up to320MPa and305MPa. With the increasing of alloying elements, the compound phases distributed on the grain boundaries which seriously affect the mechanical properties of alloys. The heat treatment process parameters of ZA74alloy are:solution350℃+28h, aging165℃+24h. The main reasons of heat treatment strengthen are solution and aging strengthening. The precipitates at the grain boundaries and within the matrix during aging treatment, which benefit to the improved mechanical properties.3. The semisolid microstructure evolution process and mechanism analysis of ZA74magnesium alloy are investigated by isothermal heat-treatment. The results indicate that the particle size has a direct relationship with the Zn content. With the increasing of the Zn content, it could obtain fine particles, and when the Zn content is greater than7%, the size of particles is less than50μm, and the particle size of ZA202alloy is28.9μm. The coarsening rate of solid particles is reduced by Al element. During partial remelting, the main mechanism of Mg-Zn-Al alloy is eutectic microstructure solution, solute diffusion, coarsening, dendrite separation, spheroidization and final coarsening. Furthermore, the factor of solute diffusion, energy fluctuation and constituent fluctuation, which result in the appearance of subboundary and root remelting, which are the separation mechanism of particle during partial remelting. When the liquid phase is less, solid particle separation mechanism is liquid phase infiltrate liquid and extend along the grain boundary and sub-boundary; and when there is a certain liquid, root remelting mechanism plays a leading role in the separation of solid particles.4. Choosing the ZA74alloy to thixotropic forming and heat treatment, it show that the semisolid forming can significantly improve the mechanical properties of ZA74alloy. The tensile strength of ZA74alloy is256MPa used thixotropic die-casting, compared to the metal forming increased by27%. The tensile strength of semisolid thixoforming ZA74alloy is352MPa after heat treatment, which is increased by38%compared to the cast alloy, and compared with AZ91D alloy (230MPa) increased by53%. |
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