Papers by Author: Suk Bong Kang

Abstract: Horizontal continuous casting (HCC) Al-18%Si alloy billets were prepared with the addition of fine-grained Al-18%Si as a master alloy. The influence of cooling conditions and this fine-grained structural materials addition on the microstructure and tensile properties were studied. The uniformity of the HCC Al-18%Si alloy billets was also analyzed. The results denote that the HCC Al-18% Si alloy billets showed finer microstructure and better mechanical properties after double cooling. Fine-grained structural material addition can effectively reduce macrosegregation and make both primary and eutectic silicon much finer. The ultimate tensile strength clearly increased with an addition of 15 percent fine-grained structural material compared to the HCC alloy with no addition of fine-grained structural material. However, a small decrease occurred after an addition of fine-grained structural material up to 30 percent.

Abstract: Aluminum alloys are commonly used as a material for heat exchangers due to their higher thermal conductivity and specific strength among various metallic materials. Twin roll strip casting process is considered to produce the high quality and low manufacturing cost aluminum alloy fin stock for automobile heat exchangers. Thermomechamical treatment has carried out to obtain optimum processes for initial cold rolling, intermediate annealing and final cold rolling, which can meet the requirements for high strength and high thermal conductivity after brazing heat treatment. Additionally the effect of copper element variation was considered to determine the optimum content of copper element in Al-Zn-Mn-Si-Fe-Cu based alloys produced by twin roll strip casting process. Mechanical properties and thermal condutivity have been evaluated before and after simulated brazing process. The nuclei of recrystallization might be formed along shear deformation bands during initial cold rolling and should be grown during intermediate annealing to enhance the permeation of molten brazings for the following brazing process. Final cold rolling has allowed strain hardening and controlling of sagging amount as fin stock materials of heat exchanger. In the present study the suitable thermomechnical treatment and optimum copper content was suggested to balance the properties of strength, thermal conductivity, brazing behaviour, corrosion resistance and sagging resistance in Al-Zn-Mn-Si-Fe-Cu based alloys produced by twin roll strip casting process.

Abstract: Evolution of texture and microstructure during deep drawing of ZK60 wrought magnesium alloys were investigated using EBSD (electron backscatter diffraction). ZK60 (Mg-5.5Zn-0.6Zr) alloys were prepared through both direct chill casting (DC) and twin-roll strip casting (TRC). Deep drawing tests were carried out under various working temperature and drawing speeds. The diameters of the sheets and punch were 74 mm and 37 mm, and overall limit drawing ratio was 2. Texture and microstructure evolution was examined with various working condition. The DC samples had larger grains than TRC samples, and thus more twins were observed in the DC samples.

Abstract: Texture and microstructure evolutions during deep drawing of AM31 magnesium alloys were investigated at various temperatures and deformation rates. Two different types of sheets were fabricated by twin roll strip and conventional ingot casting. They were warm-rolled down to 0.6mm and then fully-annealed for deep drawing. Drawing temperatures were 200oC to 350oC and punch rates, 30mm/min, 40mm/min, and 50mm/min. The blank size and punch diameter were 74mm and 37mm, respectively, and thus overall maximum drawing ratio was 2.0. Processing maps for deep drawing of both sheets at elevated temperatures were suggested. Initial textures showed typical basal fibers with an axisymmetric arrangement. Sheets made by ingot casting had larger grain size than those by twin roll casting. The basal fibers were evolved into other orientations during deep drawing, which contained both compression along the circumferential direction in the flange and tension along the drawing direction in the cup wall. Most evident reorientations were found in the flange. With deformation, finer grains increased. Necking and cup-failure were usually expected in the lower wall near the bottom.

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.

Abstract: Aluminum alloys are commonly used as a material for heat exchangers due to their higher thermal conductivity and specific strength among various metallic materials. The lightweightening heat exchangers for automobile application are requisite for reducing the evolution of CO2 and improving the efficiency of fuel. The twin roll strip casting process is considered to produce the high quality and low manufacturing cost aluminum alloy fin stock for automobile heat exchangers. Thermomechamical treatment has carried out to obtain optimum processes for initial cold rolling, intermediate annealing and final cold rolling, which can meet the requirements for high strength and high thermal conductivity after brazing heat treatment. Mechanical properties and thermal condutivity have been evaluated before and after simulated brazing process. The nuclei of recrystallization might be formed along shear deformation bands during initial cold rolling and should be grown during intermediate annealing to enhance the permeation of molten brazings for the following brazing process. Final cold rolling has allowed strain hardening and controlling of sagging amount as fin stock materials of heat exchanger. In the present study the suitable thermomechnical treatment was suggested to balance the properties of strength, thermal conductivity, brazing behavior and sagging in Al-Fe-Mn-Si-Zn based alloys produced by twin roll strip casting process.

Abstract: The microstructure of a twin roll cast (designated as TRC in short) and warm rolled Mg-4.5Al-1.0Zn (designated as AZ41 in short) alloy was investigated using OM and TEM after annealing treatment at the temperature range of 300oC~400oC. Tensile test was performed to show the influence of the annealing treatment on mechanical properties. The microstructure of AZ41 alloy sheet consisted of fibrous structure of elongated grains and shear bands along the rolling direction. Static recrystallization (SRX) was observed at and above 300oC during annealing treatment process. Shear bands, dislocations and twins play an important role during SRX and serve as nucleation sites, this leads to grain refinement. The sheet had higher strength and lower elongation after warm rolling. However annealing treatment after warm rolling induced the decrease of strength and increase of elongation. This results in the balance of strength and elongation in AZ41 alloy sheet. Annealing treatment at 400oC for 40min can be considered to be the optimum annealing treatment, and at this condition the tensile strength, yield strength and elongation are 375MPa, 285MPa and 17.7%, respectively.

Abstract: SiCp reinforced copper matrix composites with the reinforcement content of 30-50vol. % were fabricated by hot pressing using Cu-coated and uncoated SiC powder. And the microstructure and electrical conductivity of the composites were also studied. The results showed that with the increasing of SiCp particle size, the electrical conductivity of the composites also increased. And the oxides in the composites can decrease the electrical conductivity of the composites obviously. The electrical conducting property of the composites can be improved by the copper coating layer and suitable annealing treatment. It provided important data for the application of SiCp/Cu composites as electronic packaging materials.

Abstract: Using two-dimensional Eulerian formulations coupling viscoplastic flow and heat transfer, the behaviors of aluminum alloys and stainless steel during FSW were overviewed. The plastic behaviors of the materials are complicated and the flow stresses are depending on deformation rate, temperature and deformation histories. Constitutive equations considering both strain hardening from accumulation of crystal defects and softening from recovery or recrystallization were used to model the materials. Strain hardening is incorporated with a strength that evolves with deformation rate and temperature along streamlines in the flow field. Strength evolutions have a Voce-like saturation limit because of the severe plastic deformation during FSW process. The model equations for kinematic and temperature were solved using the standard finite element method. The evolution equation for the strength is integrated along streamlines. The strength and temperature distribution vary with process conditions and constitutive equations. Stainless steel and AA6061 have different strengthening mechanisms. Modified constitutive equations were applied to reflect microstructural features of each material.

Abstract: The sheet of ZK60 alloy with a thickness of 1mm was prepared from a casting ingot followed by homogenization and warm-rolling. Variations in microstructure and mechanical properties of ZK60 alloy sheets were investigated during T6 treatment. Especially artificial aging after solution heat treatment affected both precipitates distribution and mechanical properties with aging treatment. Variations of mechanical properties were related to precipitates, i.e. rod-shaped ( 1 β ′ ) or disc shaped ( 2 β ′ ) particles. Around the peak of hardness values, regularly distributed rod-shaped ( 1 β ′ ) precipitates were found. The rod-shaped ( 1 β ′ ) precipitates were oriented with a growth direction of [0001]. When over-aged, rod-shaped ( 1 β ′ ) precipitates were expected to decrease and the density of disc-shaped ( 2 β ′ ) precipitates to change. The rod-shaped ( 1 β ′ ) precipitates mainly consist of {Mg, Zn}, while disc-shaped ( 2 β ′ ) precipitates, {Mg, Zn, Zr} or {Mg, Zn}. In this study the optimum T6 treatment was determined as solution treatment at 430 °C for 6 hours and subsequently aging treatment at 175 °C for 18 hours. At this T6 condition the tensile strength, yield strength and elongation are 321MPa, 280MPa and 16%, respectively.