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Wear properties of 6061 Al-Al2O3 particulate composites were investigated and correlated with the microstructure. The peak-aged composite was more wear-resistant than the underaged composite and the plane perpendicular to the extrusion direction was more wear-resistant than that parallel to the extrusion direction. The difference of the wear resistance between two different orientations was observed to be greater than that between underaged and peak-aged matrix microstructure, suggesting that preferential orientation of particulates has a greater effect on the wear resistance than the change of the matrix microstructure. The difference of wear properties between two different orientations can be explained by the effect of preferentially orientated reinforcing particles on the shear modulus and shear deformation. The mechanically mixed layer with fine Al2O3 fragments was observed at the sliding speed of 1.14 m/s. When the sliding speed was increased to 362 m/s. the presence of the subsurface layer with fine particles was much less pronounced and the specific weal increased drastically.

keyword : Composite, Microstructure, Shear modulus, Wear resistance, Aging, Orientation, Fragment, Plastic flow

Hot torsion tests were conducted in the temperature range from 320 to 480℃ under a strain rate of 1.0/sec in order to investigate the hot workability of SiCp/AA2024 composites reinforced with SiCp reinforcements of different volume fractions(0, 5, 10, 15, 20, and 30 vol. %). Initial microstructures of the composites fabricated by powder metallurgy route were studied to analyze the effect of reinforcing volume fractions on the flow stress and work hardening rate of the composites during hot deformation. Hot restoration mechanisms, dynamic recrystallization(DRX) and dynamic recovery(DRV). of the composites were studied from the flow curves and deformed microstructures. The dislocation density in the matrix and the grain size of the composites were analysed by transmission electron microscopy(TEM). The DRX occurred in all the composites during the hot deformation at 370~430℃ under a strain rate of 1.0/sec. Also, the flow stress and the dislocation density of the composites increased with increasing the volume fraction of SiCp reinforcement, and the difference in flow stress between the composites reinforced with 15, 20, and 30 vol. % SiCp was small. Also, the hot ductility of the composites deformed at different temperatures was studied in relation to the volume fraction of SiCp.

keyword : Hot workability, Work hardening rate, Hot restoration, Dislocation density, Flow stress, Hot ductility

Microstructural change in 93W-4.9Ni-2.1Fe heavy alloy was investigated by metallurgical analyses of the fragments from penetrators after high speed impact test. Two kinds of specimens were prepared. The one was normal specimen manufactured by a typical liquid phase sintering process, and the other was the D specimen made by a double-cycle sintering process, respectively. The morphology of tungsten grain in the D specimen became irregular by the double-cycle sintering. And microstructural changes observed from the fragments after the test were local deformation in W grain and matrix phase, fracture, formation of adiabatic shear band, reaction with steel target and so on, irrespective of the specimens. However, fracture mode was changed from W-W and W-matrix interface to W cleavage when W grains became irregular, by which the penetrator could be more brittle and fragile. From the analyses, it was suggested that the microstructural change in tungsten grain and formation of inhomogeneous matrix band would be beneficial for the self sharpening effect.

keyword : Tungsten heavy alloy, Self sharpening, Adiabatic shear band, Penetration, Microstructure, Instability, Tungsten cleavage

The early stage of reactive ball milling of Al and Ti elemental powders in a hydrogen atmosphere was investigated using XRD, PSA, SEM and TEM to find out the formation time of TiH2 phase and the usefulness of this phase in processing of nanocrystalline structure. Brittle TiH2 phase formed by hydriding reaction was essential to reduce grain size and particle size. Both grain size and particle size decreased with increasing Ti content and milling time. Hydriding reaction occurred at initial milling time between 1 and 4 hours. Nano-sized grain began to form at the same time as brittle hydride formed and more milling after 10 hrs milling had a little effect on the further reduction of the grain size. The final structure of nanocrystalline Al-Ti alloy powders was the dispersion of nano-sized TiH2 in nano-sized Al matrix. The grain size observed from TEM micrograph was less than 20 ㎚. During the reactive ball milling of Al and Ti elemental powders in a hydrogen atmosphere, it is found that grain size was first refined to nano-structure and then particle size began to reduce from 45㎛ to 15㎛.

keyword : Reactive ball milling, Hydriding reaction, Titanium hydride nanocrystalline Al-Ti alloy

The precipitaion and sensitization behavior of nitrogen added Type 316L stainless steels(SS) were investigated by using specimens cold worked for 0~40%. The alloys had a variation in nitrogen content from 0.04 to 0.15%. To quantify the degree of sensitization double-loop electrochemical potentiokinetic reactivation (DL-EPR test was performed in a 0.1M H2SO4 + 0.01M KSCN solution at 30℃. The addition of nitrogen increased sensitization resistance by retarding the onset of M23C6 precipitation and shifted Time-Temperature-Sensitization (TTS) curve to higher temperature and longer time range. Cold work accelerated the M23C6, precipitation and sensitization kinetic due to the increase in dislocation density. However, the acceleration of sensitization was found to depend on the added nitrogen content in the alloys. The alloys with high nitrogen(>0.1%N) content exhibited higher accelation of the sensitization as a function of the cold work than that with low nitrogen content. From the microstructural analysis, this was found to he attributed to the development of intensive slip bands during cold work and retardation of dislocation annihilation during subsequent aging in the alloys with high nitrogen content.

keyword : Type 316L stainless steels, Sensitization, Cold work, Nitrogen addition

The electromagnetic stirring of molten metal in the continuous casting mold is one of the most effective tools to decrease various casting defects such as macrosegregation, center cracks, and to homogenize the grain structure of the ingots. For DC casting of large diameter aluminium billets, it is important to control molten metal flow and heat transfer in order to prevent the formation of center cracks and macrosegregation. A circular inductor with minimum height producing travelling magnetic field in vertical direction has been designed with the help of a 3D numerical simulation program. The electromagnetic field, fluid and heat flows have been analysed by the numerical calculation and physical model experiment. The results of calculation agreed very well with those of experiment. It has been found that the downward stirring near the mold wall seems more efficient to homogenize the melt than the upward stirring. An optimized inductor design for stirring aluminium melt in the DC casting mold of 240 mm inner diameter have been proposed based on the result of the numerical calculation.

keyword : Round DC casting mold, Traveling magnetic field, Numerical analysis, Magnetic field, Fluid flow, Heat transfer, Physical model experiment

The vacuum brazing of 6061 aluminium alloy with Al-12Si-1Mg-5Cu-xGe(x=0, 10, 20wt.%) braze alloy has been conducted at temperatures ranged 530~570℃ for 60min under brazing pressure of 1MPa to investigate the braze joint microstructure. the joint strength and the wettability of braze alloy. The liquidus temperatures of braze alloys with Ge=0, 10, 20wt.% were 572℃, 558℃ and 541℃, respectively. The wettability of braze alloy was found to be improved with increasing the brazing temperature. Braze joint microstructure was varied with the brazing temperature. Under low brazing temperatures(≤550℃, the silicon and germanium, which are expected to be the elements present in the braze alloy, are observed at the braze joint. On the other hand, these elements were not detected under the brazing temperature of 570℃. This may be due to the diffusion of the elements into 6061 aluminium alloy. The joint strength was found to be increased with increasing the brazing temperature. The maximum joint strength was obtained at 570℃ brazing temperature and its value was 73% of tensile strength of the base metal.

keyword : Vacuum brazing, 6061Aluminium, Germanium, Al-Si-Mg-Cu-Ge, Wettability, Joint strength

Air plasma spray coatings of three different WC-12%Co powders based on the powder manufacturing methods were conducted. Mixed powders composed of S&F (Sintered and Crushed) WC-12%Co which revealed the best wear resistance in this study and NiCrSiB with different ratio(25:75(B1), 50:50(B2), 75:25(B3)) were also evaluated, The best wear resistance of S&F WC-12%Co coating was mainly due to its high hardness related with large amount of homogeneously and/or uniformly distributed total carbides and low porosity. The wear resistance of blend coatings increased with increasing WC-12%Co weight percent but was not noticeably improved compared to that of WC-12%Co coatings. The dominant wear mechanisms of WC-12%Co coatings were combined actions of adhesive wear, carbide particle fragmentation, and tribo-film formation by material transfer on which fatigue crack formed and propagated under repeated loading leading to delamination. In the case of blend coatings, the dominant mechanism of B1 and or B2 coating was observed to be the following sequence : abrasive wear(ploughing or scratching) in the soft NiCrSiB coating layers → carbide extrusion and fragmentation → crack formation and propagation at the rear of abrasive mark →separation of coating layers and delamination as well as adhesive wear (adhesion). But, B3 coating which contained the 75wt% of WC-12%Co showed both the wear mechanism of B1 and or B2 coating and that of WC-12%Co coating.

keyword : Plasma spray coating, Powder manufacturing method, Wear resistance, Adhesive wear, Abrasive wear, Carbide particle fragmentation, Crack, Delamination

In order to investigate the effect of particle shape on the gaseous reduction rate of iron ore, iron oxide pellets of various shapes including the long cylindrical-and disc shape were reduced with CO-CO2 gas mixturein in the mixed-control regime. Experimental results showed that the reduction rate of the disc-shaped particle was the fastest among all and the long cylindrical particle followed. The experimental results were analyzed by the unreacted-core model for a non-spherical particle proposed in the previous study by the authors, which was intended for the particle of the non-spherical shape relatively near to the sphere. According to the analysis, this model was applicable for the cylindrical particles of which the height are in the range from a half to twice of the cylindrical diameter while the long cylindrical-and disc-shaped particle out of the above range were not. Based on the review of the experimental-and analysis results, the reaction aspect concept for the long cylindrical-and disc-shaped particle was discussed.

keyword : Shape factor, Reduction rate, Iron ore, Non-spherical particle, Non-spherical radius, Unreacted-core model

From the viewpoint of the removal of tramp element, the thermodynamic properties of arsenic in molten Fe-C alloy was investigated between 1523K to 1823K by using chemical equilibrating technique. The effects of the arsenic, carbon concentration and the temperature on the activity coefficient of arsenic were also measured. The activity coefficient of arsenic in carbon saturated iron was measured as 0.09 at 1673K. The following activity coefficient of arsenic in molten iron was derived in terms of interaction parameters and temperature. □ By combining the present experimental results and the reference for thermodynamic properties of arsenic, the possibility of the removal of arsenic from molten iron by evaporation was discussed.

keyword : Tramp element, Arsenic, Thermodynamics, Activity Coefficient, Interaction coefficient, Vaporization