Study Of Permanent Die Coating For Magnesium Alloys

There is no suitable permanent die casting coating for magnesium alloy in present industrial application. Permanent die coating for magnesium alloys has been investigated by choosing suitable refractory powders, suspensions, and binder in this paper. The causes and resultant of reaction between refractory powders and magnesium alloy melt, as well as the causes of binder failure have been analyzed. The rheological property and thermal conductivity have been tested and an appropriate rheological model was established.The results showed that, magnesium oxide could be used as refractory powders of permanent die coating for magnesium alloys. High bauxite would react with magnesium alloy melt at high temperature, which leaded to the failure of the coating and the reaction products were Mg2Al3 and Mg2Si. The reaction area increased with increasing pouring number. The coating with addition of inhibitor decreased the reactivity and the life time could be prolonged with increasing inhibitor up to 6wt.% .Result of suspend ability showed that, the best ratio of suspensions was: bentonite (3%), CMC (0.2%), P (0.3%) and the suspend rate was 100.Result of rheological property showed that, the modified Casson model could fit the rheological curve exactly, and characterize suspension property and thixotropy of the coating. In the modified Casson model,τ0 could be used to characterize the stability of suspending system of the coating. Largerτ0 indicated better suspendability. Ratio ofτ0 andη∞could be used to characterize the cutting dilutability of the coating and上上下000ττ?τcould be used to characterize the time dependability.Result of binder showed that, sodium hexametaphosphate had a good coherence at room and high temperatures, its optimum amount were 4% -7%. Aluminum dihydrogen phosphate could react with magnesium oxide for the acid water solution, which made it unsuitable for magnesium alloys coating. Sodium silicate was a good binder at room temperature, and the coherence of coating improved with increasing additions of sodium silicate. However, it showed weak binding strength when the temperature exceeded 400~450℃, which also made it unsuitable for magnesium alloys coating.A testing method of thermal conductivity of permanent die coating for magnesium alloys is studied in this paper. Immersion testing method is an effective method to directly describe and calculate the thermal conductivity of permanent die coating.