Research On Suppressing Of Porosity At The Lower Weld Seam In Fiber Laser Welding Of Stainless Steel

Laser welding has the advantages of small heat-affected zone,large aspect ratio,and small welding deformation.It has been widely used in nuclear automobiles,power,shipsand other fields.However,at present,there was still a need for in-depth research on the pore defects and suppression mechanisms that appear in the laser welding process.In the pharmaceutical field,the back of the weld of key components such as the isolator and the lyophilizer plate layer was in direct contact with the drug,so no pores can appear on the back of the weld.Based on this,this paper conducted an experimental study on the generation and suppression of pores at the bottom of the fiber laser welding seam in a 3 mm thick stainless steel butt joint.The three methods of applying an ultrasonic field,changing the welding position,and optimizing the shielding gas were used to compare the process test and high-speed camera.The bottom pore defects of laser welding were studied.The main work carried out was as follows:(1)The effect of ultrasonic frequency on the pores at the bottom of the weld was carried out,and combined with high-speed camera observation,the mechanism of the suppression of pores by the application of ultrasonic was analyzed.The results show that when the ultrasonic frequency was 0-20 kHz,the number of pores decreases first and then increases.When the ultrasonic frequency was 15 kHz,the number of pores was the least,and no bottom pores are seen.When ultrasonic was applied,the opening of the welding hole was larger and the stability was maintained for a long time,and the fluctuation of the rear edge of the hole was more stable than when no ultrasonic was applied.The application of ultrasound improves the stability of the welding holes and inhibits the generation of keyhole-induced porosity.(2)The research on the influence of welding position on the pores at the bottom of the weld was carried out,and the suppression mechanism of welding on the pores was analyzed in combination with high-speed camera observation.The results show that both horizontal welding and vertical welding can inhibit the formation of pores to a certain extent.Among them,vertical downward welding has the best pore suppression effect,and the bottom pore has the best suppression effect.In vertical downward welding,the rear edge of the small hole was more stable,and the small hole remains stable for a longer period of time compared with flat welding,thereby suppressing the occurrence of keyhole-induced porosity.(3)The research on the influence of shielding gas on the pores at the bottom of the weld was carried out,and the suppression mechanism of shielding gas on the pores was analyzed in combination with high-speed camera observation.The results show that when only side blowing protection gas was applied(N2)the number of pores was large,when only bottom protection gas was applied(N2)the number of pores was slightly reduced,when both bottom and side blowing protection gas are applied(N2)the number of pores was the least.And the bottom pores are effectively suppressed.When the bottom and side blowing protective gas are applied at the same time,the opening of the small holes was extended and the upper surface molten pool was elongated,which was conducive to the escape of bubbles,so the number of pores was reduced.