Effects Of Cooling Rate And High Pressure On The Solidification Of The Zn-Al Alloy

The castability of Zn-Al alloys is pretty bad due to their large solid-liquid phases zone, and hence good castings are seldom achieved at traditional foundry process. It tends to form casting shrinkageand porosities, which greatly deteriorates the mechanical properties of the alloys and restricts their practical applications. There needs to be an effective process method to improve the solidification of Zn-Al alloy. In this thesis, based of rapid solidification process and solidification under high pressure, ribbon and cylindrical samples with a diameter of 1 mm,2 mm and 5 mm were prepared by melt spinning and copper mould casting, respectively. Zn-22A1 and Zn-5.1Al alloys were prepared by solidification under high pressure with a rapid pressure increase mode, The microstructures and mechanical properties of the above samples were investigated.The microstructures remarkably changed with the cooling rate of solidification. The morphology of the master alloy showed coarse dendrite or chrysanthemum-like, and the morphology of the cylindrical and ribbon samples experienced a progress of fine dendrite, regular latticework and unobvious grains. Further, the lattice parameter of a phase of rapidly solidified ribbon is smaller than that of other samples solidified at a lower cooling rate, which is attributed to the retention of extra Zn atom within the Al crystalline lattice. Besides, the cooling rate also had a remarkable effect on the microhardness of samples.The microstructures of Zn-22A1 alloys changed obviously with the pressure applied during solidification.. The microstructures of the alloys changed from coarse dendrites to fine latticeworks. Further, the lattice parameter of specimen solidified at 3GPa pressure is remarkably smaller than that of specimen solidified at normal pressure. And the microhardness of samples increased with the pressure applied.Similar phenomena were also observed for Zn-5.1 Al alloys, the microstructure were refined with the applied pressure during cooling. Further, pressure did not change the phases, but changed the phase content. Also,the elements of the alloy become well-distributed because of the pressure and the cooling rate.