Study On Heating And Solidification Microstructure Of Upset AZ91D Mg Alloy In The Semi-solid State

Magnesium alloy, intitled the most promising green engineering material in the 21st century, had caught more and more international researchers' attention. SSF (Semi-solid Forming) was a new technology to manufacture near net shaped parts. The technology of the magnesium alloy semi-solid processing was studying. Moreover the manufacture of the semi-solid Mg alloy by SIMA had the unique advantage.In this paper, the Mg alloy specimens for upsetting were studied. The deformation microstructure was observed. DSC method was used to analyze the melting behaviors of Mg alloy. Semi-solid quenching was also done. Influences of parameters on microstructure of Mg alloy under the simulating production conductions were investigated. Both solidification modes of quenched microstructure and the spheroidization mechanism of Mg alloy for upsetting were analyzed. The results indicated as follows:(1) There were significant effects of deformation parameters on microstructure of Mg alloy for upsetting. The original dendrites of AZ91D Mg alloy were changed by upsetting. The dendrites became having a certain direction and got broken or bended obviously. The more deformation rate, the more the original dendrite got broken or got plastic deformation.(2) With the low deformation rate, the defects of dendrites of AZ91D Mg alloy were mostly twin crystals. With the high temperature and the high deformation rate, the deformation mode was dislocation. The distortional strain energy in the microstructureincreased.(3) The results of DSC method showed that the more deformation rate, the less eutectic melting active energy and the distortional strain energy. At 40% deformation rate, the trend of the eutectic melting active energy and the distortional strain energy decreased slightly. The melting temperature is a little lower as the deformation rate increased.(4) There were two important control factors, which were holding temperature and holding time, affecting the globalization processing. In the certain range, with the holding temperature going up, the liquid phase and the globalization of semi-solid microstructure went up. Meanwhile, holding time was shorter relatively. With the holding time prolonged, the grains changed from the equiaxed shape to the near-globular shape, but if the holding time is too long, the uniformity and the globalization decreased because of Oswald ripening mechanism.(5) The solidification modes of liquid phase of semi-solid AZ91D Mg alloy were in close relationship with the degree of supercooling determined by cooling rate. Under conditions of liquid quenching, liquid phase precipitated a-phase firstly. Some α-phase clung to primary solid grains and grew up, others nucleated independently and grew up in the liquid phase. The mayor degree of supercooling caused that a-phase grew into spear-shape or dendrite. Eutectic solidification developed by divorced growth and coupled growth. Divorced growth generated bigger β-phase in grain boundaries. Coupled growth generated lamellar structures.