Papers by Author: Heung Sub Song

Abstract: A new tungsten heavy alloy with hybrid structure was manufactured for the kinetic energy penetrator. The tungsten heavy alloy is composed of two parts: core region is molybdenum added heavy alloy to promote the self-sharpening; outer part encompassing the core is conventional heavy alloy to sustain severe load in a muzzle during firing. The fracture surfaces of the specimen is observed after ballistic tests. The core region revealed brittle behavior with W/W inter-granular fracture which activates self-sharpening. On the other hand, outer part exhibited conventional ductile fracture mode. From ballistic test, it was found that the penetration performance of the hybrid structure tungsten heavy alloy is higher than that of conventional heavy alloy. This heavy alloy is thought to be very useful for the penetrator in the near future.

Abstract: This paper focuses on the variations of static and dynamic properties of tungsten heavy alloy with heat treatment. The matrix phase of 93W-4.9Ni-2.1Fe (weight percent) has been penetrated into W/W grain boundaries during a cyclic heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. By applying the cyclic heat treatment, the impact energy of tungsten heavy alloy is increased about three times from 57 to 170 J. When the tungsten heavy alloy is cyclically heat treated at 1150 °C and then re-sintered at 1485 °C, W/matrix interface is changed from round to undulated shape. The irregularity of the interface is increased with increasing the number of heat treatment cycles. From the measurement of the residual stress of W grains by X-ray diffraction, it is found that the irregularity of the interface is closely related with strain energy stemmed from the difference of thermal expansion coefficient between W particles and matrix phase. From dynamic ballistic test, it is found that the tungsten heavy alloy with undulated W grains forms many narrow fracture bands which are preferential for the self sharpening effect, thus, for the improvement of the penetration performance.

Abstract: Fabrication of two different types of functionally graded tungsten carbide cobalt (WC-Co) coatings has been tried by using detonation gun to enhance both the hardness in the surface region and the toughness of the coating. Five layers with a continuous compositional change of 75 to 92 wt.% of WC were sprayed to obtain ‘layer-structured gradient’ coatings. Each layer was controlled to have both minimum porosity and minimum phase decomposition of WC. New types of ‘continuously graded’ coatings were also fabricated to eliminate any obvious interfaces between layers by the ‘shot control method’ of detonation gun spraying. The wear resistance of these coatings was measured and compared to that of conventional WC-Co coatings by using dry sand abrasion tests under the condition of 20kg load and 200rpm rotating wheel. Present functionally graded coatings revealed more improved wear resistance than that of conventional coatings.