The additive manufacturing (AM) of high γ′ fraction Ni-based superalloys is an area of significant interest for the fabrication of high temperature components in gas turbines. AM offers particular benefits including, the reduction in the number of individual parts, the elimination of joining/fastening, and the production of components with complex geometries, which allows access to more complicated cooling channel architectures. However, high γ′ fraction Ni-based superalloys remain challenging to process by AM. The design of novel alloys tailored to AM processes is therefore warranted. For this to be achieved, the underlying physical metallurgy of existing alloys produced by AM needs to be fully understood.

The most commonly studied high γʹ fraction superalloy manufactured by AM is CM247LC. This is due to the vast industrial experience of the alloy and the unique balance of properties that it offers, which makes it suitable for a range of applications. Despite the wealth of studies that exist on this material, few have reported success in achieving AM material in a defect free state. In addition, to ensure that suitable properties are acquired from AM material, it is common for a series of post processing steps to be applied including sand blasting, hot isostatic pressing and heat treatments. It is therefore paramount that the origins of defect formation in alloys such as CM247LC are understood both through the AM process and the post-processing operations if the design of future alloys tailored towards AM are to be successful.

In this study, the microstructural evolution of alloy CM247LC was examined in the as-deposited state and after each post-processing step, including HIP and multiple heat treatment stages. The thermal characteristics of the material were determined using differential scanning calorimetry, and detailed microstructural studies were performed using microscopy methods (SEM-EDX, EBSD, STEM). Resonant ultrasound spectroscopy was utilised to understand the elastic anisotropy. The results obtained indicated that post-processing operations were necessary for microstructural optimisation, albeit not all post-processing stages studied resulted in significant microstructural evolution, suggesting that cost savings and tailoring of the post processing methodologies is required. The microstructural analyses suggested that an improved understanding of carbide distribution control is required. The results will also be compared against thermodynamic model predictions and the implications towards future alloy development will be discussed.