Effect of current pulses on the temperature distribution and microstructure in TIG tantalum welds
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temperature distribution in the fusion zone and the HAZ is therefore necessary. A previous paper H described a mathematical model developed for the analysis of the heat flow during bead-on plate welding of thin tantalum sheets with the regular T I G procedure. The present paper presents the thermal history during welding with current pulses, calculated with a modified version of the mathematical model. The numerical results are used to explain experimental results reported previously ~~and shown in Figs. 1 and 2. The Numerical Method. In the mathematical model, described in detail elsewhere, H the heat flux from the welding arc was expressed as: Qarc = qol e
c§
[1]
where I = welding current, r = distance from the center of the arc on the welded specimen, C = characteristic constant of the arc, qo = constant depending on the welding arc and the rate of welding." In the present case, of welding with pulses in the welding current, the arc current varied with time as shown in Fig. 3. The thermal history was calculated with the value of I in Eq. [1] changing accordingly to the given pulse. This was done employing a modified form of the original finite difference solution program.l~ The input data contain the initial temperature of the specimen and the size of the mesh employed for the finite differences, the initial position of the arc, the constant parameters of the arc and the parameters of the current pulse, i.e., the high and low values of the current,/max and Imin, and the time intervals at each value,/max and tmin. In accordance with the experimental conditions ~~the arc was held stationary at the high value of the current until a temperature above the liquidus was reached at a given distance from the center of the arc. At this stage the simulation of the welding with constant linear velocity and current pulses was started. The value of the welding current was defined at each step following the given "current vs time" function. Assuming that the welding is performed along the symmetry axis of the tantalum specimen, the calculations were done for one-half of the specimen. Results. The dimensions of the specimen and the mesh employed in the calculations are given in Table I together with the value of constant C of Eq. [1]. The current pulses used in the different calculations are given in Table II, together with the corresponding values of qo. Table II also presents the heat input per unit length J corresponding to each of the considered cases. It was shown n that during regular welding, after an initial transient, the temperature distribution reaches a steady state. A similar situation was obtained from the calculation of the temperature distribution during welding with pulses of the arc current. In the following only the steady state temperature field will be considered. The theoretical model was first employed to calculate the temperature distribution in welds produced at a rate of 1.67 mm/s with the four different current pulses given in Table II. These pulses were employed experimentally in the welding of ta
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