Metal Additive Manufacturing Using Non-transferred and Transferred Type Electric Arc: Microstructural, Mechanical and Biomedical Investigations

Abstract

Recent developments in the additive manufacturing (AM) field require a process with maximum deposition rate and superior build quality while reducing the cost of production, porosity and contaminations. In such situations, wire arc additive manufacturing (WAAM) found beneficial compared to the laser and electron beam melting-based AM systems. Additionally, the WAAM system relies on wire feedstock, which makes it more advantageous to achieve high deposition at low feedstock cost and improvement in part density and mechanical characteristics. However, geometrical undulation, spatter, molten metal overflow, thermal distortions, anisotropy, and wastage of substrate material are the key points to consider during WAAM. Based on these issues, the objectives of this experimental investigation are ormulated. Initially, a non-transferred type electric arc (NTA) has been developed between tungsten and wire electrodes, which facilitates deposition even on the non-conductive substrate. The unavailability of electric arc connection with substrate, makes minimal heat input to the substrate and maximises the developed heat for wire melting. It also minimized the impingement of filler wire and molten droplets into the melt pool, enhancing the deposition efficiency while reducing the spatter. No arc connection also limits the generation of oversized molten pool on the substrate surface and later on minimises the molten metal overflow during thin-layered fabrication. The developed NTA system is also applicable in small-scale castings via drop-by- drop transfer of molten metal into a mould cavity under the plasma shielding (helps in reducing the porosity chances). It also culminates the requirement of melt furnace, sprue and risers that make the process advantageous from the cost point of view (reduction in initial setup and production cost). Moreover, the casted part shows improvements in ultimate tensile strength (UTS) compared to its wrought part. Geometrical undulations and anisotropy are primarily caused by improper thermal energy management, as most of the commercially available GMAW machines have current controlled wire feeding mechanisms. So, adequate control over arc current generation and wire feed speed (WFS) is not achieved. To mitigate such issue, an autonomous wire feeding system (AWFS) has been designed and integrated into the GMAW-WAAM, which independently controls the WFS of filler wire irrespective of the welding current values. The availability of fine-tuning options for WFS meticulously controls the flow of arc current that in turn, maintains and manages the thermal energy distributions during the deposition process. The thin-layered structures (using ER70S-6 and 316 and filer wire) fabricated through this approach depict mechanical anisotropy of <5%, indicating the isotropic nature of deposit. Moreover, the bulk texture evolution depicts similar fiber texture evolutions with limited variations in the texture intensity, which also highlights the isotropic nature of the deposit. Further, the deposited structures of 316LL SS (via AWFS-WAAM) evolve δ-ferrite structure, which is tunable with the interlayer dwell time (IDT). Its occurrences improved the corrosion resistance, preventing the release of toxic ions into the bloodstream (reduces the hemolysis rate <0.3%) and unaltered the CD spectra of plasma protein, reflecting high hemocompatibility for the WAAMed 316L SS. Also, less adherence and activation of platelets on the WAAMed deposits indicates high biocompatibility. Moreover, the reduction in contact angle (highly hydrophilic) promotes the adsorption of body fluid and proteinaceous materials that boost the adhesion, viability, proliferation, and spreading of MG63 cells (in-vitro). It also allows the growth of osteoblast cells, marrow spaces and collagen fibers (from in-vivo studies) on the WAAMed deposits. The WAAMed implant does not show any acute toxicity in the blood profiles and vital organs like liver and kidney after long-term toxicology analysis in WISTAR rats. Moreover, a double-wire WAAM (after integrating AWFS) system has been developed to fabricate Fe-based alloy fabrication, especially advanced high-strength steel (AHSS). It aims to tune the chemical composition and mechanical performance of the as-built alloy. The fabricated thin-layered alloys exhibit anisotropy within 5%, indicating the process’s effectiveness in maintaining the isotropicity. While compared to different AHSS standards, the as-built alloys exhibit superior mechanical performances in terms of strength and ductility.