Showing papers by "Tarasankar Debroy published in 1989"

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Abstract: Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.

Weld pool development during GTA and laser beam welding of Type 304 stainless steel; Part I - theoretical analysis

Abstract: A computational and experimental study was carried out to quantitatively understand the influence of the heat flow and the fluid flow in the transient development of the weld pool during gas tungsten arc (GTA) and laser beam welding of Type 304 stainless steel. Stationary gas tungsten arc and laser beam welds were made on two heats of Type 304 austenitic stainless steels containing 90 ppm sulfur and 240 ppm sulfur. A transient heat transfer model was utilized to simulate the heat flow and fluid flow in the weld pool. In this paper, the results of the heat flow and fluid flow analysis are presented.

Emission spectroscopy of plasma during laser welding of AISI 201 stainless steel

Abstract: The rates of vaporization of alloying elements from the weld pool were related to the emission spectra of the plasma during pulsed laser welding of AISI 201 stainless steel under various welding conditions. The temperature distribution in the plasma was determined from the spectra obtained from various locations in the plasma plume. The extent of ionization of the plasma was calculated from the electron temperatures To understand the role of surface active elements, emission spectra and the vaporization rate of iron that resulted from the welding of ultrapure iron samples were compared with those from the welding of oxidized samples or samples that were doped with sulfur or oxygen.

Modeling the effect of surface active elements on weld pool fluid flow, heat transfer and geometry

Abstract: The influence of sulfur on the heat flow and fluid flow and the transient development of the weld pool was quantitatively evaluated for two heats of Type 304 stainless steel containing 90 and 240 ppM sulfur, respectively. A transient heat transfer model was utilized to simulate the heat flow and fluid flow in the weld pool during stationary laser and gas tungsten arc (GTA) spot welds. A recently developed surface tension model was utilized to calculate the temperature coefficient of surface tension (d..gamma../dT) as a function of temperature and sulfur content. This allows a realistic evaluation of the effect of surface-active elements on the fluid flow and weld geometry. The computed results indicate that during stationary welding, the weld pool surface temperatures are fairly high. As a consequence, the temperature coefficient of surface tension becomes negative and a radially outward or a bifurcated flow of weld metal occurs even when the weld pool contains a significant amount of sulfur. The predictions of the model were verified by comparing the calculated and experimentally observed fusion zone geometry. The results indicate very good agreement between the predicted and experimentally observed fusion zone geometry, for both laser and gas tungsten arc (GTA) spotmore » welds, due partly to the accurate treatment of surface tension gradient driven flows. 21 refs., 7 figs., 2 tabs.« less

Emission spectroscopy of plasma during laser welding of AISI 201 stainless steel

Abstract: Les vitesses de vaporisation des elements d'alliage dans le bain de fusion ont ete correles avec les spectres d'emission du plasma au cours du soudage a l'aide d'un laser CO 2 de l'acier inoxydable AISI 201 sous des conditions operatoires variables