Abstract Scope |
Keywords:
Gas metal arc welding, Droplet resonance, Vibrating electrode, Current-independent metal transfer, Weld formation
Introduction:
For decades, the metal transfer has been an active research area in welding, recognized for its significant role in determining the spatter, arc stability, and weld quality. Drop spray provides desirable characteristics of uniform droplet transfer, little spatter, and a more uniform bead, but sufficiently high welding current exceeding spray transition current is needed, which prevents the use of GMAW in workpieces having specifically thin sections or comprised of relatively heat-sensitive materials. Separate control of metal transfer and heat input is an essential way to obtain stable metal transfer and freely adjustable heat input, although it is a difficult task considering the high coupling of the metal transfer process and the base-plate heating process.
Introducing external force may be the most effective way to achieve current-independent metal transfer. This work presents a system and experimental verification for the droplet transfer control using droplet forced resonance to achieve fully current-independent metal transfer. The droplet oscillation and forced resonance excited by the vibrating electrode are then observed. Subsequently, metal transfer under continuous electrode vibration is studied and compared with the metal transfer without mechanical assistance to experimentally verify the success in achieving the fully current-independent metal transfer.
Experimental Procedures:
The experimental system consists of the welding cell, electrode vibration device, data acquisition system, high-speed camera, and controllers. The power source worked in constant voltage (CV) mode. The coordination of the wire feeder, excitation device, and welding power source was realized by the PC central controller via a PLC. The wire was fed into the electrode vibration device by an independent wire feeder to vibrate the electrode in the axial direction. The vibrating electrode actively excited the oscillation of the pendant droplets by applying alternating mechanical force to it. The wire feeding speed and electrode vibration frequency were also controlled by the PC central controller. The welding electric signal and high-speed droplet image sequences were synchronously recorded at 2 kHz by a data acquisition system and a high-speed camera. To quantitatively analyze the oscillation of the droplets and electrodes, took the downward direction as the positive direction, a point above the wire tip (drop top) on the wire axis as the origin, the drop length and vertical coordinates of the wire tip were measured in pixels from recorded images.
All the experiments were conducted as bead-on-plate welding with 0.8 m/min wire feed rate, 12 cm/min travel speed and 15 L/min Argon-rich mixture of 95 %Ar+5 %CO2, performed under the direct current electrode positive (DCEP) condition. The welding voltage was set at 19 V. During the experiments, the welding torch was fixed, and the workpiece was moved at a constant speed. The base metal was mild steel with dimensions of 100 mm × 50 mm × 1.2 mm, placed on the surface of the motion platform. 1.2-mm-diameter ER70S-6 wire was chosen as the electrode. The distance from the contact tip to the workpiece was set at 12 mm.
Results and Discussion:
The natural frequency of the pendant droplets of different sizes was obtained by monitoring the self-damping oscillation. The results show that the natural frequency of the droplet decrease as its size grows.
The continuously vibrating electrode excites the forced axial oscillation of the droplet.
When the excitation amplitude is intentionally set to be relatively small, the oscillation amplitude of the pendent droplet increases firstly and goes down gradually, reaching peak value when the natural frequency of the droplet approximately equals the excitation frequency, i.e., the forced resonance occurs.
Current-independent was achieved owing to the considerable oscillation amplitude and concomitant download momentum when an excitation with amplitude of 0.315 mm, frequency of 60 Hz is used. The transfer is in globular transfer mode with a drop diameter of 2.42 mm. The mean welding current is about 32.5 A. Compared with the metal transfer in uncontrolled welding with the same welding parameters, the metal transfer utilizing the resonance is much faster, more uniform, more stable, and virtually no spatter.
Thanks to the improved metal transfer by utilizing droplet resonance, the bead formation is improved significantly, which looks uniform and fine. However, the bead formation in the conventional GMAW looks discontinuous and irregular. The thin base sheet is burned through by a superheated droplet in large size.
The retaining force provided by surface tension is far larger than the detaching forces consisting of the gravity force, electromagnetic force and plasma drag force considering the welding current is 33A only. It’s the downward inertial force resulting from the downward moving droplet in the resonant region that detaches the drop and results in a significant increase in transfer frequency. In addition, the droplet resonance oscillation here is independent of the welding current, so the metal transfer utilizing droplet resonance oscillation can be regarded as “current-independent transfer”.
Conclusion:
The results obtained are summarized as follows:
1.The free oscillation frequency of the pendent droplet, i.e., the natural frequency, decreases with the increase of drop size.
2.The continuously vibrating electrode excites the forced axial oscillation of the droplet. The oscillation amplitude of the droplet increases firstly and decreases later, reaching a maximum when the natural frequency is almost equal to the excitation frequency.
3.Relatively large excitation amplitude results in large inertial force produced by drop oscillation, inducing droplet detachment in advance in the resonant region. Compared with conventional GMAW at low current, the metal transfer utilizing resonance oscillation is much faster, more uniform, more stable, and virtually no spatter.
4.At low current (about 33A in this paper), the downward interior force due to droplet oscillation is the main detaching force in metal transfer utilizing resonance oscillation. The current-independent metal transfer can be produced by the vibrating electrode of which the frequency is close to the natural frequency of the pendant drop.
5.Since the metal transfer utilizing the droplet resonance oscillation is current-independent, the current waveform could be freely designed to adjust heat input or meet other requirements from specific welding tasks. |