Modelling and Measurements of Densities at Solidification of Magnesium Casting Alloys

摘要

The purpose of this study is to provide experimental measurements of volume changes in commercial magnesium pressure die casting alloys, and to present a general methodology by which it is possible to make accurate calculations of the volume changes in the solidification interval of multi-component magnesium alloys. The density and volume changes are important in simulation of porosity formation in magnesium castings. Density measurements were made in a specialised push-rod dilatometer for both solid and liquid state. The solid density depends on the alloy composition. A most significant effect is due to aluminium which increases the density of the magnesium alloy. As an example, AZ91 (～9% Al) has 2.8% higher density than AM20 (～2% Al) at room temperature. At the liquidus temperature, AZ91 has 3.8% higher density than AM20. Above the melting temperature, an average of the densities at heating and cooling respectively, have been evaluated. The liquid expansion/contraction is in the region of 0.24 ― 0.27 kg/m~3·C or 1.54 ― 1.71 vol.% per 100℃ temperature change. The not direct accessible densities are appearing in the two-phase temperature region. The shrinkage at solidification can be measured until a dendrite network has formed. The fraction solid at this point, the coherency point, is normally around 0.25―0.4, depending on the solidification rate and mode. For the alloys investigated this shrinkage varies from about 0.92 to 3.45%. The following shrinkage has been modelled due to non measurable quantities of the densities in the melt and in solid. The measurements were used to validate models for the molar volumes. The difference between measured and modelled densities is in the range of 0.5%, which is smaller than the accuracy of the experiments. The density calculations have been fully coupled to a deterministic simulation of the solidification sequence including segregation of the elements.