Process Monitoring and Control during Additive Manufacturing

Research in metallic additive manufacturing (AM) has been rapidly growing over the past 10 years, with increasing industrial relevance to the aerospace repair sector. Currently there is a lack understanding of how the processing parameters and the geometry affect the thermal condition of the part and hence the final microstructural/mechanical properties; without this understanding, industrial certification for aerospace will be difficult to achieve.
This work explores the range of microstructures possible that may be developed during AM of Inconel 718, a common alloy used in aerospace, and how in-situ monitoring and process control can be used to influence these structures. Two AM processes are explored: laser powder bed fusion (L-PBF) and laser directed energy deposition (L-DED). Printability is discussed with reference to nickel alloy design and the imposed conditions in both L-PBF and L-DED, concluding that there are different alloy design criteria for the two processes. The spread of Inconel 718 microstructures is shown to be distinct between the processes, confirming that the microstructural outcomes of the processes are fundamentally different, despite the processes being fundamentally similar.
Various in-situ monitoring techniques are tested in the L-DED process, with coaxial monitoring chosen for development of a simple feedback control loop. This improves both the thermal signature of the process and the microstructural homogeneity of the components. A level of intrinsic microstructural and mechanical homogeneity remains and this is further investigated. It is demonstrated that the geometry of components can have a significant effect on the heat flow, which results in a change in grain orientation. Additionally, process induced precipitation via in-situ heat treatment is proposed as a mechanism to explain the measured geometric hardness variation. Using advanced coaxial monitoring and harnessing the power of in-situ heat treatment, it is hypothesised that it would be possible to remove or reduce this process-induced variation in mechanical performance. This would bring the process closer to industrial certification.