The combination of the lightest structural magnesium alloys with the expendable pattern casting (EPC) process will bring a bright future for magnesium applications. In this paper, a pioneering effort has been made to fully explore the effects of vacuum and other processing parameters, including the coating permeability, the thickness of coating, the pouring temperature, the gating system structure and the foam density on the fluidity of AZ91 magnesium alloy. The results obtained indicate that vacuum is the most effective parameter to improve the fluidity, which also have a strong interactive effect with the pouring temperature and coating. The fluidity increases with adopting of high permeability coating, and increasing pouring temperature, vacuum, the pattern width and thickness; while decreases with increasing the coating thickness and foam density. When the unchoked gating system is adopted and vacuum is applied, the fluidity decreases with increasing distance between the ingate and the downsprue; otherwise, the effect of gating system structure on the fluidity is insignificant. A modified model has been established to illustrate the effect of vacuum on the mold filling, which was analyzed by optical microscopy in the paper. The vacuum greatly changes the mass and heat transfer process: vacuum applied not only determines the profile of metal-foam interface which influences the mass transfer process, but also greatly speeds up the removing rate of pattern decomposition products at the metal-coating interface; it also changes the primary heat transfer mode to heat convention and have a great influence on the casting temperature field distribution and solidification process.
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