Abstract
In this study, the effect of oxygen in the shielding gas on the material flow behavior of the weld pool surface was discussed to clarify the dominant driving weld pool force in keyhole plasma arc welding (KPAW). To address this issue, the convection flow on the top surface of weld pool was observed using a high-speed video camera. The temperature distribution on the surface along keyhole wall was measured using the two-color pyrometry method to confirm the Marangoni force activity on the weld pool. The results show that the inclination angle of the keyhole wall (keyhole shape) increased especially near the top surface due to the decrease in the surface tension of weld pool through surface oxidation when a shielding gas of Ar + 0.5% O2 was used. Due to the change in the keyhole shape, the upward and backward shear force compositions created a large inclination angle at the top surface of the keyhole. From the temperature measurement results, the Marangoni force was found to alter the direction when 0.5% O2 was mixed with the shielding gas. The shear force was found to be the strongest force among the four driving forces. The buoyant force and Lorentz force were very weak. The Marangoni force was stronger than the Lorentz force but was weaker than shear force. The interaction of shear force and Marangoni force controlled the behavior and speed of material flow on the weld pool surface. A strong upward and backward flow was observed in the case of mixture shielding gas, whereas a weak upward flow was observed for pure Ar. The heat transportation due to the weld pool convection significantly changed when only a small amount of oxygen was admixed in the shielding gas. The results can be applied to control the penetration ratio in KPAW.
Highlights
Plasma arc welding (PAW) is one of the arc welding processes that produces deep penetration and low distortion compared with conventional gas tungsten arc welding [1,2]
The two necessary conditions for welding stability include: the molten flow on both sides must bridge the gap formed at the rear part by the passage of the keyhole and the molten flow must be supported by the surface tension acting on the back surface of molten pool
As described in a review paper, the four principal forces driving the welding convection flow of welding arc processes include: (i) drag force, which is created by the plasma flow acting on the liquid surface through the keyhole and plasma flow acting on the weld pool surface; (ii) buoyance force, which is caused by the temperature difference within the weld pool; (iii) Lorentz force, which is generated by the self-magnetic field of the welding current within the weld pool; and (iv)
Summary
Plasma arc welding (PAW) is one of the arc welding processes that produces deep penetration and low distortion compared with conventional gas tungsten arc welding [1,2]. To improve the quality of welding joints, Zhang et al observed the influence of welding speed and welding current on the change in the keyhole diameter and the weld pool surface using an insulated gate bipolar transistor (IGBT) power module [8,9,10]. Using this module, welding current was controlled in pulse waveform to decrease the brightness of the main arc. The results showed that with varying shielding gas flow rate, the convection inside the weld pool changes, causing welding defects such as undercut [19]. The temperature distribution on the surface along the keyhole wall was measured to evaluate the Marangoni force acting on the weld pool
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