COLD METAL TRANSFER BASED WIRE ARC ADDITIVE MANUFACTURING OF BIMETALLIC MATERIAL

Abstract

Wire Arc Additive Manufacturing (WAAM) has emerged as a promising technology for fabricating large and complex metallic structures due to its high deposition rates, reduced material waste, and cost-effectiveness. Among various WAAM techniques, Cold Metal Transfer (CMT)-WAAM stands out for its low heat input, superior arc stability, and minimal spatter generation, making it highly suitable for the fabrication of aluminium alloy components. While CMT-WAAM is commonly used for building single-material structures, recent advancements have showcased its potential in producing bimetallic structures that are gaining attention in engineering applications for combining the advantageous properties of different alloys within a single component, thereby enhancing strength and thermal performance. This study focuses on the fabrication of bimetallic aluminium alloy walls using the CMT- WAAM process, employing ER5356 and ER4043 filler wires. ER5356 is selected for its superior strength and ductility, while ER4043 is chosen for its excellent fluidity and crack resistance. Initially, two independent current ranges: 115-125 A for ER5356 and 90-100 A for ER4043, were finalized to ensure uniform bead geometry, and consistent wall thickness. Subsequently, using a trial-and-error approach, suitable current combinations for both filler wires were determined to ensure proper interlayer bonding and uniform wall morphology, with ER5356 current = 115 A, ER4043 current = 90 A and travel speed (TS) = 6 mm/s, yielding the lowest surface roughness and porosity. To further enhance reliability, two additional current combinations for ER5356/ER4043, specifically 120 A/95 A and 125 A/100 A were selected at a constant TS of 6 mm/s, based on the visual inspection of the fabricated walls, resulting in a total of three sets of current parameters. Using these parameters, six bimetallic WAAM walls were fabricated and evaluated under both unidirectional and bidirectional deposition strategies, resulting in walls with dimensions of 150 x 100 x10 mm3. Comprehensive evaluations of microstructural characteristics, mechanical behaviour, and residual stress distributions indicated that the bidirectional deposition strategy combined with the 115 A/90 A current setting delivered the most favourable results across all performance metrics. This is confirmed by optical microstructure as well as Field Emission Scanning Electron Microscopy (FESEM), and Electron Backscatter Diffraction (EBSD) at low current combination shows equiaxed grains on the ER4043 layer, fine grains on the ER5356 layer, v and columnar-fine grains at the interface of the bidirectional wall while discontinuous dendritic grains are displayed in ER5356 layer of unidirectional wall. Further, EBSD analysis of the bidirectional wall fabricated at low current combination revealed that the ER5356 region had an average grain size of approximately 16.28 μm, whereas the ER4043 region exhibited significantly coarser grains, averaging around 118.92 μm. Energy dispersive spectroscopy (EDS) analysis indicates a main difference in weight percentage for Si and Mg contents at the interface layer of the bidirectional wall than the unidirectional wall, with X- ray diffraction (XRD) analysis specifying the intermetallic compounds like α-Al, Al12Mg17, Mg2Si, and Al3.21Si0.47 in both depositional directions. The tensile strength of the wall fabricated with the 115 A/90 A bidirectional current combination achieved the highest average value of 205.73 MPa and 18.94% elongation, showing significant plastic deformation and fracture toward the ER4043 side. Vickers microhardness was higher in the transverse direction (~82-56 HV) than in the longitudinal direction (~78-50 HV), with the upper zone showing the lowest hardness, while the wall fabricated with the 115 A/90 A bidirectional current combination exhibited the highest hardness across all layers. Residual stress analysis indicated higher stress in unidirectional deposition (S2 wall) than in bi-directional deposition, with the bottom ER5356 layer experiencing the highest stress. The study further extended to a detailed tribological evaluation of each bimetallic wall under varying loads (20N, 30N, and 40N). It is found that the wall fabricated with the current combination of 150 A/90 A exhibited the lowest wear rate (0.0019 mm3/m) and the minimum Coefficient of Friction (COF) (0.25), in the ER5356 layer. Results showed that the bidirectional walls consistently exhibited lower wear rates and COF, with wear rate increasing at higher loads due to thermal gradients and microstructural softening in upper layers. Scanning Electron Microscope (SEM) image of worn surface and wear debris analysis identified abrasion, adhesion, and delamination as dominant wear mechanisms, with larger debris observed in upper layers under high loads. Overall, this thesis presents a systematic approach to fabricating bimetallic aluminium alloy walls through the CMT-WAAM technique, ensuring enhanced mechanical performance, improved surface integrity, and superior wear resistance. The findings offer valuable insights for advancing the development of high-performance, lightweight structures in applications where both mechanical and tribological properties are essential.