Pyrometallurgical Studies of Molten Metal Droplets for the Characterization of Gas Metal Arc Welding

Pyrometallurgical Studies of Molten Metal Droplets
for the Characterization of Gas Metal Arc Welding

Stephen Liu

Center for Welding, Joining and Coatings Research

Colorado School of Mines

Golden, Colorado 80401, U.S.A.

Abstract

The presence of silicate islands on a steel weldment can result in loss of productivity. The conditions under which these silicate islands form are not totally elucidated. The mechanisms and kinetics of formation of these slag particles must be better understood for their eventual control and elimination.

Studies at the Colorado School of Mines have gained insight into the effects of welding parameters on weld pool chemical composition and silicate formation rate. Oxidation is the controlling process. To characterize the detailed mechanisms and extremely fast kinetics of oxidation may require better controlled experiments such as electromagnetic levitation and pendant drops. This paper will describe the approaches taken to examine the mechanisms of silicate island formation. Electromagnetic levitation and melting of filler metal droplets with controlled flow of shielding gases (type, flow rate, and sequence) allowed for well controlled oxidation of the molten droplets. High speed video under high intensity monochromatic LED lighting recorded the different stages of formation and coalescence of these silicate islands. FeO formation occurred early on which preceded the oxidation of other alloying elements. Controlled Pendant droplet experiments investigated both chemical and electrochemical reactions since the droplet would still be attached to the electrode. Results from both experiments confirmed that oxidation started in the beginning of the melting process. Contrarily, silicate islands only formed later in the molten droplet. As a result of the kinetic characteristics, there may be a window of opportunities to minimize silicate island formation. This presentation will also emphasize the need for more fundamental approaches in studying weld metal development.