Study On Regulation Of Microstucture In 2205 Duplex Stainless Steel Weld Processed By Fiber Laser

Duplex stainless steel has excellent mechanical and corrosion resistance,which is determined by the appropriate ratio of two phases.Due to the rapid cooling rate during fiber laser welding,weld metal is prone to imbalance two phases,which deteriorate the mechanical and corrosion resistance of the joint,and limit the application of laser welding in duplex stainless steel,so the way to improve two phases ratio of weld metal was studied in this article,including two points: welding heat cycle and austenite forming element.Firstly,analysis the effect of laser focus position to weld metal,the finite model of high power fiber laser welding was built,so the temperature field was calculated with different laser focus position,and corresponding experiments was token.Results showed that laser focus position was the key of temperature field and weld metal.When laser focus position was higher than the surface of plates,highest austenite content(25%)was found in the center section of weld metal.When laser focus position was lower than the surface of plates,highest austenite content(28%)was found in the bottom section of weld metal.When defocus amount was zero,austenite content of weld metal became more uniform.Effects of nickel on the microstructure were tested by adding nickel foil to weld,and the mechanism of nickel in the regulation of two phases was studied.Results showed that when nickel content was 7-10% in weld metal,the microstructure of weld was close to base metal,because nickel can alter the weld pool solidification mode from ferrite-mode to austenite-mode.When nickel content was over 12%,austenite became the majority phase,which also harms the balance of two phases.Effects of nitrogen on the microstructure were tested by changing the composition of shielding gas,and the mechanism of nitrogen in the improvement of austenite content was studied.Results showed that when shielding gas contained more than 50% nitrogen,nitrogen can infiltrated into the liquid metal by the molten pool surface,in addition,because of the molten pool flow,nitrogen was added into the center section of weld metal after solidification,which increase the transition temperature of ferrite to austenite.When shielding gas was pure nitrogen,the austenite content of weld metal reached 40%.The mechanical property and corrosion resistance of 2205 duplex stainless steel laser welding joint was tested.Results showed that when laser welding was shielded by pure nitrogen,the joint tensile strength was 756 MPa,impact energy was 177 J and pitting potential was close to base metal,which mean it had the optimum performance.Compare those different regulation ways of welded joint,it was found that use negative defocus,or add nickel and nitrogen elements were beneficial to improve the two-phase ratio of the weld metal,relatively speaking,the method of improving the nitrogen content in the weld metal by the protective gas has good effect,and it has the value to popularize in the production application.