# Arc Shape Characteristics with Ultra-High-Frequency Pulsed Arc Welding
## 1 Introduction  
Arc welding is a traditional process technology in industry. It has been developed through a long history of studying its mechanism and engineering applications. However, many phenomena lack a systematic approach, which includes the arc constriction. With classical theory, it was found that the parallel current lines attracted each other when they were in the same direction. This is also applicable in an arc plasma [1] with direct current welding (DC welding). Most of the time, DC arc welding creates the current pointing from the base metal to the electrode. The current is formed during the movements of electrons and positive particles. The current elements with the same direction can be recognized as the current line series. They will attract each other with a macro perspective as one is surrounded in the electromagnetic field created by another. Mutual attraction occurs between any two lines or elements. It can be deduced that the Lorentz force, which is the major electromagnetic force and only changes the direction of charged particles, would construct the arc plasma in a radial direction, which is called arc constriction. This phenomenon has been proved by experiments [2]. Arc constriction will create a pinched arc plasma [3] during the welding process. It is regularly believed to increase the stiffness, arc voltage, and energy density of the arc plasma. As a result, it will enhance the penetration and reduce the shielding area of the welding arc, which is important for the arc welding process. Firstly, constriction of the arc leads to an increased arc temperature and axial flow velocity with the increased current density [4]. Significant results can usually be obtained under the condition of a large welding current greater than 150 A [5,6] and a pulsed current [7,8]. The research [9] revealed an increased arc voltage with different shielding gases, which displayed obvious arc constriction. Secondly, some results indicate significant constriction with the pulsed welding process [10,11]. Previous work indicatesthat the root radius decreased during pulsed arc welding [12], and the attenuation coefficient of the magnetic force is defined to evaluate the intensity of the arc constriction, even though it was a direct reflection of the constriction view. Thirdly, some studies demonstrated the effect of arc constriction on the weld appearance [13,14], which was also proved with pulsed welding [15]. Previous work found an increase in the arc voltage and arc constriction with pulsed welding [16]. Such reflections could be correlated with the arc constriction during pulsed arc welding. Finally, the continued arc shapes were displayed to exhibit the arc constriction with an ultra-high-speed camera most of the time. Some experiments found the increased arc size with a large current [17,18], although that caused the arc constriction. Thus, the arc constriction is predicted to have other reflections with a pulsed arc plasma of more than 20 kHz. Furthermore, visual observation is very popular in arc plasma surveillance; it can be used to track characteristic elements and to conduct arc plasma diagnosis by imaging of the continuous spectra [19]. At the same time, it is simple and rapid to analyze the distribution of the temperature field using the method of image processing, and the dynamic change of the arc plasma can be monitored in real time by a high-speed camera, and the image processing methods are required in order to distinguish the region of the arc plasma for further study.  
Above all, the welding arc core region and the edge of the arc contain plenty of information about the welding arc physics, and it is meaningful to study the ratio of the arc core region to the total region. This paper studies the principle of arc constriction in order to experiment, conjecture, and verify. The aim is to contribute to arc diagnosis with respect to the pulsed arc constriction.