Magnesium alloys have many attractive properties, such as low density and high specific strength and rigidity. It is predicted that the applications of magnesium alloys will grow rapidly in the near future, especially in the automotive industry. In this thesis, several new joining techniques were developed on magnesium alloy sheets for automobile body construction.
Friction stir welding (FSW) was carried out on AZ91D magnesium alloy sheets. The influence of the process parameters on the FSW weld quality and microstructural development, as well as changes mechanical properties were investigated. Sound weldments was obtained at a rotating speed of 1380r/min while a rotating speed of 1960r/min was found too high. The microstructures of the zones in a weldment showed very different features depending on the thermal and mechanical conditions. The hardness and strength of the stirring zone were remarkably improved due to the fine recrystallized grain structure. The mechanical properties of FSW weldments were found superior to those of TIG (tungsten inert gas welding) weldments, and they largely depended on heat input during the welding processing.
Resistance spot welding (RSW) was carried out on AZ31B and AZ91D magnesium alloy sheets. The influences of process parameters on weld nugget width and weld defects were investigated. A signigicant amount of defects in RSW nuggets of AZ31B and AZ91D sheets were observed. Liquation cracking was widely spread in RSW joints of AZ31B, and expulsion prevailed during RSW of AZ91D.
The influence of tools and female dies in friction stir spot welding (FSSW) on metal flow patterns and the widths of fusion zone were investigated in the experiment. With probes of suitable height and left-handed screw threads the width of fusion zone could reach up 2mm, and maximal tensile-shear strength 4.6kN.
Rivets used in friction stir riveting (FSR) were designed and machined, and the process was investigated preliminarily. The fusion zones of friction riveting jionts were very narrow, and maximal tensile-shear strength was only about 3.4kN. The fusion zones of FSR jionts were wider after the shape of rivets was optimized, and the fusion zone can reach up to 1mm in width.
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