Search

Graphical abstract The influences of frequency on short-circuiting transfer characteristics in PU-GMAW process were investigated by the welding electric signal and high-speed video images. The droplet transfer processes of adding pulsed ultrasonic were more stable than that of conventional GMAW. The peak current and droplet cycle in GMAW was improved obviously by adding pulsed ultrasonic. The dimensionless parameters (γ and Δ γ) were carried out to quantify the effect of ultrasonic on droplet transfer behavior in PU-GMAW. Abstract Addition pulsed ultrasound in GAMW process, which is described as the pulsed ultrasound wave assisted GMAW (PU-GMAW), has become a new method to control the droplet transfer process. The effect of frequency on short-circuiting transfer characteristics in the PU-GMAW process was investigated. The short-circuiting transfer processes were analyzed via welding electric signal and high-speed video images. The results showed that the droplet transfer processes of the PU-GMAW with the frequency of 20Hz and 50Hz were more stable than that of the conventional GMAW and U-GMAW. The PU-GMAW at the frequency of 50Hz obtained the minimum peak current and the minimum droplet transfer cycle. The droplet size and arc shape in the PU-GMAW also had a similar conclusion. The middle electromagnetic forces of the U-GMAW and the PU-GMAW were greater than that of the GMAW. The electromagnetic force of the PU-GMAW was bigger than that of the U-GMAW when the r was less than 1.75mm. Additionally, the electromagnetic force of U-GMAW was bigger than that of PU-GMAW when the r increased to more than 1.75mm. The greater the electromagnetic force increment, the stronger the effect of ultrasound on the electromagnetic force. Therefore, the effect of ultrasound on the electromagnetic force was the biggest in the middle of the arc. Meanwhile, the influence of pulsed ultrasound on the electromagnetic force was stronger compared with the U-GMAW. [ABSTRACT FROM AUTHOR]

High-speed video images of 24 downward negative lightning flashes terminating on tall structures in Guangzhou are selected to analyse the connecting behaviour of the downward and upward leaders during the attachment process preceding the first return stroke. Three types of leader connecting behaviour have been observed: Type I, the tip of downward leader (DL) to the tip of upward connecting leader (UCL), which accounts for 42% of all the events (10/24); Type II, the DL's tip to the lateral surface of UCL, which accounts for 50% (12/24); and Type III, the combination of Types I and II, which accounts for 8% (2/24). For the two cases of Type III behaviour, each case had two junction points (one with Type I and the other with Type II behaviour) between the downward and upward leaders. Therefore, Types I and II can be viewed as the two basic types of the leader connecting behaviour during the attachment process in negative lightning. No attachment process exhibited the connection of the UCL's tip to the lateral surface of DL. The presence of multiple DL branches and their integrated effect on the development of positive UCL/UCLs are likely to be the main reasons for the Type II behaviour.