A study of joining mechanism on Al Alloy A6061-T6 Friction Stir Spot Welds

• Main Authors: Ju-Jen Liu, 劉如真
• Other Authors: Yuan-Ching Lin
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/69945937789904205260

博士 === 國立臺灣科技大學 === 機械工程系 === 100 === Friction stir spot welding (FSSW) is a derivative process of the friction stir welding (FSW), which is a solid-state joining technique, due to the process temperature below the melting point, so it therefore does not exit porosity, solidification cracking and other defects generated by traditional fusion welding. It is especially suitable for magnesium, aluminum and other light metal alloys. Results showed that, the construction of the transient constrained space is a prerequisite to the formation of a stirring area and the bonding of the welding work pieces. Friction stir spot welding (FSSW) is driven mainly by the materials flow induced by the stirring tool in a constrained space that gradually diminishes the contact interface between the upper and lower plates leaving them bonded to each other. The stirring tool has a decisive influence on the flowing behavior of the plastic and the bonding strength of the welding work pieces. During the FSSW period, except for the traction force of the tool that can directly cause the flow of plastic materials, the pressure gradient generated in the constrained space by the tool geometry that causes the axial or radial flow of the materials and the formation of a stir zone are even more key factors. The formation and growth of the stir zone will influence the bonding behavior of the plates and the effective thickness reduction trend of the upper plate. This will further influence the mechanical strength of the welding work pieces. Therefore, the appropriate design of the stirring tool can greatly improve the bonding performance of the welding work pieces. In addition, appropriate process parameters are also a prerequisite to the formation of well-welded work pieces. The triangular tool pin and the threaded cylindrical tool pin will form stir zone caused by the different plastic flow mechanisms. The threaded cylindrical tool pin has a higher performance than the smooth triangular tool pin when driving the axial flow of materials. However, the triangular tool pin has a higher performance than the threaded cylindrical tool pin when driving the radial flow. Therefore, the nuggets of welding working pieces of the two types are totally different in their geometric features. Because of the geometrical differences of the hook, its tensile shear failure modes and strength also have significant differences. The welding work pieces made by the process using the threaded cylindrical tool pin has a slightly higher tensile and shear strengths than those made by the process using the triangular tool pin. This study established the FSSW materials flow modes using different geometrical tools as well as observations from the experiments. It has also used tracer techniques to elaborate on the influences on the flow of materials and the bonding effect of the plates induced by the shoulder of the stirring tool and the geometrical shapes of the tool pin. The formation mechanism of the stirring zone was presented and the effects of the welding parameters on the welding work piece failure modes and their bonding strength were also discussed. This was done in the hope of providing more useful references for the FSSW tool design.