Study of flux-cored arc welding processes for mild steel hardfacing by applying high-speed imaging and a semi-empirical approach

Abstract

Arc welding processes with flux-cored wire electrodes are often applied for steel hardfacing. The optimal choice of the process parameters is a key issue for the process stability and the surface quality of the weld seam. However, this complex as a relation between the process characteristics, the predominant mechanisms of the arc, the material transfer, the solidification of the molten pool, the metallurgical properties and therefore the wear behaviour of the weld seam surface is not well studied. Synchronous high-speed imaging with spectral filters and different viewing angles is used for a detailed analysis of the arc attachment at the wire electrode, the wire melting and the behaviour of the weld pool. Two different gas metal arc welding processes, a pulsed process without short circuits and a modified short arc process, and different flux-cored wire electrodes have been used. All the combinations have been studied under different shielding gases: mixtures of Ar with CO2, O2 or He. Macrosections have been used to characterize seam width and dilution. Vickers hardness (HV 0.1) was tested to quantify the hardness of the different phases. The modified short arc processes have turned out to be more stable and go along with a reduced energy transfer to the substrate. As a consequence of the lower energy input, the short arc processes cause a lower dilution but a poor weld seam geometry in comparison with the pulsed processes. The choice of the shielding gas has a significant impact on the melting of the wire and the weld pool behaviour in particular in case of the modified short arc processes. A flatter and regular seam but with more coarse-grained surface structure is obtained with larger admixture of a molecular gas in the shielding gas flow. A semi-empirical approach of the correlation of power input and weld seam geometry demonstrates the potential decrease of the dilution and only smaller changes of the seam form factor by decreasing the electric power of a pulsed process.