A Contribution to Arc Length Discussion

Soldagem & Inspeção. 2015;20(3):367-380 http://dx.doi.org/10.1590/0104-9224/SI2004.06 Invited Papers A Contribution to Arc Length Discussiona Stephan Egerland1 1 Fronius International GmbH, Wels, Austria. Received: 31 Aug., 2015 Accepted: 31 Aug., 2015 E-mail: (SE) Abstract: An investigation was raising the question: “What does ‘arc length’ mean?” Actually, it is considered expressing a kind of natural relationship between arc voltage and arc column shape. Statements such as “The higher the voltage the longer the arc” or “The arc voltage proves approximately proportional to the arc length”, are frequently noticed in this conjunction. However, the author suggests that there is no general possibility of describing ‘arc length’ over the whole welding process range. Instances are represented in this paper, showing both theoretical attempts of definition and practical observations. This paper intends to contribute to a serious discussion of something trivial, indeed very well-known or used among welding experts, but actually yet hardly understood, at least as when it comes to closer examination Key-words: Arc length; Controlled GMAW; Spray arc; Electrode tapering. 1. Introduction and Theoretical Approach BS 499-1:2009 [1] describes ‘arc length’ as the “distance from the tip of the welding electrode to the adjacent surface of the weld pool”. An investigation [2] of high performance gas metal spray arcs aimed at the evaluation of differences generated by either the varying consumables or the power source type used and was also employing high-speed cinematography. One of the chosen details to determine: variation in arc length. Independently of equipment and consumables used, it was found difficult in general however, precisely to define the length of the welding arc. It often proved even impracticable to accurately determine the wire electrode tip and as such the gap between the tip and the weld pool, enveloped by the arc plasma. Aware of this, a research of the welding literature on arc length definition was conducted attempting to reveal deeper sense. Surprisingly very little information was found though on a technical term, extensively used in welding. Instances were compiled providing different viewpoints on arc length issues to stimulate a more quantitative approach to the ‘arc length’. For example, Gülsöz [3] studied the melting behaviour of coated electrodes applying direct- (straight- and reverse polarity) as well as alternating current shielded metal arc welding (SMAW). Although explicitly using ‘arc length’ to describe his observations the author provides only a ‘theoretical’ definition, as in Figure 1, which schematically depicts the arc length representing the distance between anode and cathode. Greyscale high-speed cinematography, however, proves the difficulties in transferring theoretical models to real environment, as seen Figure 2. The arc, described as “climbing” along with the cathode spot in [3], varies considerably in its shape. Also, the actual weld pool surface remains undefinable. Arc voltage is explained to have considerable or proportional effect on arc length. Increasing voltage leads to arc extension and vice versa. a This paper is based on IIW Document No. XII-2013-11 which was subjected to appropriate revision to achieve its final current form. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited. Similar to BS 499-1:2009, the ASM Metals Handbook [4] deals with arc length in shielded metal arc welding as “[...] the distance from the molten tip of the electrode core wire to the surface of the molten weld pool”. Droplet growth, depending on coating flux composition, short circuit frequency etc. are considered influencing the arc length. Bohlen [5] describes arc length based on a schematic representation of a gas tungsten arc, Figure 3. The arc is considered bridging the gap between anode (work piece) and cathode (tungsten electrode). Real observations of gas tungsten arcs provide quite different arc shapes though and, in conjunction, arc lengths, Figure 4. According to Kou [7], who has conducted own, and reviewed research from other investigators (Friedman and Glickstein [6] and Key [8]), varying electrode vertex angles are producing varying power densities and fusion profiles, Figure 5. Egerland Figure 1. Schematic arc discharge representation (after [3]). Figure 2. Greyscale images: thick covered basic electrode (after [3]). 368 Soldagem & Inspeção. 2015;20(3):367-380 A Contribution to Arc Length Discussion Figure 3. Schematic gas tungsten arc representation (after [5]). Figure 4. (a) Influence of tungsten electrode vertex angle on arc shape (after [6]); (b) different power density in dependence on electrode included angle (after [7]). Figure 5. Relationship of fusion profile and tungsten electrode vertex angle and truncation (after [7]). Soldagem & Inspeção. 2015;20(3):367-380 369 Egerland Figure 6 schematically represents the relationship between arc length and electrode geometry. Assuming the tapered electrode (Figure 6 left) is reduced incrementally from the tip (horizontal dashed lines) towards reaching a vertex angle of finally 180° (marked by the continuous horizontal line), the geometry influence decreases. Keeping the welding torch distance constant while only varying the electrode tip geometry, leads to considerable different arc length (denoted by ∆ arc _ length ). Assuming now a changing electrode geometry, a welder would attempt to manually balance the arc length by adjusting the distance between electrode and work piece surface for maintaining the ‘arc length’ constant. Wear, e.g., occurring through welding is suggested reducing the electrode length, leading to increasing arc length and thus requiring the welder to correspondingly adjust the electrode tip to work distance (ETWD) and approaching the electrode tip towards the work piece. Figure 6. Schematic representation of arc length variation in dependence on electrode geometry at constant welding torch distance and drooping power supply characteristic. Employing Gas Metal Arc Welding (GMAW), the dynamic arc behaviour drastically changes due to the continuously fed and melting wire electrode. Literature reveals only little information on an actual ‘arc length’ definition Figure 7, taken from [5] e.g. represents a typical characteristic for a larger diameter mild steel wire electrode used for pure carbon dioxide GMAW. Even though qualitative designations such as ‘long’ and ‘short’ are used to determine specific arc length regions, no substantial or quantitative information is provided in regards to how arc length is measured through welding. Scotti [10], recognising the contradictions as when ‘arc length’ is involved for quantitatively describing welding ar (...truncated)