Correlating the microstructure of the die-chill skin and the corrosion properties for a hot-chamber die-cast AZ91D magne
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I. INTRODUCTION
MAGNESIUM alloys have a high strength/weight ratio and a density of roughly two-thirds that of aluminum and onequarter that of iron.[1] These characteristics make magnesium alloys valuable in vehicle and aerospace applications.[2,3] The corrosion resistance of the alloy depends on similar factors that are critical to other metals. However, the magnesium alloy is not widely used, partly because of its high electrochemical activity,[4] which significantly amplifies the relative importance of some factors, thus reducing its corrosion resistance. Die casting is one of the most widely employed methods of producing magnesium alloy components. Two basic diecasting processes are used—hot chamber and cold chamber—differentiated by the metal-injection method.[5] The former process has some advantages over the latter, including rapid cycling, improved fluidity, the use of a lower injection pressure, etc.[5] The former process, thus, offers a favorable method for the mass production of electronic appliance housings with thin section thicknesses. However, the corrosion properties of a hot-chamber die-cast thin plate (e.g., 1.4-mm thick, in the present study) have been addressed briefly in previous studies.[6,7] Additionally, during die casting, the mold, which is clamped together by hydraulic force, is rapidly filled (in 5 to 100 ms) by forcing the molten metal through a narrow gate.[8] The metal solidifies with a high cooling rate (100 °C/s to 1000 °C/s), yielding a fine-grained material. A distinctive characteristic of this process is the structural inhomogeneity of the metal, caused by the BING-LUNG YU, Graduate Student, and JUN-YEN UAN, Assistant Professor, are with the Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted October 27, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
formation of a chilled skin on the die-cast surface. The properties of the surface layer of the magnesium alloy are an important determinant of the applications of the alloy.[9,10] For example, the yield stress of a nominally 1-mm-thick magnesium die casting falls from 186 to 160 MPa when 0.125 mm is removed from both surfaces.[11] Notably, the skin surface of a die-cast alloy corrodes first upon exposure to a corrosive environment, so a study on the microstructure of the die-chill skin and its effect on corrosion is necessary. However, the die-skin structure of a hot-chamber die-cast alloy remains unclear. Metallurgical factors that may affect the corrosion characteristics of die-cast Mg alloys include the microstructure[4,5,12–15] and the chemical composition.[5,14,16–19] The corrosion performance of a specified alloy with a specific composition (such as AZ91D) is determined by its microstructure. Song et al.[12] examined the corrosion characteristics of the AZ91D alloy die cast via the cold-chamber process. The as-cast sample had a thickness of 6 mm, and some microstructural discrepancy existed between the d
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