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

提高金属合金的力学性能是工程领域一个长期的课题。更坚固、更坚韧的金属有助于提高运输效率、提高基础设施的安全性，并减少一些工业部门的大修。改善金属力学性能的一种常用方法是赋予它们复杂的纳米级结构。无数种不同的制造工艺可以用来生产纳米结构的金属。然而，它们都有有限的可扩展性，这阻碍了它们在结构应用中的使用。在本论文中，我将探索使用激光粉末床熔合（LPBF）技术生产大块高性能纳米结构合金的可能性。LPBF是一类增材制造工艺，它依赖于金属原料的小规模熔化和凝固成三维零件。通过微调激光参数，可以操纵材料的热历史，并在纳米尺度上设计材料的凝固结构。为了展示这项技术提供的机会，我将重点介绍17-4 PH马氏体不锈钢。我将采用的LPBF策略旨在控制材料在冷却时保留的母相，以及强化富铜纳米沉淀物和亚微米凝固细胞的发生，这决定了合金的机械性能。值得注意的是，这些结构可以在传统和LPBF中形成，通过长时间的特殊热处理产生17-4 PH。然而，这种额外的后处理步骤的时间和成本不利于LPBF的可伸缩性。因此，我的部分工作也将集中在没有后处理的情况下实现这种结构控制，从而实现更具成本效益和可持续的制造方法。

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.