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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