Pressure field stabilization in high-voltage underwater pulsed metal forming using wire-initiated discharges

Electrohydraulic forming of sheet metals is characterized by the usage of large transient hydraulic pressure generated in underwater current discharges. Pulsed power underwater discharges are often categorized as being non-repeatable in terms of pressure map replication. The work described here presents the improvements made in terms of pressure stabilization based on process parameter optimization pertaining to impact electrohydraulic forming. The work consists of a comparative study showing the differences obtained in terms of pressure fields, when discharges initiated by high-voltage breakdown (wireless discharges) and initiated by copper and aluminium wires at otherwise equal test conditions are compared, for a conical discharge chamber. Characteristic pressure maps belonging to the three analyzed discharge conditions are presented and the criteria for a quantitative comparison are set: maximum pressure value, relative pressure scatter and arithmetic mean deviation for a test pressure field. The maximum pressure value characterises the limitation in the sheet material thickness that could be formed at this pressure. For all 3 types of tests the obtained maximum pressure value is nearly equal with a little bit higher level for discharges initiated by aluminium wire. The relative pressure scatter provides information about uniformity of the pressure distribution along loaded area. While wireless tests showed low uniformity with average relative pressure scatter of 33 %, the application of copper and aluminium wire reduced non-uniformity down to 28 and 24 % respectively. The most important effect of the wire introduction has to do with the great increase of stability (repeatability) of pressure fields observed, characterised by a decrease of the arithmetic mean deviation of pressure along a pressure loaded area.