An investigation on interfacial reaction between in-situ melted AZ91D magnesium alloy and ceramic shell mold during investment casting process

The reaction between ceramic shell investment mold and AZ91D magnesium alloy as well as the related mechanism involved during investment casting process using in-situ melting technique were explored. AZ91D granules were melted in a ceramic shell investment casting mold at 750 degrees C in an argon protected environment and a melting flux. The interface of adhered investment-AZ91D cast alloy and the residues that appeared on the surface of the castings, as the mold-metal reaction products, were analyzed to determine the morphology, elements and compounds that may have developed due to the reactions. It was discovered that the high process temperature and high affinity of magnesium with oxygen developed cracks in the ceramic shell investment mold. Penetration of molten metal through the cracks also occurred and caused adherence of investment on the casting surface. The results showed that the black residue with a granular morphology has the same microstructure as that of AZ91D alloy and also comprises of MgO and Mg2Si on its surface. The findings revealed that two types of products formed on the shell surface due to the mold-metal reaction. The first product formed on the surface as a result of the reaction between AZ91D alloy and the binder producing MgO and MgAl2O4. The second product formed because of the penetration of Mg into the ceramic shell investment mold followed by reaction with oxygen bearing materials forming MgO and Mg2Si. (C) 2012 Elsevier B.V. All rights reserved.