Evaluating shielding gas-filler wire interaction in bi-metallic wire arc additive manufacturing (WAAM) of creep resistant steel-stainless steel for improved process stability and build quality

Abstract

This study investigates the effect of shielding gases (namely, 98%Ar-2%CO2 and 96%Ar-3%CO2–1%O2) on the bi-metallic wire arc additive manufacturing (WAAM) of creep resistant steel (CRS) and stainless steel (SS). Contrary to the conventional practice of using material-specific gas, i.e., two gases for bi-metallic deposition, the aim here is to find a common shielding gas that creates a favorable mix with alloying elements of each filler wire to facilitate smooth arcing and metal transfer. The appropriateness of the shielding gas is broadly evaluated in terms of two criteria, viz. (a) arc stability and (b) bead uniformity and the penetration pattern. The stability of the arc (in terms of cyclograms, probability density distribution, voltage signal analysis), the accuracy of the bead (visual inspection, X-ray radiography, geometric deviation), and the quality of the deposit (penetration pattern) are first evaluated for single bead-on-plate deposits for the two candidate material and shielding gas pairs. The result of the analysis suggests the appropriate gas, which is then validated by depositing and investigating actual bi-metallic walls of CRS and SS. The change of the gas without alteration of other parameters significantly affects the bead shape, metal-transfer pattern, chemical composition of the welding arc, and bead penetration, all of which contribute to the ability of materials to fuse without any defects at the bi-metallic interface. Higher CO2 and O2 contents in the shielding gas stabilize the welding arc by increasing the metal vapor (i.e., more ionized particles) because of exothermic reaction and reduced surface tension, respectively. The stability leads to bead uniformity and an increase in the weld penetration at the edges and weld dilution, which negates the lack of fusion when two materials are deposited side-by-side to create a bi-metallic wall. The presence of O2 in the gas increases molten metal fluidity, which minimizes the flatness variation on both sides of the bi-metallic wall. The reported results are fundamental and have high significance, serving as guidelines for a wide array of multi-material service components fabricated using WAAM.