발간논문

Home > KJMM 논문 > 발간논문

An optimal binder system was selected for the successful application of the metal powder injection moulding(MIM) with thermoplastic binders using sieve size Fe-powder. The thermal analysis for the binders(high density polyethylene, low density polyethylene, carnauba wax, paraffin wax) and the binder mixtures were taken using DSC and TGA. An optimal binder content for MIM using sieve size powder(-270mesh) was experimentally determined from the mixing and moulding behaviour. A relatively good mixing and moulding behaviour could be obtained by using a 45% paraffin wax-15% carnauba wax-30%low density polyethylene-10%stearic acid mixture. The optimal amount of powder loading was 59-60vol%, the critical value being 61vol%. The binder could be nearly removed by wicking and thermal decomposition.

keyword :

An investigation has been performed on the effect of working on densification and microstructure of powder forged AISI 4640 steels. The densification by powder forging was remarkably improved as the degree of working increased. e. g. the highest density near full densification (∼98%) was achieved in the 50% deformed specimen. The microstructure of prior austenite grains revealed that the full densification by powder forging results from the pore elimination by material flow into pores. The line broadening analysis of X-ray diffraction patterns indicated that the considerable effect of HTMT (high temperature thermomechanical treatment) can be induced by powder forging process. This phenomenon was confirmed by the refinement of martensite structure in the forged specimens. The decrease of hardness during tempering was appreciably retarded with increasing working degree. This is believed to be mainly due to the activated precipitation of fine alloy carbide and its related refinement of tempered martensite structure by HTMT effect.

keyword :

Secondary hardening embrittlement (SHE) in the 6W-3Ni(WN3) and 6W-6Ni(WN6) steels isothermally aged in the range, 550-650℃, was studied to analyze the effects of isothermal aging conditions, which cause variations in carbide morphology, impurity segregation, and hardening or softening of matrix, on the impact toughness and fracture behavior. In underaged conditions, the impact toughness reached the upper shelf energy (USE) levels with increasing test temperature, and mainly ductile dimples were observed on the fracture surfaces. In overaged conditions, on the other hand, the impact toughness was not recovered but remained at the low levels, 2-5 J, even at 250℃, and the fracture surfaces showed intergranular mode. In peak-aged conditions, the impact toughness reached USE levels and fracture surfaces were mostly represented by dimples, similar to those of underaged conditions. However, their USE levels were lower than those of underaged conditions by 10-15 J, or the impact toughness was not recovered and the fracture surfaces exhibited the intergranular mode, similar to those of overaged conditions. In underaged and peak-aged conditions showing the recovery of impact toughness, the relatively easy occurrence of transgranular dimples in the USE regime was caused by the coarse cementite and the intrinsic hardening with fine W_2C precipitation. In contrast, in overaged and peak-aged conditions showing no recovery of impact toughness, the easy occurrence of intergranular fracture is believed to be caused by the impurity segregation at the grain boundaries.

keyword :

Al-10wt% Ti alloy has been produced by mechanical alloying (high energy ball milling) and subsequent hot extrusion. By means of thermal analysis of mechanically alloyed powder and X-ray analysis of sintered specimen, it has been found that Al₃Ti was formed at 260℃ during the degassing process. The fact that Al₃Ti was thermally stable was obtained from the results of TEM, SEM, X-ray, hardness test of as-extruded specimens which are exposured at various time (1-40hr.) and temperature (300 ℃, 400℃, 500℃). This was conincided with low solubility and diffusivity of Ti in Al matrix. Room temperature and high temperature (300℃, 500℃) tensile strengths of mechanically alloyed Al-10wt% Ti alloy were superior to those of conventional precipitation-hardened alloy (2014-T6 and 2219-T81). This improvement of tensile strength was due to homogeniously and finely dispersed Al₃Ti, oxide and carbide With increasing Ti and PCA contents, tensile strength increased while ductility decreased. Oxide and carbide revealed stronger effects on high temperature strenght than those of Al₃Ti. High temperature properties unchanged after heat treatment.

keyword :

The relationship between mircrostructural changes during deformation and shpae memory effect(SME) has been studied in Fe-15Mn-5Cr-5Co-3Si alloy. Dislocations generated as well as ε-α` transformation during deformation were revealed to deteriorate the SME. Teh γ→ε transformation contributing to SME was hindered by the multiplication of the dislocations during the deformation. The transformation of stress-induced ε into α` was found to be another cause for making SME worse due to the reduction in the amount of the ε. It is proposed that SME could be improved not only by the increase in strength but by the prevention of ε→α` transformation during the deformation.

keyword :