Flashing phenomena in flash welding of mild steel are studied by oscillogram using comparatively small size test pieces.
The secondary no load voltage of welding transformer is adjusted in a range of 5-30 V. See Fig. 1. Secondary current of transformer is directly measured by using co-axial cylindrical shunt shown in Fig. 2.
For simplicity of experiment, a straight cam is used and the platen velocity is adjusted to 1.75-28 mm/sec under the experimental condition.
In the early stage of flashing, alternation of two distinct periods is observed, one being the period of alternation of short circuit and arcing and the other being the no current period. See Fig. 8.
The above mentioned alternation of successive short circuit and arcing is the special character when steel is used in air, the cause being CO gas produced in molten metal in arcing period. See Fig. 11-13.
The volume expansion of molten metal due to CO gas results in a short circuit and the contraction of CO gas results in the breaking of the short circuit. It is observed the short circuit is apt to occur more frequently when the instantaneous current becomes close to zero. The reason can be of course explained by the decreasing of repulsive arc force which acts on the molten metal.
For secondary no load voltage of 5 V, arc decays quickly, while for no load voltage larger than 15 V, arc is sustained for a long time and results in great heating of the test piece.
For no load voltage of 5 V, frequent short circuit and arc of small current occur as shown in Fig. 15, because the molten metal is not flashed off as in the case of large heat input and results in the short gap for a given platen velocity.
The circuit constant L/R affects also the arcing mode. For small values of L/R, arc energy is naturally small and frequent short circuits are observed as shown in Figs. 16 and 18. It must be noticed that the duration of a short circuit becomes very short, because the high current density of the contact point heats up the contact point and makes early breaking of the contact.
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