Evaluation of microstructure, mechanical properties, and corrosion resistance of additively manufactured materials: Directed Energy Deposition (DED)

Abstract

Metal additive manufacturing has experienced significant growth in recent years due to its ability to produce complex components through the layer-by-layer deposition of material directly from digital models. Among the different available technologies, Directed Energy Deposition (DED) processes, and particularly Wire Arc Additive Manufacturing (WAAM), stand out for their high deposition rates and their capability to manufacture large-scale metallic components. As a starting point for this research, a comprehensive state-of-the-art review of these technologies is presented. On the one hand, existing applications of metal additive manufacturing for the production of large-scale components are analyzed, highlighting the strong potential of DED processes for the fabrication of functional parts across various industrial sectors. On the other hand, a review of the scientific literatura concerning the corrosion behavior of steels produced by different additive manufacturing techniques is conducted, identifying how the manufacturing proces influences the final material properties and revealing several research gaps that still require further investigation. The experimental studies of this thesis focus on the use of WAAM technology for the fabrication and characterization of both monomaterial and multimaterial components. First, the influence of the welding process employed in WAAM on the corrosión resistance of austenitic stainless steel 316L is investigated by comparing the GMAW and PAW techniques. The results show that materials produced by both processes retain high corrosion resistance. However, the PAW process exhibits slightly improved behavior due to the better process control and the lower number of manufacturing defects. Additionally, PAW-fabricated walls show slightly higher hardness values and greater microstructural homogeneity. A second monomaterial study investigates the influence of different GMAW-based WAAM deposition modes on the microstructure, hardness, wear, and corrosion behavior of Al5356 walls. The results reveal that, although magnesium segregation at grain boundaries promotes susceptibility to intergranular corrosion regardless of the deposition mode, the selected transfer mode significantly affects the resulting porosity and grain size. In particular, the pulsed AC mode leads to lower porosity and coarser grain size, resulting improved corrosion resistance compared with pulsed GMAW and Cold Arc conditions. Hardness values remain similar for all deposition modes, although pulsed AC exhibits slightly lower and more homogeneous values due to its coarser microstructure. No significant differences are observed in the tribological behavior among the tested deposition modes. Subsequently, the potential of material hybridization is explored through two different strategies: in-situ alloying and the fabrication of bimetallic structures through sequential multimaterial deposition. In the first approach, in-situ alloying of 316L stainless steel and ER70S-6 carbon steel is studied using the WAAM-GMAW-HW process, varying the relative proportion of both feedstock materials. This strategy enables the generation of new compositions during the deposition process. The results indicate that using stainless steel as the main wire hinders proper mixing of the materials, leading to defects and heterogeneous microstructures with variable hardness values. Conversely, when ER70S-6 acts as the primary wire, a more homogeneous mixture is obtained, characterized by a predominantly martensitic microstructure with high hardness. In both cases, the corrosion resistance of the resulting materials lies between that of stainless steel and carbon steel. Finally, the feasibility of producing bimetallic structures by sequential multimaterial deposition of Inconel 718 and 316L stainless steel using WAAM is evaluated, employing two deposition strategies: overlapping beads and alternating beads within the same layer. The results demonstrate that the fabrication of these multimaterial walls is technically feasible. However, the interface between both materials is identified as a critical region where microcracks and discontinuities may form. From a mechanical perspective, the properties of the bimetallic walls are comparable to those of WAAMprocessed 316L stainless steel. Nevertheless, their corrosion resistance is reduced compared to the base materials due to the presence of interfacial defects and the formation of galvanic couples, which promote localized corrosion. Overall, the results highlight the potential of WAAM technology for the fabrication of tailored materials and multimaterial structures, as well as the importance of optimizing process parameters and component design, particularly the distribution of materials within the structure, to ensure adequate corrosion performance in demanding industrial applications.