Effects of Friction Stir Processing Parameters and In Situ Passes on Microstructure and Tensile Properties of Al-Si-Mg C
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THE depletion of natural resources and the deterioration of our environment are two of the most serious problems of our current world. Using lightweight and high strength materials in fields such as the automobile manufacturing industry will help reduce resource consumption and pollution. Therefore, it is vital to incorporate high strength and high performance alloys into these industries. A356 aluminum alloy is widely used to produce automobile wheels and other parts due to its relatively good casting properties, mechanical properties, corrosion resistance, and physical properties.[1,2] The high strength of the A356 alloy is mainly derived from the precipitation hardening of fine Mg2Si phases,[1,2] which is dependent on heat treatment conditions.[3–5] Furthermore, the mechanical properties of the cast A356 alloy are also controlled by factors such as the volume fraction and dimension of shrinkage,[6,7] the shape and distribution of eutectic Si particles,[8,9] and Fe-rich intermetallic phases. Among these, porosity is a main factor harmful to mechanical properties, particularly fatigue properties.[10,11] Furthermore, it was also G.R. CUI, formerly Postgraduate with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China, is now Lecturer with the Harbin Institute of Technology at Weihai, No.2, Wenhua West Street, Weihai 264209, P.R. China. D.R. NI, Associate Professor, and Z.Y. MA and S.X. LI, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] Manuscript submitted January 17, 2014. Article published online August 14, 2014 5318—VOLUME 45A, NOVEMBER 2014
reported that the strength and the crack propagation resistance of the A356 alloy would benefit from the refinement of dendrites and the uniform distribution of Si particles.[12–15] In order to enhance the mechanical properties of A356 castings, it is necessary to modify the microstructure. However, traditional solutions through chemical modification or heat treatment[1,7,16,17] have their particular limits. For example, chemical modification would introduce defects at high temperature or unsustainable modification effect; whereas heat treatment could not exert effect on the pores. Recently, the technique of friction stir processing (FSP), a development based on friction stir welding (FSW),[18] has been demonstrated to be effective in overcoming the above shortcomings. It was reported that FSP on the casting materials resulted in the breakup of brittle secondary phases, the refinement of matrix grains, the dissolution of precipitates, and the elimination of porosity, thereby significantly improving the mechanical properties of cast alloys.[19–28] For cast A356 alloy, FSP has been demonstrated to produce significant enhancement of tensile properties[22,23]: the yield strength (YS), ultimate tensile strength (UTS), and total elongation was 132, 169 MPa, and 3 pct for t
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