Effects of oxygen and sulfur on alloying element vaporization rates during laser welding
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INTRODUCTION
O N E of the major problems in the use of a high power density laser beam for the welding of many important engineering alloys is the loss of volatile alloying elements. The inadequate control of weld composition and properties are familiar difficulties in the welding of several aluminum alloys and high manganese stainless steels.[~'2'3] The vaporization of alloying elements from the weld pool is influenced by various factors such as the temperature and concentration distributions at the weld pool surface, the extent of surface coverage by the surface active elements, interracial turbulence, and the modification of the nature of the interface due to the presence of plasma r41 in close proximity of the vaporizing interface. In a recent paper, tSl we have demonstrated that the rate of alloying element vaporization during conduction mode laser welding of stainless steels is controlled by plasma induced intrinsic vaporization at the weld pool surface. Surface active elements such as oxygen and sulfur are known to influence the weld pool fluid motion and aspect ratio which, in turn, affect the vaporization rate] 6'71 Heatto-heat variations in the concentrations of oxygen and sulfur in several commercial alloys are thought to be responsible for the lack of reproducibility of weld geometry and properties. Previous emission spectroscopic investigations by Savitskii and Leskov I81 indicated that during GTA welding of steels, the rates of vaporization of iron and manganese increased with the increase in the sulfur and oxygen concentrations in steel. Similar effects were observed by Dunn, Allemand, and Eagar, Igj who found that the presence of sulfur in the base plate enhanced the intensity of emission of Fe, Cr, and Mn peaks. Although alloying element vaporization is regarded as an important problem during laser processing and the presence of surface active elements is known to affect vaporization, no systematic investigation
R SAHOO and M. M. COLLUR, Graduate Students, and T. DEBROY, Associate Professor of Metallurgy, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted March 16, 1988. METALLURGICAL TRANSACTIONS B
of the role of oxygen and sulfur on the alloying element vaporization rates has been undertaken so far. The work reported in this paper was aimed at understanding the roles of oxygen and sulfur on the rates of vaporization of alloying elements during laser welding. Since most commercial alloys contain a large number of elements, ultrapure iron samples, doped with oxygen or sulfur, were welded with a carbon dioxide laser in the pulsed mode to determine the effects of dopants. The time average vaporization rates were compared with the rates determined from the welding of ultrapure samples. The nature of the various species present in the plasma during welding was monitored by emission spectroscopy. The presence of surface active elements such as sulfur or oxygen in the base metal affects the surface area a
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