• Journal of the Korean Society for Heat Treatment
• /
• v.17 no.1
• /
• pp.17-22
• /
• 2004

In the present study, newly developed spark plasma sintering(SPS) technique was introduced to refine the grain size of ${\gamma}$-based TiAl intermetallic compounds. Ti-46Al-1.5Mo and Ti-46Al-1.5Mo-0.2C(at%) prealloyed powders were produced by mechanical milling(MM) in high-energy attritor. The mechanically milled powders were characterized by XRD and SEM for the microstructural evolution as a function of milling time. And then, the MMed powders were sintered by both spark plasma sintering and hot pressing in vacuum (HP). After the sintering process, MM-SPSed specimens were heat-treated in a vacuum furnace (SPS-VHT) and in the SPS equipment(MM-SPS) for microstructural control. It was found from microstrutural observation that the microstructure consisting of equiaxed ${\gamma}$-TiAl with a few hundred nanometer in average size and ${\alpha}_2-Ti_3Al$ particles were formed after both sintering processes. It was also revealed from hardness test and three-point bending test that the effect of grain refinement on the hardness and bending strength is much higher than that of carbon addition. The fully lamellar microstructures, which is less than $80{\mu}m$ in average grain size was obtained by SPS-VHT process, and the fully lamellar microstructure which is less than $100{\mu}m$ in average grain size was obtained by MM-SPS for a relatively shorter heat-treatment time.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.9 no.6
• /
• pp.1555-1559
• /
• 2008

The characteristics of 304 (Fe-18%Cr-12%Ni) and 316L (Fe-18%Cr-13%Ni-2.4%Mo) austenite stainless-steel compacts sintered with $5{\sim}15{\mu}m$ powder were investigated and the results led to the following conclusions: (1) When the sintering time was 3.6ks, the relative density of sintered compacts was $95{\sim}98%$, regardless of any other sintering condition. (2) When a vacuum sintering was done with $5{\mu}m$ stainless steel powders, almost fully-dense sintered compacts were obtained at is = 57.6ks. (3) The amount of residual oxygen in 304 and 316L sintered compacts was $0.5{\sim}0.6%$, regardless of sintering atmosphere. (4) The amount of residual oxygen in the vacuum sintered compact decreased more than 0.3 % due to addition of carbon powder, thereby reducing the formation of oxides. Furthermore, the addition of carbon improved the density of sintered compact, which enables us to make a fully-dense high performance sintered compact.

• Korean Journal of Materials Research
• /
• v.20 no.3
• /
• pp.125-131
• /
• 2010

In this study, Ti powders were fabricated from Ti scrap by the Hydrogenation-Dehydrogenation (HDH) method. The Ti powders were prepared from the spark plasma sintering (SPS) and their microstructure was investigated. Hydrogenation reactions of Ti scrap occurred at near $450^{\circ}C$ with a sudden increase in the reaction temperature and the decreasing pressure of hydrogen gas during the hydrogenation process in the furnace. The dehydrogenation process was also carried out at $750^{\circ}C$ for 2 hrs in a vacuum of $10^{-4}$ torr. After the HDH process, deoxidation treatment was carried out with the Ca (purity: 99.5%) at $700^{\circ}C$ for 2 hrs in the vacuum system. It was found that the oxidation content of Ti powder that was deoxidized with Ca showed noticeably lower values, compared to the content obtained by the HDH process. In order to fabricate the Ti compacts, Ti powder was sintered under an applied uniaxial punch pressure of 40 MPa in the range of $900-1200^{\circ}C$ for 5 min under a vacuum of $10^{-4}$ torr. The relative density of the compact was 99.5% at $1100^{\circ}C$ and the tensile strength decreased with increasing sintering temperature. After sintering, all of the Ti compacts showed brittle fracture behavior, which occurred in an elastic range with short plastic yielding up to a peak stress. Ti improved the corrosion resistance of the Ti compacts, and the Pd powders were mixed with the HDH Ti powders.

• Proceedings of the Korean Powder Metallurgy Institute Conference
• /
• 2006.09b
• /
• pp.814-815
• /
• 2006

The removal of oxygen during sintering by carbothermic reduction was studied for steel compacts Fe-Cr-Mo-C and Fe-Mo-C prepared from prealloyed powders. The compacts were prepared by pressing at 600 and 1000 MPa and sintering at 1100 and $1300^{\circ}C$ in vacuum. It showed that for the Cr-Mo steel, deoxidation strongly depends on the sintering temperature, in contrast to the plain Mo steel; at $1300^{\circ}C$ very low oxygen levels were measured with the standard density compact while at high density still significant oxygen is contained. This indicates inhibition of final deoxidation by pore closure, but apparently without adverse effect on the mechanical properties.

• The Transactions of the Korean Institute of Electrical Engineers
• /
• v.40 no.7
• /
• pp.684-689
• /
• 1991

Magnetic properties of sintered Fe-Si-P alloys have been investigated as a function of sintering condition and composition. Sintering was carried in the temperature range from 1100ø C to 1400ø C in vacuum. As the sintering temperature increases, the magnetic properties of specimens were improved mainly due to the easy movement of domain wall because large pores and large grains were formed during the sintering process at high temperature. When sintered at 1400ø C, Fe-2w/o Si-0.5w/o P compact had the best mgnetic properties, but more phosphorus addition degraded magnetic properties. It appears that the degradation was caused by the formation of non-magnetic compounds such as Si P, Fe3P in the compacts with high phosphorus contents.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 1999.11a
• /
• pp.117-120
• /
• 1999

The relationship between the DC degradation characteristics of the ZnO varistor and the ambient sintering-process is investigated in this study. ZnO varistors made of Matsuoka's composition were fabricated by standard ceramic techniques. The ambient sintering-process is performed at the extraordinary electrical-furnace which is equipped with the vacuum system. The Gas of sintering process was oxygen, nitrogen, argon, air respectively. The microstructure of ZnO varistors be made use of SEM equipment. The condition of DC degradation tests were conducted at $115\pm2^{\circ}C$ for periods up to 13 h. Current-voltage analysis is used to determine nonlinear coefficients($\alpha$). Resistance-frequency and capacitance-frequency analysis are accomplished to the understanding of electrical properties as DC degradation test. From above analysis, it is found that the ZnO varistor sintered in oxygen atmosphere showed superior properties at the DC degradation test.

• Journal of Powder Materials
• /
• v.22 no.4
• /
• pp.283-288
• /
• 2015

Two sintering methods of a pressureless sintering and a spark-plasma sintering are tested to densify the Fe-TiC composite powders which are fabricated by high-energy ball-milling. A powder mixture of Fe and TiC is prepared in a planetary ball mill at a rotation speed of 500 rpm for 1h. Pressureless sintering is performed at 1100, 1200 and $1300^{\circ}C$ for 1-3 hours in a tube furnace under flowing argon gas atmosphere. Spark-plasma sintering is carried out under the following condition: sintering temperature of $1050^{\circ}C$, soaking time of 10 min, sintering pressure of 50 MPa, heating rate of $50^{\circ}C$, and in a vacuum of 0.1 Pa. The curves of shrinkage and its derivative (shrinkage rate) are obtained from the data stored automatically during sintering process. The densification behaviors are investigated from the observation of fracture surface and cross-section of the sintered compacts. The pressureless-sintered powder compacts show incomplete densification with a relative denstiy of 86.1% after sintering at $1300^{\circ}C$ for 3h. Spark-plasma sintering at $1050^{\circ}C$ for 10 min exhibits nearly complete densification of 98.6% relative density under the sintering pressure of 50 MPa.

• Journal of Powder Materials
• /
• v.22 no.5
• /
• pp.356-361
• /
• 2015

Fe-30 wt% TiC composite powders are fabricated by in situ reaction synthesis after planetary ball milling of (Fe, $TiH_2$, Carbon) powder mixture. Two sintering methods of a pressureless sintering and a spark-plasma sintering are tested to densify the Fe-30 wt% TiC composite powder compacts. Pressureless sintering is performed at 1100, 1200 and $1300^{\circ}C$ for 1-3 hours in a tube furnace under flowing argon gas atmosphere. Spark-plasma sintering is carried out under the following condition: sintering temperature of $1050^{\circ}C$, soaking time of 10 min, sintering pressure of 50 MPa, heating rate of $50^{\circ}C/min$, and in a vacuum of 0.1 Pa. The curves of shrinkage and its derivative (shrinkage rate) are obtained from the data stored automatically during sintering process. The densification behaviors are investigated from the observation of fracture surface and cross-section of the sintered compacts. The pressureless-sintered powder compacts are not densified even after sintering at $1300^{\circ}C$ for 3 h, which shows a relative denstiy of 66.9%. Spark-plasma sintering at $1050^{\circ}C$ for 10 min exhibits nearly full densification of 99.6% relative density under the sintering pressure of 50 MPa.

• Journal of the Korean Ceramic Society
• /
• v.47 no.6
• /
• pp.485-490
• /
• 2010

In our previous studies, continuous SiC fiber-reinforced SiC-matrix composites ($SiC_f$/SiC) had been fabricated by two different slurry infiltration methods: vacuum infiltration and electrophoretic deposition (EPD). 12 wt% of $Al_2O_3-Y_2O_3$-MgO with respect to SiC powder was used as additives for liquid-phase assisted sintering. After hot pressing at $1750^{\circ}C$ under 20 MPa for 2 h in Ar atmosphere, a high composite density could be achieved for both cases, whereas the problems such as large grain size and non-uniform distribution of liquid phase were observed, which was resulted in the relatively poor mechanical properties of composites. Therefore, efforts have been made to reduce the grain growth during the sintering, including the optimization for hot pressing condition and utilization of spark plasma sintering using a SiC monolith. Based on the results, spark plasma sintering was found to be effective method in decreasing the amount of sintering additive, time and grain growth, which will be explained in comparison to the results of hot pressing in this paper.

• Journal of the Korea Institute of Military Science and Technology
• /
• v.15 no.3
• /
• pp.231-240
• /
• 2012

During the past two decades, Metal Injection Molding(MIM) has become a very competitive technology to fabricate small, precise and complex-shaped parts in large quantities. In this research, the applicability of MIM technology in the mass-production of the high precision fuze parts to save manufacturing cost was investigated. The water-atomized 17-4PH stainless steel powder, one of the best corrosion-resistant high strength materials, was injection-molded into real-shape fuze part and flat tensile specimens. The injection-molded parts were thermally debound in hydrogen gas flow without solvent extraction. Sintering of the debound parts was carried out in vacuum at temperatures ranging from $1150^{\circ}C$ to $1370^{\circ}C$. The sintering behavior, mechanical properties, dimensional precision, corrosion resistance of the MIMed 17-4PH stainless parts were investigated. It was found that almost all the properties of the MIMed parts were comparable to those of the mechanically machined parts. Also, actual military field tests using both MIMed and mechanically machined fuze parts were performed as well and were found to be very successful.