Investigation On Closed-loop Control System For Deep-Penetration PAW And Stainless Steel Welding Processes

In the process of keyhole plasma arc welding(K-PAW)for medium-thickness steel plates,a high energy density arc can achieve one-side welding with back formation,which has significant advantages such as low cost and high efficiency.However,the narrow welding parameter window and poor welding stability limit its wide application in industry.As the thickness of the workpiece increases,the interaction between the plasma arc and the liquid molten pool in the thickness direction of the workpiece becomes more intense,the molten pool and keyhole becomes more unstable,and welding defects are easier to appear.In order to broaden the range of weldable thickness,increase welding penetration and improve the stability of the welding process,a fast-response K-PAW closed-loop control system has been developed.which is very important for the application of K-PAW in industry.In the new closed-loop control system,the programmable logic controller(PLC)is used as its control unit and the efflux plasma voltage is served as the real-time feedback signal of the keyhole status,which achieves a pulse adjustment frequency above 4 Hz and a response speed within 10 ms,and satisfies the control of the welding processes.The "pulse key-holing" current waveform is designed for 6?10 mm thick plates,its core idea is that the welding current is adjusted in real time with a higher frequency based on the minimum penetration current.The results show that the adaptive pulse current not only makes the opening and closing process of the keyhole rapid and stable,but also reduces the welding heat input and improves the ability of system to control the heat input.By analyzing the dynamic behavior and force characteristics of the molten pool during the deep-penetration welding processes,the research proposes a "Controlled quasi-stable key-holing" current control strategy for 304 stainless steel with a thickness of 12 mm or more.The core of the strategy is to keep the keyhole open,and through the alternate action of the peak current and the base current,the keyhole size only has a slight and controllable change,which overcomes the difficulty of control in the final stage of penetration.The pulse current no longer controls the opening and closing state of the keyhole,but furthest makes the molten pool stable by keeping the keyhole open.Welding experiments are carried out on 304 stainless steel with a thickness of 12 mm and 14 mm.The results verified that this control strategy can reduce welding defects and obtain high-quality welded joints.In addition,the butt welding tests with a reserved gaps and alignment tolerance are carried out.The welding processes are stable and the molten pool is not burned through,which showed the good adaptability of the control system.In view of the common undercut defects of the topside weld,two methods of filler wire welding and TIG cover welding were used for optimization experiments.The results show that both welding processes can eliminate the undercut defects of the front weld.The results of the mechanical properties test of the welded joints show that both the bead-on-plate welds and butt welds have obtained satisfactory performance indicators.The tensile strength of the welded joints can reach to 94%of the base metal,the impact toughness at room and lower temperatures are equivalent to that of the base material,and both the angles of face and root bending of the welded joint can reach to 180°.The corrosion resistance of the welds is tested by the electrochemical potentiokinetic reactivation measurement using double loop method and the corrosion test of stainless steel with 65%nitric acid.The results show that the corrosion resistance of the weld is better than that of the base metal and the metal near the fusion line.