Abstract Scope |
Gas Tungsten Arc Welding (GTAW) is a widely used welding process in industrial manufacturing for its high weld quality and steady process. However, the welding productivity is limited in high speed welding with high current, due to the appearance of the welding bead defects, such as undercutting, humping, parallel humping etc. It is believed that the excessive arc pressure in high current exerted on the weld pool surface is the primary reason for the depressed surface that generates the defects. By using magnitude scaling method, further research suggested that at high current and welding speed conditions, plasma shear force of the plasma imposed on the free surface determines the flow of liquid metal under the arc, while Marangoni force can be negligible. Magnitude scaling method cannot give the mechanisms on the weld pool dynamics and the change in the driving forces of the weld pool. For a more accurate model for the weld pool dynamics, the arc properties can be derived from the arc model in terms of the real condition, rather than premising arbitrarily. Therefore, the present molten pool dynamics during high speed GTAW is simulated from arc property.
A weld pool model for high current GTAW with high speed is developed based on the arc properties including heat flux, current density, arc pressure and plasma shear force at the weld pool surface. These arc properties are calculated through solving an arc model with flat weld pool surface and severely depressed weld pool surface, respectively. The arc properties in the former case are used to the initial time and those in latter case are used during the arc moving when there is severely depressed weld pool occurs. The VOF technique is employed to track the free surface of the weld pool. Buoyance, Lorentz force, plasma shear force, surface tension, arc pressure and hydrostatic force were all taken into account for the weld pool. We applied this model to high speed GTAW of stainless steel SUS 304 with a thickness of 5 mm. The welding current is 350 A, welding speed is 20 mm/s; for arc model, arc length is 3 mm and diameter of the tungsten electrode is 3.2 mm with a 60 degrees conical angle and 0.3 mm tip radius. The model was solved by FLUENT software package. The results are validated by comparing with the experimental data.
During high speed GTAW, free surface of the weld pool is severely depressed by the large arc pressure, with convex appearing around the concave surface, especially that adjacent to the weld pool rear, which implies a hump will form. Weld pool is elongated greatly and the elongation becomes more apparent when the arc moves.
The temperature of the weld pool is much lower than that in low current, the temperature at the convex part of the weld pool is higher than that at the bottom of the depressed weld pool surface. Since the convex part of the weld pool is more close to the tungsten electrode tip, more current pass through between these two locations and more heat are transported from the arc to the weld pool by electron absorption, which is the main part of the heat to the weld pool. The maximum temperatures are a little above the liquidus ones, thus the freezing time of the weld pool is very short. The reasons for the low temperature can be attributed to two factors. 1. Large deformation of the weld pool leads to the dispersed heat flux to it, compared with that in the case of nearly flat weld pool in low current. 2. High moving speed of the arc results in short time for heating, and the temperature increase of the weld pool is limited accordingly. The molten metal flow within the weld pool is found to be outward from the depressed bottom to the outside, then flow towards the weld pool rear, where it is blocked by the solidified weld pool and becomes contraflow. As a result, metal flow to the weld pool rear and the contraflow results in accumulation of the liquid metal, and further becomes a hump as the weld pool freezes quickly.
The conclusion can be drawn as follows:
1. Considering arc properties is necessary in understanding the weld pool dynamics during high speed welding.
2. Low temperature of the weld pool and strong flow of liquid metal to the weld pool rear are two key factors responsible for the formation of the hump in high speed welding.
3. The low temperature of the weld pool results from dispersed arc heating to a remarkably depressed surface, and from high moving speed of the arc; the strong molten metal flow to the weld pool tail is caused by the large plasma shear force.
Keywords
Arc properties; Weld pool; Free surface; Humping; Backward flow |