Pulse Current Auxiliary Sintering

Abstract

In the first part of this chapter, the technique of PCAS was firstly introduced in detail, including its origin, development, classification, fundamental working mechanism and a lot of advantages over other traditional material preparation technique. The second part paid special attention to the application of the technique to synthesis ofγ- TiAl alloys. On self-built pulse current auxiliary sintering apparatus (PCAS), influences of different parameters, including heating-up rate, sintering temperature and duration time, on obtained microstructures in sintered products were studied. After systematic researches on heating rates of 2~9°C ⋅ s −1, sintering temperatures of 1200~1300℃, dwell times of 5~15min, it was determined that 1250℃ as the temperature and 5min as the duration time were more beneficial for generating dense, even and fine γ-TiAl based alloys. When powders were synthesized in the oven at 1250℃ for 5min with heating rates of 4.5, 3, 2.5, 2℃ ⋅ s −1, near gamma, duplex, fine fully lamellar and coarse fully lamellar microstructures were obtained, correspondingly. After comparison studies among these alloys, it was found that grain size, lamellar volume fraction and lamellar spacing played an important effect on room temperature mechanical properties of the sintered products. In general, duplex alloys possessed better comprehensive behaviors. Their ductility displayed at room temperatures could get to 4.51%. Thus, brittleness problem was solved to some extent. In tensile tests at various temperatures on sintered TiAl alloys with near γ and duplex microstructures, it was observed that the one with equi-axed fine grains possessed more superior elevated temperature tensile properties. In temperature range of 950℃~1000℃ and strain rate range of 2.083×10-4s-1,the obtained elongation was over 240%. The results demonstrated its ability to display superplastic behavior at relatively low temperatures. Based on calculation of the activation energy Q for Superplastic deformation being 212.6~252.2kJ•mol-1, the superplastic deformation mechanism for the alloy was determined as the grain boundary sliding accommodated by grain boundary diffusion Based on the success in the synthesis of dense γ-TiAl alloys with fine microstructures, improved room-temperature and ideal high-temperature tensile properties, more studies are under way for processing industrial parts with good quality and improved mechanical properties directly by PCAS technique. These relative studies will explore more prospects for the application of the technique.