Parametric Development of Cold Metal Transfer Wire Arc Additive Manufacturing
Type of DegreeMaster's Thesis
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Current commercial metal additive manufacturing techniques offer great accuracy and detail in small parts but are characterized by low deposition rates, high cost, and limited scale. This research seeks to develop, review, and establish new standards for the Wire Arc Additive Manufacturing (WAAM) technique utilizing a Fronius Cold Metal Transfer (CMT) welder. CMT is a new and proprietary Gas Metal Arc Welding (GMAW) process that provides increased process stability and control and while reducing excess heat input in deposition. WAAM uses an electric arc to melt and deposit metal alloys at much greater rates than other additive manufacturing techniques. This technology can be scaled to produce large parts, using a wide variety of filler metals, directly from 3D CAD drawings. This technology offers an alternative to low-volume casting or subtractive manufacturing, reducing the time to manufacture, reducing wasted material, and saving on tooling. Wire Arc Additive Manufacturing (WAAM) as a process has been considered since 1925, with Cold Metal Transfer (CMT) based WAAM being the most technologically advanced iteration of this process. This study builds off a prior research at Auburn University on GMAW based WAAM using a 3-axis gantry CNC. In the integration of the Fronius CMT welder, several new feedback control loops were added to increase process stability and deposition accuracy. Problems with varying contact tip to work distance are solved by the use of a probing loop. To evaluate the capabilities of deposition, multiple geometries were printed. These include thin and thick-walled structures as well as complex geometries. These objects were printed using three filler metals, a carbon steel (ER70S-6), an aluminum alloy (ER4043), and a stainless steel (ER308). Optimal weld parameters were studied with respect to thick-walled and thin-walled deposition with the goal of finding the best practices for welding the different geometries. The surface roughness was evaluated by correlating it to the percent yield, calculated as the ratio of the net size after machining to material deposited. The tensile strength was evaluated in multiple orientations for ER70S-6, ER308, and ER4043. The tensile strengths were determined to be at or above manufactures specifications and isotropic, or independent of direction, for carbon steel and aluminum. However, the tensile strength for ER308 stainless steel did not show isotropic results, with the transverse direction and longitudinal direction at 95 and 100% manufacturers spec, respectively. The microstructure was examined for ER70S-6, ER308, and ER4043. Layer interfaces could be seen on the edges of ER308 and throughout the ER4043 sample. A uniform microstructure was observed throughout ER70S-6. Little to no porosity or void space was noticed throughout all the samples evaluated. A fine equiaxed grain structure was observed for all the materials.