Improving current window in keyhole welding with double-layers hybrid arc heat source

Abstract

Keyhole welding can be easily achieved with a plasma arc, but the process window is narrow because too high arc pressure will damage the keyhole stability. Double-layers hybrid arc heat source was developed by compositing an outer layer of free arc into the center plasma arc jet, the arc heat power can be adjusted in wide range with suppression of arc pressure changing. Keyhole stability was tested to be improved with 6mm-thick stainless steel in previous works, but the stability principle is not yet discussed. In this paper, keyhole welding tests were carried out with thicker steel plates to evaluate the usable welding current window improvement and the weld pool stabilization principle. Results show that the usable current window with the hybrid arc is nearly double wider over that with plasma arc in the 6mm-, 8mm- and 16mm-thick plates. The added free arc heat is deposited into the keyhole bottom with limited ratio, the backside weld width increases much more (about 2.4–6 folders) slowly by increasing the free arc current than by increasing the plasma arc current. Keyhole weld pool stabilization principle is outlined: if the plasma arc is given, arc pressure is almost fixed, surface tension in backside weld surface decreases slowly with the free arc current, weld pool force balance can be achieved in much wider range of welding current. Smooth front and backside weld surfaces are easily obtained with hybrid arc. The research indicates that, for application of the developed hybrid arc system in keyhole welding field, the plasma arc should get the fully penetration threshold, the free arc current is used as the process adjuster, the usable welding current range is very wide.