발간논문

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Low carbon steel was welded with martensitic stainless steel by CO₂ laser and microstructures, mechanical and corrosion properties were compared with those obtained by GTA welding method. In laser welding process, since more heat was transfered through the low carbon steel side compared with the stainless steel due to the difference of thermal conductivities between two steels, the size of grain growth region in the heat-affected area was larger in the low carbon steel side as compared to that in the stainless steel side. Meanwhile, since more heat was accumulated in the stainless steel side of the weld pool compared with the low carbon steel, more clear dendritic structures were developed in stainless steel side, and its cooling rate was calculated to 6500˚K/sec. When as-received steels were welded by laser beam, porosities were formed in the weld pool and in the boundary region between weld pool and low carbon steel side. Meanwhile, if steels were heat-treated before laser welding, porosities were not formed. In tension tests, laser welding resulted in a little larger tensile strength and elongation compared with those of GTA welding. In Rever bending tests, laser welding resulted an increase of about 10-50% in bending repetition number compared to that in GTA welding, and fractures were occured in the grain growth region of low carbon steel side for both welding processes. Immersion tests of welded specimens in ferric chloride solution resulted that pitting corrosion took place in the boundary region between low carbon steel and weld pool. Corrosion rate was smaller in laser welded specimens compared to GTA welded ones for non-heat treated specimens, and the results were reversed for heat-treated specimens.

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Al deposited Si was electrostatically bonded to glass with different coefficient of thermal expansion. Bondings were achieved around 150℃ and cracks were observed in specimens bonded above 200℃ whereas no bondings were achieved at 100℃. This paper investigates the effect of bonding temperature and applied voltage on the bonding property of Al-deposited Si/glass bonded couple and the method of low temperature electrostatic bonding.

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SiC_p/2024 Al composites were prepared using various processing techniques, such as the spray forming, powder metallurgical hot pressing, thixoforming, and compocasting. The interfacial characterizations of these composites were performed using scanning electron microscopy, Auger electron spectroscopy, transmission electron microscopy, and X-ray diffraction on reaction products extracted using the electrochemical dissolution. The value of these combined techniques in elucidating the morphologies of the interfacial reaction products was also demonstrated. The influence that each fabrication process has on the extent of the interfacial reaction was studied. Adequate process parameters to fabricate SiC_p/Al alloy composite were proposed based on thermodynamic considerations.

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The Ti-Ni/Al Composites were fabricated by the squeeze casting method. They were fabricated using Ti-50.2at.% Ni wire with diameter of 0.3, 0.5 and 0.7㎜ and with volume fraction of 5, 10 and 15 under the applied stress of 80㎫ at 473K. The as-cast structure and interface reaction were observed Optical microscope(OM) and Scanning electron microscope(SEM). The microstructures of shape memory treated(ice quenching after heat treated for 30minutes at 773K) composites were: fined more than as-cast structure and the micro structures fined more larger volume fraction than the smaller volume fraction. In the case of shape memory treated Ti-Ni/Al composite, Al was diffused to Ti-Ni wire. In the long fiber, the interface reaction was not affected by pressure. When shape memory treated composites were tensed at 353K, occurred compressed stress by restoring force of shape memory alloy. The strength and elongation of Ti-Ni/Al composites increased with decreasing Ti-Ni wire diameter.

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One of the major issues in fabricating the Al alloy composites reinforced with SiC particulates is to avoid interfacial reaction products, i.e., Al₄C₃ and Si The formation of such interfacial reaction products can be supressed by the presence of adequate amount of Si within the matrix of the composite. According to the calculations, Si contents required to prevent the interfacial reactions in the SiC/Al composite were found to increase with increasing temperature. In addition, there exists a transient temperature near 620℃, at which a sudden increase in the equilibrium Si contents is required to prevent the formation of Al₄C₃. Experimental method to verify the theoretical results is also demonstrated.

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Further densification of the Al₂O₃-Al₄B₂O_9 ceramic composites has been carried out by infiltration of alumina sol, colloidal silica, and chromic sad into these porous SLS (Selective Laser Sintering) parts followed by heat-treatment at high temperature (1600℃). The infiltration with colloidal silica into the composites yielded mullite(3Al₂O₃·2SiO₂) and excess amorphous silica phase which did not react with alumina. The maximum density of this composite was about 75% of the theoretical density and the maximum bend strength was around 33㎫. A solid solution of Al₂O₃,-Cr₂O₃ was obtained by infiltration with chromic acid into the composites. The density was about 80% of the theoretical density and the bend strength was around 15㎫. Finally, single phase alumina parts with 50% of the theoretical density were obtained by the infiltration of alumina sol into the composites. The bend strength after firing was found to be about 20㎫.

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The billets of four powder metallurgy high speed steels. ASP30, T15, Rex20, and 10V, have been manufactured by hot isostatic pressing(HIPping). The microstructures of the four alloys have been characterized in the gas-atomization condition, after the HIPping treatment, and after the subsequent heat-treatment. The mechanical properties such as hardness, bend strength, and sliding wear resistance have been also evaluated. In the atomized condition, the solidification microstructure of the 10V alloy shows primary MC carbides embedded inside fine equiaxed dendrites while those of ASP30, T15, and Rex20 alloys exhibit eutectic MC or M₂C carbides in the interdendritic region of primary austenite. M₂C carbides have been decomposed into M_6C carbides during the HIPping treatment. MC and M_6C carbides are present in a martensite structure except the 10V alloy which has only MC carbides after heat-treament. The bend strength have been severely scattered regardless of the alloy composition due to the formation of internal crack by thermal impact and the inclusions such as MnS formed along the prior powder boundaries. The wear resistances of 10V and Rex20 alloys have been found to be superior to those of ASP30 and T15 alloys.

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