Investigation On Deep-penetration Plasma Arc Welding Technology

Thanks to its unique arc characteristics and keyhole effect,the keyhole plasma arc welding can achieve one-sided welding and two-sided forming welding process under the circumstances of no grooving,filling wire or not.The welding depth and width ratio are large,the heat affected zone is narrow,and the mechanical properties are excellent.In addition,as one of the three high-energy beam welding methods,compared with electron beam welding and laser welding,the cost of plasma arc welding equipment is lower,the preparation work before welding is less,and the welding production efficiency is high.Therefore,the technology of plasma arc welding has a wide application prospect in many industrial fields such as aerospace.However,with the increase of the thickness of the workpiece to be welded,the tendency of the molten pool to collapse increases,and the welding process range becomes narrower,which greatly limits the application range of plasma arc welding.Based on the stress analysis of the weld pool in the welding process,the direction of the plasma arc pressure acting on the weld pool is changed by adjusting the space attitude of the welding gun,so as to reduce the spatter and the tendency of the weld pool collapse.Through a large number of experiments,the best angle of the welding gun is 2.75 degrees.By comparing the effects of different pulse waveforms on weld formation,it is concluded that the waveforms used in deep penetration plasma arc welding need to have three characteristics:fast adjustment speed,continuous change and small change range.By comparing the influence of different control strategies on the weld forming,especially the influence caused by the selection of control quantity and controlled quantity.It is concluded that the control strategy should meet the two characteristics:the controlled quantity can accurately and timely reflect the hole state,and the control quantity can timely adjust the welding heat input.Based on the above analysis,the controlled keyhole sustaining technology of welding 14 mm stainless steel with keyhole plasma arc welding is proposed,and the keyhole is always in the open state after the first perforation.In order to maintain the stability of the keyhole,the adaptive voltage threshold is used to determine the size of the efflux plasma arc voltage,and the frequency conversion pulse is used to adjust the heat input in time.Through the welding experiment of 14 mm thick stainless steel plate with plasma arc welding,the weld with beautiful appearance and excellent performance is finally obtained.In this paper,the effects of the following structural changes on the shape of the arc and the formation of the weld are studied.It mainly includes the shape of the tungsten extreme part,the direction of the guide groove on the surface of the porcelain sleeve and the end structure of the cavity in the nozzle.The results show that the circular platform at the end of tungsten electrode makes the arc slightly divergent,the spiral guide groove can significantly increase the arc penetration,and the nozzle with improved structure can further restrain the arc.After using the improved nozzle,the maximum year-on-year contraction of arc diameter is 30.23%,the maximum year-on-year reduction of arc diffusion angle is 45.89%,and the maximum year-on-year increase of weld penetration is 30.43%.The welding experiment was carried out with the improved nozzle and tungsten electrode with platform.At last,with the average welding current reduced by 14.6%,the plasma arc welding of 14mm thick stainless steel was completed with less welding heat input.The front and back of the weld were well formed.This will lay a foundation for the realization of plasma arc welding with larger thickness and deep penetration.