Droplet Transfer Induced Keyhole Fluctuation and Its Influence Regulation on Porosity Rate during Hybrid Laser Arc Welding of Aluminum Alloys

Hybrid laser arc welding (HLAW) features advantages such as higher welding speed and gap tolerance as well as smaller welding deformation and heat-affected zone than arc welding. Porosity in hybrid laser arc weld due to keyhole fluctuation tends to be the initial source of crack propagation, which will significantly diminish the weld performance. A high-speed imaging technique was adopted to record and analyze the droplet transfer and keyhole fluctuation behavior during hybrid laser arc welding of aluminum alloys. A heat transfer and fluid flow model of HLAW was established and validated for a perspective of the evolution process of droplet transfer and keyhole fluctuation. The relationship between keyhole fluctuation and weld porosity was also revealed. During the droplet transfer stage, liquid metal on the top surface of the weld pool flows toward the keyhole originated by globular transfer, and the keyhole fluctuates and decreases significantly, which has a higher tendency to form a bubble in the weld pool. The bubble evolves into porosity once trapped in the mush-zone near the trailing edge of the weld pool. Therefore, globular transfer during HLAW is the principal origin of keyhole fluctuation and weld porosity. Welding current has a significant influence on keyhole fluctuation and weld porosity rate. Droplet transfer frequency, keyhole fluctuation, and porosity rate increase with higher welding current under the globular transfer mode. The porosity rate shows a nearly positive correlation with the standard deviation of keyhole fluctuation.

Influence of Polarity Arrangement of Inter-Wire Arc on Droplet Transfer in Cross-Coupling Arc Welding

In order to reduce the influence of polar zone effect in cross-coupling arc by changing inter-wire arc (IWA) configuration, the influence of polarity arrangement of the IWA on droplet transfer was studied. The change of voltage-current waveform and the process of droplet transfer were recorded and analyzed by a high-speed camera and electric signal synchronous acquisition system. The results show that when the IWA polarity is arranged as anode on the bottom and cathode on top, the anode spot force always promotes the droplet transfer and reduces the critical current value of spray transfer. However, with the increase in the input voltage of the IWA, the resistance of the cathode spot force becomes obvious, which hinders the droplet transfer. While the IWA polarity is arranged as anode on top and cathode on the bottom, increasing the input voltage of the IWA obviously reduces the plasma arc voltage. The critical current of spray transfer increases in anode droplet, while the cathode droplet is mainly globular transfer, and there is no spatter explosion process. Through a comprehensive comparison of the droplet transfer process of anode and cathode under the different IWA polarity arrangement, the process of anode and cathode in the IWA polarity arrangement of the anode on top and the cathode on the bottom is more stable than that in the IWA polarity arrangement of the anode on the bottom and the cathode on top, mainly because the cathode spot force under high current do not hinder the cathode droplet.

Prediction Model of Droplet Size in Low-Current GMAW

A low-current GMAW experimental system is established using a constant current power supply and a controlled wire feed system. The metal transfer behavior and the droplet growing process in low-current GMAW are investigated by using a high-speed camera, and the welding current and voltage are monitored by a data acquisition set. The metal transfer in low-current GMAW can be classified into three modes: short-circuiting transfer, long-arc short-circuiting transfer and large globular transfer. The arc voltage is the primary factor that affects the metal transfer mode. The droplets growing in low-current GMAW are observed and analyzed. The maximum detaching size of the droplet and the corresponding growing time with different welding current are calculated from the recorded image sequences of metal transfer. Based on such results, a prediction model of the droplet size is thus established, which will be used in the future research on pulsed laser-enhanced GMAW.

A Way to Weld Sheet Metal with Double-Electrode GMAW

This paper designed a double-electrode GMAW(gas metal arc welding) system. This system includes main arc and bypass arc. Main arc (Base metal current: Ibm) is supplied with Constant Current power (CC mode) and bypass arc (bypass current: Ibp) is Constant Voltage power (CV mode). Main arc electrode used common carbon wire, bypass arc electrode used water cool copper. Welding experiment shows this DE-GMAW can change common metal transfer into spray transfer with lowest critical total current (Itotal) 200 amps. When Ibm decreases and bypass voltage increases, this critical current will increase and it is less than 230 amps when keeping spray transfer. High speed video proved that metal transfer is changed from spray transfer to globular transfer.

Effect of Arc Length on Weld Appearance and Metal Transfer in Twin-Wire GMAW Process

The experiments were performed with the twin-wire GMAW machine of Fronius product. The metal transfer process and the arc shapes were observed by use of shadowgraph technique with a high-speed camera. When the arc length increases during twin-wire CO2 arc welding, the mode of metal transfer changes from short-circuiting transfer to globular transfer, and to spray transfer. But it is difficult to form a stable spray transfer process, and the weld appearance becomes worse. The short-circuiting transfer in twin-wire CO2 arc welding can improve the process stability.