Printing Mechanism and Microstructure Control in Wire-laser Directed Energy Deposition of Metallic Materials

Wire-laser directed energy deposition (DED) is a metal additive manufacturing (AM) technology widely used by various sectors to fabricate large-scale components due to its fast production rate, high material utilization, and low manufacturing cost. The emerging multi-laser printhead with a coaxial wire feeder can significantly reduce the directional dependence in the printing process; however, the fundamental knowledge of this new process is still limited. For this purpose, this dissertation aims to develop a deep understanding of the coaxial wire-laser DED process and offer fresh insights into the microstructure control of as-printed samples, further expediting the wide adoption of this technique in industrial production.
This dissertation employed various characterization approaches and high-fidelity multi-physics simulations to investigate the coaxial wire-laser DED of 316L stainless steel and Inconel 718 alloys. This dissertation comprises three research chapters: Chapters 3, 4, and 5. Chapter 3 focuses on a wire-laser DED process of Inconel 718 alloy with intentionally constrained energy input. Operando synchrotron x-ray characterization and multi-physics modeling were employed in synergy to investigate the printing behavior and microstructure evolution in the melt pool. This study revealed that, under this specific printing condition, the feedstock wire only partially melted as it entered the melt pool. Despite the wire continuing to be heated and melted by the surrounding melt pool, solid particles, such as MC-carbide, were released and remained in the bottom region of the rear melt pool, consequently resulting in microstructure heterogeneity in the as-printed part. Chapter 4 introduces a wiggle deposition strategy to control the solidification microstructure in wire-laser DED by generating an unstable melt pool. A certain level of melt-pool instability altered the crystallographic texture and, consequently, the mechanical anisotropy of printed samples. Chapter 5 focuses on an “abnormal” columnar-to-equiaxial transition and icosahedral-short-range-ordering-mediated solidification in metal AM. Operando high-energy synchrotron x-ray diffraction and total-scattering analysis were adopted to investigate the atomic structure of the melt pool and the role of atomic ordering in the rapid solidification of metal AM processes. This chapter presents a comprehensive understanding of the “abnormal” columnar-to-equiaxial transition in metal AM and provides experimental insights to enrich the modern solidification theory.