Influence of Process Parameters on Microstructure and Mechanical Properties of Friction-Stir-Processed Mg-Gd-Y-Zr Castin
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INTRODUCTION
AS the lightest metals, magnesium alloys have attracted increasing interest in recent years for potential applications in the aerospace, aircraft, and automotive industries.[1] However, the low strength and poor creep resistance at increased temperatures restrict their wide applications. Recently, Mg-RE alloys (RE represents rare earth elements) were developed to meet the demand for high temperature applications. The addition of RE elements can produce thermally stable second phases in the Mg matrix, leading to high strength at a high temperature as well as good creep resistance.[2] Currently, the Mg-RE alloy parts are produced mainly by casting, and the microstructure of the castings is characterized by coarse grains and coarse eutectic networks at the grain boundaries. This microstructure limits the mechanical properties of the Mg-RE castings, especially the properties at a high temperature. Therefore, a modification of the microstructure is needed to enhance the mechanical properties and broaden the applications of Mg-RE alloys. Conventionally, heat treatment and various plastic deformation methods are applied to modify the microstructure of the Mg castings.[3,4] However, the heat treatment at high temperature for lengthy time not only is time consuming but also results in coarse grains. Furthermore, a lengthy heat Q. YANG, Postgraduate, and B.L. XIAO and Z.Y. MA, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 10, 2011. Article published online February 10, 2012 2094—VOLUME 43A, JUNE 2012
treatment is also needed before plastic deformation occurs as a result of the poor formability of magnesium alloys. Friction stir processing (FSP), which was developed based on the principle of friction stir welding (FSW), has been demonstrated to be an effective method for the microstructural modification of castings.[5,6] During FSP, a processed zone is obtained in just several minutes without preheating for the cast alloys. FSP of conventional cast magnesium alloys, such as AM60B, AZ80, and AZ91, resulted in significantly enhanced tensile and fatigue properties because of the grain refinement, the breakup and dissolution of second phase particles, and the elimination of porosity.[6–9] The influences of FSP on the cast Mg-RE alloys were studied recently. Tsujikawa et al.[10] achieved the ultrafine grains and refined lamellar structure in cast Mg-5.5Y-4.3Zn by FSP; increased hardness was obtained. Freeney and Mishra[11] obtained a fine-grained microstructure with dispersed nanosized particles in EV31A casting by FSP, and the mechanical properties were improved significantly. Our previous study on cast MgGd-Y-Zr indicated that FSP resulted in significant grain refinement and the dissolution of coarse eutectic Mg5(Gd,Y) phase, thereby enhancing the mechanical properties of Mg-Gd-Y-Zr alloy significantly.[12] However, the alternating fine- and coarse-grained
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