ADVANCED MATERIALS & PROCESSES | MARCH 2025 15 although it is not as conspicuous. It is likely that multiple larger voids coalesced more readily in the C3 sample, and cannot be observed in Fig. 5d, as they are all along the fracture surface itself. MICROSTRUCTURAL CHARACTERIZATION XCT is a powerful tool for observation of the void space inside of additively manufactured material, or a specific component. Ideally, every component can be examined by XCT post-build as a qualification tool to determine the existence of any limiting process anomaly induced voids that could hinder the material’s performance. OM has been a verification tool for XCT at the smaller scale because process induced voids and voids from creep cavitation can coalesce and become larger problems. Optical image analysis requires destructive testing as samples must be sectioned for metallographic preparation, but can observe smaller features as well as give insight into their location in the microstructure. In LPBF, the types of voids observed in the microstructure can be broken down in three simplified categories: (1) spherical pores due to the vaporization of the melt, forming a keyhole with high penetration into the material that can potentially result in entrapped gas during solidification due to the fast solidification rates not providing the time for gas to escape; (2) irregular voids formed due to incomplete melting or lack of fusion (LOF); (3) line voids that are either related to cold or hot cracking from solute segregation or high residual strains that cannot be accommodated by the material, or another form of LOF voids[4]. Figure 6 shows the cross-section of the C3 sample near the fracture surface in the ruptured condition. In Fig. 6a, multiple voids including the three common types mentioned above (indicated by arrows), are observed. Many factors can cause incomplete melting and the formation of LOF voids, but it is likely that the high degree of spatter particles deposited on the C3 are the reason for numerous voids seen in Fig. 6a. Keyhole pores are stochastic and form depending on the local environmental conditions and the instantaneous behavior of the laser, making them unstable and difficult to predict. Therefore, keyholes are sporadically distributed throughout Fig. 6a. The chemically etched version of Fig. 6a is shown in Fig. 6b, where the irregular and line LOF voids appear to lie across multiple grains; keyhole pores are no longer easily distinguishable in the etched surface. Cracking formation during creep testing was observed to nucleate from both LOF voids, as shown in Fig. 6c, but expected creep damage was also observed, as seen in Fig. 6d. Even though LOF voids are stress concentrators that are conducive to cracking, stress fields still accumulated elsewhere, finding creep cavitation to still be a possibility in contributing to creep failure. SUMMARY The AMMT program has identified LPBF AM processing as a potential route for rapid qualification of new materials for use in nuclear reactor design. Process anomalies such as spatter particles can induce LOF voids that may adversely affect the mechanical performance of the material. To assess the impact of LOF voids on creep resistance, this study used LPBF to fabricate nickel superalloy 282 with a high density of components in a single build, to intentionally induce process anomalies that may cause LOF voids. In-situ monitoring revealed a higher Fig. 5 — 3D renderings with voids and compressed pseudo crosssections reconstructed from XCT data for the (a) T3 reference, (b) T3 ruptured, (c) C3 reference, and (d) C3 ruptured samples. The arrows indicated tracked voids that grew in size during creep testing, likely due to coalescence of smaller voids. Note: Change in diameter between the reference and rupture conditions is due to the necking of the gage section during creep testing. Fig. 6 — Optical images of the cross-section for the C3 sample near the gage section a er rupture, (a) as-polished and (b-d) a er chemical etching. Distinguishing between cracks nucleated from (c) LOF voids or (d) a result of creep deformation. (c) (d) (b) (a) (a) (b) (c) (d)
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