In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by lowering the short circuit current to a low level just before the rearcing period. The reduction of spatter requires an improvement in the accuracy of predicting the rearcing by stabilising the metal transfer and improving the robustness of the accuracy against disturbances. The controlled bridge transfer (CBT) process optimises the accuracy of predicting the rearcing in real time in response to the metal transfer, in order to realise spatter reduction and stable arcs in non-pulsed GMAW. Traditionally, GMAW is carried out using electrode positive polarity. However, this polarity is not sufficient for welding extrathin steel sheets, specifically those thinner than 1?0 mm. With an electrode negative (EN) CBT process, although some slight arc voltage fluctuation occurs caused by the behaviour of the cathode spots on the tip of the wire during in EN polarity GMAW, instantaneous voltage is used, through a command computation, to improve the transient response against the disturbance. Consequently, a stable arc can be obtained without increasing the number of short circuits in a unit time to obtain spatter free welds.
The initial yield surface of a superplastic material was investigated by using a combined loading of axial force and torque. The thin-walled tubular specimen made of Zn-22 wt.%A1 alloy was used in the experimental part of this study. Tests were carried out at room temperature (293 K) and at elevated temperature (523 K). The experimental results show that the malerial tesled did not deform superplastically at 293 K exhibiting a yield surface that can be described by the second invariant of stress deviator (by Mises' criterion). On the other hand, the material was found to deform superplastically at the higher temperature of 523 K with a yield surface more complex compared to that observed at 293 K. This difference may be attributed to the difference in major deformation mechanism at two applied temperatures; that is, the slip within the grains at 293 K and the grain boundary sliding at 523 K.
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