br Conclusions br Acknowledgements br Introduction

Conclusions
Acknowledgements
Introduction Magnetically impelled arc butt (MIAB) welding glycogen synthase kinase 3 a solid state welding process involving rotation of arc around the tube to aid the uniform heating of faying surfaces. The schematic diagram of the MIAB process is shown in Fig. 1. The tubes to be welded in MIAB welding are held in aligned position by a clamping arrangement and are influenced by a constant magnetic field around the weld region. Heat is generated in the faying edges through electrical discharge and rotation of arc by application of the external magnetic field. After sufficient heating, the tubes are forged to expel the molten metal and other impurities from the faying surface resulting in solid state joining of tubes [1–4]. MIAB welding process is a single shot process and can readily be automated as no manual skill is involved, and is very fast with a welding time of 22 s for a pipe thickness of 6 mm. It produces a solid state bonding which is conducive to excellent mechanical properties. It also opens up to the possibility of dissimilar metal joints. MIAB welding requires no part rotation unlike friction welding, and hence is much simpler and less expensive. Other potential benefits of MIAB welding include less internal flash, shorter weld time, less metal loss, uniform heating and reduced machine maintenance. The amount of current used for MIAB is around 500 A, and hence MIAB is more economical than the flash butt welding process. MIAB welding process is widely used in power, defence, oil and gas sectors and is seen as an effective replacement to friction, flash, resistance and butt welding [5]. MIAB welding process is characterized in six stages (Fig. 2) providing arc initiation, beginning of arc rotation, arc transitory rotation, arc stable rotation with each other arc instable rotation, and tube upsetting [6]. The tubes are initially made to be in contact and arc is initiated by creating a small gap between the abutting surfaces. The arc is influenced by the constant magnetic field, and the interaction leads to the rotation of the arc around the circumference of the tube (Fig. 2, Phase I). The next phase is identified as the beginning of the arc rotation phase (Fig. 2, Phase II). The speed of the arc rotation keeps increasing, and the arc transitory phase (Fig. 2, Phase III) records this phase with an abrupt change in the speed of arc rotation. The stabilization of arc velocity (Fig. 2, Phase IV) is visually recognized by the formation of an arc ring between the abutting surfaces of the tubes. The arc rotation heats up the surfaces of the tubes to form a thin layer of molten metal along the surfaces. The formation of molten metal bridges in the arc gap creates instability in arc rotation and is characterized by high amplitude fluctuations at arc velocity. Longer time spent in this phase of arc instable rotation (Fig. 2, Phase V) is likely to lead to quenching of the arc itself. Hence, an upset force is to be provided to fuse the molten edges in the stage (Fig. 2, Phase VI) of tube upsetting. These stages could be combined in one or more descriptions to form different set of stages, and in this paper, MIAB welding is seen as a combination of four stages, viz., arc initiation, arc stabilisation, arc rotation and upsetting, without affecting the dynamics of process description. MIAB welding is also known as ROTARC (ROTATING ARC) welding. Georgescu et al. reported the working of pneumatically operated ROTARC equipment analogous to MIAB welding process with technical designing details [7]. In MIAB welding, the magnitude of electromagnetic force controls the weld quality and depends on the magnetic flux density around the tube circumference. Many researchers have modelled the magnetic flux density distribution in MIAB welding. Kim et al. proposed a two dimensional finite element model for the analysis of magnetic flux density distribution in MIAB welding [8]. The magnetic flux density between the pipes increases with the increase in exciting current and the decrease in gap size of the pipes. Arungalai et al. reported the simulation of electromagnetic force distribution in MIAB process [9,10]. The study emphasizes the magnetic flux density distribution and the effect of electromagnetic force in controlling the arc rotation speed.