Study On Fresnel Absorption In The Keyhole During Deep Penetration Laser Welding Of Aluminum Alloy 6016

In this thesis,the essential feature of deep penetration laser welding,i.e.,the keyhole effects,is expounded.Moreover,the status and difficulties of research on keyhole shapes and Fresnel absorption are analyzed.Due to the opacity of welding materials,it is difficult to observe the actual keyhole in the present experimental studies,and it is also difficult to be convinced for the results from the research of Fresnel absorption based on the highly simplified keyhole model.In order to solve the related problems,the keyhole profiles and Fresnel absorption in deep penetration fiber laser welding were studied systematically and experimentally in this thesis.Firstly,keyhole was first observed simultaneously from side view and top view during deep penetration fiber laser welding of aluminum alloy by using a specially designed experimental setup.The results show that the keyhole profile is slightly bent and elongated in the opposite direction of welding.Based on the experimentally measured results,the laser power density distribution was calculated and determined.Secondly,under the assumption that the cross section of the keyhole at any depth is similar to the keyhole profile on top surface,a 3D keyhole in deep penetration fiber laser welding was reconstructed.The laser beam was discretized using the finite element method and the multiple reflections of laser lights in the 3D keyhole was tracked using the geometric optical approximation method.The calculation formula of the Fresnel absorption on the keyhole wall was deduced.Then.the distribution of power density on the keyhole wall by Fresnel absorption was calculated and its influencing factors were analyzed.The results showed that the laser energy is very unevenly distributed on the keyhole wall.The laser intensity on the left and right keyhole wall is higher than that the front and rear keyhole wall in the upper part of the keyhole because of the elongation of the keyhole.In the middle and lower part of the keyhole,the total laser intensity near the front wall is obviously higher than those near the rear wall and the side wall due to the bending of the keyhole.Because the laser energy is transmitted to the bottom of the keyhole by multiple reflections of the laser beam,the laser intensity absorbed from Fresnel absorption on the wall in the bottom of the keyhole is much larger than that of the rest part of the keyhole.In other words,multiple reflections of the laser is the most important factor affecting the distribution of laser intensity by Fresnel absorption.Through Fresnel absorption,the laser intensity on the wall in the bottom of the keyhole is very high,so the material in the bottom of the keyhole is vaporized drastically,which forms huge recoil pressure.Under the action of the recoil pressure,the pressure and energy balance in the keyhole can be maintained and the existence of keyhole have been guaranteed.In other words,Fresnel absorption plays a key role during deep penetration laser welding.