| Summary: | 碩士 === 國立虎尾科技大學 === 材料科學與工程系材料科學與綠色能源工程碩士班 === 104 === Friction stir spot welding (FSSW) is a kind of solid state welding method developed on the basis of the friction stir welding (FSW) process. The working temperature of FSSW is generally lower than melting temperature of metals, and this process is quite suitable for joining magnesium (Mg) and aluminum (Al) alloys. Light-weight Mg alloys have some advantages, such as high specific strength,high specific stiffness, well recyclability and radiation absorption of electromagnetic waves. The aim of present study is to investigate the microstructure variation, phase composition, microhardness, welding strength and fracture morphologies of FSSW butt-joined AZ91/LZ91 and AZ61/LZ91 dissimilar Mg alloys. Experimental results show that grain size and Mg17Al12 precipitate particles are significantly refined within the welding zone (WZ) due to the intense plastic flow after FSSW joining process. The microhardness with WZ region is obviously higher than the base metals. The increasing microhardness is resulted from the grain refining effect during the FSSW joining process. Tensile testing results show that the average welding strength of A9-L9 joints is about 63.7 MPa. However, the average welding strength of A6-L9 joints is generaly higher than 100 MPa. SEM observations for fracture morphologies show that the fracture of dissimilar joints is occurred from the butt-joining interface. Through the statistical analysis of the Weibull distribution function, FSSW-joined A9-L9 and A6-L9 dissimilar Mg alloys with a wear-out failure model are recognized as reliable joints for further engineering application.
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