In welding 6061-T6 aluminum alloy, softening caused by the dissolution of strengthening ␤ Љ (Mg 2 Si) precipitates occurs in heat-affected zones (HAZs). Laser beam welding is advantageous in view of narrower softened regions. The width of the softened region in a laser beam weld with a welding speed of 133 mm/s is 1/7 that of a tungsten inert gas (TIG) weld with a speed of 5 mm/s. The hardness distributions and width of softened regions in the HAZ have been quantitatively predicted to characterize the laser beam welding process. To this end, a kinetic equation describing the dissolution of age precipitates has been established and has been applied to 6061-T6 aluminum weldments. The hardness profiles and the width of softened zones have been successfully predicted in both welding processes. Prediction of the width of softened regions with varying power inputs and welding speeds reveals that a high energy density and a high welding speed in laser beam welding result in significantly narrower softened regions, in which the width is insensitive to variations in welding parameters compared to that of TIG welding.

Dissimilar material joining of polyamide 6 and polyethylene plates to a plain carbon steel (SPCC) plate was performed using friction lap joining. The polyamide 6 and SPCC plates could be directly joined by friction lap joining, whereas the polyethylene and SPCC plates could not. Corona discharge treatment of the polyethylene surface enabled the joint formation with SPCC. The tensile shear fracture load of the SPCC/polyamide 6 and SPCC/corona-discharge-treated polyethylene joints increased with the joining speed up to 600 mm min − 1 , beyond which it decreased. These joints were fractured at the base material of the plastic plate at optimal joining speeds in the tensile shear test. Continuously joined interfaces of these materials were observed via cross-sectional microstructure analysis. Transmission electron microscopy and selected area diffraction patterns indicated that these materials were joined through the surface oxide layer of SPCC composed of Fe 3 O 4 . The relation between the tensile shear fracture loads and the results of XPS analysis indicated that polar groups such as amide, hydroxyl, and carboxyl groups on the plastic surfaces were highly effective for joint formation of the SPCC/plastic joints.

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The Ni-Fe alloy consumable that applies to the welding of 9% nickel steel for LNG storage tanks is highly susceptible to solidification cracks. To evaluate susceptibilities such as the brittleness temperature range (BTR), a Trans-Varestraint test is generally conducted. However, it is difficult to evaluate the minimum strain value for the BTR and the real temperature at both ends of a solidification crack in a conventional Trans-Varestraint test because these values are measured indirectly. In this study, we propose determining the temperature range by conducting in-situ observations during a Trans-Varestraint test using a highspeed camera and two-color pyrometry so that the temperature range can be measured directly from the temperatures at both ends of the crack. Furthermore, we measured the augmented strain from the time elapsed since the initiation of bending. This method allowed us to successfully measure the augmented strain and the temperature range in the Trans-Varestraint test and to determine the BTR more accurately.