Calculation of weld metal composition change in high-power conduction mode carbon dioxide laser-welded stainless steels
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I.
INTRODUCTION
D U R I N G laser beam welding of many important engineering alloys, pronounced vaporization of alloying elements takes place from the weld pool surface. As a consequence, the composition of the solidified weld pool often differs significantly from that of the alloy being welded. For example, significant changes in the composition of the weld metal have been reported t~ 41 in the laser welding of high-manganese stainless steels and various aluminum alloys. The problem of composition change is particularly pronounced in the welding of thin sheets ~4~ where lasers are most commonly used. Currently, there is no comprehensive theoretical model to predict, from fundamental principles, laser-induced metal vaporization rates and the resulting weld pool composition changes. Because of its importance, alloying element vaporization from the weld pool has been investigated both experimentally and theoretically. Apart from the examination of the weld metal composition and structure to evaluate the direct effects of vaporization, much of the previous experimental work was based on in s i t u monitoring of the alloying element vaporization by emission spectroscopy. [5,6,7] It was found that during welding of stainless stems, the most dominant species in the vapor phase were iron, manganese, nickel, and chromium. Block-Bolten and Eagar ~8~ used calculations based on the Langmuir equation to demonstrate that iron and manganese were the most prominent vapor species in the welding environment. Although the rates calculated from the Langmuir equation are useful for obtaining relative vaporization rates
K. MUNDRA, Graduate Student, and T. DEBROY, Professor, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted February 26, 1992. METALLURGICAL TRANSACTIONS B
of various alloying elements, tS1 the calculated vaporization rates are significantly higher than the actual vaporization rates under commonly used welding conditions. Even at low pressures, of the order of 2 0 0 / z m of Hg, the vaporization rates of pure metal drops were found 191 to be about an order of magnitude lower than the values calculated from the Langmuir equation. The main difficulties in the calculation of the alloying element vaporization rate are the estimation of the condensation rate of the vapor species on the surface of the pool t~~ and the determination of the effect of plasma t5'121 in the suppression of the vaporization rate. When a metal is irradiated with a very high-power density laser beam, a significant amount of vapor condensation can take place, and the kinetics of vapor condensation must be taken into account in the calculation of the net vaporization rate. Anisimov and Rakhmatulina tw] and Knight tlq derived the equations for the calculation of the vapor condensation rates for pure metals by solving the equations of conservation of mass, momentum, and energy in a thin layer adjacent to the liquid-vapor interface, known as the Knudsen layer. C
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