Pear-shaped bead cracking occurs in carbon steel when P/W, the ratio of penetration depth to bead width, increases. Pear-shaped bead cracking is a type of solidification cracking. In a previous study, we achieved the reduction of BTR in carbon steel by appropriate Ti addition. However, there are very few studies that pear-shaped bead cracking of carbon steel was prevented by reducing BTR. In this study, the effect of BTR reduction by Ti addition on the prevention of pear-shaped bead cracking was investigated. Narrow gap GMA welding was carried out using 0.15%C as the standard material and 0.15%C-0.95%Ti as the countermeasure material. In the weld metal microstructure of 0.15%C-0.95%Ti, there was a formation phase at dendrite boundaries and columnar grain boundaries. According to the EPMA analysis results, Ti-based oxides and MnS were formed in 0.15%C and in addition to these, TiC was formed in 0.15%C-0.95%Ti. The results of the theoretical investigation of BTR indicated that BTR was significantly reduced by Ti addition when Ti content was reduced to 0.74% in 0.15%C-0.95%Ti. Weld cracking was observed in the center of the weld metal in 0.15%C, but not in 0.15%C-0.95%Ti. Observation of the fracture surface of 0.15%C indicated that the cracking was solidification cracking. The occurrence of solidification cracking was determined by the weld shape ratio (P/W ratio). The solidification cracking at 0.15%C can be uniformly determined by the P/W ratio, and the cracking occurs when the P/W ratio exceeds 1.0. On the other hand, in 0.15%C-0.95%Ti, no solidification cracking occurred even when the P/W ratio exceeded 1.0. In other words, it was found that pear-shaped bead cracking (solidification cracking) of carbon steel can be prevented by reducing BTR.

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This paper investigates the wear of friction stir spot welding tools, focusing on the shape change of each tool part as wear loss. In this study, an accelerated wear test using a metal matrix composite was carried out, because a wear test primarily generates only mechanical wear. When the shape changes of each part of the tool obtained from the results of the wear test are considered as wear loss, in all parts of the friction stir spot welding tool, the tool life was able to be evaluated using the Taylor’s life equation for cutting tools. The tensile-shear strength of A5052/A6061 friction stir spot lap joints made using a worn tool in the final stage of the wear test decreased to about 70 % of the strength when made using a non-worn tool. From the observation results in a cross section of friction stir spot welds, the reason for the decrease in tensile-shear strength of A5052/A6061 friction stir spot lap joints made using a worn tool is suggested to be due to the change in the shape of the plastic flow region, especially in the lower sheet, resulting in a decrease in the shear area and the formation of a stress concentration area due to the shape change of the tool. Since the results of the estimation of tool life using the Taylor's life equation and the life curve based on joint strength are around the same, it is suggested that the shape changes in the probe edge, which are considered to have a significant effect on the plastic flow, are appropriate for the evaluation of tool life.

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The solidification microstructure of the weld metal greatly affects the material properties. The process of solidification microstructure formation is closely related to the occurrence of solidification cracking. Therefore, it is important to predict the solidification microstructure of the weld metal. Since the solidification microstructure of the weld metal is affected not only by the microstructure formation associated with melting and solidification but also by the grain growth in the heat-affected zone, this study uses a Monte Carlo method to predict the formation of microstructure not only in the weld metal but also in the weld zone. The aim of this study is to reproduce the microstructure of the weld zone, especially the solidification microstructure of the weld metal, for broad range of welding conditions that have not been investigated. The calculated weld bead shapes under each welding condition were in good agreement with the experimental results. The relationship between Monte Carlo step (MCS) and real time was clarified from the results of the experimental investigation of grain growth of SUS310S and the grain growth calculation by the Monte Carlo method. The microstructure morphology of the weld zone was reproduced by the Monte Carlo method. The calculated solidification microstructure of the weld metal was compared with the experimental results by EBSD analysis, and the calculated and experimental results were in good agreement. In conclusion, it was shown that the coupled prediction method developed in this study can predict the weld bead shape and the microstructure morphology of the weld zone, especially the solidification microstructure of the weld metal.

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The automotive industry is striving to improve the fuel efficiency of internal combustion engine vehicles and extend the possible cruising range of electric vehicles, and reducing the weight of the vehicle body is an important issue. Aluminum alloys are being applied to panels and structural members as a means of reducing vehicle body weight. In addition, technologies have been developed to die-cast large parts in batches to achieve high composites and a reduction in the number of parts. It is required to join aluminum castings and rolled material by resistance spot welding, which is the most widely used joining technique in the automotive industry. However, aluminum castings have poor resistance spot weldability, and when joined with aluminum rolled material, there may be no penetration into the rolled material. In this study, the joint strength of resistance spot welded joints between castings and rolled aluminum alloy without penetration into the rolled aluminum alloy was measured, and the joining principle and strength development mechanism were investigated. The joint strength was measured by tensile shear test and cross-tension test, and the fracture morphology was observed. It was confirmed that the strength of the joint was comparable to that of spot-welded joints of rolled aluminum. Furthermore, cross-sectional observation and interfacial microstructural analysis revealed diffusion bonding between the molten α-Al of the casting aluminum alloy and the non-molten rolled aluminum alloy.

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