microstructure is favorable for barrier effect suppress- ing the galvanic effect β-phase. It is, thus, expected that AM80 alloy has similar corrosion resistance to commer- cial AZ91 and AM60 alloys.
Conclusions
1. The microstructure and mechanical property test- ing suggested an optimized aluminum content of 7-8% in Mg-Al-Mn based alloys which offer bal- anced tensile properties (strength and ductility).
2. The AM80 (Mg-8%Al-0.3%Mn) alloy castings produced by super-vacuum die casting process show very good properties, i.e., yield strength of 157 MPa, ultimate tensile strength of 252 MPa and elongation of 5.6% in the as-cast condition, which compare favorably with the workhorse aluminum die casting alloy A380. The aging treatment (T5) does not provide improvement in mechanical prop-
erties due to the grain growth offsetting the precipi- tation hardening effect.
3. Both T5 (artificial aging after as-cast condition) and T6 (artificial aging after solution and water quench) provide significant strength improvement of Mg-Al based alloys containing more than 7% Al in gravity casting. T5 is recommended due to its similar heat treatment response compared to T6, but eliminating the solution treatment process time reduces energy consumption as well as the risk of distortion during quenching.
Acknowledgments
The authors wish to acknowledge Willie Dixon for assis- tance in metallography and mechanical testing, Jason Traub for gravity casting experiments. The die casting samples were provided through the USAMP Magnesium Front End Research and Development project.
Figure 8. Optical micrographs showing the T5-aged (5 hours at 232C [450F]) microstructure of AM80 alloy (SVDC): (a) overall section; (b) skin and core sections; and (c) core section.
International Journal of Metalcasting/Fall 10 57
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