Papers by Author: Taek Kyun Jung

Abstract: This paper described extrudability and bonding strength of copper (TPC) clad aluminum alloy (pure Al, Al3003, Al5005, and Al7072) composites produced by indirect extrusion at 350°C with extrusion ratio of 21.39. Conical typed die with semi angle of 30°. Carbon oil was used to reduce the friction between billet/sleeve and billet/die. Maximum extrusion pressure was estimated to 491MPa for the Cu/Al, 714MPa for the Cu/Al3003, 820MPa for the Cu/Al5005, and 743MPa for the Cu/Al7072 alloy composites. No surface fracture was observed. From SEM observation, diffusion layer between the sheath material and the core material of extruded composites is observed and its thickness was measured to about 1.5㎛. The bonding strength was estimated to 65MPa for the Cu/Al, 89.3MPa for the Cu/Al3003, 70MPa for the Cu/Al5005, and 75MPa for the Cu/Al7072 alloy composites.

Abstract: The microstructures and mechanical properties of the bulk Al-Fe-(Mo, V, Zr) alloy produced by melt spinning process and subsequent hot extrusion at 693K in the extrusion ratio of 25 to 1 were investigated. TEM observation revealed an equiaxed grain structure with the average grain size of 200 nm for the extruded bulk alloy. Extremely fine dispersoids based on Al-Fe phases, Al-Fe-(Mo, V) phases and Al-Zr phases were observed to be distributed uniformly within grains and at grain boundaries. The size distribution of the binary Al-Fe and the Al-Fe-(Mo, V) phases were ranged from 20 nm to 50 nm, whereas the Al-Zr phase was less than 10 nm. The very high tensile strength of about 800MPa was achieved at room temperature for the extruded bulk alloy.

Abstract: Bulk Al-8Fe-2Mo-2V-1Zr (wt.%) alloys were produced by melt spinning which can give rise to develope a nano crystalline structure in terms of rapid cooling and subsequent hot extrusion. The bulk alloys exhibited multi-phase microstructures consisting of ultra fine equiaxed grains with the average grain size of 100nm and a fine intermetallic Al-Fe, Al-V and Al-Zr based phase having less than 50nm in particle size. From compression test, it was revealed that the bulk alloys have very high yield strength at both room temperature (942MPa) and elevated temperatures (651MPa at 473K, 500MPa at 573K, respectively).

Abstract: Three types of composite materials, Al-10Ni-6Ce (at%)/pure Al (Vf=0.3), Al-10Ni- 6Ce/Al-3.6Mn (Vf=0.3) and Al-10Ni-6Ce/Al-5.5Mg (Vf=0.3), and monolithic Al-10Ni-6Ce alloy were successfully fabricated to a fully dense rod-shaped bulk form having a diameter of about 10mm by adopting a powder forging or extrusion process using amorphous Al-Ni-Ce powder together with crystalline pure Al, Al-Mn and Al-Mg powders. The monolithic Al-Ni-Ce specimen forged at 648K showed a very high compressive strength of 1.3GPa without exhibiting any compressive plastic strain. All of the composite specimens forged at 648K gained a compressive plastic strain with the considerable sacrifice of strength. In contrast, Al-Ni-Ce/Al-Mg composite specimen extruded at 648K showed a noticeably high compressive strength of 1.2GPa with the compressive plastic strain of 0.5%. The extruded Al-Ni-Ce/Al-Mn composite specimen also exhibited a considerably high compressive strength (1.1GPa) accompanied with plastic strain (0.2%).

Abstract: Al-8Fe-2Mo-2V-1Zr alloy powders were prepared by gas atomization and melt spinning method. In melt spinning technique, melt spun ribbons were pulverized by a speed rotor mill to make a powder shape. In order to produce a bulk form, powders were canned and hot extruded in the extrusion ratio of 25 to 1 at 693K. For the gas atomization and hot extrusion processed bulk material, equiaxed grains with the average size of 400 nm and finely distributed dispersoids with their particle sizes ranging from 50nm to 200nm were observed to display a characteristic nano-structured feature over the entire region. For the melt spun and hot extrusion processed alloy, a refined microstructural feature consisting of equiaxed grains with the average size of 200 nm and fine dispersoids with their particle sizes under 50 nm appeared to exhibit a difference in microstructure. Yield strength of the latter alloy was higher than that for the former alloy up to elevated temperatures. The maximum yield strength was measured to about 800 MPa at room temperature for the latter alloy.

Abstract: Two atomized alloy powders, those chemical compositions are Al-10Si-5Fe-1Zr and Al- 10Si-5Fe-4Cu-2Mg-1Zr, were pre-compacted by cold pressing with 350MPa and subsequently hot forged at temperatures ranging from 653K to 845K and at an initial strain rate of 10-2/s in order to produce bulk cylindrical type alloys with the diameter of 10 mm. The addition of Cu and Mg into the present alloy causes a decrease in the eutectic reaction temperature of Al-10Si-5Fe-1Zr alloy from 841K to 786K and results in a decrease of flow stress at the given forging temperature. TEM observation revealed that in addition to Al-Fe based intermetallics, Al2Cu and Al2CuMg intermetallics appeared to display the alloying effect additionally. The volume fraction of intermetallic dispersiods increased by the addition of Cu and Mg. Compressive strength of the present alloys was closely related to the volume fraction of intermetallic dispersoids.

Abstract: High strength Al-8Fe-2Mo-2V-1Zr (wt.%) alloys fabricated by a melt spinning and a hot extrusion process were produced to correlate the microstructure and mechanical property. Melt spun ribbon prepared by single roll melt spinner showed a cellular structure with an average size of 10nm and Al-Fe based intermetallic dispersoid of less than 10nm in particle size. The melt spun ribbon obtained was then pulverized to make a powder shape followed by hot extrusion at 648K, 673K, 723K and 773K in extrusion ratio of 5 to 1, respectively. Equiaxed grain structure containing Al-Fe based intermetallic phase was observed in all extruded specimens. According to increasing extrusion temperature, the grain size increased and particle size of intermetallic dispersoid. The lattice parameter increased from 0.4051nm to 0.4059 nm with increasing extrusion temperature from 648K to 773K, those values were larger than that obtained in pure Al (0.4049nm). Yield strength of the specimen extruded at 648K measured to 956MPa at room temperature, 501MPa at 573K and 83MPa at 773K, respectively. With increasing extrusion temperature yield strength decreased significantly at room temperature and even in the intermediate temperature range, while no noticeable difference in yield strength was observed at 773K.

Abstract: The rod-shaped bulk composites consisting of Al-10Ni-6Ce and Al-4Fe-0.6Mo-1.1V- 0.3Zr alloy (mixing ratio; 0.7:0.3, 0.5:0.5 and 0.3:0.7) and corresponding monolithic alloys were produced to a full density via powder forging process. The process involved pre-compaction of rapidly solidified alloy powders and subsequent isothermal forging at 673K. The forged Al-10Ni- 6Ce alloy exhibited nano-scaled crystalline particles, such as fcc-Al, Al3Ni, Al4Ce and Al11Ce3 phase, coexisting with an amorphous phase. In the case of the forged Al-4Fe-0.6Mo-1.1V-0.3Zr alloy, an equiaxed grain structure was observed to exist with uniformly distributed nano-scaled Al- Fe based intermetallics. The monolithic Al-10Ni-6Ce alloy had a considerably high maximum compressive strength (MCS) of 1.35 GPa without showing any compressive plastic strain (CPS). In contrast, the monolithic Al-4Fe-0.6Mo-1.1V-0.3Zr alloy possessed noticeably high CPS of 25% with the MCS of 0.71GPa. The composites acquired the CPS varying from 1 to 5.8 % and the MCS from 1.26 to 0.74 GPa, with increment of the volume fraction of Al-4Fe-0.6Mo-1.1V-0.3Zr alloy from 0.3 to 0.7.

Abstract: The mechanical properties of pre-sintered Al-10Si-5Fe-1Cu-0.5Mg-1Zr (wt%) alloy were investigated in the temperature range from 673K to 813K and at initial strain rates from 10-4 to 100 s-1. In the high temperature range of 793K and 813K, the strain rate sensitivity index was close to that for superplasticity (0.3). Stress exponent was estimated to be 2 and 5 in the temperature range from 793K to 813K and from 673K to 773K, respectively. The activation energies for the plastic flow were calculated to 77kJ/mol for the n=2 region and 127kJ/mol for the n=5 region. These values were close to that for grain boundary diffusion and self-diffusion in pure aluminum, respectively. We found that a dynamic recrystallization (DRX) and grain boundary sliding (GBS) occur depending on the test temperature and stain rate. A filament-like phase containing Cu and Mg was observed in the cracked surface of the specimen deformed at 793K and 813K.

Abstract: The bulk Al84Ni10Ce6 alloy was fabricated by a powder forging process. The process involved pre-compaction of amorphous powder by cold pressing and subsequent isothermal forging at temperatures from 523 to 823K with the strain rate of 10-2 s-1. The porosity decreased rapidly with increasing forging temperature up to 648K, and a fully dense bulk specimen with the porosity less than 1% was achieved when the forging was carried out at and above 648K. TEM observation on the fully dense bulk alloys revealed a mixed structure consisting of nano-scaled crystalline particles and amorphous matrix. It was also revealed that the size and volume fraction of the crystalline phases increased with increasing forging temperature. Noticeably high compressive fracture strength of 1355MPa and Vickers hardness number of 530 were obtained at room temperature for the fully dense bulk specimen forged at 648K, which contains the refined crystalline particles (average size: 28nm, volume fraction: 44%) in an amorphous matrix.