Complex reaction behaviour of ceramic mould with the molten AZ91 alloy during investment casting

Abstract

The effects of different pouring temperatures on the reaction behaviour of the ceramic shell with the molten Mg-9%Al-1%Zn-0.4%Mn (all are in wt-%) (AZ91) alloy during investment casting were studied. The scanning electron microscopy (SEM) micrographs with energy dispersive spectroscopy (EDS) analysis illustrated a complex interaction behaviour of the alloy with the mould components at various pouring temperatures. The MgO + MgAl2O4 compounds were formed on the surface of investment cast alloy at the pouring temperature of 600°C. With increasing the temperature to 650°C, the MgO compounds with the Al2O3 particles were observed at the interface of metal/mould. The Al2O3 particles were the main compound on the surface of alloy at the pouring temperature of 700°C. The formation of Al2O3 particles instead of the MgO compounds on the surface was related to the increase of magnesium evaporation by enhancement of the pouring temperature and also diffusion of Al2O3 particles from the ceramic shell to the surface of the alloy.

Production and Application of Magnesium Alloys Castings

“…Another specific feature of the production of magnesium alloy castings is the high sensitivity to corrosion in many operating environments [4]. Therefore, the surface treatment of Mg castings is one of the most effective and economical corrosion protection methods, where a suitable protective coating acts as a barrier between the magnesium alloy and the outdoor environment.…”

“…The final coatings on magnesium alloys are formed at much higher anodizing voltages and also have an irregular porous structure, compared to aluminium, where the porosity is regular. Hard anodizing coatings for magnesium alloys include, in particular, coatings of the HAE, Dow 17, and Tagnite type [4]. Anodic coatings are most commonly used on magnesium castings of gearbox housings, interior doors, and engine blocks in the transport industry.…”