ADVANCED MATERIALS & PROCESSES | MARCH 2025 12 Nuclear reactor designs have only seen incremental technological advancements since the 1950s. Materials systems used to build reactors still include Zr-alloys as fuel cladding, stainless steels for structural components, and some use of Ni-based super- alloys in the coolant pumps and turbines, despite the large body of development in metal alloys[1]. The challenge lies with materials qualification, a rigorous process requiring exhaustive mechanical testing, and in some cases, irradiation data needed for confidence in the material’s performance. For example, the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) requires extensive materials testing that could take more than 10 years to satisfy all the requirements needed[2]. Section III, Division 5 of the ASME BPVC currently only lists six alloys that have met all the requirements for qualification: SS304, SS316, Fe-2.25Cr-1Mo, Fe-9Cr-1Mo, Alloy 800H, and the most recent addition, IN617, which was added in 2020[2,3]. BACKGROUND AND MOTIVATION Safety-oriented industries such as nuclear have adopted a design–manufacture–test approach to qualification that provides continuous feedback. Additive manufacturing (AM) techniques have realized the ability to fabricate custom designed components to nearnet shape from a wide variety of materials[4]. AM enables reactor designers to optimize complex geometries, not thought possible with conventional manufacturing techniques; moreover, the fast turnaround of the AM process allows for rapid prototyping of the design before moving on to the test stage. The U.S. Department of Energy (DOE) Office of Nuclear Energy established the Advanced Materials and Manufacturing Technologies (AMMT) program to build a rapid qualification framework that takes a more manufacturing and microstructure approach to evaluating new materials and their performance and eventual deployment in nuclear reactors[5-7]. The AMMT program has identified multiple AM and other advanced techniques, but has taken special interest in laser powder bed fusion (LPBF) AM due to the high geometric resolution (<100 µm) that can be achieved across a variety of materials. A schematic of the LPBF process is shown in Fig. 1. During the LPBF process, powder is dispensed into the system and wiped over a starting substrate (i.e., build plate), before the laser selectively melts the profile of the desired component to the base; powder layers are typically <100 µm, requiring hundreds to thousands of consecutive layers to build components[4]. The repetitive local melting of the laser is conducive to high cooling rates, on the orders of 105–107 K/s[8], which can greatly vary the microstructure and generate high residual strains. Moreover, the complex melting and solidification can result in the formation of many process anomalies such as pores, cracks, and layer delamination. In-situ monitoring capabilities integrated with software such as Peregrine, developed at Oak Ridge National Laboratory, is trained to observe process anomalies related to the printing process such as part over-melting or recoater streaking and digging that may cause incomplete melting[9-11]. One process anomaly that can be detected with in-situ capabilities is the formation of spatter particles due to the recoil pressure associated with the laser interacting with the powder bed, which is quite violent, sending particles hurdling upward in the chamber[12], as shown in Fig. 1. An inert gas shroud is often used to sweep over the top of the powder bed and carry spattered particles across to the side, since these particles can oxidize with any remaining oxygen in the build chamber, or coalesce with other particles and become much bigger than the starting powder particle size[11]. However, spatter particles do not always make it to the side of the build plate and redeposit on the build volume. Spatter particles have been directly correlated to the formation of lack of fusion (LOF) voids, among other process anomalies, characterized by inconsistent melting, forming irregular shaped voids[11]. This work investigates the processing of Ni282 by LPBF with attempts to induce process anomalies and potentially more LOF voids. A Ni282 build was set up with a high density of parts located near one another in attempts to induce process anomaly variations across the build volume. The experiment focused on spatter particle process anomalies because they were easy to identify with in-situ monitoring; however, the LOF void variation could Fig. 1 — Schematic of a typical LPBF build chamber as well as dramatization of the spatter particle formation.
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