Microstructure Evolution and Mechanical Properties of Rheo-Squeeze Cast Mg-9Al-1Zn Alloy by Experiments and Thermodynami
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INTRODUCTION
MAGNESIUM alloys, as the lightest structural metals, are receiving increased attention in the automotive application, triggered by the demand for vehicle weight reduction. However, these alloys have a large solidification interval and a high susceptibility to casting defects, resulting in poor mechanical properties and creep resistance.[1] Investigations have shown that the casting defects like segregation, porosity, and hot tearing can be overcome by refining microstructure or through the morphological transition from dendritic grains to spherical ones.[2,3] And many chemical and physical approaches for the grain refinement of magnesium alloys have been developed.[4] Moreover, casting defects can also be reduced by means of slower die filling and solidification under high pressure. Currently, the preferred casting route for Mg alloys is high pressure die-casting (HPDC). However, the HPDC components contain a substantial amount of porosity and hot tearing.[5] Squeeze casting is another effective casting route and currently is being evaluated.[6] So one of great challenges for magnesium alloys is to use grain refining approaches to develop solidification processing technologies to produce components with fine and uniform microstructure and free of cast defects, offering high mechanical properties. HONG-MIN GUO, Professor, and HUA-LAN JIN, Lecturer, are with the School of Materials Science and Engineering, Nanchang University, 999 Xuefu Road, Nanchang, Jiangxi 330031, P.R. China and also with the Key Laboratory of Near-Net Forming in Jiangxi Province, Nanchang University, 999 Xuefu Road, Nanchang, Jiangxi 330031, P.R. China. Contact e-mail: [email protected] SHU-GUO ZHANG, Ph.D. Candidate, XIANG-JIE YANG, Professor, and XU-BO LIU, Associate Professor, are with the Key Laboratory of Near-Net Forming in Jiangxi Province, Nanchang University. Manuscript submitted December 17, 2014. Article published online March 4, 2015 2134—VOLUME 46A, MAY 2015
Rheo-squeeze casting (R-SQC) is an innovative semisolid metal processing, where produced semi-solid slurry is cast into near-net parts using conventional squeeze casting (SQC) machine. It has been found that the R-SQC process is available for the production of aluminum alloy parts that are suitable for heat treatment to enhance mechanical properties.[7–9] But little information is available about magnesium alloys under R-SQC conditions.[10] The objective of this research is to assess the mechanical properties of conventional squeeze cast and rheo-squeeze cast AZ91 magnesium alloy and to correlate these properties with the microstructure and the casting process. Based on the developed two-stage thermodynamic calculation procedure, the solidification path and phase formation in R-SQC will be investigated deeply.
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EXPERIMENTAL PROCEDURES
The experimental alloy, AZ91 containing 8.9 wt pct Al, 0.75 wt pct Zn, 0.27 wt pct Mn,
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