# Altering the Supply of Shielding Gases to Fabricate Distinct Geometry in GMA Additive Manufacturing
## 2 Results and Discussion
### Dimensional Accuracy  
Height comparisons of the scanned wall structures are presented in Figure 5. Each scanned wall (Figure 5, yellow color) is compared with the original 132 \(\times\) 250 \(\times\) 13 mm computer-aided design (CAD) geometry (Figure 5 blue color). Although identical parameters were used, the binary blend underbuilt the height, while the ternary blend approached the height of the CAD model. This height difference is critical to the design phase of WAAM to achieve dimensionally accurate parts. There have been some technological advancements made that allow the deposition of additional layers in order to achieve dimensionally accurate parts with closed loop control [23]; however, in this study, we used open loop control to investigate the effect of using different shielding gases.  
A single bead-on-plate experiment was performed to evaluate the bead characteristics with the binary and ternary blends. As seen in Figure 6, the ternary blend had a deeper and narrower bead profile, while the binary blend had a shallower and broader penetration, which led to the difference in height seen in Figure 5.  
The total area of the deposition, including the dilution and bead, was 20.6 mm\({}^{2}\) for the binary blend and 22.2 mm\({}^{2}\) for the ternary blend. The dilution area below the surface was 5.7 mm\({}^{2}\) for the binary blend and 6.6 mm\({}^{2}\) for the ternary blend. The lower dilution area associated with the binary blend indicates that the total transferred heat was lower than  
Figure 5: Comparison between CAD geometry (in blue) and scanned build part (in yellow) for the two gas blends.  
Figure 6: Bead-on-plate experiment using a binary (**left**) and ternary blend (**right**).  
that achieved with the ternary blend. It is obvious from the height difference in Figure 6 that the penetration and melting efficiency were higher with the ternary blend.  
The arc pressure is inversely proportional to the square of arc radius, as given by \(\mathrm{P_{arc}}=(\mu_{0}\,\mathrm{I^{2}})/(4\pi\mathrm{r^{2}})\), where I is the welding current, \(\mu_{0}\) is the permeability, and r is the arc radius. Since the welding current was identical in both gases, the narrower weld indicates that the arc pressure was higher for the ternary blend than the binary blend.  
Cross-sections from both the binary- and ternary-blend walls were cut and mounted on a conductive mount. Figure 7 shows the thickness of the two walls. The thickness of each wall was measured at four different heights. The binary blend resulted in a significantly thicker wall (average thickness 12.5 \(\pm\) 0.3 mm) than the ternary blend (average thickness 10.1 \(\pm\) 0.1 mm). This difference is consistent with the previous observation of the penetration width from the bead-on-plate experiments (Figure 6).  
In a further experiment, the shielding gas was altered from the ternary to the binary blend midway through the build. As seen in Figure 8, the width increased when the binary blend was used. This is essential information because fabrication by the wire arc additive process is based on a trial-and-error approach. The advances in the technology have allowed us to print dimensions parallel to the print direction accurately with closed-loop control. However, we still need to adjust process parameters to change the dimensions perpendicular to the print direction.  
Figure 8: Side and cross-sectional views of wall deposited by altering the shielding gas from ternary blend to binary blend midway through the build.  
Figure 7: Thickness of wall cross-sections deposited using binary (**left**) and ternary (**right**) gas blends.