Novel 3D printing strategies to produce high performance, nanostructured steels

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Improving the mechanical performance of metal alloys is a longstanding effort in engineering. Stronger and tougher metals are conducive to more efficient transportation, more secure infrastructure, and reduced overhauling in several industrial sectors. One common approach to improve the mechanical behaviour of metals is to impart them with a complex, nanoscale structure. A myriad of different manufacturing processes may be employed to produce nanostructured metals. However, they all suffer from limited scalability, which hampers their use in structural applications. In this thesis, I will explore the possibility of using laser powder bed fusion (LPBF) technology to produce bulk high-performance nanostructured alloys. LPBF is a class of additive manufacturing processes which relies on small scale melting and solidification of a metal feedstock into a 3-D part. By fine-tuning the laser parameters it is possible to manipulate the thermal history of the material and engineer the material's solidification structure at nanoscale. To showcase the opportunities offered by this technology, I will focus on 17-4 PH martensitic stainless steel. The LPBF strategies which I will employ will aim at controlling the parent phase that the material will retain upon cooling, as well as the occurrence of the strengthening copper-rich nanoprecipitates and submicron solidification cells which dictate the alloy's mechanical performance. Notably, these structures can be formed in conventional and LPBF produced 17-4 PH through long specialised heat treatments. However, the time and cost of this additional post processing step is prohibitive to the scalability of LPBF. Part of my work will thus also focus on achieving this structural control without post-processing, leading to a more cost-effective and sustainable manufacturing approach.