Microstructure,Property And Thermodynamic Calculation Of Mg-Al Alloys For Rheo-squeeze Cast

Rheocasting is one of the most promising net shaping technology of light alloy complex structure in the 21 st century.Magnesium alloys as a kind of light metal material,have extremely extensive application prospects in the defence industry.It is an important research to find new magnesium alloys that suitable for semi-solid forming.In this dissertation,the two-stage thermodynamic calculation for rheocasting is established firstly,and researching the processing properties of AZ91 alloy for rheocasting combining the experiment.Then designing and optimising the special composition of Mg-Al-Sr al oy for rheocasting on this basis.According to the operation procedure of rheocasting,the two-stage thermodynamic calculation procedure of rheocasting is edtablished.This procedure can calculate phase formation,lidiud composition,phase content,solidification range,processing window and temperature sensitivity of fraction solid.Thermodynamic calculation is a scientific and useful approach to investigate processability of alloys in semisolid state.This paper systematically investigated the solidification behaviour,phase formation rules,mechanical properties and semisolid formablility of AZ91 alloys for rheo-squeeze cast.The study found that R-SQC can effectively reduce the formation of porosity in AZ91,refine microstructure,and improve mechanical properties.With the increase of solid content in semi-solid slurry,the grain size and the volume content of ?-Mg phase increased,and the content of ?-Mg₁₇Al₁₂ phase also increased,it leads to a decrease of strength and ductility.Based on the developed two-stage thermodynamic calculation procedure,the increasing of fraction solid at TF of Mg-9Al-1Zn alloy leads to an increase of concentration level of ?-Mg₁₇Al₁₂.It shows that Mg-9Al-1Zn may not be suited for R-SQC using semi-solid slurry with high fraction solid.This kind of alloy might be used for R-SQC using semi-solid slurry containing lower fraction solid.Based on the developed two-stage thermodynamic calculation procedure,the increasing of Al and Sr leads to adecrease of solidification range,an increase of processing window,and decreasing the temperature sensitivity of fraction solid.Also,with the increase of Al,the concentration level of ?-Mg₁₇Al₁₂ increased;but with the increase of Sr,the concentration level of ?-Mg₁₇Al₁₂ decreased.The suitable composition range of Mg-Al-Sr al oys is Mg-?5⁷?Al-?3⁵?Sr aloys.