The characteristics of the application of the friction stir spot welding process to a keyhole which was produced at the friction stir spot welding, to refill the hole and make it disappear, were investigated. This paper focused on the material flow caused by a rotational tool, using a scrolled groove tool without a probe that could produce a material flow toward the rotating centre. The keyholes in lap joints of aluminium alloy disappeared when the FSSW conditions were a tool rotational speed of 1500 rpm, a plunging depth of 0.7 mm and a dwell time of 4 s, although gaps in the bottom of the keyhole was observed in some of them. From the observation results of the material flow conditions in this case, to eliminate the non-refill space and to transmit the rotary torque to the stir zone, it is suggested that it is important that the material flow to refill a keyhole and the deformation of the keyhole edges reach to the bottom of the keyhole at approximately the same time.
Joint microstructures, mechanical properties and fatigue behavior of ferritic stainless steel SUS430 welds with different filler metals were investigated. Two filler metals with different chemical compositions, Filler I and II, had been used for the MIG butt welding of SUS430. Filler I showed coarser grains in the microstructure than Filler II including Al and Ti. In both welded joints, the hardness of heat affected zone (HAZ) was the highest due to the formation of hard needle-like martensite. The welding heat input of Filler I welded joint was higher than that of Filler II. Consequently, martensite content of Filler I was more numerous and bigger than the Filler II. Fully reversed axial fatigue tests had been performed using smooth specimens of welded joints and base-metal specimens at ambient temperature in laboratory air. In both welded joints, fatigue fracture took place at base metal region due to the higher hardness in weld metals and HAZ regions than the base metal.
In place of the 18%Cr overlay welding material, which has been used for furnace water wall tubes of recovery boilers, a 25%Cr overlay welding material with good corrosion resistance has been newly developed. In consideration of dilution with a carbon steel tube, the Cr content of the new welding materials are designed to be more than 20%Cr. And in order to fine the grains of the weld metal, some elements, such as Nb, Al and Ti, are added to these welding materials. Mainly two types of welding materials, which have Austenite + Martensite + Ferrite of materials for the three-phase and fully ferritic materials, are examined. As results of weldability tests and bending properties, the fully ferritic welding materials are superior to the three-phase materials, respectively. On the point of the corrosion test, which are exposed into some smelt ashes obtained from the recovery boilers, the corrosion rate of the full ferritic material was confirmed to be the about 1/2 of that of 18%Cr overlay welding material. The 25%Cr overlay welding material was found to exhibit excellent corrosion resistance. This study introduces the development of 25%Cr ferritic welding material and methods for reliable overlay welding and examines the weldability, the bend property and the corrosion resistance of the 25%Cr overlay welded metal on tube.
Ductility-dip cracking susceptibility in the laser multipass weld metal of alloy 690 was quantitatively evaluated using different filler metals varying the contents of P and S. In order to synthesize a ductility-dip crack in the laser multipass weld metal, the cross-bead longitudinal-Varestraint test with laser welding was applied under the various welding speed. The ductility-dip temperature range (DTR) was increased with an increase in P and S contents in the weld metal, and S was more effective to enhancing the DTR compared with P. Furthermore, the DTR was decreased with an increase in the welding speed. A numerical simulation of grain boundary segregation of P and S revealed that the segregated concentrations of P and S at grain boundaries were increased with an increase in the P and S contents in the weld metal, and that those were slightly decreased with an increase in the welding speed due to the rapid heating and cooling rates. It follows that the changes in ductility-dip cracking susceptibility with the filler metal composition and welding speed were consistent with the grain boundary segregation behaviours of P and S during laser multipass welding process. According to the regression analysis of the DTR, ductility-dip cracking in the laser multipass weld metal would be inhibited by reducing the segregated concentration at grain boundary (P+2.42S) to less than approx. 8at%.
The effect of nitrogen on the solidification cracking susceptibility of austenitic stainless steels (SUS316L and SUS310S) was quantitatively evaluated by transverse-Varestraint test. The amount of nitrogen in Ar shielding gas during GTA melt-run welding was varied in order to vary the nitrogen content in the weld metals. The solidification brittle temperature range (BTR) of SUS316L was enlarged with an increase in the nitrogen content in the weld metal, while that of SUS310S was very slightly increased with an increase in the nitrogen content. Numerical analysis of solidification segregation during welding revealed that the solid-liquid coexistence temperature range of SUS316L was enlarged with an increase in the nitrogen content in the weld metal, whereas that of SUS310S was almost constant regardless of the nitrogen content. A nitrogen itself essentially had a negligible effect on the solidification cracking susceptibility of fully-austenitic stainless steels, because a nitrogen neither solidification-segregated in the weld metal nor reduced the solidified temperature during welding. However, a nitrogen increased the solidification cracking susceptibility of austenitic stainless steels with two-phase solidification, because the solidification segregation of P and S was promoted by reducing the amount of δ-ferrite in the weld metal.
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