Optimization of Process Stability and Deposition Quality in CMT Arc Welding of Pure Copper on Cast Iron using He-Ar Gas Mixtures

This study investigates the arc characteristics and process stability of Cold Metal Transfer (CMT) welding for fabricating copper (Cu) overpacks of spent nuclear fuel disposal canisters. Applying arc deposition processes to pure Cu poses significant challenges due to the material's high thermal conductivity, which inherently induces arc instability. To address this, the influence of shielding gas composition (100% Ar vs. 75% He - 25% Ar) on arc physics, metal transfer regularity, and bead geometry was systematically analyzed. In the pure argon (Ar) environment, the arc exhibited an expanded bell-shape with erratic root wandering, resulting in a random and unstable nature of metal transfer with a high Vilarinho regularity index (IVsc) of 0.43 and excessive spatter generation. Conversely, the addition of 75% helium (He) induced a strong thermal pinch effect, constricting the plasma into a highly regular and stable columnar arc morphology. This morphological change effectively anchored the arc root, significantly improving process stability with a reduced IVsc of 0.17 and eliminating current overshoot-induced spatter. Consequently, the He-rich condition produced beads with superior dimensional uniformity compared to the irregular deposition observed under pure Ar. These findings demonstrate that utilizing the thermal pinch effect via optimized shielding gas is critical for overcoming the intrinsic instability of Cu-Fe system, providing a fundamental basis for high-quality canister fabrication.