Shock synthesis and densification of tungsten based heavy alloys

摘要

Tungsten heavy alloys have been traditionally manufactured by thermal sintering were the compaction is achieved by heating under pressure the pre-compacted powders, to elevated temperatures, up to 3000 deg C. Additives such as dopants are usually incorporated in the original powder mixtures in order to control the grain sizes, and mechanical properties of the sintered microstructures. This process is capable of producing high-density tungsten heavy alloys but has its own limitations. Shock induced synthesis (SIS) and shock compaction (SC) or dynamic compaction (DC), are processes used for the manufacture of advanced materials. In shock synthesis high detonation velocity explosives are used for the synthesis, whereas in shock compaction low detonation velocity explosives, elevated temperatures and prior coating of the powders are used to generate the shock waves required for optimal densification. Due to the high shock wave velocity, the temperatures within the capsule raise above the melting points of some or all the elements present in the powder mixtures. The liquid mixtures are then quenched in microseconds, which results in novel phases. Shock compaction offers the possibility of producing high temperatures necessary for strong metallurgical bonding within the particle surface regions where they are most required, while the internal powder mass, within the particles remain at relatively lower temperatures. A novel approach for producing dense and fine grain tungsten heavy alloys has been investigated. This approach combines reaction synthesis and dynamic consolidation. By combining these two processes tungsten heavy alloys with 98 percent of the theoretical density have been produced. In this work four tungsten-iron-nickel heavy alloys and one tungsten-copper-nickel heavy alloy were manufactured by shock synthesis and densification. The elemental powders were mixed and pre-compacted at 20 deg C by packing them in cylindrical steel capsules of 25 mm diameter. Ammonite with a detonation velocity of 3.6km/s was used as the main explosive. The samples were preheated to temperatures between 300 and 1000 deg C before the application of the shock wave. The specimens were then characterized by optical microscopy, SEM analysis, EDS quantitative analysis, quantitative image analysis, X-ray diffraction, hardness and microhardness testing. This paper will discuss the effects of the shock parameters on the microstructure and strength of these five tungsten heavy alloys and compares the results obtained with those of alloys of similar compositions manufactured by thermal sintering.