Friction stir spot welding (FSSW) is one of the solid state welding methods, and it is getting attention as a dissimilar metal joining method which does not require auxiliary materials. We have developed a new dissimilar material joining method—scrubbing refill FSSW (Sc-RFSSW)—, and Sc-RFSSW provides higher joint strength than conventional FSSW methods in joining the aluminum alloy and non-coated mild steel. This new method is based on the concept of rapid formation of a newly formed surface by the interaction of high-hardness tool with the surface of lower plate. In this study, the mechanical properties and metallurgical joining interface of an aluminum alloy and an ultra-high strength steel joint by Sc-RFSSW were investigated. It was found that the joint strength of Sc-RFSSW was higher than that of the conventional RFSSW. In addition, stop action tests were conducted to observe the formation of the newly formed surface of the steel plate, and finally the mechanism of the formation of newly formed surface was discussed.

　In magnetic pulse welding, the joint interface often shows characteristic wavy morphology. The wavy morphology mainly depends on impact angle and impact velocity. However, there is a chance that other factors affect the wavy morphology. In this study, pure Al and pure Cu plates were magnetic pulse welded under the same impact angle and impact velocity but different flyer plate thickness so that we could focus on the effect of kinetic energy. The experimental results showed that thick flyer plate led to a larger wave size. A series of numerical simulation was conducted in order to investigate the wavy interface formation in detail. The simulation results showed good agreement with the experimental result. It was confirmed that the increase in collision energy due to the increase of flyer plate thickness enlarged the reentrant metal jet emission and the local fluid-like deformation of Al and Cu behind the collision point, which resulted in the enlarged wave size.

　Friction stir welding of A1050 pure aluminum and CFRP comprised of PA6 and carbon fiber was carried out. The effects of polishing and silane coupling treatment before welding on bonding strength were investigated. Bonding of CFRP and A1050 were achieved by using a silane coupling agent, irrespective of polishing on an A1050 plate surface. Sound joints were obtained by utilizing a combination of polishing and silane coupling treatment. The fracture of the joints occurred at CFRP base plates during tensile shear tests. Covalent binding should be formed between A1050 and PA6, which leads to high joint strength.

　Friction stir spot welding (FSSW) is one of the solid state welding methods, and it is getting attention as a dissimilar metal joining method. We have developed a new dissimilar material joining method —scrubbing refill FSSW (Sc-RFSSW)—, and this new method is based on the concept of rapid formation of a newly formed surface by the interaction of high-hardness tool with the surface of lower plate. In this study, weld bonding with Sc-RFSSW was performed and joint strength and adhesive discharge process were analyzed. In the strength evaluation, it was suggested that the Sc-RFSSW spot strength did not decrease even if the adhesive was placed between the upper plate and lower plate, and the sufficient strength per welded area was obtained in the Sc-RFSSW joint. In addition, it was shown that the adhesive was sufficiently discharged from the interface.

　The strength of magnesium alloys can be effectively increased by refinement of crystal grain size. This study was made to investigate the strength of Mg-Al-Zn-Ca alloys processed with friction stir processing (FSP), which is a method for refining crystal grains. The relationship between mechanical properties and process conditions was investigated. The grain size became finer and more uniform by FSP. The hardness of the FSPed material tends to decrease slightly compared to that before processing. As for tensile properties, FSP processing decreased the value of 0.2% proof stress, and plastic deformation started at lower stress in FSPed specimens. However, the ductility was improved by FSP. In this study, specimens processed with high-pressure torsion (HPT), was also studied. The grain size of the specimens was significantly refined by HPT, and the relation between hardness and grain size followed the Hall-Petch relation, indicating that grain size reduction is effective in increasing the strength of Mg-Al-Zn-Ca alloys used in this study. The effect of change in crystal orientation on the strength required for plastic deformation is also discussed.

　In this study, characteristics of the metallographic structure of the explosively welded clad material of AZ31 magnesium alloy/A6005C aluminum alloy were investigated. Microstructure observation revealed that the wavy interface including the intermediate phase layer and partially molten metal structure were formed near the interface after explosive welding. The thickness of the interlayer was 1 µm or less, and the presence of γ-Mg₁₇Al₁₂ phase was confirmed. In addition, adiabatic shear bands were observed near the interface at AZ31 side. Crystal grains were refined as 1 µm or less near the interface at AZ31 side.