Microwave sintering: A new approach to fine-grain tungsten - I

Recently, there has been a renewed interest in fine-grain dense objects of refractory metals containing no liquid-phase sintering aids. This has been the motivation for exploring new sintering techniques, and using ultrafine powders as the starting material In the work reported here, two different types of commercially available and two experimental grades of tungsten powders were microwave sintered in the absence of additives. The effects of primary particle size, phase purity, aggregate size, and morphology on densification behavior have been studied. There was no direct correlation between the characteristics of the as-received powders and the sintered density; for example, a smaller initial particle size did not lead to higher sintered densities, even with higher green densities. The densification behavior was governed by a combination of particle characteristics. In contrast to conventional sintering, it was found that in microwave sintering, the degree of densification increased with increasing aspect ratio. Densities ≳98.5% of the pore-free level were achieved with a grain size in the range of 2-4 μm by microwave sintering followed by hot isostatic pressing (HIPing) at a temperature as low as 1,500°C. In the absence of HIPing, the density of microwave sintered tungsten was 12%-18% higher than the density of conventionally sintered tungsten.