발간논문

Home > KJMM 논문 > 발간논문

The comparison of fatigue lifes between C-Mn(SC42) cast steel and HSLA cast steels and the effects of addition elements on thermal fatigue properties of HSLA(High Strength Low Alloy) cast steels have been investigated by a thermal fatigue test machine. The thermal fatigue resistance of HSLA cast steels was superior to that of SC42 cast steels and this was caused by higher thermal conductivity and tensile strength of HSLA cast steels. In case of HSLA cast steels, the thermal fatigue life was increased as the Ti contents were increased. On the contrary, The fatigue resistance was decreased as the C contents were increased. HSLA cast steels with combined addition of Nb, V and Ti had more excellent thermal fatigue life than those of individual addition of them. Therefore the optimum composition of HSLA cast steels for thermal fatigue resistance was 0.1%C-0.65%Mn-0.05%Nb-0.05%V-0.05Ti having the highest tensile stress.

keyword :

High temperature creep behaviour of Al-5 wt.% Mg alloy matrix composite reinforced with 7.5 vol.% BN flakes was studied at temperatures of 300, 330, and 300℃ under the applied stress range of 15-80 ㎫. The composite was manufactured by the pressureless infiltration technique in a nitrogen atmosphere. The composite showed high apparent stress exponent and activation energy. Threshold stress was also shown, which was strongly temperature dependent. True stress exponent of the composite was measured as 5 and true activation energy was found to be 131 KJmol^(-1), which is close to the self-diffusion activation energy of pure Al. The major creep mechanism of the composite was found to be lattice self-diffusion of the matrix alloy. Aluminum nitride(AlN) and other compounds such as AlB₂, which were formed in the matrix during the pressureless infiltration process, seemed to be the cause of the threshold stress and the high creep resistance of the matrix alloy.

keyword :

This study aimed to investigate the change of microstructures and mechanical properties with the solidification rate of AZ91D magnesium alloy. In older to change the solidification rate of AZ91D melt, the pouring temperature and amount of inoculant(C₂Cl_6) were controlled and the permanent mold of cone type was used. The microstructure of as-cast specimens was investigated by optical microscope and scanning electron microscope(EDS) analysis and the grain size was measured by planimetric method. The variations of hardness and tensile properties on the grain size were investigated, and the deformed specimen was observed by TEM. In case the solidification rate was below about 20℃/sec, the grain refinement increases markedly as the solidification rate is increased, but the degree of grain refinement was diminished to higher solidification rate. The Hall-Petch relationship between grain size and hardness, yield strength was held good and the values of 53.0, 419.6 for σ_o and k were obtained, respectively.

keyword :

In an attempt to develop a thermally stable solder system, an in-situ Pb-Sn solder composite reinforced with Cu_6Sn_5 dispersoids was investigated for its thermal stability. The stability was evaluated mainly by measuring the growth rate of intermetallics at in-situ composite solder/BLM(ball limiting metallurgy) interface as a function of the number of reflow soldering cycles and aging time. The rates were compared with those of the eutectic Pb-Sn and Sn-Ag solders. After the thermal treatments, the solder joints were tested for their shear strengths. The results indicated that the in-situ composite solder has a higher shear strength and better thermal stability than the eutectic Pb-Sn solder.

keyword :

Metal matrix composites(MMCs) are known to be widely applicable for parts of transportation devices such as automobiles and aircrafts. Especially Al matrix composites using SiC particles as a reinforcement are spotlighted because of low cost, superior specific modulus, specific strength, wear resistance and high temperature stabilities. But Al₄C₃ formed by interfacial reaction between Al and SiC weakens the interfacial bonding strength. It also was known to be unstable in the water soluble atmosphere. In this study, the passive oxidation of SiC powder is used as protective layer against the reaction between matrix Al and SiC particle. We investigated the changes in interfacial product of the composites and mechanical properties such as interfacial bonding strength, tensile strength in terms of the oxidized layer thickness of reinforcement.

keyword :

The corrosion behavior and mechanical properties were investigated for new Zr-based alloys which were prepared by vacuum arc remelting and then received different thermo-mechanical treatments. Corrosion tests were carried out in a mini autoclave containing each of 360℃ water, 400℃ steam and 360℃ aqueous LiOH(70 ppm) solution. Microstructures of the tested alloys were analyzed using optical microscopy, SEM and TEM. All of the tested alloys showed higher corrosion rates in 400℃ steam than in 360℃ water or in the aqueous LiOH solution. As for the corrosion behavior in 400℃ steam, the alloy of low contents of Nb and Sn showed an enhanced corrosion resistance whereas the alloy with high Sn content exhibited a transition in corrosion rate as early as 80 days exposure. The yield strength, tensile strength and hardness decreased as the final annealing temperature increased in the range between 460 and 530℃. It was concluded from TEM study that mechanical properties and corrosion behavior would be related to the microstructure of the new Zr-based alloys.

keyword :

Superalloys, such as IN738 and IN738LC, have been used in the hot section as gas turbine blade materials. In order to preserve surface stability and mechanical properties of these materials, a protective oxide scale must develop on the surface. However, such oxide scales often crack or even fail as a result of the accumulation of growth and thermal stresses, thus the thermal and mechanical properties of oxide scales must be understood. IN738 and IN738LC have slight difference in carbon content and showed different oxidation behavior. From the isothermal oxidation experiment between 900 and 1050℃ IN738LC revealed better oxidation resistance than IN738. These materials both had outer scale consisted of mainly Cr₂O₃ with some TiO₂ and discontinuous internal oxides of Al₂O₃. It appeared that the additional Cr carbide in Cr₂O₃ resulted in IN738 suffering more cracks and failure at high temperature than IN738LC. This phenomenon was also seen in the case of cyclic oxidation experiments at 900-1100℃.

keyword :

The corrosion behaviors with grain refinement were investigated on the AZ91D magnesium alloy. The grain size was controlled by the inoculation and the change of pouring temperature, the use of cone-type permanent mold. The salt spray test was conducted for 480 hours following ASTM B 117 specification. And the corroded surface and corrosion products were analyzed by SEM and XRD, respectively. Both the corrosion rate and the corrosion depth were markedly decreased by the fine distribution of second phase due to the grain refinement. The relationship between corrosion rate and grain size in the present study could be well predicted the corrosion rate of as-cast AZ91D alloy that have a range of grain size in commercial castings. The corrosion products were consisted of Mg(OH) ₂ and chlorides mainly.

keyword :

Activation and mass transfer process was studied using rotating cylinder electrode regarding the flow-accelerated corrosion of low alloy steel (ASME SA106 Grade C) in room temperature alkaline solution. The reducing agent diffused easily as rotating speed increased, and caused corrosion potential to shift upward. Passive film formed from pH 9.8 according to the mechanism of oxidation and subsequent precipitation of ferrous species. Above pH 10.4, film formation process was active and stable. Corrosion current showed increment with rotating velocity in pH 6.98, while it soon saturated in above pH 9.8. On the other hand the limiting current, which governs pure mass transfer phenomenon of reducing species, increased with the rotating speed regardless of pH value. This indicates that activation reactions, which constitute oxidation of ferrous ion and formation of passive film on the bare metal surface, controls the entire corrosion process.

keyword :

A numerical algorithm which simulates a three-dimensional casting process in a rectangular mold was developed and tested. To analyze filling process, the VOF method is used with the SIMPLER algorithm and, to analyze solidification process, the Equivalent Specific Heat method is used also with the SIMPLER algorithm. Though the complete symmetric plane assumption in z-direction was adopted in a two-dimensional analysis, it was shown that flow patterns during filling, a point of time when the residual flow decayed and the intensity of natural convection differed in each plane in a three-dimensional analysis due to z-directional velocity components and boundary conditions. In addition, the heat transfer analysis in a mold were also done and results showed that the mold temperature varied in considerably wide range which was different from the convectional analysis of constant heat sink temperature assumption.

keyword :