Microstructure Evolution in 304L Stainless Steel Cladding Produced by Additive Friction Stir Deposition

Additive friction stir deposition (AFSD) is a solid-state additive manufacturing and repair process that provides a way to rapidly repair and/or fabricate components using similar feedstocks as the repaired substrate without melting the feedstock or substrate material. In AFSD, the material is deposited using a high-shear thermomechanical deposition process resulting in a near-net shape product. In this work, we evaluate the viability of the AFSD process for depositing stainless steel 304L (SS304L) feedstock onto the same stainless steel substrate. This is considered as a potential crack repair or cladding repair methodology, especially at high deposition rates, for dry cask storage containers (DCSCs) used for spent nuclear fuel. The SS304L DCSCs are susceptible to chloride-induced pitting and stress corrosion cracking, which demands periodic maintenance and repair. Therefore, AFSD, due to its non-melting deposition process provides low heat input and wrought-like mechanical properties, is evaluated here for SS304L through a combined microstructural and mechanical analysis of the deposited layers and substrate material. Specifically, electron backscatter diffraction identified equiaxed grains with a mean size of ~ 3–4 µm in the deposit, similar to the average grain size of 3.5 µm in the wrought substrate, with regions of refined grains near the tool/deposit and deposit/substrate interface. The microhardness at the mid-thickness of the deposit (280 Hv) is comparable to the wrought substrate hardness of 278 Hv. The dense, wrought-like SS304L deposit suggests further applying the promising AFSD technique to repair structures like the DCSCs.