Papers by Author: Yun Mo Yeon

Abstract: The fatigue properties of friction stir (FS) and TIG welded Al 6N01-T5 alloys were compared in the present study. The low cycle fatigue (LCF) test was performed under total strain amplitudes in the range of ± 0.6 ~ ± 1.5% and with a strain rate of 3 x 10-3/s. During low cycle fatigue, the base metal showed little cyclic hardening or softening, whereas both the FS and TIG welded zones showed a large amount of cyclic hardening until fracture, although the fatigue life of the TIG welded zone was much shorter than that of the FS welded zone. The fatigue crack growth rate (FCGR) was retarded in the heat affected zone (HAZ) on the advancing side of the FS welded zone, due to the compressive residual stress in this region. The differences in the fatigue properties in these two welded zones were discussed in terms of the microstructural changes during fatigue.

Abstract: The grain growth behavior and mechanical properties in the friction stir weld zone after post weld heat treatment (PWHT) have been investigated. As PWHT temperature increased, a normal grain growth of as-welded equaxied grains ceased and abnormally grown grains with elongated shape coarsened. Huge elongated grains changed into smaller equaxied grains at 500°C. In case of lower heat input condition, abnormal grain growth initiated faster due to smaller initial grain size. The weld zone with bigger initial grains had advantages to maintain the thermal stability at high temperature. The hardness near the weld zone was almost recovered to the 95% of the unaffected base metal at 500 °C and the weld zone under lower heat input condition resulted in the homogeneous recovery through the whole weld zone.

Abstract: The microstructure and mechanical properties of spot friction stir welded A 5052 alloy were investigated with insertion depth of welding tool. As the insertion depth of welding tool increased, the size of stirring zone increased and the thickness of upper sheet decreased. The value of shear load was the lowest at the shallowest insertion depth and increased to the highest value of 3.35 kN at a 1.6mm of insertion depth. An increase in the pin insertion depth beyond 1.6mm did not result in further increase in the lap shear load. Spot friction stir welded joints showed shear fracture mode at shallower insertion depths and fracture mode changed to plug fracture mode as the insertion depth was deeper.

Abstract: Friction stir welding of dissimilar formed Mg alloys(AZ31/AZ91) was successfully carried out at the limited welding conditions. In a sound joint, SZ was mainly consisted of AZ31 Mg alloy which was located the retreating side. Dynamic recrystallization and grain growth occurred and β intermetallic compounds of AZ 91 Mg alloy was not observed in SZ. BM had a higher hardness than that of the weld zone. The fracture location was not weld zone but BM of the AZ91 Mg alloy in tensile test.

Abstract: Friction stir welding of similar Mg-Zn-Y alloy was successfully carried out under various welding conditions. The external shape of the joints showed good quality and the weld zone perpendicular to the welding direction didn’t also include any defects. In the weld zone, the strengthening particles were homogeneously dispersed in the Mg matrix. The morphology of these particles was slightly varied with the welding speed and with the location such as SZ, TMAZ, HAZ and BM. The shear band caused by the severe deformation was observed in the TMAZ. SZ had a higher hardness than that of the base metal. Though UTS of the joints slightly varied with welding speed, it reached the equal value with that of BM.

Abstract: The hardness distribution related to the precipitates behaviors as friction stir welded and PWHT (post weld heat treated) 6061 Al alloy have been investigated. Frictional heat and plastic flow during friction stir welding created a fine, eqiuaxed and elongated microstructure near the weld zone due to dynamic recovery and recrystallization. A softened region which had been formed near the weld zone couldn't be avoidable due to the dissolution and coarsening of the strengthening precipitates. PWHT (SHT+ Aging) homogeneously recovered the hardness distribution over that of the base metal without softening region, resulted from non-homogeneously distributed hardness only aging treated. 36ks aging followed by SHT gave a higher hardness overall weld than that of the base metal due to a higher density of the spherical shaped precipitate.;