Characterization of fold defects in AZ91D and AE42 magnesium alloy permanent mold castings

• Published in: Materials characterization Vol. 61; no. 3; pp. 296 - 304
• Main Authors: Bichler, L., Ravindran, C.
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01.03.2010; Elsevier
• Subjects: CASTING; CASTING MOLDS; COMPUTERIZED SIMULATION; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Fold; INTERMETALLIC COMPOUNDS; Magnesium alloy; MAGNESIUM BASE ALLOYS; MATERIALS SCIENCE; MICROSTRUCTURE; OPTICAL MICROSCOPY; OXIDATION; PARTICLES; Permanent mold casting; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; RARE EARTHS; SCANNING ELECTRON MICROSCOPY; SEGREGATION; Solidification; SOLUBILITY; SOLUTES; STREAMS; Fold; Oxidation; Permanent mold casting; Segregation; Magnesium alloy; Magnesium base alloys; Permanent mould casting; Permanent mold; Foundries; Microstructure; Cast alloy; Defects
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2009.12.011

Casting premium-quality magnesium alloy components for aerospace and automotive applications poses unique challenges. Magnesium alloys are known to freeze rapidly prior to filling a casting cavity, resulting in misruns and cold shuts. In addition, melt oxidation, solute segregation and turbulent metal flow during casting contribute to the formation of fold defects. In this research, formation of fold defects in AZ91D and AE42 magnesium alloys cast via the permanent mold casting process was investigated. Computer simulations of the casting process predicted the development of a turbulent metal flow in a critical casting region with abrupt geometrical transitions. SEM and light optical microscopy examinations revealed the presence of folds in this region for both alloys. However, each alloy exhibited a unique mechanism responsible for fold formation. In the AZ91D alloy, melt oxidation and velocity gradients in the critical casting region prevented fusion of merging metal front streams. In the AE42 alloy, limited solubility of rare-earth intermetallic compounds in the α-Mg phase resulted in segregation of Al 2RE particles at the leading edge of a metal front and created microstructural inhomogeneity across the fold.