Quench Sensitivity of an Al-7 Pct Si-0.6 Pct Mg Alloy: Characterization and Modeling
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NTRODUCTION
CAST Al-7 wt pct Si-Mg alloys are in-situ metal matrix composites, due to the presence of eutectic Si particles. They have been used successfully in a variety of automotive and aerospace applications. Heat treatment of these alloys involves a solution treatment, subsequent quenching, and finally natural or artificial aging. During solution treatment, the Mg2Si (b) phase is dissolved, the Al matrix is homogenized, and the eutectic (Si) phase becomes dissociated and spheroidized. During artificial aging, b† precipitates form, which subsequently increase the yield strength. Quenching the castings at the highest rate possible, as in cold water, retains more solute and vacancies in solution, which increases the yield strength attainable after aging. However, high cooling rates associated with water quenching result in the generation of thermal stresses leading to distortion and residual stresses, especially in castings with complex geometries. Conversely, low cooling rates that provide reduced levels of thermal stress issues produce nonstrengthening quench precipitates, such as b¢ and b in the Al-Mg-Si system, which ultimately reduce the strength attainable after aging. Therefore, process engineers strive to design cooling MURAT TIRYAKIOG˘LU, Professor of Engineering, is with the Department of Engineering, Robert Morris University, Moon Township, PA 15108, USA. Contact e-mail: tiryakioglu@rmu. edu RALPH T. SHUEY, Senior Technical Consultant is with the Alcoa Technical Center, Alcoa Center, PA 15069, USA. This article is based on a presentation made in the symposium entitled ‘‘Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,’’ which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee. Article published online April 3, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
processes with an optimum balance of strength and thermal stresses. To accomplish this task, the effect of different cooling paths on the strength needs to be characterized and modeled. Furthermore, the dependences on composition, homogenization, and aging need to be understood and ultimately modeled. To attain this manifold, quantitative mastery of process-property relations, investigation should follow the intermediate microstructure, i.e., the precipitates formed during quench. Several studies on the quench sensitivity of cast Al-7 pct Si-Mg alloys have been reported.[1–7] In only three of these[1,2,3] was microstructure investigated for quench precipitates. The literature on quench sensitivity of wrought Al-Mg-Si alloys is relatively voluminous, but use for Al-7 pct Si-Mg casting alloys requires identifying and understanding the differences. The following advances are now reported. Quench sensitivity of D357, an aerospace cas
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