In this study, the effect of titanium on the solidification brittle temperature range (BTR) of alloy 617 was evaluated, and the mechanism by which the eutectic reaction of titanium carbide influences the solidification cracking susceptibility of alloy 617 was clarified. The BTR was evaluated by the Varestraint test. Initially, the BTR decreased with increasing titanium content, reaching its minimum at 1.4% Ti, but further increased with additional Ti. Microstructural analysis of the quenched solidification microstructure of weld metal revealed the crystallization of Ti carbides during solidification. To elucidate the mechanism by which Ti carbide formation during solidification influences the solidification cracking susceptibility of Alloy 617, a theoretical approach was conducted to understand the solidification brittle temperature range of Alloy 617. Ti carbide formation in the terminal stage of solidification reduced C concentration in the residual liquid and increased the solidification completion temperature. Excessive Ti addition causes an increase in Ti concentration in the terminal stage of solidification, which increases the BTR. Therefore, the existence of an appropriate range for the improvement of BTR by eutectic reaction of carbide using Ti was clarified. For the alloy 617 used in this study, the solidification cracking susceptibility was minimized when the Ti/C mass% ratio was around 30.

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Ultrasonic bonding process of A6061 alloys is investigated focusing on the growth of micro-bonds initially formed on the bonding interface. The interfacial condition is controlled by partially anodic oxidizing to the surface. The growth of the micro-bonds locally formed in the initial stage of joining in the absence of an anodic oxide film is considered from the viewpoint of the relative motions of the bonding materials and the shear deformation caused by the relative motions. Dispersed initial bonds formed on the non-anodized surfaces hinder the relative motion between the bonding materials. By applying partial anodic oxidizing, the formation of the initial bonds is concentrated in the non-anodized areas. The localized bonds expand with shear deformation and develop into plastic flow, leading to the increase of joint strength. The shear force applied by the ultrasonic horn tip and anvil contributes to the plastic deformation in the later stages of joining. It is revealed that　the formation of the localized initial bonds by means of the partial anodic oxidizing promotes the relative motion of the bonding materials and the resultant growth of the initial bonds. Localization of the initial bonds can be an effective means of increasing joint strength and reducing joint strength variation.

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Friction stir welding (FSW) performed by the simultaneous and reverse rotation of a pair of tools with pre-heating process by means of high frequency induction heating, namely pre-heated double-sided FSW, was studied in an attempt to enhance the travel speed for welding of automotive advanced high strength steels (AHSS). The experiments were conducted on butt joint of 1.6 mm thick 1180 N/mm2 grade AHSS sheets. The maximum travel speed free from weld incompletion was 4.0 m/min without pre-heating and increased to 6.0 m/min with pre-heating, fairly comparable with laser beam welding, one of the conventional automotive welding processes. According to the metallographic observation and mechanical testing of experimental welds of double-sided FSW without pre-heating, it is considered that the weld needs to be heated homogeneously minimizing the thermal gradient at high temperature where the yield strength is sufficiently low in order to promote plastic flow and extend stir zone, which results in the soundness of weld quality. When the travel speed was higher and/or the tool rotation speed was higher than appropriate welding conditions, a thermal gradient developed across the thickness direction in the weld and the extension of stir zone was inhibited; then the weld became incomplete. On the other hand, when the pre-heating with high frequency induction was applied, temperature in the weld raised homogeneously prior to welding, which can assist to minimize the thermal gradient in the weld; thus the plastic flow was conceivably promoted extending the stir zone, leading to the enhancement of travel speed.

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