Seam-tracked, remote laser beam welding with beam oscillation

Transcription

1 Seam-tracked, remote laser beam welding with beam oscillation Effecting the weld seam surface structure, the weld seam geometry and the gap bridging capability A.Müller 1 ; S.F. Goecke 1 ; F.Albert 2 ; P.Sievi 2 ; S.Baum 2 ; Rethmeier, M. 3 1 Brandenburg University of Applied Science, Brandenburg 2 Scansonic MI GmbH, Berlin 3 Berlin TU, Institute for machine tools and factory operation (Institut für Werkzeugmaschinen und Fabrikbetrieb), Berlin Compared to conventional laser beam welding, remote laser beam welding produces significant increases in productivity. Improved workpiece accessibility and optimised travel paths shorten the cycle time. Up until now, remote applications were almost exclusively limited to welding of square-groove (I) seams on lap joints. The reasons for this are the accuracy of positioning and movement which can be achieved combined with the remote-typical long focal distances of the optical system, in the main, though, the workpiece tolerances from the manufacturing process. The development of processing heads for remote laser beam welding with intelligent sensors and seamtracking as well as new movement units to widen / stabilise the process window, as well as new application areas with the option to fillet weld have contributed to productivity increases. These are expanded upon in this paper. 1 INTRODUCTION New areas of application are continuously being opened up for robot-guided remote laser beam welding. In this process, the laser beam is moved by the robot's movements and also diverted by a fast moving mirror integrated into the remote processing head. These are found at a large working separation from the workpiece - i.e. remote. This produces a reduction in the time taken to position the system and the cycle times, improves access to the components and reduces the risk of contamination of the processing optical system. A disadvantage of the classical remote processing is, however, that exact positioning of the laser beam on the workpiece is not achievable due to the large working separation. Therefore, the main application area of remote laser beam welding was limited up until now for welding square-groove (I) seams on lap joints. Both component tolerances and also the inaccuracy of the guide machines only play a minor role here. 2 OPTICAL SEAM TRACKING DURING REMOTE LASER BEAM WELDING As it is now possible always to accurately position the laser beam on the component assemblies, even with a large working separation and taking tolerances and inaccuracies into consideration, meaning that fillet seams on lap joints or even square-groove (I) seams on butt joints can be produced. Overly large overlapping areas can be reduced as a result and weight savings can be achieved. Critical material combinations, which require a defined beam offset, can be joined in the same way. Figure 1 shows a seam-tracked, remote laser beam welding solution which was the first to achieve the tasks described above. Thanks to a light-sectioning sensor system, the butt joint is detected as it passes into the process. This means that the laser beam is precisely guided to the area of the butt joint independently of the path the robot is taking through the working area. This means that a defined lateral page 1 of 8

2 separation to the joint can be used, for example, to weld alongside the butt joint. Inaccurate robot programming or workpiece tolerances can be compensated for in this way. Processing optical system Scansonic RLW-A Light section to detect the butt joing Deflected laser beam Workpiece Figure 1: Scansonic remote processing head with RLW-A optical seam tracking and a representation of the joint detection running forward 3 beam lines to detect the butt joint, trailing: laser beam at the butt joint. 3 LASER BEAM OSCILLATION IN CONJUNCTION WITH OPTICAL SEAM TRACKING In addition, the system technology shown in figure 1 combines the optical seam tracking with a laser beam oscillation using a highly-dynamic galvanometer scanner. This means that the remote joining can be done on various joint geometries on 3D components. The beam oscillation, for example, also means that ejections from the melted material are reduced by a circular shaped oscillation in the X-Y axes in the welding feed direction [1]. This is caused by the influence of the melted material dynamics as well as the modification of the steam channels produced during welding to an oval or oblong shape [2]. Furthermore, thanks to the beam oscillation's influence on the melted material geometry [3] and the reference to the effects on the level of efficiency [4]. The beam oscillation also influences the surface structure of the weld bead. 4 EFFECTING THE WELD SEAM SURFACE STRUCTURE, THE WELD SEAM GEOMETRY AND THE GAP BRIDGING CAPABILITY 4.1 Welding with optical seam tracking Figure 2 shows the results of seam-tracked laser welding based on weld seam images or micrographs. On the left is the result of a correction of the weld seam position on a curved sheet. The guiding machine in this example moves linearly between 2 points with a speed of v = 8 m/min over the workpiece. Using a light-sectioning sensor system, the curves are detected and the following laser beam is diverted to the correct position using the galvanometer scanner. In the middle of figure 2 it can be seen that the seam tracking itself caused the creation of a welded in square-groove (I) seam on the face here as an example two sheets of 22MnB3 welded at v = 4 m/min. When welding galvanised steel sheets, this allows page 2 of 8

3 the possibility of precisely setting fillet welds even with full penetration; welding errors, like ejections or burn throughs, are significantly reduced in comparison to square-groove (I) seams on lap joints. It is clear that seam tracking has a great influence on the weld seam geometry and the surface structure. Figure 2: Experimental results of remote laser beam welding with optical seam tracking - left: meander welding seam formed during a point to point movement of the guiding machine with seam tracking, centre: welded in square-groove (I) seam created on the face, right: fillet weld seam on galvanised sheets with seam tracking 4.2 Welding with optical seam tracking and beam oscillation By using beam oscillation overlaid locally to the feed speed, the possibilities to influence the weld seam formation are significantly increased. The oscillation profile can be varied along with the oscillation direction and its amplitude as well as the oscillation frequency. To be able to produce comparable results from experiments, in what follows the overlap degree of the welding focus along the welding feed axis at the oscillation distance will be considered. If a sine-wave oscillation with frequency (f), the feed speed/welding speed (VS) and the focus diameter used (FD) are taken, the overlap length (OL) is determined as follows: 1 ÜDDDD = FF D f V 2 S and the above-specified overlap degree (OD) as follows: ÜDDDD = ÜDL F D 100 With this to assist, the feed speed and frequency parameters for a defined welding focus diameter are comparable. For oscillation amplitudes (A) where A = 0.5 FD,an overlap degree of at least 50% is recommended for welding steel base materials. Larger amplitudes require correspondingly larger overlap degrees. Figure 3 shows the dependence of the focus diameter's required frequencies to achieve particular overlap degrees with welding speeds v = 3 m/min and v = 6 m/min. page 3 of 8

4 Overlap degree v = 3 m/min Overlap degree v = 6 m/min Frequency in Hz Frequency in Hz Figure 3: Frequency compared to overlap degree / welding focus diameter, welding speed left = 3 m/min, right = 6 m/min. The calculation approach serves as the first approximation to form a model of the experimental parameter field. Material characteristics (such as melting and boiling points, molten material viscosity, heat conductivity, coating, etc.) must be separately considered. It is possible to change the temperature gradients in the molten pool in connection with the molten bath flow which can be influenced by the oscillation frequency. The molten pool geometry can therefore be controlled within certain limits. Figure 4 shows the formation of the intensity profile on the weld surface as well as the weld cross-section in two examples. Intensity in the process field Intensity profile Intensity Path X Profile Y Intensity in the process field Intensity profile page 4 of 8

5 Path Y Intensity Pat Figure 4: Path X Profile Y Intensity on the process field (left) and intensity profile (right) for various selected beam oscillation parameters. There are almost no limits to set the various intensity profiles. For example, the oscillation in the system shown in figure 1 can range up to a frequency, f, of 1 khz, waveforms are sinus, square or triangular, amplitudes can range from less than 1/10 mm to several mm. What can be achieved with this? Weld seam surface structure The optical appearance of the weld bead can be influenced by the laser beam oscillation. This effect is particularly evident when welding aluminium alloys. Figure 5 shows the first results based on two sheets of a 6000 alloy in a fillet weld on the lap joint. As an example, you can see that oscillation frequencies from approx. f = 600 Hz with amplitudes under A 0 1 mm and using a sinusoidal oscillation shape, significantly smooth out the weld seam surface. It must be pointed out here that this was done without the use of a process gas. For the results shown in figure 5 a disk laser with λ = 1,030 nm and a maximum power, P, of 5 kw was used. Welding was carried out with a speed of v = 5 m/min. Figure 5: Weld bead formation on an aluminium alloy based on the oscillation frequency, λ = 1,030 nm, P = 3.7 kw, v = 5 m/min, F D = 560 µm, oscillation shape: sinusoidal, A = 1.0 mm Weld seam geometry As well as the surface structure influences described above, the weld seam geometry can be controlled by the beam oscillation overlaid on the feed direction. Micrographs of the welding results on construction steel S355, t = 5 mm thickness, illustrate the initial results, compare figure 6. Depending on the oscillation shape selected, the oscillation frequency and amplitude, the welding depth, the weld seam cross-section and the weld seam width can be adjusted. The oscillation in this example was transverse to the feed direction. The focus diameter was 300 µm. page 5 of 8

6 Sinusoidal swinging frequency Frequency (triangular oscillation) Amplitud Amplitud Figure 6: Micrographs of weld trials on 5 mm thick construction steel varying the oscillation shape, the oscillation frequency and the amplitude, λ = 1,030 nm, P = 3 kw, v = 3 m/min For practical use, the adaptability of the welding depth and weld seam width via the selected amplitude is interesting. So, for example, the welding depth in the example shown can vary from complete penetration welding to a welding depth of 0.8 mm with the same feed speed and laser power Gap bridging The possibilities arising from the beam oscillation regarding the defined settings of the weld seam geometry can also be used to expand the typically narrow process window for fillet welding over a lap joint. In practice at this butt joint there are frequently gaps of up to 1 mm [5] and these can lead to faulty weld seams. According to [6], a beam offset of y = 0.4 mm is needed in the direction of the upper sheet is needed to bridge gaps of 0.8 mm. To transform the results coming from the welding trials using a 600 µm focus diameter, the hypotenuse c in the right-angled triangle shown in figure 7 is calculated using the step height (hs). The hypotenuse length corresponds to the required deflection of the laser beam with perpendicular incidence. Calculated with the real lateral angle of incidence of the laser, the side length a describes an irregular triangle (A, B, C`) with the required deflection. page 6 of 8

7 Top sheet Bottom sheet Figure 7: Beam displacement on the top sheet depending on the step height measured by laser triangulation. This procedure is used for fillet welds on sheets made of MSW ZE with thicknesses, t, of 1.5 mm with a set gap of 0.8 mm and a lateral welding angle of α l = 10 from the perpendicular. The amplitude determined is A = 0.41 mm. With a overlap degree OD = 77%, good welding results are achieved with amplitudes from A = 0.35 mm to A = 0.55 mm. In this range you can already observe welding behaviour ranging from through welding to just welding in, compare figure 8. Figure 8: 5 OUTLOOK Micrographs of fillet welds welded using beam oscillation λ = 1,030 nm, P = 4 kw, v = 4 m/min, F D = 300 µm, oscillation shape: sinusoidal, f = 480 Hz, left: A = 0.35 mm; right: A = 0.55 mm) In principle, seam tracking remote processing optical systems can produce significant process improvements. The required processing time is reduced compared to systems with tactile seam tracking and is only slightly larger when compared to classical remote optics without seam tracking. The potential of the system and the process engineering is still far from being exhausted. Currently in the Laser Application Center of Scansonic MI GmbH, work is being done on mixed materials where initial results show that the material and the component geometry can be matched to the oscillation strategy to provide advantages in terms of the weld seam properties. When combined with the seam tracking, the remote laser beam welding will be revolutionised. The authors would like to thank the Federal Ministry for Education and Research (Bundesministerium für Bildung und Forschung BMBF) for the support for parts of the work published in this paper in the project "ECOWELD" as part of the "Training of young engineers in Technical Universities" a part of the "Research at Technical Universities" program. [1] N.N.: page 7 of 8

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