Chemical Behaviour of Copper in the Application of Unconstrained Cr-Ni-Al-Cu Metal Powders in Submerged Arc Welding: Gas Phase Thermodynamics and 3D Slag SEM Evidence

Abstract

Unconstrained metal powders of Cu, Cr, Ni and Al were applied to submerged arc welding (SAW) to clarify the chemical behaviour of copper in this modified SAW process. Aluminium metal is avoided in SAW because it is easily oxidised. Excessive aluminium oxides in the form of slag or inclusions in the weld metal will lead to poor weld metal materials properties. Aluminium is an effective deoxidiser and can be used to prevent Cr and Ni loss to the slag by preventing oxidation of these metals. The results show that carbon steel was alloyed to 5.3% Cr, 5.3% Ni, 3.6% Al and 5.2% Cu at 80% Cr yield, 81% Ni yield, 54% Al yield and 79% Cu yield. BSE (backscattered electron) images of the three-dimensional (3D) post-weld slag sample show 3D structures within the slag dome. The 3D structures contain features of vapour formation and recondensation. In addition, nano-strands appear in the 3D structures and confirm the vaporisation and recondensation of fluorides. The chemical behaviour of copper metal powder added in SAW is to vaporise as metallic copper and incorporate in the Al-Si-Mg-Ca-Mn-Fe-Cu-Na-Cr-Ni fluoride. Copper, in combination with aluminium, has a stabiliser effect in SAW due to its formation of an initial alloy melt of low liquidus temperature, thus decreasing the temperature required to melt high-melting-point metals such as Cr into the weld pool. Although Al and Cu have similar vapour pressures at specific temperatures, it appears that Cu does not substitute for Al in the gas phase. Gas-slag-alloy thermochemical equilibrium calculations confirm the partial oxygen pressure lowering effect of aluminium and the vaporisation of copper as metallic copper with very little copper-fluoride species expected to form. The quantity of metallic copper vaporisation calculated in the gas-slag-alloy thermochemical equilibrium is much higher than the vaporisation quantity measured in welding. This may be due to recondensation of vaporised copper which is not accounted for in the equilibrium calculation at the set arc cavity temperature, as well as the effect of surface-active elements such as sulphur and oxygen in limiting the vaporisation reaction of copper.