Effects of oxygen and sulfur on alloying element vaporization rates during laser welding

Abstract

Inadequate control of weld metal composition due to vaporization of volatile alloying elements is a serious problem in the welding of many important engineering alloys. Effectiveness of surface active elements such as oxygen or sulfur in blocking vaporization sites on the weld pool surface was investigated. Several iron samples doped with oxygen or sulfur were exposed to a carbon dioxide laser beam in pulsed mode. The time average metal vaporization rates and the emission spectra were compared with those obtained from ultra pure iron samples. Since the weld pool surface area and temperature distribution are affected by oxygen and sulfur, the true effects of these elements on metal vaporization rates cannot be easily evaluated from welding data. Therefore, rates of isothermal vaporization of iron and copper drops doped with oxygen or sulfur were determined both in the presence and the absence of low pressure argon plasma. These rates were compared with the rates of vaporization of ultrapure metal drops. Presence of sulfur or oxygen in metals always resulted in increased metal vaporization rates. The results are analyzed on the basis of interfacial phenomena.