Investigation of Gas Metal Arc Welding as a Potential Method for Additive Manufacturing of Magnesium Alloys

Gas Metal Arc Welding (GMAW) was investigated as a method for the rapid Wire Arc Additive Manufacturing (WAAM) of magnesium alloys. Magnesium AZ61a deposition wire was used to build multilayer walls, large blocks, and hollow cylinders using both high and low input-energy-rate (IER) parameters. The printed structures were analyzed to determine mechanical properties, microstructure, and porosity. Multilayer-wall samples printed at the same torch travel speed (TTS) showed a material yield strength (YS) of 120 MPa, independent of print orientation in relation to the applied tensile test pull force. The samples that were printed at a faster TTS showed the same response to loading conditions, but had a lower YS of 106 MPa, thus demonstrating how an increase in TTS lowers the YS of the deposited material. The stress-at-fracture for all these samples was between 260 MPa and 270 MPa. For the large multi-layer/multi-row (MRML) samples a YS of 120 MPa was also obtained but with lower stress-at-fracture points between 150 MPa and 220 MPa depending on print orientation, due to the presence of larger internal defects caused by bead overlap issues. Scanning Electron Microscope (SEM) analysis was performed on the fracture surface, showing ductile behavior in the fused regions and also uncovering material defects in the MRML samples such as trapped spatter, trapped gas bubbles, and cracks. Optical micrographs were obtained to analyze the microstructure of the samples in the heat affected zone (HAZ) as well as in the bulk material. Grain refinement from 38 µm pre-weld down to 10 µm and 28 µm post-weld was determined for MRML blocks and multilayer walls, respectively. Multilayer hollow cylinders were printed to test the ability of the method to produce closed-shape parts. These cylinders were produced at both high and low IERs and yielded parts with post-deposition machined wall thicknesses ranging from 1.5 mm to 4.5 mm. X-ray Computed Tomography (XCT) was performed to determine the porosity of these parts. The three sections analyzed showed a total-part percent porosity of 0.04 %, 0.039 %, and 0.07%. Larger individual defects, particularly at the closure-of-bead zone were detected, with a maximum single layer percent porosity of 0.8 %. Lastly, a Finite Element Analysis (FEA) model was created to simulate the deposition of the beads and the heat transfer throughout the process. The element activation feature in COMSOL Multiphysics was combined with the simulated torch path to model the deposition of the material. Heat transfer modes of conduction, radiation, and convection were conditionally assigned to the boundaries of the substrate and of the beads as functions of time and material deposition. The Goldak double-ellipsoid heat source was used as the input heating method for the substrate. To simulate the true-to-life GMAW process, where already molten material drops onto the substrate, a bead pre-heating function was created and applied to the inactive elements of the bead before it gets deposited during the simulation so that when the elements are activated above the weld pool they are at the correct temperature.