The Simulation Research On The Keyhole Effect Of Deep Penetration Laser Welding Based On A Novel "Sandwich" Method

Laser beam welding is becoming a relatively standard process in the modern technology. CO2 laser welding have increased its role due to its precision, low distortion, efficacy with difficult-to-weld materials, high production rate and process flexibility capabilities. During laser welding, a high-intensity (e.g. with intensity higher than 106 W/cm2) laser beam directed upon the a metal surface usually causes localized heating and evaporation of welded material and results in the formation of a slender capillary into the workpiece, denoted as a keyhole, as well as violent plasma generation characterized by high charge densities. Laser-induced plasma resides inside the keyhole (refer to hereafter as keyhole plasma) and above the workpiece surface (referred to hereafter as plasma plume). The welding process is influenced by the plasma produced by laser irradiation. Plasma plume above the keyhole is the bright, often bluish, flash frequently seen during laser welding. Excess plasma can block, reflect, or refocus the laser beam, resulting in insufficient penetration, burn -through, irregular weld shape, or contamination of beam delivery optics. However, keyhole plasma plays a significant role in coupling between laser beam and materials. Keyhole Effect is the essential characteristic of deep-penetration laser welding. However, very little work has concentrated on the observation of the keyhole profile and the corresponding measurements of the keyhole plasma due to its inconvenient observation. In this paper, a novel method was inventively used to take measurements of the keyhole and the light emission of the keyhole plasma, based on deep -penetration laser welding aluminum films clamped in between two pieces of GG17 glass that we called it a "sandwich" sample. The main designed setup included: 1) The PHC-1000 CO2 laser unit is truss-folded and quasi-sealed, and the laser mode is TEM00; 2) Spectrum analysis instrument which was employed to measure electron temperature and electron density of the keyhole plasma inside the keyhole when laser welding is made up of prism spectroscope, CCD camera and postprocessing software. 3) Specially welding jig setup can well avoid the disturbance of the plasma plume above the welding sample. First of all, a whole keyhole profile can be clearly observed by a high speed camera and a specially designed experimental setup, as well as electron temperature and electron density of keyhole plasma inside the keyhole can be measured by spectral analysis method. Experimental results show that: 1) After testing various specification of glass, GG17 glass was finally chosen as the sample materials to observe keyhole light emission with no disturbance when laser welding, in virtue of its continuous spectrum and same permeability of keyhole light emission for various wavelength chosen to calculate keyhole temperature. 2) Specially welding jig setup can well avoid the disturbance of the plasma plume above the welding sample. 3) Based on the welding depth criterion, defocus was the most influential and welding velocity was the least influential for the range of level setting chosen for the parameters; likewise on the melting width criterion, the flux of the shielding gas was the most influential and welding velocity was the least influential. 4) Microscopic parameters of keyhole plasma inside the keyhole were dissimilar to the plasma plume outside the entrance of the keyhole. For our welding conditions, electron temperature inside the keyhole ranged between 14000K and 18000K, and electron density varied from 1.2×1017 cm-3 to 2.5×1017 cm-3 with different welding process parameters. 5) Low density of keyhole plasma is beneficial to couple laser energy to the target material, but excessive keyhole plasma inside the keyhole will suppress laser energy absorption by target material, even resulting in collapse of keyhole. Secondly, IB (inverse Bremsstrahlung) absorptivity of keyhole plasma and Fresnel absorptivity of the material was obtained based on the experimental observation. Then multi-reflection absorption and IB absorption due to keyhole plasma was calculated. The calculation results indicate that: 1) IB absorptivity of the keyhole plasma decreased with the increase of electron temperature, and increased with the enhancement of electron density. 2) The intensity absorbed distributed non-uniformly on the keyhole wall, in respect that the intensity absorbed on the front wall of the keyhole was much greater than that on the rear wall of the keyhole, and the maximum intensity absorbed appeared on the bottom of keyhole profile. The intensity absorbed on the front wall of the keyhole mainly stemmed from Fresnel absorption by reason of multi-reflection, while the intensity absorbed on the rear wall of the keyhole chiefly rooted in IB absorption due to keyhole plasma. 3) Defocus exerted the earth influence on the intensity absorbed on the front wall of the keyhole, but little influence on that on the rear wall. Positive defocus reduced the laser radiation intensity on the material, decreasing the intensity absorbed on thekeyhole wall. While negative defocus increased the laser radiation intensity on the material, making the intensity absorbed on the keyhole wall enhanced. 4) By comparing the heat flux lost on the keyhole wall and laser intensity absorbed on the keyhole wall through Fresnel absorption and IB absorption, both energy distribution on the keyhole were uniform. The keyhole adjusted its heat flux loss and the laser intensity absorbed by self-consistent method, finally making itself reaching energy balance. Finally, on the basis of the above actual keyhole obtained by experiment, a two-dimensional and a three-dimensional heat transfer model with a cylinder surface heat source have been developed in view of temperature dependant material properties and the convection in the welding pool. The two models were numerically solved by the finite element method using ANSYS software. The calculation results show that: 1) Because of the welding speed, the temperature gradient and the velocity gradient on the front wall of the keyhole is much greater than those on the rear wall of the keyhole. 2) By comparing considering the convection in the weld pool or not, the width and length of the weld pool increased. Likewise, by comparing considering the axial flows in the weld pool or not, the width and length of the weld pool increased as well.