Papers by Author: Cha Yong Lim

Abstract: The 3-layers accumulative roll bonding process (ARB) has been attempted to increase the strength of copper alloy (Cu-0.02wt.%P) by refining grain size. The 3-layers accumulative roll bonding was conducted up to 7 cycles at room temperature without lubrication. Microstructural evolution of the copper alloy with the number of the 3-layers ARB cycles was investigated by optical microscopy (OM), transmission electron microscopy (TEM), and electron back scatter diffraction (EBSD). The average grain size has been refined from 20 μm before ARB to 170 nm after 7 cycles of 3-layers ARB. More than 70% of ultrafine grains formed by 3-layers ARB were composed of high angle grain boundaries. The average misorientation angle of ultrafine grains was 30.7 degrees in the center of the specimen. Tensile strength after 7 cycles of 3-layers ARB was 605 MPa, which is about 3.2 times higher than the initial value.

Abstract: High strength Al-Mg alloy strips with high Mg contents (5-10wt%Mg) were successfully fabricated by twin roll casting. In order to get a good surface quality of Al-Mg strips, an optimum process condition was investigated in this experiment. The morphology of the cast nozzle and the roll separate force during twin roll casting was important to improve the surface quality of the strip and homogeneity of the cast structure through the thickness. The size of intermetallic particle like Al-Fe compounds was reduced down to 1~2m due to a high cooling rate of Al melt during strip casting. In addition, the dendrite structure was very fine and the segregation of Al8Mg5 phase between grains was remarkably reduced. Therefore, the Al-Mg strips have good workability during additional cold/warm rolling processes. After annealing, the rolled sheets have superior tensile properties to a commercial high strength Al-Mg alloy sheet.