The Effects of Processing Temperatures on the Laser Powder Bed Fusion of Nickel-Base Superalloys

Abstract

Sustainable utilisation of materials whilst improving performance and capability is the baseline of additive manufacturing (AM). AM and laser powder bed fusion (LPBF) has seen rapid growth in industrial adoption and acceptance. The range of metals processable on LPBF machines includes alloys of Steel, Aluminium, Cobalt, Titanium and Nickel etc. Benefits such as design freedom and high part complexity are typical of the LPBF process. With these benefits, there is a growing demand to manufacture safety-critical parts such as turbine blades, vanes, turbomachinery impellers, engine igniters, rockets & exhausts, etc., using LPBF. Existing literature has identified that most of the superalloys used to manufacture these parts are difficult to weld. One of these alloys is the directionally solidified CM247LC which has comparable properties to the more expensive single crystal superalloys.
CM247LC is a nickel-base superalloy strengthened by the precipitation of γ’ in a coherent γ/γ’ matrix. The occurrence of various crack mechanisms during the processing of CM247LC using LPBF is a significant challenge. To date, parametric studies have been employed to reduce these cracks, and post-processing is required to achieve desirable crack free parts.
This thesis reports the effects of processing temperatures on the LPBF of CM247LC, with temperatures between room temperature and 1000°C. Microstructure and crack mechanisms of the samples are characterised using optical microscopy, scanning electron microscopy and image analysis software. As this is not a parametric study, the LPBF parameters employed are determined through the design of experiments at normal processing conditions (RT – 200°C).
The experiments were carried out on a LPBF machine developed by the Authors for processing at 1000°C. The results show that processing at 1000°C is insufficient to eliminate all cracks mechanisms in CM247LC. Experimenting at temperatures higher than 860°C, however, resulted in the elimination of liquation, strain age and ductility dip cracks. Whilst this was the case, at processing temperatures of 860°C and 1000°C, the only cracks observed occurred as zipper-like solidification cracks propagating through the height of the sample.
Comparing the results to the thermomechanical properties of CM247LC indicates that higher processing temperatures are required. The requirement for higher processing temperatures raises sustainability concerns on process energy consumption and process cost. The cost is also associated with a question on reusability of the CM47LC powder, with the morphology and microstructural changes observed at processing temperatures higher than 850°C.
Based on these findings, further research is suggested on exploiting alternative heating capabilities to cross the 1000°C barrier and further reduce the cooling rates. Whilst the ability to manufacture legacy and new parts from existing materials is an AM attraction, the research findings indicate a need for research into designing alloys specifically for the AM process.

Date of Award	22 Jul 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Stephen Newman (Supervisor)