African Fusion June 2015

Cover Story: GMAW: cur ent progress

This paper by Yoshinori Hirata of the Graduate School of Engineering at Osaka University, Japan, was presented at a plenary season of the IIW’s 67 th International Assembly and Conference in Seoul South Korea and highlights key current developments in quality and productivity for the gas metal arc welding (GMAW) processes. Current progress in the science and technology of GMAW processes Yoshinori Hirata

R esearch and development of high productivity and/or high quality welding processes have been conducted for several years. The gas metal arc welding (GMAW) process is widely used in various production fields because it provides high deposition rate and/or it is suitable for auto- matic/roboticwelding. In theGMAWprocess, theelectrodewire melts due to arc heating and forms a molten metal droplet at the wire tip. Then it detaches and transfers from the wire tip to the weld pool. The series of wire melting and subsequent molten drop transfer processes results in deposition of the metal required to complete butt welds and/or fillet welds. So the stability of the gas metal arc and the associated metal transfer dominates weld quality and productivity. This technical issue has been prompting welding machine mak- ers, consumable makers and gases suppliers to improve and develop high performance welding processes. In this paper, current progress in the science and technology related to GMAW processes is discussed based on the author’s personal research history. Introduction In the Materials and Manufacturing Science division of the Graduate School of Engineering at Osaka University, we have been investigating welding phenomena associated with MIG/ MAG and TIG welding with both steady current and pulsed current using analogue transistor controlled power sources constructed by ourselves since around 1977[1][2]. Analogue transistor power sources enable us to explore various current waveforms. Then, in order to clarify the role of pulsed current parameters such as pulse peak current, pulse width, pulse frequency for rectangular pulse waveforms, metal transfer and/or weld pool oscillation phenomena were analysed with aid of high speed photography. In particular, we showed that the detachment time of metal droplets from the wire tip is significantly dependent on the magnitude of pulse peak cur- rent, which is important for the ‘one droplet transfer per pulse’ control transfermodewidelyused invarious sectors of industry at the present [3]. At the same time, our domestic welding machine makers have confirmed that ‘one pulse-one droplet’ transfer is very effective for spatter reduction [4][5]. Following this development, welding machines under transistor and/or inverter control promptly spread, together with applications that take best advantage of pulsed current welding processes. Over the past few decades, current control using devices such as power transistors, MOSFETs and IGBT have advanced significantly. In addition, inverter circuitswith high-speeddigi- tal control have been significantly improved. Separately from

gettinghighperformance frompower supplies, themechanical control of wire feeding has been progressing. Accordingly, very fast and precise control of current, voltage and wire feeding are now available. In recent years, commercial digitally controlledarcwelding machines have been spreading for the practical applications in various sectors of industries in all over the world. On the other hand, recent progress in visualisation and simulation of welding processes has been making it possible to observe, measure and analyse welding phenomena at high resolution in terms of both of time and space. The change from qualitative understanding to quantitative is sure to result in an evolution of welding processes towards better reliability and higher productivity. Today, high performance digital cameras have been spreading intoproduction sites and applied toprac- tical fabrication: detection of electrode position and groove shape,monitoring thearc/weldpool duringweldingoperation, etc. The implementation of these systems and the resolution of the challenges posed will be more actively pursued as the availability of more explicit knowledge becomes available. In this article, the science and technology of theGMAWpro- cess are described in the light of current progress in relevant and allied technologies. Present state of GMAW In the GMAW process, the filler wire is the electrode. The elec- trodewire ismeltedby the arc heat and amoltenmetal droplet forms at the wire tip. Then it detaches and transfers from the wire tip to the weld pool, so the arc length and/or the arc root locationare varied intermittentlywith eachmetal transfer. This means that, in comparison with the TIG arc, the gas shielded metal arc is less stable as a heat source in both time and space. Also, the stability of metal transfer dominates weld quality and productivity. These problems have been driving welding machinemakers, welding consumablemakers, gas suppliers, universities and institutes towards research and development of welding processes for improved quality and productivity. Various GMAW processes have been delivered to the market corresponding to the needs of welding related products. In order to highlight specific advantages of processes, these have been taggedwith various catch phrases, such as ‘high stability metal transfer andwelding arc’, ‘improved stiffness (directivity) of welding arc’, ‘high deposition’, ‘deep penetration’ and ‘sound welds’ , but the understanding of the specific or quantitative effects of the newly developed processes is not always obvi- ous, nor are the reasons for the improvement of weld quality and/or productivity in practical use on fabrication sites. This

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June 2015

AFRICAN FUSION