Experimental Study On Characterisation Behavior Of Keyhole During High Power Fiber Laser Deep Penetration Welding

High power fiber laser with its comprehensive advantages,such as high power density and high beam quality,low operating cost and high flexibility,has attracted worldwide attention in the field of welding.In recent years,lots of researchers have found that high power fiber laser welding are prone to defects such as fluctuations of penetration,splash and humps,etc.This kind of welding defects are closely related to the dynamic behavior characteristics of the keyhole.For a comprehensive understanding of keyhole dynamic characteristics and revealing the defects processed during high power fiber laser welding,fluctuation characteristics of keyholes morphology as well as the influencing factors,the interaction between laser beam and the keyhole wall and energy coupling way in the keyholes are investigated in this article.Keyhole morphology fluctuates transiently during laser welding,the obvious effect of both laser power density and duration(welding characteristic time)on keyholes' morphology is confirmed.The plume in the keyhole is much brighter than the one out of it.The white luminous body on the front wall of the keyhole is significantly stronger than the one on the back wall;The gas-liquid interface on the front wall of the keyhole is not a smooth straight line and a certain recession is found below the white light in the front wall;the recession(and white light emitter)has a tendency to periodically move downwards.The small fluctuations in the front wall of the keyhole are closely related to the dynamic behavior of the keyhole.Splashes are observed to erupt along the direction of welding as the welding velocity is low and splashes erupt in the negative direction of welding during high velocity welding.In the welding,the laser beam directly acts on the front wall of the keyhole,and the white luminous body on the front wall actually emits intense vaporized light at the center of the spot and periodically moves downward in the beam direction with a period of 0.3 to 0.8 ms.The strong vaporization caused by laser radiation moved down along the front wall in keyhole wall is actually a dynamic process of the interaction of the laser beam with the front wall of the keyhole.The recoil pressure generated by the steam acts on the front wall so that it(the bottom of the white light)is slightly recessed in the welding direction.The front wall of the keyhole has a melted thickness of about 0.5 mm.The formation of keyhole in the welding process is found to be a periodic punching process in which the center of the laser spot is on the front wall of the keyhole.The direction of the drilling is along the beam direction,and the drilling cycle is between 0.3 and 0.8 ms.The evaporation characteristics on the hole wall reveal that in the high power fiber laser welding,the coupling mode of the laser energy in the hole is multiple reflection absorption,but what determines the penetration of the weld is the punching motion caused by the first incident absorption of the incident laser beam on the front wall of the keyhole.Based on the laser welded "sandwich" model,the interaction between the laser beam and the keyhole wall during high-power fiber laser welding can be indirectly obtained by using the strong vaporization vapor as the medium for observing the interaction between the laser beam and the keyhole wall.According to the close relationship between the hump/splash formations,the changing angle of the front wall of the keyhole and the eruption of plume.The preliminary analysis considers that the fluctuation of the keyhole is related to the fluctuation of the keyhole front wall and the fluctuation of the front wall is determined by the change of the absorption near the Brewster's angle rate of the incident laser on the front wall.That the hump/splash originating from the evaporation of vapor from the front wall promotes posterior wall formation.The main reason for the change of the weld pool and the hump is the fluctuation caused by the fluctuation of front wall interacted with the laser.When the incident angle of the laser beam on the front wall approaches the Brewster's angle,the absorption rate of the laser material reaches the maximum,the front wall evaporation vapor has the greatest thrust on the back wall,so that a hump/splash is formed,which means that the formation of a hump/splash corresponds to an angle of the light beam incidence on the front wall close to the Brewster angle.