METHOD AND PROCESSING MACHINE FOR PORE DEFECT MONITORING OF A LASER WELDING PROCESS FOR WELDING A PLURALITY OF BAR CONDUCTORS AND ASSOCIATED COMPUTER PROGRAM PRODUCT

Abstract

A method is provided for monitoring a laser welding process for welding two workpieces of metallic material, particularly copper or aluminum, preferably bar conductors, by using a laser beam, particularly for monitoring a plurality of identical laser welding processes for welding two identical workpieces with the same laser power and the same welding duration of the laser beam. During the welding, the laser beam is directed onto adjacently disposed end faces of the workpieces to melt a fusion spot at the end faces then solidifying to form a weld bead. During the welding, the solidification duration from turning off the laser beam until solidification of the fusion spot is determined, the determined solidification duration is compared with a setpoint solidification duration predetermined for pore defect-free welding, and if the determined solidification duration falls below the predetermined setpoint solidification duration, the solidified weld bead is classified as defective.

Claims

1. A method for pore defect monitoring of a laser welding process for welding two workpieces or bar conductors made of metallic, copper or aluminum material by using a laser beam, or for monitoring a plurality of identical laser welding processes for welding two identical workpieces with the same laser power and the same welding duration of the laser beam, the method comprising: during welding of two workpieces, directing the laser beam onto adjacently disposed end faces of the workpieces in order to melt a fusion spot at the two end faces then solidifying to form a weld bead; during the welding of the two workpieces, determining a solidification duration from turning off the laser beam until solidification of the fusion spot; comparing the determined solidification duration with a setpoint solidification duration predetermined for pore defect-free welding; and classifying the solidified weld bead as defective upon the determined solidification duration falling below the predetermined setpoint solidification duration.

2. The method according to claim 1, which further comprises starting from turning off the laser beam, continuously recording locally resolved digital images of the fusion spot by using a detector or a camera, and generating intensity-scale pixel images or grayscale pixel images from the locally resolved detector images.

3. The method according to claim 2, which further comprises determining an intensity scale value averaged over all pixels of the pixel image or an established image section of the pixel image for each pixel image, and determining the solidification duration aided by a temporal profile of the averaged intensity scale values.

4. The method according to claim 2, which further comprises recording the detector images with a recording frequency of at least 100 Hz.

5. The method according to claim 2, which further comprises recording the detector images with a recording frequency of at least 1 kHz.

6. The method according to claim 2, which further comprises respectively evaluating the detector images in an image section or in an annular image section around a midpoint of the fusion spot.

7. The method according to claim 1, which further comprises upon classifying a weld bead as defective, automatically rewelding the weld bead or instigating another action or a warning message.

8. The method according to claim 7, which further comprises instigating the rewelding or the other action depending on how much the determined solidification duration deviates from the predetermined setpoint solidification duration.

9. A processing machine for laser welding two workpieces or bar conductors made of metallic, copper or aluminum material, the processing machine comprising: a laser beam generator for generating a laser beam; processing optics for directing the laser beam onto mutually adjacently lying end faces of the two workpieces in order to melt a fusion spot at the two end faces then solidifying to form a weld bead; a locally resolving detector for locally resolved detection of the fusion spot and recording of locally resolved detector images; an image processing unit for evaluating the locally resolved detector images recorded by said detector in order to determine a solidification duration from turning off the laser beam until solidification of the fusion spot; and a pore defect monitoring device monitoring or classifying the solidified weld bead with respect to pore defects being aided by the determined solidification duration.

10. The processing machine according to claim 9, wherein said image processing unit includes: an intensity-scale pixel image generating device for generating intensity-scale pixel images from the recorded detector images, and an evaluation device for evaluating the intensity-scale pixel images in order to determine the solidification duration from turning off the laser beam until solidification of the fusion spot.

11. The processing machine according to claim 10, wherein said detector is disposed coaxially with the laser beam.

12. A non-transitory computer program product with instructions stored thereon, that carry out the steps of claim 1 when executed on a machine controller of a processing machine.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0023] FIG. 1 is a diagrammatic representation of a processing machine according to the invention for the laser welding of two bar conductors by using a laser beam;

[0024] FIGS. 2A-2C are images of a fusion spot generated on the end faces of the two bar conductors during the laser welding of two bar conductors directly after turning off the laser beam (FIG. 2A), during the solidification (FIG. 2A) and after the solidification (FIG. 2C);

[0025] FIG. 3 is an image of the liquid fusion spot with an annular image section around the midpoint of the fusion spot; and

[0026] FIGS. 4A and 4B are a temporal profile of the radiation intensity of the thermal radiation emitted by the fusion spot (FIG. 4A) and the temporal profile of the gray values of the images, respectively averaged over predetermined image pixels of the recorded images (FIG. 4B).

DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatically illustrated processing machine 1 used for the laser welding of two workpieces made of metallic material, in this case by way of example in the form of two bent bar conductors 2 (hairpins) made of copper, by using a laser beam 3. The two bar conductors 2 have the same end face 4 to be welded, with the same cross section, and are disposed adjacently, with their end faces 4 preferably at the same height.

[0028] The processing machine 1 includes a laser beam generator 5 for generating the laser beam 3, a processing head 6 with processing optics 7 for directing the laser beam 3 onto the end faces 4 of the two bar conductors 2 in order to melt a fusion spot or zone 8 on the end faces 4, a locally resolving detector directed onto the fusion spot 8, for example in the form of a camera 9, an image processing unit 10 for evaluating the digital images recorded in a locally resolved fashion by the camera 9, and a monitoring device 11 which monitors the fusion spot 8 solidified to form a weld bead 8′ with respect to pore formation with the aid of the evaluated camera images.

[0029] The laser beam 3 generated by the laser beam generator 5 strikes a beam splitter (for example in the form of a dichroic mirror) 12, which is reflective for the wavelength of the laser beam 3. Through the use of the beam splitter 12, the laser beam 3 is reflected through a focusing device not shown herein (for example a focusing lens) onto the processing optics 7, and is directed by the latter onto the two end faces 4. The processing optics 7 may, for example, be a laser scanner which has two mirrors respectively rotatable about mutually perpendicular axes, in order to deflect the laser beam 3 two-dimensionally.

[0030] Image beams 13 coming from the fusion spot 8, which travel through the processing optics 7, the beam splitter 12 which is transmissive for the image beams 13 and a further beam splitter (for example in the form of a dichroic mirror) 14, which is reflective for the image beams 13, to the camera 9 and form the image of the fusion spot 8 there, are registered by the camera 9. As shown, the camera 9 is aligned coaxially with the laser beam 3 by using the further beam splitter 14. An optical filter 15 and a collimation lens 16 for focusing the image beams 13 are optionally also disposed between the further beam splitter 14 and the camera 9. The optical filter 15 blocks the wavelength of the laser beam 3 so as to transmit only the process radiation coming from the fusion spot 8, but not the laser beam 3 reflected at the workpieces 2. The camera 9 may be configured to record individual images, or as a video camera for recording a video sequence, the recording frequency preferably being at least 100 Hz.

[0031] In order to monitor a laser welding process, in particular a plurality of identical laser welding processes respectively on two identical workpieces 2 with the same laser power and the same welding duration of the laser beam 3, the following procedure is adopted.

[0032] After the workpiece processing, i.e. beginning with turning off the laser beam 3, images 17a-17c of the fusion spot 8 are recorded continuously by the camera 9 (FIGS. 2A-2C), with the fusion spot 8 appearing light in the recorded images 17a-17c. The image 17a shows the liquid fusion spot 8 directly after turning off the laser beam 3 and before the solidification, the image 17b shows the liquid fusion spot 8 during the solidification and the image 17c shows the solidified weld bead 8′.

[0033] In a grayscale pixel image generating device 10a of the image processing apparatus 10, grayscale pixel images with pixel values between 0 (dark) and 255 (light) are respectively generated in an x-y pixel grid from the recorded images 17a-17c. With a temperature of the fusion spot 8 decreasing more greatly, the grayscale value changes from light to dark. In the pixel images, an identical image section (region of interest (ROI)) 18 is respectively defined by the image processing apparatus 10 (FIG. 3), for example in the form of an annular image section around the midpoint M of the fusion spot 8. In an evaluation device 10b of the image processing apparatus 10, the pixel images are evaluated in the ROI 18 in order to determine the solidification duration Δt from turning off the laser beam 3 until solidification of the fusion spot 8.

[0034] FIG. 4A shows the temporal profile of the radiation intensity I of the thermal radiation emitted by the fusion spot 8 after turning off the laser beam 3 at the instant t=0. The radiation intensity I decreases after turning off the laser beam 3 and remains at a plateau value from a few milliseconds before the solidification of the fusion spot 8 until the solidification (instant tE), before decreasing to zero.

[0035] After turning off the laser beam 3, the gray value profile is registered in a temporally resolved fashion inside the locally averaged ROI 18. For this purpose, by using the evaluation device 10b, the gray value G averaged over all pixels of the ROI 18 is determined for each pixel image and the temporal profile shown in FIG. 4B of the averaged gray values G is evaluated. With the aid of the characteristic temporal profile of the averaged gray values G, the instant tE of the solidification and therefore the solidification duration Δt may be determined clearly.

[0036] For repeated identical laser welding processes, in which the laser beam 3 always has the same laser power and the same welding duration, respectively on two identical bar conductors 2 with the same end faces 4 and the same bar cross section, in the case of pore defect-free welding, all welding instances have the same solidification duration Δt.

[0037] The solidification duration Δt determined in this way is compared in the monitoring device 11 with a setpoint solidification duration ΔtS predetermined for a pore defect-free weld bead 8′. If the solidification duration Δt determined falls below the predetermined threshold value ΔtS (Δt<ΔtS), the solidified weld bead 8′ is classified as defective (“pore defects present”). In the case of excessive deviations, automated rewelding may be initiated or any other desired action may be instigated.

[0038] In order to illuminate the fusion spot 8, the processing machine 1 may have an illumination laser 20, the illumination beam 21 of which is coupled through the two beam splitters 12, 14, which are transmissive for the wavelength of the illumination beam 21 in this direction, coaxially with the laser beam 3 into the processing head 6 and are directed onto the fusion spot 8. The illumination beam 21 reflected at the workpiece 2 travels on the reverse path back to the further beam splitter 14, which is reflective in this direction and deflects the illumination beam 21 onto the camera 9. In this case, the fusion spot 8 appears dark in the recorded images and illuminated solid material appears light.