A Theoretical And Experimental Investigation On Keyhole Effects In Deep Penetration Laser Welding

After widely analyzing the existing research works on deep penetration laser welding, in view of the existing problems, the keyhole effects in deep penetration laser welding are theoretically and experimentally studied in this paper.First of all, GG17 glass is chosen as the sample material, which has excellent heat-resistance and a great deal of differences between the softening point and the vaporization point. A whole keyhole can be clearly observed by a high speed camera and a specially designed experimental setup. The laser spot diameter and its intensity distribution are measured with a scanning pinhole. The effects of such factors as focal spot, focal position, welding speed and the plasma on the shape and size of the keyhole are experimentally studied. The results are as follows: Because the conductivity of GG17 glass is far less than that of most metals, the diameter of the keyhole depends greatly on that of the laser spot relevant to the effective laser intensity (corresponding to the intensity that can vaporize the sample material), but little on the welding speed. The keyhole shape is like an cone, whose vertex angle decreases as keyhole depth increases. If the keyhole is deep enough, the keyhole diameters at different depth vary little. In mis case, the keyhole can be treated as a cylinder. The curve of the keyhole relates closely to the welding speeds. The greater the welding speed, the greater the curve of the keyhole. Increasing the welding speed and the defocus, or decreasing the laser power, the keyhole depth decreases. Because the ionization energy of GG17 glass is much higher than that of most metals, it is difficult to form plasma in laser welding of GG17 glass. So the main energy absorption mechanism is Fresnel absorption.Secondly, the effects of Fresnel absorption on laser energy absorption in deep penetration laser welding are systematically studied. After measuring the geometrical data of the keyhole from the keyhole photos, the equations of the front and the rear wall of the keyhole can be obtained by the method of polynomial fitting. Then, on the basis of measuring the reflectivity of the sample, according to geometrical optics theory, the intensity absorbed by cylindrical, conical and actual keyhole wall through Fresnel absorption are analyzed and calculated by means of tracing a beam light. The results show: the multiple reflections on the keyhole walls can only influence the intensity distribution on the bottom of the keyhole. However, the intensity distribution on the top of the keyhole is mainly decided by the direct incidence. To the actual bending keyhole, not only the intensity distribution on the front wall but also mat on the rear wall depend on the intensity distribution after multiple reflections of the laser beam that irradiates directly on the front wall. After multiple reflections, the intensity absorbed on the front wall of the actual keyhole can be pretty uniform. However, the intensity absorbed on the rear wall of the actual keyhole can't be well-distributed, most part of which even has no beam illuminated. Thismmeans that the laser energy is mainly absorbed by the workpiece on the front wall of the keyhole.Thirdly, the effects of inverse Bremsstrahlung absorption of the plasma in the keyhole on laser energy absorption in deep penetration laser welding are systematically investigated. According to geometrical optics theory, the intensity absorbed by cylindrical, conical and actual keyhole wall through inverse Bremsstrahlung absorption of the plasma existed in these kinds of keyholes are analyzed and calculated by means of tracing a beam light. The calculation results show that the intensity absorbed on the keyhole walls is mainly determined by the intensity absorbed by the plasma through inverse Bremsstrahlung absorption from direct incidence, inverse Bremsstrahlung absorption of the plasma from multiple reflections has a little influence on the intensity distribution only on the bottom of the keyhole. The attitude and the distribution of me intensity ab...