Interfacial phenomena in welding: Vaporization and gas dissolution

During welding appreciable changes in the composition and properties of weldment can occur due to pronounced vaporization of alloying elements from the weld pool. A theoretical model was developed to predict vaporization rates and composition changes occurring during high density beam welding processes. The velocity distribution functions of gas molecules at various locations above the weld pool and the heat transfer and fluid flow in the pool were coupled to model the rates of vaporization during conduction mode laser welding of iron, titanium, and stainless steels for low and high powers. Computed values of the vaporization rates, the vapor composition and the weld metal composition change were found to be in good agreement with the corresponding experimental values reported in literature.;The role of plasma in influencing the vaporization rates was studied. Controlled physical modeling of vaporization from the weld pool surface was conducted with ultra-pure iron in the presence and absence of plasma under various conditions. The plasma was characterized using optical emission spectroscopy and plasma parameters were determined from the spectral data. The vaporization rates in the presence of plasma were considerably lower than when no plasma was present. In the absence of plasma, the decrease in vaporization rates with pressure was consistent with decreasing mass transfer rates. In the presence of plasma, the increase in vaporization rate with pressure was explained on the basis of changes in the relative magnitudes of the space charge effect.;The partitioning of gases between the weld pool and its environment can also significantly affect the weldment properties. During welding the gas concentration in the weld metal is considerably higher than that calculated by Sieverts' law. This is due to the dissociation of diatomic gas molecules to atomic and ionic species in the plasma. In order to seek a better understanding of the dissolution process, a two-temperature model was developed. Based on results of the model, the enhanced solubility in the presence of plasma could be explained on the basis of monatomic species present in the plasma.