WIRE-BASED DIRECTED ENERGY DEPOSITION: ANALYSIS OF INCONEL 718, THERMAL DISSIPATION, AND BIMETALLIC RADIAL ARRANGEMENTS

Lile Squires

doi:10.7273/000006470

Advances in metal additive manufacturing have led to an exciting range of specialized solutions that remove traditional research, design, and production limits. The ability to produce complex geometry layer-by-layer using various materials is significant but the use of wire feedstock presents unique advantages, capabilities, and challenges for directly manufactured useable metallic products. One of the most significant advantages held by wire-arc directed energy deposition (DED) is a high deposition rate and capability for large-scale structures. Unfortunately, the high energy input that enables large rate-enabling layer thicknesses also prolongs cooling times, thereby increasing deposition duration. Rate advantages are further eroded as prolonged cyclical cooling periods produce significant variations in microstructural and mechanical properties. A method is therefore conceived and proposed to shorten the deposition time while regulating microstructure in situ during Arc DED-based additive manufacturing (AM) of metals. A conceptual mechanism designed to transfer heat from upper layers as they are deposited is discussed and experimentally validated using cold metal transfer (CMT) of Inconel 718 (IN718). Tensile strength, microstructure, and phase development in as-processed and heat-treated conditions with natural cooling are compared against structures produced using active Conformal Upper Layer Heat Removal (CULHR). Results highlight a 45% reduction in process time, the possibility of manipulating the as-processed state to promote homogeneous grain growth, uniform phase development, overall morphological improvement, and a solutionizing effect with evidence that minimizes the role of specimen orientation after heat treatment. Efficient, high-quality material depositions naturally lead to deeper focus on high-performance combinations of material. Bimetallic wire arc additive manufacturing (AM) has been traditionally limited to depositions characterized by single planar interfaces. Herein, a more complex radial interface concept is explored, with in situ mechanical interlocking and as-built properties suggesting a prestressed compressive effect. A 308L stainless core is surrounded by a mild steel casing, incrementally maintaining the interface throughout the Z-direction. A small difference in the thermal expansion coefficient between these steels creates residual stresses at their interface. X-ray diffraction analysis confirms phase purity and microstructural characterization reveals columnar grain growth independent of layer transitions. Hardness values are consistent with thermal dissipation characteristics, and the compressive strength of the bimetallic structures shows a 33% to 42% improvement over monolithic controls. These results demonstrate that biomimetic radial bimetallic variation is feasible in high-rate, high-energy input wire arc-DED, with improved mechanical response over monolithic compositions. A basis is therefore provided for advanced structural design and implementation, which is demonstrated in the annular combination of AA5356 and SS308L. A viable mechanical coupler is formed without unique alloying or sophisticated technology other than a concentric radial deposition pattern with simultaneous cooling. Vast differences in the material properties of these materials cause major compatibility issues in fusion welding with traditional equipment, but the residual pressure created between concentric material bands creates a workable mechanical bond despite the absence of reliable metallurgical bonding. Interfacial characterization showed a 300x reduction in crack width for concentrically constrained interfaces with narrowed diffusion zones. Coupler specimens sustained 732.96 Nm in torsion, 34.17kN in tension, and a maximum of 475 MPa in compression. Fracture modes confirm the relevance and importance of concentric residual loads in creating the mechanically viable joint. A proof-of-concept article demonstrates the practicality of joining aluminum to steel in this manner using the bimetallic coupler and ordinary welding processes. It is expected that this work will hold real benefit for researchers and practitioners as they seek to leverage the rapidly expanding geometrical and material capabilities of metal AM with specific focus on wire-arc DED, and are confronted with the inevitable choice between wire and powder form feedstocks.

WIRE-BASED DIRECTED ENERGY DEPOSITION: ANALYSIS OF INCONEL 718, THERMAL DISSIPATION, AND BIMETALLIC RADIAL ARRANGEMENTS

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