Effect of Friction Stir Processing on Microstructure and Tensile Properties of an Investment Cast Al-7Si-0.6Mg Alloy
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RISING fuel costs and tougher environmental restrictions are encouraging increased use of lighter metals and alloys in the automotive and aerospace industries. Cast aluminum alloys have received particular attention as reliance on nonferrous metals and alloys has grown. Cast Al-7Si-Mg alloys with a Mg content of 0.25 to 0.65 wt pct (A356 and A357 alloys) are popular because they offer a combination of high strength and good casting characteristics.[1–4] The tensile properties of cast Al-Si alloys, in particular ductility and toughness, are limited by three major microstructural features (i.e., porosity, Si particles, and primary aluminum dendrites).[5–8] Refinement of the cast microstructure is necessary to address these limitations. Techniques currently available for refining a cast Al-Si microstructure can be classified broadly into two categories. The first category changes the Si particle morphology through chemical or thermal modification techniques.[8–10] Chemical modification is done by adding small amounts of sodium, strontium, or antimony (also known as eutectic modifiers) to the melt.[9,10] These elements change the shape of coarse acicular eutectic Si particles, making them fine globules. Of these three additives, strontium is the most effective and thus the SAUMYADEEP JANA, formerly Graduate Student, Missouri University of Science and Technology, Rolla, MO 65409, is now Postdoctoral Research Associate, Pacific Northwest National Laboratory, Richland WA 99352. RAJIV S. MISHRA, Curator’s Professor, is with the Missouri University of Science and Technology. Contact e-mail: [email protected] JOHN A. BAUMANN, Technical Fellow, is with Boeing Research and Technology, St. Louis, MO 63166. GLENN J. GRANT, Staff Scientist, is with the Pacific Northwest National Laboratory. Manuscript submitted September 30, 2009. Article published online June 23, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
most widely used, but it suffers from increased gas pickup and associated porosity.[11] During thermal modification, the casting is held at the solution treatment temperature for a long time to spheroidize the eutectic Si particles.[8] The second group of techniques for refining cast Al-Si microstructure acts on the coarse primary aluminum phase through heat treatments. Heat treatment at 850 K (577 C) for 8 minutes caused considerable refinement of aluminum dendrites in a semisolid processed A356 alloy.[3] These techniques do not eliminate porosity effectively or uniformly redistribute refined Si particles in the aluminum matrix. Therefore, a more effective modification technique is desirable. Recently, friction stir processing (FSP) has been examined as a method to alter the microstructure in cast metals. This technique, based on friction stir welding (FSW),[12] involves inserting a rotating tool into a cast component. The tool is traversed along the desired path to cover the region of interest modifying the local cast microstructure. Plastic deformation in the vicinity of the tool eliminates both the dendritic microstructure and the porosit
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