Research On Investment Castingand Heat Treatment Of Ti-48Al-2Cr-2Nb Alloys

In this paper, the oxide ceramic mould for TiAl-based alloys and the effect of heat treatment on investment casting Ti-48Al-2Cr-2Nb alloys have been investigated. A simple analysis on the mechanism of the binder on the role of oxide ceramic mould, interfacial reaction between investment casting Ti-48Al-2Cr-2Nb alloy and zirconia ceramic mould, and the effect of heat treatment on the microstructures and mechanical properties.Studies have shown that the Si-O bond generated by the phase transformation to improve strength of mould in the the roasting process of the binder. Alternating with silica sol and ethyl silicate binder as the binder using on the back layer of zirconium oxide ceramic can improve the mould strength, while providing a good deformability of the mould. Also by experiments, the best route of using alternating silica and ethyl silicate binder prepared as a background layer of zirconium oxide ceramic shell is roasting for one hour in 1000℃.The original microstructure of the investment casting Ti-48Al-2Cr-2Nb alloy is thickness full lamellar structure, and the mechanical properties of the alloys at room temperature also very poor. Through the interfacial reaction between the alloys and zirconia ceramic mould, we can see, the thickness of interfacial reaction layer is about 80um.The compounds formed by the interfacial reaction and solid solution of O atoms strengthens interfacial reaction layer and allows a significant increase in hardness.In the technology of quenching-tempering,a large number of non-equilibrium microstructure, such as dislocations, stacking faults, such as reverse domain wrong, generated in the quenching process, these microstructures contain a large number of defects, providing a large number of nucleation sites, refined Alloys and lamellar grain size. The technology of cyclic heat treatment refining the alloys is mainly by discontinuous coarsening reaction through the tempering process. Times of cycles and holding time in the technology of cyclic heat treatment has significant impact on the effect of heat treatment.Quenching - tempering heat treatment technology and cycle heat treatment technology improves the mechanical properties of investment casting Ti-48Al-2Cr-2Nb alloy at room temperature, through refining the microstructures of the alloys. For instance, through quenching - tempering technology the maximum tensile strain at room temperature of the alloys increased to 0.88 %, while after cyclic heat treatment the value of the maximum tensile strain of the alloys reached to 0.68%.According to compression tests and fracture analysis of microstructure produced by heat treatment, The alloys are inter-laminar fracture and there was no significant yield.