Papers by Author: Tae Kyo Han

Abstract: The characteristics of Nd:YAG laser welded 600MPa grade TRIP(transformation induced plasticity) and DP(dual phase) steels with respect to hardness, microstructures, mechanical properties and formability was investigated. A shielding gas was not used, and bead-on-plate welding was performed using various welding speeds at a power of 3.5kW. In the case of TRIP steel, the hardness was the highest at the fusion zone(FZ) and HAZ near the FZ and decreased as the base metal was approached. The maximum hardness increased with increasing welding speed to 3.6m/min and then remained. The microstructures of FZ and HAZ near the FZ consisted of ferrite and bainite for all welding speeds. In the case of DP steel, the maximum hardness was obtained at the HAZ near the FZ. It increased rapidly to 2.1m/min and then showed nearly the same value. The difference between the maximum hardness of HAZ and that of FZ increased with decreasing welding speed. The microstructure of FZ was composed of acicular ferrite but the HAZ near the FZ contained bainite and ferrite at a low welding speed. Both steels had similar tensile properties and formabilities. In a perpendicular tensile test to the weld line, all specimens were fractured at the base metal, and the strengths were somewhat higher than those of raw metals. In a parallel tensile test, the strengths of the joints were higher than those of raw materials but the elongations were lower. Formability was determined to be approximately 80% as compared with raw material at the optimum conditions. Differences in hardness near the welded zone was dependant on the relative contents of ferrite.

Abstract: In the case of transient liquid phase diffusion bonding with Ni base superalloy GTD-111, the bonding temperature was sustained at 1403K ~ 1453K. Thus, the microstructure of specimens heated at 1403K ~ 1453K was examined. In the raw material, γ-γ' eutectic phases, platelet η phases, MC carbide and PFZ were clearly observed in interdendritic regions or near the grain boundary and the size of primary γ' precipitates near the interdendritic regions were larger than the core. The primary γ' precipitated in the dendrite core dissolved early in the bonding process. γ' precipitated near the interdendritic regions were partially solubilized and their shape was changed. The dissolution rate increased with increasing temperature. Phases in the interdendritic regions or near the grain boundary changed continuously with time at the bonding temperature. At a bonding temperature of 1403K, the eutectic phases remained, but η phases were transformed from a platelet shape to a needle morphology and the PFZ region widened with time. The interdendritic region and near the grain boundary became partially liquid at 1423K and fully at 1453K by the reaction of η phases and PFZ. The interdendritic region and near grain the boundary became liquid and new phases which were mixed with η phases, PFZ and MC carbide crystallized during cooling at 1453K. Crystalline η phases were transformed from a rod shape to a platelet shape with increasing holding time.

Abstract: The effect of a mixed powder on the wide gap transient liquid phase diffusion bonding of a directionally solidified Ni base superalloy, GTD-111 was investigated. The mixed powder consisted of a mixture of a powdered Ni base filler (GNi-3) and powdered base metal (GTD-111). The range of the base metal powder was 40 to 70wt%. Bonding was performed at a temperature of 1463K, using various holding time. In the case of a lower 50wt%, the base metal powders completely melted and base metal mating at the interface dissolved at an early time, and extent of dissolution of base metal decreased with increasing mixing ratio. Liquid was eliminated by isothermal solidification, which was controlled by the diffusion of B into the base metal. The solids in the bonded interlayer grew epitaxially from the mating base metal inward from the insert metal and the number of grain boundaries formed at the bonded interlayer corresponded with those of the base metal. The finishing time for isothermal solidification was about 74ks. In the case 60wt% and higher, the base metal powders partially melted and remained in the vicinity of bonded interlayer. The solid was formed from the remaining powder and base metal mating at the interface. Finally, the bonded interlayer underwent the poly-crystallization when isothermal solidification was complete. The contents of Al and Ti in the bonded interlayer with a holding of 74ks were equal to that of the base metal.

Abstract: The effect of welding conditions on defects, microstructure, mechanical properties and formability of CO₂welded 800MPa grade TRIP steel was investigated. Bead-on-plate welding was carried out under the various welding speed in 6kW with a shield gas of Helium. Extensive porosity was observed in the joints produced at a 4m/min of welding speed but was not observed in the joints when a welding speed greater than 6m/min was used. The maximum hardness was obtained at the weld metal and the HAZ near the weld metal and was the same regardless of the welding speed. The HAZ hardness near the base metal was reduced due to retained ferrite but was higher than that of the base metal. In a parallel tensile test to weld axis, the strength of the joints was higher than that of the base metal. Elongation was found to be lower than that of the raw material because a crack was initiated in the bead at the strength levels corresponding to the tensile strength of the base metal and was propagated perpendicular to tensile direction. The formability of joints was found to be 63% that of the raw material.