The Effect of Processing Parameters on Structure Evolution during Laser Additive Manufacturing and Post-processing of Niobium-Silicide Based Alloys

Thermal efficiency is one of the most important concerns of aero-engines, both in terms of environmental protection and financial incentive. One approach to improve efficiency is increasing the operating temperature of the turbine segment. However, the approach is presently restricted by the temperature capability of currently used nickel superalloys, because current operational temperatures in the high-pressure stages of commercial gas turbines approach the melting temperatures of Ni-case alloys. Nb-based alloys contain a blend of a strong silicide phase and tough solid solution matrix that has been regarded as a possible replacement for current turbine materials. These new alloys can significantly increase the working temperature of gas turbine engines, with a binary Nb-Si system possessing a solidus temperature close to 1,900°C. However, challenges remain for the application of such alloy systems since processing routes and alloy chemistry have not been optimised for any industrial applications.

In this study, I explored the use of a laser additive manufacturing method to process the Nb-based alloys, which were then post-processed using hot isostatic pressing and heat treatment to optimise their microstructures. The effect of scanning strategy, scanning speed and substrate pre-heating on microstructure evolution during the laser additive manufacturing and post-processing of different Nb-Si alloys were studied. It is found that a short scanning pass coupled with substrate pre-heating can prevent cracking formations, while the solidification structure can be refined with a faster scanning rate. The effect of alloying elements (Hf, Cr, Ti) on phase transformation of Nb-based alloys was investigated. It is found that Hf additions promote the formation of HfO₂ inclusions during laser forming stage and the inclusions coarsen during hot isostatic pressing and heat treatment; Cr additions promote the formation of α-Nb5Si3 and C15 Laves phases during laser forming; Ti additions promote the formation of a Ti-rich Nb₅Si₃ phase. The most important remaining challenges for Nb-Si alloys is to obtain a combination of optimum mechanical properties at room and high temperatures through further optimisation of process variables and alloy chemistry.