Tensile behavior of friction-stir-welded AZ31-H24 Mg alloy
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Since the development of the friction stir welding (FSW) process in 1991 by The Welding Institute (TWI) in the United Kingdom, this joining method has gathered a great amount of interest in a variety of applications, particularly in aerospace and automotive industries.[1,2,3] The advantages of this solid-state joining process encompass better mechanical properties, low residual stress and deformation, weight savings, and reduced occurrence of defects compared to conventional welding methods.[4] At present, research on the FSW process has mainly focused on the joining of Al alloys,[5,6,7] and the FSW of Al alloy is now in the early stage of commercial usage. Magnesium alloys are the lightest structural alloys commercially available, and the interest in friction stir welding of Mg alloys is rapidly increasing.[8] However, the tensile ductility of Mg alloys has been reported to be greatly reduced with FSW compared to that of Al alloys, and the degrading mechanism has not been completely established.[9] The objective of the present study was therefore to examine the tensile behavior of friction-stir-welded AZ31-H24 Mg alloy. The mechanism for the deterioration of tensile property was proposed based on the Auger electron spectroscopy (AES) data and the microstructural observations documented by optical microscope and scanning electron microscope (SEM). The 4-mm-thick AZ31(Mg-3.6Al-1Zn-0.6Mn in wt pct)H24 Mg alloy plate was used in the present study. Friction stir welding was conducted at varying rotating speeds of
SUNGGON LIM, Graduate Student, and SANGSHIK KIM, Professor, are with Division of Materials Science and Engineering, Engineering Research Institute, Gyeongsang National University, Chinju 660-701, Korea. Contact e-mail: [email protected] CHANG-GIL LEE and SUNG JOON KIM, Principal Researchers, and CHANG DONG YIM, Senior Researcher, are with Institute of Materials Science and Technology, Korea Institute of Machinery and Materials, Changwon 641-010, Korea. Manuscript submitted September 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
1200, 1400, and 1600 rpm and welding speeds of 0.1 and 0.2 mpm (m/min). Tensile specimens were prepared with the tensile direction perpendicular to the welding direction, so that the weld zone is located in the middle of the specimen. Tensile tests were performed at a nominal strain rate of 1 103/s on an S2 model R&B (Daejeon, Korea) universal testing machine. The side and fracture surfaces of tested specimens were examined using a scanning electron microscope (SEM). The Mg alloys are known to have a relatively thick oxide layer on the surface, and these oxides could be entrapped into the weld zone during the friction-stir-welded process. Since these oxides in the weld zone would affect the tensile property of friction-stir-welded AZ31-H24, Auger electron spectroscopy (AES) analysis was conducted to measure the oxygen profile on the tensile-fractured surface. Figure 1 represents (a) the typical cross-sectional macrograph of the friction-stir-welded AZ31-H24 alloy, and the opti
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