발간논문

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Laminated glass fiber-reinforced plastic (GFRP) composites were applied to an insulating structure of a magnet system for a nuclear fusion device. Decreased inter-laminar strength by a strong repulsive force between coils which is induced a problem of structural integrity in laminated GFRPs. Therefore, it is important to investigate the inter-laminar characteristics of laminated GFRP composites in order to assure more reliable design and better structural integrity. Three types of the laminated GFRP composites using a high voltage insulating materials were fabricated according to each molding process. To evaluate the grade of the fabricated composites, mechanical tests, such as hardness, tensile and compressive tests,were carried out. The autoclave molding composites satisfied almost of the mechanical properties reguested at the G10 class standard, but the vacuum impregnation (VPI) and Prepreg composites did not.

keyword : GFRP, composites, nuclear fusion, mechanical properties, tensile test

Thermal degradation behavior of a WO3-TiO2 monolithic catalyst was investigated in terms of structural, morphological, and physico-chemical analyses. The catalyst with 4 wt.% WO3 contents were prepared by a wet-impregnation method, and a durability test of the catalysts were performed in a temperature range between 400℃ and 800℃ for 3 h. An increase of thermal stress decreased the specific surface area, which was caused by grain growth and agglomeration of the catalyst particles. The phase transition from anatase to rutile occurred at around 800℃ and a decrease in the Brønsted acid sites was confirmed by structural analysis and physico-chemical analysis. A change in Brønsted acidity can affect to the catalytic efficiency; therefore, the thermal degradation behavior of the WO3-TiO2 catalyst could be explained by the transition to a stable rutile phase of TiO2 and the decrease of specific surface area in the SCR catalyst.

keyword : porous materials, powder processing, grain growth, raman spectroscopy, catalytic acid-site

The study investigated the influence of current density on the corrosion-protection properties of an AZ91 Mg alloy subjected to plasma electrolytic oxidation coating. The present coatings were carried out under an AC condition at three different current densities, i.e., 100, 150, and 200 mA/cm2. From microstructural observations, the micro cracks connecting each micro pore were pronounced on the oxide surface of the samples coated at current densities higher than 150 mA/cm2 since increasing the current density in this study led to an increment in the relative volume fraction of the MgO compound. Based on potentio dynamic polarization and immersion tests, the sample coated at a current density of 100 mA/cm2 showed superior corrosion resistance.

keyword : Mg alloy, coating, plasma electrolytic oxidation, current density, corrosion

Nanopowders of Mo, Ti and Si were made by high-energy ball milling. A dense nanostructured (Ti,Mo)Si2 compound was sintered by the pulsed current activated combustion method within two minutes from mechanically activated powder of Mo, Ti and Si. A highly dense (Mo,Ti)Si2 compound was produced under simultaneous application of 80 MPa pressure and a pulsed current. The mechanical properties and micorostructure were investigated. The hardness and fracture toughness of the (Mo,Ti)Si2 were 1030 kg/mm2 and 4.9 MPa·m1/2, respectively. The mechanical properties were higher than monolithic TiSi2.

keyword : compound, nanostructure, sintering, mechanical properties

Nanopowders of MgO, Al2O3 and SiO2 were made by high energy ball milling. The rapid sintering of nanostructured MgAl2O4-Mg2SiO4 composites was investigated by a high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid for the application of nanomaterials. Highly dense nanostructured MgAl2O4-Mg2SiO4 composites were produced with simultaneous application of 80MPa pressure and induced output current of total power capacity (15 kW) within 2min. The sintering behavior, gain size and mechanical properties of MgAl2O4-Mg2SiO4 composites were investigated.

keyword : composite, nanostructure, high-frequency induction heated combustion, Mg2SiO4, mechanical properties

The poisoning effect of Zr in aluminum alloys was investigated by analyzing the filtered cakes of aluminum alloy melt taken with the Prefil footprinter through a variety of analytic instruments, SEM/EDX, Auger, and TEM. Experimental results indicated that the morphology and chemical composition of the aluminum alloys were not modified with the addition of Zr, which is to previous belief that Zr poisoning is caused by modification of (Ti1-xZrx)Al3. On the other hand, TiAl3 surroundig TiB2 particles was modified and its lattice parameter was more mismatched by increasing Zr content, leading to less nucleation rate. This is also supported by the observation that the poisoning effect is reduced when Ti is added, resulting in a lower content ratio of Zr to Ti. These results suggest that extra Ti should be added to eliminate the poisoning effect of Zr in aluminum alloys containing Zr.

keyword : Al alloys, solidification, nucleation, image analysis, poisoning effect

SA508 Gr.4N Ni-Cr-Mo low alloy steel, in which Ni and Cr contents are higher than in commercial SA508 Gr.3 Mn-Mo-Ni low alloy steels, may be a candidate reactor pressure vessel (RPV) material with higher strength and toughness from its tempered martensitic microstructure. The inner surface of the RPV is weld-cladded with stainless steels to prevent corrosion. The goal of this study is to evaluate the microstructural properties of the clad interface between Ni-Cr-Mo low alloy steel and stainless weldment, and the effects of post weld heat treatment (PWHT) on the properties. The properties of the clad interface were compared with those of commercial Mn-Mo-Ni low alloy steel. Multi-layer welding of model alloys with ER308L and ER309L stainless steel by the SAW method was performed, and then PWHT was conducted at 610℃ for 30 h. The microstructural changes of the clad interface were analyzed using OM, SEM and TEM, and micro-Vickers hardness tests were performed. Before PWHT, the heat affected zone (HAZ) showed higher hardness than base and weld metals due to formation of martensite after welding in both steels. In addition, the hardness of the HAZ in Ni-Cr-Mo low alloy steel was higher than that in Mn-Mo-Ni low alloy steel due to a comparatively high martensite fraction. The hardness of the HAZ decreased after PWHT in both steels, but the dark region was formed near the fusion line in which the hardness was locally high. In the case of Mn-Mo-Ni low alloy steel, formation of fine Cr-carbides in the weld region near the fusion line by diffusion of C from the base metal resulted in locally high hardness in the dark region. However, the precipitates of the region in the Ni-Cr-Mo low alloy steel were similar to that in the base metal, and the hardness in the region was not greatly different from that in the base metal.

keyword : alloys, welding, microstructure, hardness test, heat-treatments

The effect of high shear speed on shear force, shear energy and fracture surface was investigated for the solder joint of a Sn-3.0Ag-0.5Cu ball. For both ENIG and OSP pads, the shear force increased with an increase in shearing speed to 0.3 m/s. However, for an ENEPIG pad, the shear force increased with an increase in shear speed to 0.6 m/s and kept almost constant afterward. The shear energy decreased with an increase in shearing speed for ENIG and OSP pads. For the ENEPIG pad, however, the shear energy almost remained constant in a shearing speed range 0.3-3.0 m/s. The fracture mode analysis revealed that the amount of brittle fracture for the ENIG and the OSP pads increased with shearing speed, and a complete brittle fracture appeared at 1.0 m/s for ENIG and 2.0 m/s for OSP. However, the ENEPIG pad showed only a ductile fracture until 0.25 m/s, and a full brittle fracture didn`t occur up to 3.0 m/s. The fracture mode matched well with the shear energy. The results from the high speed shear test of SAC305 were similar to those of SAC105.

keyword : electrical/electronic materials, soldering, mechanical properties, scanning electron microscopy, SEM, high speed shear

This paper describes an experimental study on ultrasonic welding of similar and dissimilar metals. There are optimum welding conditions which are found for welding of Al/Al and Al/Cu. It evaluated weldability using tensile test, SEM observation and EDX-ray analysis. Both ultrasonic welding of Al/Al and Al/Cu have amplitude as the variable factor. Al/Cu welding was examined again with welding time as variable factor to find the best conditions. The more welding time or amplitude increase, the better weldability. The optimum conditions for ultrasonic welding of Al/Al were formed at pressure 0.25 MPa, welding time 0.25 sec, amplitude 90%. Pressure 0.25 MPa, welding time 0.4 sec, amplitude 80% are optimized for Al/Cu ultrasonic metal welding and solid-state diffusion generated by ultrasonic vibration and frictional heat is confirmed at the welded interface.

keyword : metals, welding, diffusion, ultrasonic, weldability

The present study evaluated the microstructures and mechanical properties of severely deformed Ni-30Cr alloys. Cross-roll rolling (CRR) process was introduced as a severe plastic deformation (SPD), and Ni-30Cr alloy sheets were cold rolled to 90% thickness reduction and subsequently annealed at 700℃ for 30 min to obtain the recrystallized microstructure. Electron back-scattering diffraction (EBSD) was introduced to analyze grain boundary character distributions (GBCDs). The application of CRR to the Ni-30Cr alloy was effective in enhancing the grain refinement through heat treatment; consequently, the average grain size was significantly refined from 33㎛in the initial material to 0.6㎛. This grain refinement directly improved the mechanical properties, in which yield and tensile strengths significantly increased relative to those of the initial material. We systematically discuss the grain refinement and accompanying improvement of the mechanical properties, in terms of the effective strain imposed by CRR relative to conventional rolling (CR).

keyword : metals, cold working, grain refinement, electron backscattering diffraction, EBSD, mechanical properties