Research on novel soft variable polarity plasma arc welding technology for aluminum alloys in horizontal position

Considering the imperative need of horizontal welding technology for aluminum alloys with medium thickness and its bottleneck problems, keyhole mode variable polarity plasma arc(VPPA) welding technology is studied. Ordinary VPPA horizontal welding experiments for aluminum alloy thickness 8 mm plates are carried out. It is found that the keyhole is very difficult to be established. The forces acting on keyhole and the fluid flow in weld pool are analyzed to seek the cause of the problem. An inequality equation is induced to describe the relationship of surface tension, arc pressure and the gravity. When the inequality equation is satisfied, the closure of keyhole can be achieved. Based on the above theory, a novel soft VPPA is proposed. Under the keyhole mode welding condition, the decrease of arc pressure makes the critical radius of keyhole on back side larger and promotes the stable establishment of keyhole. Using this novel technology, keyhole mode horizontal welding of 2219 aluminum alloy plates thickness 8 mm is achieved and its weld formation is very good. Porosity welding defect can be eliminated mostly by preheating method and scraping oxide film before welding.