Keyhole welding with double-layer coaxial hybrid arc torch: a primary work

Keyhole welding has an advantage in high efficiency, but the narrow process window for stable keyhole is a critical issue for application. Double-layer coaxial hybrid arcing technology is developed to decouple the heat and pressure properties in an arc source; this will improve the controllability of the weld pool thermal-force state. In this research, an arcing torch, designed by embedding an outer ring tungsten into the plasma arc nozzle to form a double-layer coaxial hybrid arc, was experimentally tested for its application possibility in the keyhole welding process. From the arc image and arc pressure results, stable hybrid arcing process was successfully achieved in the torch when the outer arc current is lower than 30 A. The arc pressure/current ratio in the outer arc current is about half to that in the center plasma arc current. Applying a 10-A outer arc current, keyhole welding can be achieved at a given center arc current of 110 A, which is lower than the threshold current for keyhole welding in an 8.3-mm thick 304 stainless steel plate. In the keyhole welding process in a 4.0-mm thick 304 stainless steel plate, keyhole behavior and the resultant weld have invisible change between the welding process with the hybrid arc torch and the ordinary plasma arc torch. The experimental test results give a promising method to improve the keyhole welding process window.