MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALUMINUM-MANGANESE CAST ALLOYS
A. Luo and A. Sachdev General Motors Research and Development Center, Warren, MI, USA Copyright © 2010 American Foundry Society Abstract
Mg-Al-Mn (AM) based cast alloys were optimized for balanced tensile properties (strength and ductility) and response to heat treatment. The microstructure and mechanical property tests suggest that an aluminum content of 7-8% Al is optimum in these alloys for structural applications. AM80 (Mg-8%Al- 0.3%Mn) alloy castings produced by super-vacuum die casting process show very good properties; yield strength of 157 MPa, ultimate tensile strength of 252 MPa and elongation of 5.6% in the as-cast condition, which compare
Introduction
Vehicle weight reduction is a critical enabler to meet fu- ture competitive and mandated fuel economy requirements. Magnesium is the lightest of all structural metals and magne- sium alloys have higher mass saving potential compared to advanced high-strength steel (AHSS), aluminum, polymers, and glass fiber reinforced polymers (GFRP) for equal stiff- ness or strength.1
Magnesium castings can provide small or
large complex parts which are dimensionally more accurate than fabricated and joined steel or aluminum components.2
There are currently two major alloy systems, Mg-Al-Zn (AZ) and Mg-Al-Mn (AM), for automotive casting appli- cations. AZ91 (Mg-9%Al-1%Zn) alloy has high strengths but low ductility, is generally used for non-structural parts that are strength dominated and exposed to ambient tem- peratures like brackets, covers, cases and housings; pro- viding essentially the same functionality with significant mass savings. For structural applications such as instru- ment panels, steering systems and radiator supports, where crashworthiness is important, AM50 (Mg-6%Al-0.3%Mn) or AM60 (Mg-6%Al-0.3%Mn), offer unique advantages due to their higher ductility (10-15% elongation) and high- er impact strength compared to die cast A380 alloy.1
How-
ever, the strength of AM50/60 alloy is too low for critical structural applications.
With magnesium alloys expanding into more critical struc- tural applications such as the new Corvette Z06 structural cast magnesium crossmember (cradle),2
there is a continuing need to optimize existing cast alloys and develop new alloys International Journal of Metalcasting/Fall 10
favorably with the workhorse aluminum die casting alloy A380. Both T5 (artificial aging after as-cast condition) and T6 (artificial aging after solution and water quench) provided a similar improvement in strength for gravity-cast AM alloys containing more than 8% Al, but minimal strengthening in high pressure die castings.
Keywords: solidification, magnesium alloys, casting, microstructure, mechanical properties
with improved mechanical properties, especially fatigue strength and crashworthiness.
Several alloy systems, Mg-Al-Si (AS), Mg-Al-Ca (AX), Mg- Al-Sr (AJ) and Mg-Al-RE (AE), have been developed for el- evated temperature applications where creep resistance is the primary concern.3
misch-metal rare earth7
New magnesium alloys containing tin,4-6 and individual rare earth elements8,9
have been reported for structural applications; such as Mercedes 7-speed Tiptronic automatic transmission case,10
which uses
AS31 (Mg-3%Al-1%Si); Chevrolet Corvette Z06 engine cradle,2 which uses AE44 (Mg-4%Al-4%RE); and the BMW composite engine block,11
which contains a hypereutectic Al-Si alloy inner block around which is cast the AJ62 (Mg-6%Al-2%Sr) alloy.
This study aims at optimizing the current Mg-Al-Mn alloy system for improved mechanical properties in as-cast and heat-treated conditions. Casting samples of AM alloys were made using gravity permanent mold casting (GPMC) and super-vacuum die casting (SVDC) processes for microstruc- tural analysis and mechanical property testing.
Experimental Procedures Alloy Preparation and Casting
For each alloy, the melt was heated to a desired casting tem- perature between 670C (1238F) and 720C (1328F), which is
/CO2 51
Commercial AM20, AM50 and pure aluminum were used to make the Mg-Al-Mn alloy compositions (AM20 to AM100) shown in Table 1, for GPMC. These alloys were prepared and melted in a 30 lb. steel crucible under SF6
protection.
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