Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing
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THE Al-7 wt pct Si-Mg alloys with Mg contents in the range of 0.25 to 0.65 wt pct (A356 and A357 alloys) are widely used to cast high-strength components in the aerospace and automobile industries because they offer a combination of high strength[1,2,3] with good casting characteristics.[4] However, some mechanical properties of cast alloys, in particular, ductility, toughness, and fatigue resistance, are limited by three microstructural features: porosity, coarse acicular Si particles, and coarse primary aluminum dendrites.[5–8] In the past two decades, various modification and heattreatment techniques have been developed to refine the microstructure of cast Al-Si alloys. The first category of research is aimed at modifying the morphology of Si particles. Generally, chemical modification and thermal treatment have been adopted to modify the coarse acicular Si particles to fine and globular particles.[8,9,10] Chemical modification methods involve adding very small amounts of sodium, strontium, or antimony, known as eutectic modifiers.[9,10] Sodium is a good modifier and traditionally has been used to spheroidize eutectic particles. However, the benefits of sodium fade rapidly on holding at high temperature and the modifying action practically disappears after only two remelts. On the other hand, the modifying effect of strontium does not fade on holding at elevated temperature and its use has become more widespread. However, dissolution of strontium is difficult and a longer holding Z.Y. MA, Professor, formerly with the Department of Materials Science and Engineering, Center for Friction Stir Processing, University of Missouri, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. S.R. SHARMA, formerly with the Department of Materials Science and Engineering, Center for Friction Stir Processing, University of Missouri, is with Intel Corporation, Portland, OR 97124. R.S. MISHRA, Professor, is with the Department of Materials Science and Engineering, Center for Friction Stir Processing, University of Missouri, Rolla, MO 65409. Contact e-mail: [email protected] Manuscript submitted January 8, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
time is required at 750 °C, resulting in increased gas pickup.[11] Furthermore, the density of microshrinkage porosity is also higher after the addition of strontium owing to the dissolution difficulty and a depression in the eutectic transformation temperature.[12] Finally, although antimony has no fading effect and the improvement in elongation and impact strength is greater than that achieved by sodium,[9] environmental and safety concerns have precluded its use in most countries. Thermal modification involves heat treatment of cast alloys at high temperature, usually at the solid solution temperature around 540 °C for long times.[8] Solution heat treatment results in a substantial degree of spheroidization of Si particles and also coarsens Si particles. However, solution treatment at high temperature for long time increases
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