Papers by Author: Dong Yim Chang

Abstract: The effect of prior T4 heat treatment and subsequent microstructural evolution on tensile behavior of friction stir welded (FSWed) AZ31B-H24 alloy was examined in this study. Selected AZ31B-H24 plates were prior T4 heat treated at 400
for 24 hours and subsequently friction stir welded. The tensile properties, optical micrographs and SEM fractographs for FSWed AZ31B-T4 specimens were compared with those for the H24 counterparts. Prior T4 heat treatment tended to decrease the tensile ductility reduction in FSWed AZ31B-H24 specimen. The tensile ductility reduction mechanism as associated with prior T4 heat treatment is discussed based on detailed micrographic and fractographic observations.

Abstract: The effects of CeCl3-containing fluxes on RE element loss, the high temperature mechanical properties, microstructure and corrosion properties of magnesium alloy containing RE have been studied. The results showed that a certain amount of CeCl3 in purification fluxes restrained the reaction between the Ce element and fluxes to decrease the loss of Ce in alloys. With the flux containing 9% CeCl3, the loss rate of alloy element Ce can be reduced from 26.9% to 3.4%, and the σb and δ of the alloys at 150°C counld be improved from 160.9MPa and 6.2% to 176.5MPa and 7.9% compared with that of conventional MgCl2-containing flux , respectively. Besides, the coarse β phase and rod-like RE phase were refined effectively by CeCl3-containing fluxes.

Abstract: AZ31 magnesium plates with 1mm in thickness were fabricated by casting and subsequent hot rolling process. Both wheel-band continuous casting (modified Properzi) and horizontal continuous casting (HCC) machine were designed and employed to produce AZ31 alloy plates. The plates with 5x50mm in size produced by modified Properzi process exhibited good surface qualities that were sustained during following rolling process. Surface qualities of HCC plates with 120x30mm in size were very sensitive depending on the processing parameters. Homogenization treatment or surface machining before rolling was effective to get good thin plate. By means of rolling process, microstructures were refined effectively to the size of 7~10µm.

Abstract: In this study the effects of Ca addition on the rollability, microstructures and tensile properties of AZ31 alloy were evaluated. In Ca containing alloys, reduction ratio and preheating temperature are the most important variables to make good sheet without any crack. The number and length of side crack increased with increasing Ca content. With increasing the reduction ratio per pass and decreasing the preheating temperature, crack initiates easily at the side. The dendritic grains were changed to fine equiaxed grains through hot rolling by dynamic recrystallization. The Al2Ca precipitates which were segregated to grain boundaries during solidification were rearranged parallel to the rolling direction. The tensile properties of hot-rolled AZ31-xCa sheets were decreased slightly by addition of Ca.