| Semisolid processes have become more popular for industrial and commercial production because of their excellent advantages such as low temperature, low forming force, good quality of parts and so forth, over conventional casting and deforming processes. The aim of this paper is to investigate the microstructure evolution of extruded Mg-2Zn-xY (X=0.5,1、2at.%) alloy and extruded Mg-8Al-0.7Zn alloy after semisolid isothermal heat treatment in different stages. Further more, microstructure evolution of extruded Mg-2Zn-0.5Y alloy and extruded Mg-8Al-0.7Zn alloy in semi-solid electromagnetic induction heat treatment process is also investigated.The microstructure of Mg-2Zn-0.5Y alloy consists of fine and equiaxed a-Mg and dispersed second phase (I-Phase and W-Phase). The microstructures of both Mg-2Zn-1Y and Mg-2Zn-2Y alloys consist of fine and equiaxed a-Mg and dispersed W-Phase. In these three kinds of Mg-2Zn-xY alloys, a-Mg grain size of extruded Mg-2Zn-1Y alloy is the minimum. When the semi-solid isothermal time is 10min, with increasing temperature, a-Mg solid phase grains in Mg-2Zn-xY alloy are gradually spheroidized, and separated by liquid phases. Otherwise, the liquid phase contents locate both at the grain boundaries and in the grain interior significantly increase. With the increase of isothermal time (≤30min) at 580 ℃, Ostwald ripening mechanism acts a major role during microstructure evolution of semi-solid Mg-2Zn-0.5Y alloy. However, Ostwald ripening mechanism and solid particle melting mechanism act major role at the same time during microstructure evolution of semi-solid Mg-2Zn-l Y and Mg-2Zn-2Y alloys.The microstructure of cast Mg-8Al-0.7Zn is coarse dendritic, and it consists of a-Mg and β-Mg17A112 phase which along the grain boundaries and have discontinuous net shape. The microstructure of extruded Mg-8Al-0.7Zn alloy is composed of a single phase a-Mg. The average grain size of primary a-Mg is about 18p.m. When choose extruded Mg-8Al-0.7Zn alloy as the raw material, fine and spherical particles can be acquireed by semi-solid electromagnetic induction heat treatment. During semi-solid electromagnetic induction heat treatment, particle coalescence mechanism is the main mechanism before the liquid phase appearing, while particle coarsening mechanism and solid particle remelting mechanism both play major roles after liquid phase appearing. The firstly remelt areas in the alloy were the place which have low melting point phase and Al and Zn element gathering, then permeate along the grain boundary, finally the semi-solid structure which is liquid phase surrounded by solid-phase particles is formed. Meanwhile, the liquid phase can be observed both at the grain boundaries and in the grain interior.Through observation of semi-solid micro structure made by different methods, we find semi-solid billets made by electromagnetic induction heat treatment have fine, spherical and uniform particles, and this method need less time and power. While through observation of semi-solid microstructure made by isothermal heat treatment, we find the semi-solid particls are more spherical, but the average particle size is bigger, which means the mechanical property will be poor. What’s more, we have also researched the method of overheat treatment making semi-solid billets. We find the semi-solid microstructure made by this method is very small, and this method need less heat time. But because of too high temperature, the semi-solid billets are easy to be burnt and the microstructure is nonuniform. So this method is difficult to industrial achievement. Multistage heat treatment method combined the advantages of isothermal heat treatment and electromagnetic induction heat treatment. The semi-solid microstructure made by this method is very small, spherical and uniform, and this method need shorter heat time and less power. So the method of multistage heat treatment has a good application value in industry. |
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