Kyung-Sik Kim

Microstructural Characterization of Al-Zr Alloy with Nano-Sized Grains

The mechanical alloying processes was employed to fabricate Al-4at.%Zr alloy with nano-sized grains and very fine Al3Zr compounds. The phase transformations and the stability of the phases formed during mechanical alloying and heat treatment processes were investigated. The grain sizes of the alloys immediately after milling and following the subsequent heat treatment at 550°C were 54.2nm and 106.4nm, respectively. Some of Zr atoms were dissolved into the Al matrix and most of them reacted with hydrogen produced by decomposition of PCA(process control agent) to form ZrH2 during mechanical alloying process. These ZrH2 hydrides decomposed gradually after the heat treatment. Stable Al3Zr with a DO23 structure was formed by heat treatment at temperature of more than 4500C. The hardness of the Al-4at.%Zr alloy was more than two times higher than those of other Al-based alloys.

Identification of Phases in the Wide-Gap Region Brazed with BNi-3 Filler Metal Powder Using Electron Backscatter Diffraction

The phases formed in the wide-gap region brazed with BNi-3 filler metal powder in IN738 superalloy were investigated by electron backscatter diffraction (EBSD). The morphology and chemical composition of the phase were investigated by field emission scanning electron microscopy (FESEM) and Auger electron spectroscopy (AES). The wide-gap region brazed with BNi-3 filler metal had a microstructure consisting of primary Ni3B, binary eutectic of Ni3B-Ni solid solution and ternary eutectic of Ni3B-Ni-Ni3Si structure. EBSD pattern analysis revealed that the Ni3B had orthorhombic structure with lattice parameter of a=0.439, b=0.522 and c=0.662 nm, and the Ni3Si phase had cubic structure with lattice parameter of a=0.350 nm.

A Study on the Formability of AZ31B Magnesium Alloy Sheet with Experiment and Simulation

This paper is concerned with formability of AZ31B magnesium alloy sheets at various temperatures. In order to acquire the forming limit curve for quantifying the formability of the magnesium alloy sheet, hemispherical punch forming tests are performed under various temperatures. The initial shape of the blank is rectangular and six different types of specimens are used with their length along the rolling direction of 175mm and widths along the transverse direction of 25, 50, 75, 100, 120 and 175mm. With the equipment of on a SIMADZU 100-ton universal testing machine, hemispherical punch forming tests are performed and a forming limit diagram is constructed at the temperature of 100°C, 200°C and 250°C. FE analyses of hemispherical punch forming tests are also conducted by considering ductile failure model in order to compare the forming limit curve obtained from the experiment and numerical analysis. The comparison examines problems in the current material model and it suggests a future research direction.

High-Temperature Thermal Cyclic Oxidation Behavior of Wide-Gap Brazed IN738 Superalloy

The microstructure and thermal cyclic oxidation resistance of the wide-gap region brazed with different filler metal powder (BNi-3 and DF 4B) comparing with that of Ni-based IN738 alloy were investigated. The microstructure characterization showed that Cr borides with a blocky morphology were existed in the brazed region in both filler metal powder. The normalized weight gain with cyclic oxidation showed that weight loss of the specimen brazed with BNi-3 filler metal occurred after 600 cycles. However, the specimen brazed with DF 4B filler metal had no obvious weight loss until 700 cycles. It was observed that the oxidation kinetics of the all oxidized specimens followed the quasi-parabolic law, and the oxide layer was mainly composed of NiO, Al2O3 and NiCr2O4.

Wide-Gap Brazing of IN738 and the Bonding Strength Using Ni-Based Filler Metal Powders

The microstructure and bonding strength of the wide-gap region brazed with different filler metal powder (BNi-3 and DF 4B) and various powder mixing ratios of additive powder to filler metal powder were investigated. The microstructure characterization showed that Cr borides with a blocky morphology were existed in the brazed region in both filler metal powder. The bonding strength of the wide-gap region brazed with 60 wt.% IN738 additive and 40 wt.% DF 4B powder exhibited 92% tensile strength of IN738 superalloy at room temperature. The Cracks in the wide-gap brazed region initiated at the intermetallic compound and eutectic structure, and then propagated through them.

Effect of Additive Powder on Microstructural Evolution in the Wide-Gap Brazed Region by In Situ High Temperature ESEM

This study investigated the characterization of the additive powder on microstructural evolution during the heating of the powder mixture of additive and filler metal powder by in-situ high temperature environmental scanning electron microscopy (HT-ESEM) up to 1200°C. The IN738 powder (additive) and BNi-3 powder (filler metal) were used for wide-gap brazing process. A field emission gun environmental scanning electron microscope (XL 30 ESEM-FEG, FEI) equipped with a 1500°C hot stage was used for in-situ gaseous secondary electron imaging at high temperature (HT-ESEM image). The melting of filler metal powder initiated at 1224K and was spread on the IN738 additive powder with increasing temperature. After cooling, the IN738 additive powder was increased from 75μm to 100μm. It was found that the additive powder added to the wide-gap brazed region avoided the possibility of directional solidification.