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Bimetallic wire arc additive manufacturing (AM) has traditionally been limited to depositions characterized by single planar interfaces. This study demonstrates a more complex radial interface concept, with in situ mechanical interlocking and as-built properties suggesting a prestressed compressive effect. A 308 L 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. Our results demonstrate that biomimetic radial bimetallic variation is feasible with improved mechanical response over monolithic compositions, providing a basis for advanced structural design and implementation using arc-based metal AM.

 

Additive manufacturing (AM) of metals attracts attention because it can produce complex structures in a single step without part-specific tooling. Wire arc additive manufacturing (WAAM), a welding-based method that deposits metal layer by layer, is gaining popularity due to its low cost of operation, feasibility for large-scale part fabrication, and ease of operation. This article presents the fabrication of cylindricalshaped mild steel (ER70S-6) samples with a gas metal arc (MIG)—based hybrid WAAM system. A mechanism for actively cooling the substrate is implemented. Deposition parameters are held constant to evaluate the impact of active cooling on deposition quality, inter-pass cooling time, and internal defects. Surface and volume defects can be seen on the cylindrical sample fabricated without an active cooling setup. Defect quantification and phase analysis are performed. The primary phase formed was α-iron in all samples. Actively cooled deposition cross section showed a 99% decrease of incomplete fusion or porosity, with temperature measured 60 s after deposition averaging 235°C less than non-cooled. Microstructural analysis revealed uniformity along the build direction for actively cooled deposition but non-uniform microstructures without cooling. Hardness decreased by approximately 22HV from the first layer to the final layer in all cases. Property variation can be attributed to the respective processing strategies. The current study has demonstrated that active cooling can reduce production time and porosity while maintaining uniform microstructure along the build direction. Such an approach is expected to enhance the reliability of WAAM-processed parts in the coming days. 

Recent advances in the processing of wear-resistant calcium-phosphate reinforced CoCrMo composites for articulating surface applications has necessitated further investigation of performance in biological conditions relevant to patient applications. To this end, CoCrMo composites containing calcium phosphate in the form of hydroxyapatite (HA) were manufactured to study the influence of the reinforcing phase on the tribofilm formation in biologically-relevant conditions. The CoCrMo-HA composites were processed using a laser engineered net shaping (LENS™) additive manufacturing (AM) system with three distinctive compositions: CoCrMo-0%HA, CoCrMo-1%HA, and CoCrMo-3%HA. Extensive wear testing of the CoCrMo-HA composites was carried out in DMEM (cell media) and DMEM + Hyaluronic acid (found naturally in synovial fluid). Wear tests were performed at loads ranging from 5N to 20N, and wear media was measured post-test using ICP-MS techniques for the release of Co and Cr ions. During testing, all coefficients of friction remained in the 0.15-0.25 range, which was lower than the previously reported 0.50-0.75 range in DI water, indicating that the DMEM + hyaluronic acid media plays a significant role in reducing frictional contact. At loads higher than 15N, the HA-tribofilm exhibited a breakdown resulting in higher wear rates but still lower overall ion release in comparison to the CoCrMo control composition. Our results indicate that CoCrMo alloys with HA addition can significantly reduce wear rates and ion release even in the presence of naturally-occurring synovial-fluid friction-reducing constituents. 

Emulating the unique combination of structural, compositional, and functional gradation in natural materials is exceptionally challenging. Many natural structures have proved too complex or expensive to imitate using traditional processing techniques despite recent advances. Recent innovations within the field of additive manufacturing (AM) or 3D Printing (3DP) have shown the ability to create structures that have variations in material composition, structure, and performance, providing a new design-for-manufacturing platform for the imitation of natural materials. AM or 3DP techniques are capable of manufacturing structures that have significantly improved properties and functionality over what could be traditionally-produced, giving manufacturers an edge in their ability to realize components for highly-specialized applications in different industries. To this end, the present work reviews fundamental advances in the use of naturally-inspired design enabled through 3DP / AM, how these techniques can be further exploited to reach new application areas and the challenges that lie ahead for widespread implementation. An example of how these techniques can be applied towards a total hip arthroplasty application is provided to spur further innovation in this area.