METHOD FOR PRODUCING AT LEAST TWO
MULTI-DIMENSIONAL SHEET STRUCTURES ONE
VEHICLE BODY BY MEANS OF A FOCUS ANGLE IN
AREA BETWEEN 2 AND 7 [deg.] EXPOSIVE LASER BEAM
The invention relates to a method for producing vehicle bodies with multi-dimensional sheet-metal structures by means of a laser beam according to the preamble of claims 1 and 3 and to a vehicle body according to the preamble of claims 13 and 14.
A method for increasing the seam quality during laser welding of motor vehicle components with complex 3D structures is known from DE 196 05 888 A1. In this method, a weld seam with a large seam depth is produced by means of the inclination of the processing optics and thus the laser irradiation at a defined angle with respect to the surface.
A device for optimizing the machining of a workpiece by means of a laser beam through a modified focus angle shows the JP 2000-263269 A.
JP 2000-263267 A and JP 2000-254792 A also show a device with variable laser beam focus angle.
For welding, the laser beam is focused on the surface of the sheet metal structures to be welded in such a way that the intensity of the laser beam in the region of the energy input into the material of the elements is as large as possible. In other words, the energy of the laser beam is to be limited as possible to a small area in order to effect a rapid melting of the joint area and to achieve an economical process speed, which is why a lens with a short focal length is used. Since the intensity of the transmittable energy depends on the diameter of the laser beam in the focal point, the aim in automated welding is to exactly maintain the distance of the laser optics to the element to be processed so that the focal point of the laser beam is in or near the weld to be formed.
The movement tolerances of the welding robot are determined by the wear of the robot, in particular resulting play in joints and gears, the welding speed and the dynamic forces from the direction changes of the robot. The welding seams produced by such a robot are at least partially wedge-shaped in a cross-section to the welding direction.
Against this background, the invention has the object to perform a method of the type mentioned above, that are compensated by movements of the robot tolerances in the distance between the laser optics and the surface of the element to be processed and an uninterrupted and as constant as possible energy input into the Element is enabled.
This object is achieved by a method according to the features of patent claims 1 and 3 and with a vehicle body according to the features of patent claims 13 and 14. The subclaims relate to particularly expedient developments of the invention.
According to the invention, therefore, a method is provided in which the focal angle of the laser beam is set in a range between 2 [deg.] And 7 [deg.] To the laser beam axis. This makes it possible that with tolerance variations of the system, the change in the diameter and thus the cross-sectional area of the laser beam in the welding plane is low. When welding with a large focus angle, at least 10 [deg.] - 20 [deg.] Is usual, a tolerance variation of the distance of the laser optics to the welding plane of a few millimeters leads to a multiplication of the cross-sectional area of the laser beam in the welding plane. When welding with a sharp focus angle, for example 5 [deg.], The cross-sectional area of the laser beam hardly changes in the welding plane. Old and inexpensive systems often have large movement tolerances.
By compensating the movement tolerances, precise laser welding is possible with older and more cost-effective systems, in higher welding speed ranges and / or with a higher process stability. Furthermore, component fluctuations in the workpieces to be welded are easier to compensate.
To generate the laser beam with a focal angle according to the invention, diode-pumped lasers have proved to be particularly suitable, such as, for example. B. Yb: YAG laser. This laser is assigned a corresponding optics with a long focal length of about 300 - 600 mm, so that with a focus diameter of about 300 to 600 [mu] m a focus angle of 2 [deg.] To 7 [deg.] Is achieved. To achieve a higher intensity, focus diameters of about 200 to 300 μm are adjustable with a correspondingly reduced focal length. Lasers with a power of 4 to 20 KW have proved particularly advantageous for cost-effective use.
This power when the laser beam from the glass fiber, d. H. can be measured from the last interface of the structural unit of the laser is reduced due to edge radiation that is not captured by the downstream optics. Usually arise during laser welding per optical element, d. H. Lens or mirror, power losses. These power losses have so far been limited by a strong focusing optics. The proposed focus angle of 2 [deg.] To 7 [deg.] With a relatively large focal length optics of 400 to 600 mm refers to the laser beam which strikes the sheet structure after emerging from the last optical element to form a weld. Accordingly, this focus angle does not include the edge radiation and includes a lower laser power compared to the laser exit power.
With the method according to the invention, particularly high-quality and secure welded connections can be produced at a focal point position of +/- 10 mm above the sheet-metal structure. If the laser beam impinges on the sheet metal structure, a welding lens forms as a result of the energy input while the material melts. Since the energy / light radiation of the laser penetrates the melt to a certain extent, the laser beam can act on the lower side of the lens / base of the lens, thus melting lower layers. In this case, the laser beam at the side areas of the welding lens, d. H. reflected at the phase boundary melt / solid and deflected into deeper areas.
In a conventional laser welding method with a focus angle of about 17 °, therefore, a characteristic weld seam is formed which, in a section through the weld seam, can be divided into three sections perpendicular to the seam path. In the first, uppermost section almost parallel weld seam edges form. This first section is followed in a second section by tapering, at an angle converging edges of the weld seam until they converge in a third section to form a dome.
With the laser welding method with a focus angle of 2 [deg.] To 7 [deg.], It is now possible to produce particularly advantageous weld seams with parallel or almost parallel weld seam flanks of the welded connection. Due to the higher parallelism of the laser beams at a small focus angle, they are refracted less often at the boundary layer forming at the weld seam edges. As a result, a higher percentage of the radiation energy can be conducted into deeper layers. With a corresponding adjustment of the focus angle from 2 [deg.] To 7 [deg.], The focus position and the entire thickness of the sheet metal layers to be welded, parallel or almost parallel weld seam edges of the welded joint are achieved. In this case, an average value of the weld seam edges, i. H. the phase boundary melt / solid assumed.
Since local overheating forms in the molten bath due to currents / vortices, the phase boundary never runs as e.g. B. in a hole exactly rectilinear but undulating. In addition, transition areas are formed at the sheet metal structure transitions, sheet metal top surfaces and undersides. These transition areas arise due to heat build-up, which cause a rounding of the weld seam edges and by melt, which flows into the joint gap. If two sheet metal structures overlap, this creates a gap, into which melt flows. In this area, a widening of the weld seam width by at least 150%, usually at least 180%, arises compared to the forming weld seam edges in the sheet metal structure.
These transitional areas extend approx. 2 mm into the sheet metal structure and are not taken into account for checking the parallelism. The parallelism of the weld seam edges ends at an end region, which accounts for approximately one third of the lowest sheet metal structure thickness. Normally, this corresponds to 4 mm, preferably 2 to 3 mm.
Due to the guiding of the laser beam with a focus angle in a range between 2 [deg.] And 7 [deg.], An almost constant weld seam width thus results over the weld seam depth, whereby the largest weld seam width in the lower third of the weld seam depth is at least 0.2 mm away from a sheet structure surface (top sheet metal structure, sheet metal underside) in relation to the distance in the middle of the uppermost sheet metal structure greater than 0.7, in particular greater than 0.8, is.
The method according to the invention is furthermore particularly advantageous when blown through sheet metal structures. With a focus angle of 2 [deg.] To 7 [deg.] And a focus position matched to the entire thickness of the sheet layers to be laminated, a uniform energy input over the depth of the weld is possible. Thus, formed by a constant energy input in each plane / depth of the weld borhole weld edges of the weld, which are spaced at a distance from each other, wherein the distance or Lasernahtbreite is insignificantly greater in the inlet region of the laser beam than in the exit region of the laser beam / weld. In other words, the width of the weld at the top and bottom of the weld hardly varies, the deviation is less than 20%, in particular less than 15%.
This makes it possible to produce welds with particularly high quality.
The method has proven particularly advantageous for sheet metal structures to be welded, in particular sheets having a total thickness of at least 3.2 millimeters, wherein the thickness of the individual sheet metal structure can vary between a thickness of 0.6 millimeter to 2.8 millimeters , The acute focus angle of the laser beam allows a deeper welding into the sheet metal structures to be joined, which substantially increases the penetration resistance, and makes it possible that with unchanged movement tolerances of the robot, the fluctuations of the energy input are much lower. As a result, an interruption-free and almost constant energy input into the sheet metal structures is possible. The energy input decreases only slightly over the total depth of the weld in comparison to conventional methods.
A decrease in the energy input into the weld over the entire weld depth of 25%, in particular of 20%, can be achieved.
Due to the improved weld seam geometry, higher welding speeds are possible. For example, with a total sheet thickness of at least 3.5 mm and a laser power of at most 5 kW used for welding, it is possible to travel at a feed rate of at least 4.5 m / min. Alternatively, at a higher laser power of at most 6 kW, it is possible to travel at a feed rate of at least 6 m / min. If the laser system is guided over a robot arm and the laser can still be moved in addition to the movement of the robot arm via a further device, for. As a scanner, welding speeds of more than 11 m / min are possible with the inventive method with appropriate coordination of the laser power and the entire sheet metal structure thickness.
The proposed method can be further improved by replacing the usual today transmissive optics by a reflective optics such. B. a mirror. This application is particularly suitable for laser powers over 6 kW. By using a reflective optic, the position of the focal point can be additionally stabilized. The change in the focal length due to thermal effects of transmissive optics during the welding process is thereby prevented, which has a positive influence on the quality of the weld.
It is known from the prior art to produce a seam in which the focal point of the laser beam in the seam region is set to a mean diameter of 600 micrometers, the diameter of the focal point being at least 300 micrometers and at most 1000 micrometers.
Alternatively, it is proposed to produce several weld seams simultaneously and / or one after the other in the seam region, the diameter of the focal point being substantially smaller. It is favorable that the focal point of the laser beam to produce the welds is set to a mean diameter of 200 microns, wherein the diameter of the focal point has at least 100 microns and a maximum of 300 microns. With a diameter of the focal point of 200 to 300 [mu] m and a depth of the weld seam that comes close to the exit interface, or a weld through, it is possible to transmit the same or a greater force with a smaller total seam area.
The invention allows numerous embodiments. To further clarify its basic principle, some of them are shown in the drawing and will be described below. This shows in
Fig. 1 is a schematic representation of a laser beam with a large focus angle;
2 shows a schematic representation of a laser beam with a small focus angle according to the invention;
3 is a schematic representation of two different seam areas;
4a shows a cross section through a Lasemaht perpendicular to the welding direction with a large focus angle.
4b shows a cross section through a laser seam perpendicular to the welding direction with a small focus angle according to the invention;
5a shows a cross section through a laser seam of a three-layered sheet metal structure with a large focus angle;
5b shows a cross section through a laser seam of a three-layered sheet metal structure with a small focus angle according to the invention. FIGS. 1 and 2 show a laser beam 1 during the welding of two sheet metal structures 3, 4. The laser beam 1 is focused on a focal point 2 such that the laser beam 1 has a minimum diameter 14 in the desired welding plane 12.
FIG. 1 shows this laser beam 1, as known from the prior art, with a large focus angle 10 of 20 °. When changing the distance of the laser optics to the element 3, due to tolerant movements of the robot, the laser beam 1 strikes the element 3 in a different welding plane 13, wherein the distance of the focus point 2 to the laser optics is unchanged. In the deviating welding plane 13, the diameter 15 of the laser beam 1 is greater than in the desired welding plane 12. The intensity of the energy input into the elements 3, 4 is significantly reduced, so that a suspension of the welding process is possible.
FIG. 2 shows the laser beam 1 according to the invention with a small focus angle 10 of 5 [deg.]. If the laser beam 1 hits the element 3 in the deviating welding plane 13, the diameter 15 of the laser beam 1 is increased much less. The intensity of the energy input into the elements 3, 4 is reduced only insignificantly, an interruption-free and almost constant energy input into the elements 3, 4 is possible.
FIG. 3 shows a seam region 5 with a weld seam 6 with a mean diameter of 600 micrometers and a seam region 5 with a plurality of weld seams 7, 8, 9 with an average diameter of 180 micrometers. Although the total area of the welds 7, 8, 9 is smaller than the area of the weld 6, they may transmit the same or a greater force. The welds 6, 7, 8, 9 are generated by the laser beam in a welding direction 11. In this case, the welds 7, 8, 9 can be generated simultaneously and / or sequentially.
FIG. 4 a shows a cross section 11 through a welded seam 6, which connects two sheet metal structures 3, 4 to one another. The cross-section 11 is perpendicular to a direction of welding R, wherein the weld seam 6 is produced with conventional technology with a focus angle 10 of 20 [deg.]. The weld 6 can be divided into three sections. On the side facing the laser beam entrance, a first section 16 is formed, whose weld seam flanks 17 are formed almost parallel. This is followed by a second section 18, in which the weld seam flanks 17 are tapered to one another. The weld seam flanks 17 then converge toward one another in a third section 19 in the form of a dome formed as an end region. The welding energy introduced into the weld 6 corresponds approximately to the molten volume.
As is thus very clearly visible from FIG. 4 a, much more energy has been introduced into the upper sheet-metal structure 3 than into the lower sheet-metal structure 4. In the welded connection, therefore, a tensile compressive stress arises, which can lead to an angular distortion of the sheet-metal structures 3, 4. r
In contrast, Figure 4b shows a cross section 11 through a weld 6, which is generated with a laser beam with a focus angle of 5 °. The cross section 11 is perpendicular to a direction of welding R, wherein in turn two sheet metal structures 3, 4 are interconnected. The weld seam is made with the same focus diameter of the laser beam as 500 μm in FIG. 4a. The two sheet-metal structures 3, 4 have, analogously to the sheet-metal structures 3, 4 in FIG. 4a, the same thickness of 1.2 mm. The weld seam flanks 17, 20 of the laser seam are formed almost parallel. The right-hand, real weld seam edge 20 approximately reproduces the course of a welding flank, which arises as a result of turbulent flows and other thermal influences in the molten bath.
If the irregularities of the real weld seam flank 20 are smoothed, a line is obtained which runs parallel to the weld seam flank 17 shown in idealized form.
Due to the small focus angle of the laser of 5 [deg.], The laser beams are not reflected as often at the phase boundary formed by the weld seam edge 17 compared to a conventional focus angle of about 20 [deg.] And the energy is almost uniform over the entire Welding depth T, which here corresponds to the total sheet thickness t distributed. For the same or lower energy input, therefore, a through-welding of the sheet metal structures 3,4 is possible. Furthermore, due to the almost uniform energy distribution over the entire depth t, the welded connection is virtually free of stress, which is why an angular distortion of the sheet metal structures 3, 4 can be reliably avoided.
In Fig. 5a and Fig. 5b, a three-day welding connection is shown with a respective sheet metal structure thickness of 1, 2 mm. FIG. 5a shows, analogously to FIG. 4a, a cross section 11 through a weld seam 6, which connects three sheet metal structures 3, 4, 4 '' 'to one another. The cross-section 1 1 is perpendicular to a direction of welding R, wherein the weld seam 6 is produced with conventional technology with a focus angle 10 of 20 [deg.]. Due to the larger total sheet thickness t and consequently a greater energy input into the weld seam 6, the weld seam 6 has an initial upper weld seam width 21 of approximately 1000 μm. Otherwise, the welded connection again shows the characteristic sections described in FIG. 4a.
FIG. 5b again shows a cross section 11 through a weld seam 6, which is produced with a focus angle of the laser of 5 [deg.]. The cross section 11 is perpendicular to a direction of welding R, wherein three sheet metal structures 3, 4, 4 'are interconnected. Due to the effect already described, the energy introduced into the weld seam 6, with the exception of one end region 26, is distributed uniformly over the entire weld depth T. Consequently, approximately the same amount of energy is introduced into the section formed by the first sheet-metal structure 3 as in the section formed by the middle sheet-metal structure 4.
Thus, parallel welding flanks 17 with a constant weld seam width 22 are formed, which are interrupted by fillets 23 having a larger weld seam width 25 at the transition areas (sheet-metal structure transitions / sheet-metal structural surfaces). The fillets 23 are formed due to the reduced heat dissipation in the joint gap 24 and on the sheet structure surface 27. These transition areas with greater weld width 25 arise due to heat accumulation, which cause a rounding of the weld seam flanks 17 and by melt, which flows into the joint gap 24. The transition region is normally <1/6, in particular <1/8 of the total sheet thickness t. The parallel or nearly parallel weld seam flanks merge into an end region 26.
The end region 27 is a maximum of 1/3, in particular a maximum of 1/4, the welding depth T 'in the lowermost sheet metal structure and / or a maximum of 1/4, in particular a maximum of 1/5 of the total welding depth T.
LIST OF REFERENCE NUMBERS
1 laser beam
2 focus point
3 sheet metal structure
4 sheet metal structure
5 seam area
7, 8, 9 welds
10 focus angle
11 cross section
12 target welding level
13 welding level
16 First section
17 weld seam edge
18 Second Section
19 Third Section
20 Real weld seam flank
21 Weld seam width
22 Weld width
24 joint gap
25 weld width
26 end area
27 sheet structure surface
R Welding direction t Total sheet thickness
T weld depth
T 'weld depth in the bottom sheet metal structure