Process Parameter Optimization for Crack Mitigation in CM247LC processed by Electron Beam Powder Bed Fusion

Abstract

Powder bed fusion is a class of additive manufacturing (AM) technology capable of fabricating complex geometrical designs and empowering users with a variety of benefits. Two main examples are laser powder bed fusion (LPBF) and electron beam powder bed fusion (EB-PBF) known as electron beam melting (EBM). Both techniques follow a layer-by-layer fabrication manner whereby each layer is melted according to individual cross-sectional slices of a computer aided design model. Despite its escalating enticement and prominence in both research and industry, the application requires significant research to understand the interplay between process parameters and materials. An example of a group of alloys are nickel superalloys, which can be further classified as weldable and non-weldable nickel superalloys. CM247LC falls in the latter and is associated with a higher volume content of γ' precipitation strengthening phase. The greater γ' (gamma prime) content enhances the mechanical properties for high temperature applications, but also paradoxically contributes to its proclivity for cracking.

The processability of CM247LC via LPBF has been investigated by different researchers from academia and industry. A number of traditional and creative strategies were explored to minimize the crack susceptibility of CM247LC but this was at the cost of undesired compromises. Conversely, the differences in the deposition process and conditions primes the less explored EBM to be an appealing processing alternative. The current work explores the processability of CM247LC by EBM with the desired target of mitigation the material susceptibility towards cracking. The process optimization requires a fundamental understanding of process parameters and the solidification process of the deposited material. This study follows a systematic ground up approach to address the knowledge gap. This started with the basic deposition unit of a single-track and later progressed with multi-tracks.

An experimental single-track study, structured within a design of experiment (DoE) framework, was conducted to isolate the primary EBM process parameters and evaluate their effects on the track stability, microstructure, and cracking behaviour. A process map was developed to identify parameter combinations that yielded coherent and uniform tracks. Relationships among EBM parameters, melt pool morphology, grain microstructure, and cracking tendencies were established, revealing specific track conditions and inherent characteristics that mitigate cracking.

The lack of an integrated characterization system within the build chamber complicates direct analysis of solidification conditions for these single-tracks. To address this, a simplified thermal model by finite element (FEM) method was developed to provide an alternate approach to study the solidification conditions. The thermal profiles for each experimental single-track were simulated, enabling extraction of key solidification parameters, such as thermal gradient, and cooling rate, and analysis of their spatial-temporal evolution. Furthermore, the relationship between process parameters, thermal conditions, and cracking behavior were defined, identifying certain thermal profiles that reduce cracking severity.

Building on this foundation, the study progresses to single-layer deposition involving multiple tracks extending the single-track findings. Two distinct single-track process parameter recipes were selected to assess heat accumulation effects originating from the lateral stacking of tracks. The lateral arrangement was modulated using independent variables: line offset and track number. The impacts of these variables on deposited layer quality, multi-track microstructure, and cracking behaviour were assessed. The different single-track parameter recipes indicated contrasting multi-track deposition nature as reflected by surface topography analysis and subsequent assessment of the multi-track cross section. Moreover, a distinct spatial pattern of defects was observed. While these established relationships and findings point toward optimal multi-track conditions, further refinement and follow-up strategies are necessary to fully optimize the process.