Effect of Mg addition of microstructure of 319 type alloys

The present study was undertaken to investigate the effect of Mg on the occurrence of incipient melting in experimental and industrial 319 alloys using thermal analysis, tensile testing, microstructural analysis and porosity measurements. Castings were prepared from experimental and industrial alloy melts containing Mg levels of 0-0.6 wt-%. The addition of Mg leads to the segregation of the copper phase, resulting in the formation of the block-like form of the CuAl2 phase rather than its finer eutectic-like form. This makes it more difficult to dissolve the CuAl2 phase during solution heat treatment. The addition of Mg to 319 alloy, irrespective of the alloy source, modifies the Si particle morphology. This effect is observed very clearly at 0.6 wt-% Mg, with a corresponding decrease in the Al-Si eutectic temperature compared to the base alloy. As expected, the modification effect of Mg is not very obvious at low Mg addition. The addition of Mg also leads to the precipitation of the Al5Mg8Cu2Si6 phase. This phase normally precipitates after the CuAl2 phase. Nevertheless, when the addition of Mg exceeds 0.4 wt-%, the precipitation of the Al5Mg(8)Cu(2)Si(6) phase also takes place in another reaction, before the precipitation of the CuAl2 phase. The morphology of the Al5Mg8Cu2Si6 phase particles in this case is script-like rather than the irregular shaped particles normally observed.