Microstructure and Mechanical Properties of Extruded Magnesium-Aluminum-Cerium Alloy Tubes
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MAGNESIUM castings are increasingly used for automotive lightweight applications due to their low density, high specific strength, and good castability.[1] Magnesium extrusions, especially hollow sections, offer attractive mass savings opportunities for automotive lightweighting with potential applications in instrument panel beam, seat frame, roof frame, bumper beam, radiator support, engine cradle, and subframe.[1] However, wrought magnesium alloys are not widely used due to their low room-temperature formability and crashworthiness compared to steel or aluminum alloys as a result of the limited slip systems present in magnesium. Recently, there have been many attempts[2–9] to develop new magnesium extrusion alloys with enhanced workability and ductility for structural applications. The research so far seems to suggest that texture modification and grain refinement are the most promising approaches in improving the ductility of wrought magnesium alloys. Polmear[10] reported that magnesium alloys containing yttrium (Y) and neodymium (Nd) developed a more random texture during extrusion. Recently, Mishra et al.[11] reported that a significant increase in elongation in pure magnesium is obtained due to the addition of only 0.2 pct cerium (Ce), and ALAN A. LUO, GM Technical Fellow, RAJ K. MISHRA, Staff Researcher, and ANIL K. SACHDEV, Lab Group Manager, are with General Motors Global Research & Development, Warren, MI 480909055. Contact e-mail: [email protected] WENYUN WU, Postdoctoral Candidate, LI JIN, Researcher, and WENJIANG DING, Professor, are with the National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China. Manuscript submitted January 21, 2010. Article published online June 23, 2010 2662—VOLUME 41A, OCTOBER 2010
attributed this increase to a change in texture in the extruded rods that favored basal slip. However, the strength of the magnesium (Mg)-0.2 pct Ce alloy rods remains low for structural applications. This study considered the addition of aluminum for improving alloy strength. The Mg-Al-Ce ternary alloy system was investigated and compared with the pure Mg, Mg-Ce, and Mg-Al binary systems. The experiment was designed to evaluate the properties of hollow extrusions (tubes) as opposed to the rod extrusions in the previous work.[11] The microstructure and mechanical properties of the experimental alloys were studied at different extrusion conditions in search of the optimal process parameters for these alloys.
II.
MATERIALS AND EXPERIMENTAL PROCEDURES
A. Materials Pure Mg (99.9 pct), three binary alloys (Mg-0.2 pct Ce, Mg-0.5 pct Ce, and Mg-3 pct Al), and two ternary alloys (Mg-3 pct Al-0.2 pct Ce and Mg-3 pct Al-0.5 pct Ce) were prepared using pure Mg, pure Al (99.9 pct), and Mg-25 pct Ce master alloy. Other impurities in these materials included trace levels of Mn, Si, Zn, Cu, and Fe. These alloys were cast into billets 100-mm diameter and 200-mm long, at a speed of 180 to 200 mm/min using a direct-chill casting machine. A pre
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