Sung-Mo Hong

Fabrication of cast carbon steel with ultrafine TiC particles

The carbon steels dispersed with ultrafine TiC particles were fabricated by conventional casting method. The casting process is more economical than other available routes for metal matrix composite production, and the large sized components to be fabricated in short processing time. However, it is extremely difficult to obtain uniform dispersion of ultrafine ceramic particles in liquid metals due to the poor wettability and the specific gravity difference between the ceramic particle and metal matrix. In order to solve these problems, the mechanical milling (MM) and surface-active processes were introduced. As a result, Cu coated ultrafine TiC powders made by MM process using high energy ball milling machine were mixed with Sn powders as a surfactant to get better wettability by lowering the surface tension of carbon steel melt. The microstructural investigations by OM show that ultrafine TiC particles are distributed uniformly in carbon steel matrix. The grain sizes of the cast matrix with ultrafine TiC particles are much smaller than those without ultrafine TiC particles. This is probably due to the fact that TiC particles act as nucleation sites during solidification. The wear resistance of cast carbon steel composites added with MMed TiC/Cu-Sn powders is improved due to grain size refinement.

Quantitative Study on the Refinement Behaviors of TiC Powders Produced by Mechanical Milling Under Different Impact Energy

This study investigated refinement behaviors of TiC powders produced under different impact energy conditions using a mechanical milling process. The initial coarse TiC powders with an average diameter of 9.3 were milled for 5, 20, 60 and 120 mins through the conventional low energy mechanical milling (LEMM, 22G) and specially designed high energy mechanical milling (HEMM, 65G). TiC powders with angular shape became spherical one and their sizes decreased as the milling time increased, irrespective of milling energy. Based upon the FE-SEM and BET results of milled powders, it was found initial coarse TiC powders readily became much finer near 100 nm within 60 min under HEMM, while their sizes were over 200 nm under LEMM, despite the long milling time of up to 120 min. Particularly, ultra-fine TiC powders with an average diameter of 77 nm were fabricated within 60 min in the presence of toluene under HEMM.

Consolidation of Segment Powder for Diamond Tool by Magnetic Pulsed Compaction

This article presents the successful consolidation of the mixed Co and Diamond powders for a drilling segment by the combined application of magnetic pulsed compaction (MPC) and subsequent sintering, and their properties were analyzed. Homogeneous hardness (Hv 220) and density (97%) of sintered bulks fabricated by MPC were obtained by the new technique, where higher pressure has been employed for short period of time than that of general process. A fine microstructure and homogeneous hardness in the consolidated bulk were observed without cracks. Relatively higher drilling speed of 9.61 cm/min and life time of 6.55 m were found to the MPCed specimens, whereas the value of the specimens fabricated by general process was 11.71 cm/min and 7.96 m, respectively. A substantial improvement of mechanical properties of segment was achieved through this study.

An Investigation of the Stability of Y 2 O 3 and Sintering Behavior of Fe-Based ODS Particles Prepared by High Energy Ball Milling

Abstract Fe-based oxide dispersion strengthened (ODS) powders were produced by high energy ball milling, fol-lowed by spark plasma sintering (SPS) for consolidation. The mixed powders of 84Fe-14Cr-2Y 2 O 3 (wt%) weremechanically milled for 10 and 90 mins, and then consolidated at different temperatures (900~1100 o C). Mechani-cally-Alloyed (MAed) particles were examined by means of cross-sectional images using scanning electron micros-copy (SEM). Both mechanical alloying and sintering behavior was investigated by X-ray diffraction (XRD) and highresolution transmission electron microscopy (HR-TEM). To confirm the thermal behavior of Y 2 O 3 , a replica methodwas applied after the SPS process. From the SEM observation, MAed powders milled for 10 min showed a lamellastructure consisting of rich regions of Fe and Cr, while both regions were fully alloyed after 90 min. The results ofsintering behavior clearly indicate that as the SPS temperature increased, micro-sized defects decreased and the den-sity of consolidated ODS alloys increased. TEM images revealed that precipitates smaller than 50 nm consisted ofYCrO

Milling Behaviors of Al-B 4 C Composite Powders Fabricated by Mechanical Milling Process

In the present work, Al- composite powders were fabricated using a mechanical milling process and its milling behaviors and mechanical properties as functions of sizes ( , 500 nm and 50 nm) and concentrations (1, 3 and 10 wt.%) were investigated. For achieving it, composite powders and their compacts were fabricated using a planetary ball mill machine and magnetic pulse compaction technology. Al- composite powders represent the most uniform dispersion at a milling speed of 200 rpm and a milling time of 240 minutes. Also, the smaller particles were presented, the more excellent compositing characteristics are exhibited. In particular, in the case of the 50 nm added compact, it showed the highest values of compaction density and hardness compared with the conditions of and 500 nm additions, leading to the enhancement its mechanical properties.

Dispersion Behaviors of Y 2 O 3 Particles Into Aisi 316L Stainless Steel by Using Laser Cladding Technology

The present work investigated the dispersion behavior of Y2O3 particles into AISI 316L SS manufactured using laser cladding technology. The starting particles were produced by high energy ball milling in 10 min for pre- alloying, which has a trapping effect and homogeneous dispersion of Y2O3 particles, followed by laser cladding using CO2 laser source. The phase and crystal structures of the cladded alloys were examined by XRD, and the cross section was characterized using SEM. The detailed microstructure was also studied through FE-TEM. The results clearly indi- cated that as the amount of Y2O3 increased, micro-sized defects consisted of coarse Y2O3 were increased. It was also revealed that homogeneously distributed spherical precipitates were amorphous silicon oxides containing yttrium. This study represents much to a new technology for the manufacture and maintenance of ODS alloys.

Effect of B 4 C Content on the Sintering Characteristics of 6061Al-B 4 C Composite Powder

In the present work, 6061 Al- sintered composites containing different contents were fabricated and their characteristic were investigated as a function of sintering temperature. For this, composite powders and their compacts with various contents from 0 to 40 wt.% were fabricated using a planetary ball milling equipment and cold isostatic pressing, respectively, and then they were sintered in the temperature ranges of 580 to . Above sintering temperature of , real density was decreased due to the occurrence of sweat phenomena. In addition, it was realized that sinterability of 6061Al- composite material was lowered with increasing content, resulting in the decrease in its real density and at the same time in the increment of porosity.

Microstructure and Characteristics of Ag-SnO 2 Contact Materials Prepared by Magnetic Pulsed Compaction

In this study, we reported the microstructure and properties of Ag- contact materials fabricated by a controlled milling process with subsequent consolidation. The milled powders were consolidated to bulk samples using a magnetic pulsed compaction process. The nano-scale phases were distributed homogeneously in the Ag matrix after the consolidation. The relative density and hardness of the Ag- contact materials were 95~96% and 89~131 Hv, respectively.