Effect of process gas mixtures on weld material characteristics and bead geometry for wire-arc directed energy deposition

Shielding gases are used in welding technologies to prevent contamination and protect the metallic melt from disadvantageous effects that air could cause on the weld. While argon is mostly used for gas metal arc welding of aluminum, this paper investigates the use of mixtures with traces of different gases. Various properties of the weld seams are assessed: Effects on bead geometry, microstructure, defects, and mechanical characteristics of the resulting material. Investigations were performed for single welds as well as directed energy deposited wire-arc specimens. For this purpose, single bead on plate with CO₂, N₂, and O₂ in the mixture and wall geometry samples with N₂ and O₂ were manufactured and subsequently analyzed. Nitrogen in the gas mixture is reducing the bead and deposit width and decreasing the grain size compared to the reference sample. This grain size reduction is due to the formation of nitrides in the weld material acting as nucleants for new grains. Nitrides were identified by energy dispersive X-ray spectroscopy. Furthermore, nitrogen is reducing the number of pores but not significantly their volume fraction. A similar effect is reported for used amounts of O₂ on smaller scale. The characteristic mechanical strength values are comparable to reported data, but the elongation is reduced when nitrogen is present in the shielding gas mixture.