발간논문

Home > KJMM 논문 > 발간논문

Effects of shot peening on the residual stresses and fatigue properties of 300M steel were studied. The fatigue strength was enhanced by 90 ㎫ and the fatigue life was improved approximately by 800% with the shot peening at 24 amp. of impeller current for 2 min. Excessive shot peening caused surface defects and reduced the improvement of fatigue properties. The fatigue crack initiation site was found to move to subsurface mainly near non-metallic inclusions, with the increase of shot peening intensity and residual stress, but the defects on the surface was dominant crack initiation site in the excessively shot-peened specimen. The mechanism of improvement in fatigue properties in case of the subsurface fatigue crack initiation was discussed.

keyword :

In this study, we analysed acoustic emission signals obtained in fatigue and tensile test of high strength fire resistance steel for frame structure with time-frequency analysis methods. The results obtained are summaries as follows; Fatigue life of TL direction specimen was shorter than LT specimen due to easiness of crack propagation. There was a little effect to fatigue crack propagation rate by N addition and with changing rolling condition, but fatigue life was increased by B addition. The main frequency range is different in the noise and the fatigue crack propagation. It could be classified that it were also generated by composite fracture mechanics of cleavage, dimple, inclusion separation etc.

keyword :

The strength and electrical conductivity of Cu-14Fe-2X(X = Ag or Cr) microcomposite plates obtained by cold rolling combined with intermediate heat treatments have been investigated. During cold rolling the primary and secondary dendrite arms are aligned along the rolling direction and elongated into filaments. The addition of Ag was found to reduce the filament spacings at the given rolling throughout the deformation processing. The ultimate tensile strength of the Cu-14Fe-2Ag microcomposites was higher than that of Cu-14Fe-2Cr microcomposites, suggesting the refinement of the filaments is more effective than the strengthening of the filaments in strengthening the microcomposites. The strength of Cu-Fe-X microcomposites is dependent on the spacing of the Fe filaments in accord with a Hall-Petch relationship. The fracture surfaces of all the specimens showed ductile-type fracture and iron filaments occasionally observed on the fracture surfaces.

keyword :

Green sheets were prepared using tape casting method to characterize thermal barrier property according to thickness control and layer design in functionally graded materials (FGM), which was fabricated by Plasma Activated Sintering(PAS). The prepared green sheets were stacked and then sintered at 1300℃ for 2 min under the pressure of 30 ㎫ in vacuum, which were used for thermal barrier tests. Effect of zirconia layer thickness on thermal barrier property of FGM was observed. In the other type of FGM prepared with the mixed composition of partially stabilized zirconia and monoclinic zirconia, effect of micro-crack and pore in zirconia layer on thermal barrier property of FGM was also observed. After maintaining the surface temperature of zirconia layer at 1500℃ for 20 min, the surface temperature of NiCrAlY layer was measured with thermocouple, in which thermal barrier effect was about 760℃. Thickness variety in zirconia layer and micro-crack and pore in zirconia layer did not affect thermal barrier property. After thermal barrier tests, new type of crack was not investigated in FGM.

keyword :

Thixoforming process is a novel forming process that has some advantages compared to die casting. In this study, thixoforming process was selected for net shape of magnesium composites. This present work was mainly concerned with the formability and the mechanical properties of thixoformed silicon carbide particulate reinforced AZ91HP magnesium composites, which were manufactured by Rotation-Cylinder Method. The fulfilling behaviors of the composite slurries with thixotropic microstructures showed that the complete fulfilling was achieved over 0.5 effective liquid fraction regardless of the fraction of the reinforcement. Moreover it was observed that all slugs filled the die cavity with essentially laminar flow, unique flow behavior of thixoforming, even in incomplete mold filling at lower temperature. The results of mechanical properties of the composites showed significant improvement in YS and a slight increase in UTS had been achieved in the composite with 5 ㎛ SiC particulate, and wear properties of them were dependent on the size of particulate compared to the fraction of reinforcement.

keyword :

The purpose of this study was to evaluate the microstructure evolution during partial remelting as a function of isothermal holding temperature and time in the semisolid state. SiC particle reinforced magnesium composites were manufactured by Rotation-Cylinder method(RCM) and fully solidified as the billet, and then the microstructure evolutions of them were evaluated through simple reheating procedure. The results showed that the presence of SiC particles led to the fine solid globules compared to those of unreinforced magnesium alloys, and the finer solid globules were achieved with increasing SiC particle fraction. Coalescence was reduced since the solid globules were more isolated with respect to each other by the presence of the particle in grainboundary regions, and Ostwald ripening was also altered by the reduction of the effective diffusion coefficient in the liquid. The average diameters of the solid globules were independent of holding temperature but the standard deviation of the average diameters and the number of the solid globules per area were reduced as temperature increased, mainly due to high liquid fraction. Based on the previous results, it might be suggested that thixoforming of the particle reinforced composites should be performed after short isothermal holding at a relatively high temperature.

keyword :

To improve the room temperature ductility, we have tried to produce nanocrystalline Al₃Zr alloys with a metastable cubic L1₂structure. We have also studied the effects of the addition of Cu, Ni, Mn on the formation and the thermal stability of nanocrystalline structure and L1₂phase. It was found that the final powders synthesized by planetary ball milling were nanocrystalline with L1₂phase after 3h of milling in the Al-Zr binary alloy and after 3h, 6h of milling in the Ni, Cu added powders, respectively. Blended elemental Al 25 at.%Zr-12.5 at.%Mn powder were mechanically alloyed for 6h to an amorphous phase. Thermal analysis showed that L1₂-Al₃Zr is stable up to 625℃. However, the addition of Ni, Cu stabilized L1₂ phase up to 850℃, over 900℃ respectively, although it appears that a transformation to an equilibrium tetragonal phase is initiated during extended heat treatment. The amorphous phase formed with the addition of Mn is transformed to L1₂phase at 770℃, which is stable over 900℃.

keyword :

The influence of pulsed-dc plasma oxidation and nitriding treatment on the surface microstructure with variation of duty factors of cast iron is studied. The phase compositions of the layers were evaluated by using X-ray diffraction. OES(Optical Emission Spectroscopy) have been applied to analyse the active species during plasma oxidation. Surface properties of plasma oxidation and nitriding treated specimens were summarized. Oxide composition was not changed with duty factor (0.1∼0.9, pulse on-off time : 100∼900 ㎲, pulse frequency 1,000 Hz), but the retained α-Fe reduced significantly with increasing duty factor. The intensity ratio of oxygen plasma spectra shows that the formation density of oxygen radical reached maximum at duty factor of 0.5 and etching depth was 1,840 ㎚ under that condition.

keyword :

The mechanism of formation and the breakdown of anodic film in dilute NaOH solutions were examined, amount of dissolved aluminum ions was by using ICP-AES, image of anodic oxide films by using FE-SEM and TEM. Anodic oxide films were formed on aluminum in 5×10^(-5)∼5×10^(-3) M NaOH solutions at different current densities. During anodizing, anodic oxide films grew thicker, and the film growth was accompanied by the formation and repairing of many voids and imperfection in the oxide film. At the final stage, the growth of anodic oxide films was ceased by film breakdown, and the film breakdown potential became higher at lower solution concentration and lower current densities. The breakdown potential at 5×10^(-5) M NaOH solution was 1550V.

keyword :

During the fluxless solder bumping by the radiation of CO₂ laser, temperature distribution in the solder bump was modeled by finite element method(FEM) with energy input rate and heating time as variables. The peak temperatures at the interface between solder and pre-tinned Cu pad were calculated and compared with experimental data. When Gaussian intensity distribution of laser beam was assumed in the axisymmetric coordinates, the calculated and measured temperature profiles were similar to each other. It was demonstrated experimentally and theoretically that the reaction layer at the interface could change from Cu_6Sn_5(η-phase) to Cu₃Sn(ε-phase) as a function of energy input rate and heating time during the reflow solder bumping.

keyword :