Viual Sensing Of Welding Pool And Arc Behaviors In Deep Penetration Keyhole Plasma Arc Welding

Keyhole Plasma Arc Welding(K-PAW)has unique advantages in welding medium and thick plates.Because of its large arc stiffness,high arc temperature,and high energy density,it can realize one-side welding with back formation procedure without making a groove before welding and has a high production efficiency.Deep Penetration Keyhole Plasma Arc Welding has a narrow process window and low adjustable margin,so problems such as splashing and collapse of liquid metal in the molten pool are prone to occur in the process of welding,and welding defects such as pores and burn-through appear in the welded joint.Based on the visual inspection of of the molten pool and arc of large penetration plasma arc welding,this paper summarizes the variation law of welding current parameter window with thickness of the work pieces,analyzes the variation reasons of welding parameter window,verifies the superiority of "Controlled Constant Perforation" control strategy,and reveals the variation law of molten pool and arc when welding defects appear.This study has important theoretical and practical significance.Aiming at the process of deep penetration plasma arc welding,a video signal synchronization acquisition system with high-speed cameras and data acquisition cards as the core was built.Through the calibration of the acquisition system and the edge extraction of the molten pool image,the characteristic size of the molten pool was obtained.In the experiment of constant workpiece thickness,through observing the variation laws of molten pool's characteristic size during the welding process under different welding currents.It is found that as the thickness of the welded workpiece increases,the length and width of the molten pool increase to varying degrees.In addition,the experiment also explored the variable law of the reasonable parameter window range of the current under different process conditions.The results shows that when the thickness of the workpiece is 6mm and 8mm,and the welding speed is within the range of 100?300mm/min,the higher the welding speed,the larger the welding current parameter window.When the welding speed is constant,as the thickness of the workpiece increases,the welding current parameter window decreases.When the workpiece thickness reaches 14mm,the welding current parameter window is less than 5A,and it is difficult to obtain a good weld formation with full penetration.Welding experiments are carried out for a workpiece with a thickness of 14mm,the process parameters affecting the arc penetration force are analyzed,and the parameter range is determined.Through the verification and analysis of the control strategy of "Constant Back Perforation",it is found that the control strategy could maintain the stable state of the molten pool well and has good adaptability when welding 14mm thick workpieces.At the same time,the visual inspection method is used to analyze the behavior of the molten pool in the process of deep penetration plasma arc welding,and the arc behavior of the molten pool when welding defects occur.According to the changing law of molten pool size,the welding process can be divided into stable blind hole stage,unstable blind hole stage,perforation stage and maintenance stage.The time of unstable blind hole stage becomes shorter with the increase of welding current;and the welding defects are always accompanied by the fluctuations of the arc and the size of the molten pool,for example,the appearance of welding pores is often accompanied by arc instability,and the burn-through is accompanied by the characteristics of tailing in the arc shape.The visual inspection method was used to analyze the droplet transfer behavior and the flow behavior of the molten pool during the wire-filled perforated plasma arc welding.In fill wire perforation plasma arc welding,the main form of droplet transfer is large-drop free transition.Compared with the method no-wire filling,the flow velocity of the molten pool is lower when the wire is filled,from the arc area to the tail area of the molten pool,the flow velocity of the molten pool changes in accordance with the rule of "increase-decrease-increase-decrease".