Study On Keyhole Effect In Deep Penetration Laser Welding With A Layer-by-Layer Direct Observation Technology

As a welding method of high quality and efficient,deep penetration laser welding has many advantages,such as fast welding speed,large depth to width ratio,and small Heat-affected zone,etc.It is widely used in nuclear power,automobile,ship,rail transit,aerospace and other fields.The essential feature of deep penetration laser welding is the existence of keyhole,which plays a crucial role in coupling laser energy to the material.The formation and maintenance of keyhole is the key for deep penetration laser welding.For deep penetration laser welding of metal material,due to the opacity of the material,it is difficult to obtain the keyhole shape and keyhole plasma information,and it is also difficult to detect the pressure balance inside the keyhole.Aiming at solving such two important scientific problems as keyhole effect and keyhole formation mechanism in deep penetration laser welding,two key technology breakthroughs in the direct observation of keyhole shape and the global diagnosis of keyhole plasma are made in this paper by proposing a "Layer-by-Layer" method which based on the idea of "layering".By using a specially designed composite workpiece and experimental device,combined with a high-speed camera and spectrometer,the cross-sectional of keyhole in the metal and the spectrum information of the keyhole plasma were obtained during deep penetration laser welding of metal.Then,a 3D keyhole in the material was rebuilt by using the cross-sectional of the keyholes which obtained from experimental observation.By tracking the multiple reflection paths of the laser beam in the 3D keyhole,the Fresnel absorption and inverse bremsstrahlung absorption mathematical models in the keyhole were established,and the relevant algorithms were developed to study the energy absorption in the keyhole.The experiment of deep penetration laser welding was carried out,and the influence of welding process parameters on welding quality was studied.A method for detecting the pressure balance in a keyhole is proposed.The formation process of the keyhole was studied and the pressure balance detection in the keyhole was realized,which explores a new idea for studying the pressure balance of the keyhole in real deep penetration laser welding.The main research work of this paper is as follows:(1)In order to directly observe the keyhole shape in deep penetration laser welding metal,the Layer-by-Layer Imaging Method is proposed,and the keyhole observation experimental platform is designed,then the series of cross sections of a single keyhole in deep penetration laser welding metal are obtained.The spatial position relationship between adjacent keyhole sections was analyzed,and an algorithm for reconstructing 3D keyhole was developed.The MATLAB program was compiled to process the acquired keyhole cross section images.The keyhole contours represented by feature points were obtained,and the 3D keyhole inside the metal was rebuilt.The results show that the shape of the reconstructed 3D keyhole is curved,and the bending direction is opposite to the welding direction,which conforms to the actual bending form of the keyhole in deep penetration laser welding.Along the depth direction of the 3D keyhole,the size of the keyhole section shows a trend of gradual reduction,but fluctuates to some extent,which is related to the instability of the keyhole.The accuracy of the reconstructed 3D keyhole is affected by the number of keyhole sections.(2)In order to obtain the keyhole plasma information in deep penetration laser welding metal,the Layer-by-Layer Diagnosis Method is proposed,and the spectral diagnosis experimental platform is designed to diagnose the keyhole plasma spectrum inside metal in deep penetration welding with optical fiber laser.The results show that the corresponding spectral lines on different cross sections of the keyholes are similar,and the spectrum is mainly continuous,and the number of line spectra is rare.The theory of atomic ionization was analyzed,and it is confirmed that the metal vapor in the keyhole has extremely low ionization degree during deep penetration welding with optical fiber laser in the experiment.The diagnostic spectrum is mainly thermal radiation spectrum.(3)The vector was used to represent the laser beam,and the ray tracing of the laser beam multiple reflection in the keyhole was analyzed theoretically.According to the characteristics of Fresnel absorption and inverse bremsstrahlung absorption,the mathematical model of laser energy absorption in the keyhole was established,and the energy absorption at different depths in the reconstructed 3D keyhole based on fiber laser deep penetration welding was analyzed.The results show that the absorbed laser energy is different along the depth of the keyhole,and a large amount of laser energy is gathered at the bottom of the keyhole,which is related to the laser multiple reflection in the keyhole.(4)The experiment of deep penetration laser welding was carried out,and the influence of process parameters on welding quality in deep penetration laser welding was studied.The single-factor experiment and multi-factor experiment were carried out,then the order of influence of defocus,welding speed,laser power,gas flow on keyhole depth,weld width and depth-width ratio was determined.(5)In order to detect the pressure balance in the keyhole,the Gas Jet Impingement Method is proposed,and the pressure balance detection platform in the keyhole is designed to observe the formation of the keyhole in liquid under the action of gas-liquid two-phase flow.The effects of dynamic pressure,surface tension and static pressure on the formation and maintenance of keyholes were analyzed by numerical simulation and model experiments,and the magnitude of each force was calculated.The experiment of observing the keyhole in deep penetration laser welding metal by the Sandwich Method was carried out,and the keyhole in gas-liquid two-phase flow and the keyhole in laser welding were compared.The results show that the dynamic pressure tends to open the keyhole,while the effect of surface tension and static pressure is the opposite.The effect of surface tension in pressure balance is far greater than that of static pressure.The dynamic pressure increases gradually along the direction of the keyhole depth.The dynamic pressure in the keyhole inside Ga-In-Sn liquid alloy is less than 2% of the atmospheric pressure.The keyhole in gas-liquid two-phase flow is similar to that in actual deep penetration laser welding,which is helpful to further study the pressure balance mechanism in the keyhole in deep penetration laser welding.