Abstract
The disclosure relates to methods and systems for joining at least two workpieces, including forming a weld joint by moving a machining beam, e.g., a laser beam, and the at least two workpieces relative to one another along a feed direction, wherein the movement of the machining beam and the two workpieces relative to one another is superimposed with a periodic movement in a movement path, e.g., a two-dimensional movement path, which extends in a transverse direction perpendicularly to the feed direction and, e.g., additionally in the feed direction. The movement path has, between two reversal points in the transverse direction, at least one stop point at which a speed component of the periodic movement in the transverse direction is zero. The invention also relates to computer program products and systems for carrying out the methods.
Claims
1. A method for joining at least two workpieces, the method comprising: forming a weld joint by moving a machining beam relative to the at least two workpieces along a feed direction, wherein the movement of the machining beam relative to the two workpieces is superimposed with a periodic movement in a movement path that extends in a transverse direction perpendicularly to the feed direction, wherein the movement path is a two-dimensional movement path that additionally extends along the feed direction, wherein the movement path between two reversal points in the transverse direction has at least one stop point at which a speed component of the periodic movement in the transverse direction is zero, and wherein the two-dimensional movement path has two arced path sections, which contact one another at the at least one stop point, wherein the two arced path sections, upon reaching the at least one stop point, have a speed vector tangential to the feed direction.
2. The method of claim 1, wherein a non-continuous change in direction of the two-dimensional movement path takes place at the at least one stop point.
3. The method of claim 1, wherein the at least one stop point of the periodic movement is positioned along a joint center of the weld joint.
4. The method of claim 1, wherein the at least one stop point forms a front reversal point, in the feed direction, of the periodic movement along the two-dimensional movement path.
5. The method of claim 1, wherein the movement path has a further arced path section, which connects two end points, remote from the at least one stop point, of the two arced path sections to one another.
6. The method of claim 1, wherein the two-dimensional movement path in the transverse direction progresses symmetrically or asymmetrically with respect to a center axis progressing in the feed direction, containing the at least one stop point.
7. The method of claim 1, wherein the power of the machining beam is changed during the periodic movement along the movement path.
8. The method of claim 7, wherein the machining beam has a maximal power at the at least one stop point.
9. The method of claim 1, wherein the weld joint forms a fillet joint or a lap joint between the two workpieces to be joined.
10. The method of claim 1, wherein the machining beam is a laser beam.
11. A tangible computer-readable medium storing instructions that, when executed by a data processing system, cause the data processing system to carry out the steps of the method of claim 1.
12. The method of claim 1, wherein the two arced path sections each form a quarter circle.
13. A system for joining at least two workpieces, comprising: a machining head for aligning a machining beam with the two workpieces; a movement device for moving the machining beam relative to the at least two workpieces in a feed direction to form a weld joint; a beam deflecting device for deflecting the machining beam; and a control device for controlling one or both of the beam deflecting device and the movement device to generate a periodic movement of the machining beam, superimposed on the relative movement between the machining beam and the two workpieces in the feed direction, along a movement path that extends in a transverse direction perpendicularly to the feed direction, wherein the movement path is a two-dimensional movement path that additionally extends along the feed direction; wherein the control device for controlling the beam deflecting device is configured to generate a movement path, which has, between two reversal points in the transverse direction, at least one stop point at which a speed component of the periodic movement in the transverse direction is zero, and wherein the two-dimensional movement path has two arced path sections, which contact one another at the at least one stop point, wherein the two arced path sections, upon reaching the at least one stop point, have a speed vector tangential to the feed direction.
14. The system of claim 13, wherein a non-continuous change in direction of the two-dimensional movement path takes place at the at least one stop point.
15. The system of claim 13, wherein the beam deflecting device comprises at least two scanner mirrors for deflecting the machining beam.
16. The system of claim 13, wherein the machining beam comprises a laser beam.
17. The system of claim 13, wherein the two arced path sections each form a quarter circle.
18. A method for joining at least two workpieces, the method comprising: forming a weld joint by moving a machining beam relative to the at least two workpieces along a feed direction, wherein the movement of the machining beam relative to the two workpieces is superimposed with a periodic movement in a movement path that extends in a transverse direction perpendicularly to the feed direction, wherein the movement path is a two-dimensional movement path that additionally extends along the feed direction, wherein a portion of the two-dimensional movement path between two reversal points in the transverse direction has at least one stop point at which a speed component of the periodic movement in the transverse direction is zero, wherein the two-dimensional movement path has a first stop point and a second stop point, wherein the first stop point forms a reversal point in the feed direction of the periodic movement along the two-dimensional movement path, and wherein the second stop point is connected to the first stop point by a linear path section, and wherein the movement path forms a three-quarter circle with the second stop point as a circle center point.
Description
DESCRIPTION OF DRAWINGS
(1) FIG. 1 is a schematic illustration of an embodiment of an apparatus for joining two workpieces, in which a laser machining head is moved in a feed direction and a laser beam is aligned with the two workpieces to form a weld joint.
(2) FIGS. 2A and 2B area position graph and a time/speed graph of a periodic movement of the laser beam, superimposed on the movement in the feed direction, along a one-dimensional movement path in a transverse direction perpendicularly to the feed direction.
(3) FIGS. 3A and 3B area position graph and a time-dependent laser power during a periodic movement of the laser beam along a two-dimensional, circular movement path.
(4) FIGS. 4A and 4B are graphs analogous to the graphs in FIGS. 3A and 3B, of a movement path that has two circular-arc-shaped path sections, which meet at a stop point at which a speed component of the periodic movement in the transverse direction is zero.
(5) FIGS. 5A and 5B are graphs analogous to the graphs in FIGS. 4A and 4B, in which the movement path additionally has a semi-circular path section, which connects the two circular-arc-shaped path sections.
(6) FIGS. 6A and 6B are graphs analogous to the graphs of FIGS. 3A and 3B, in which the movement path forms a three-quarter circle with two stop points.
(7) In these drawings, identical reference signs are used for identical or functionally identical components.
DETAILED DESCRIPTION
(8) FIG. 1 shows an example of a construction of an apparatus 1 for joining two workpieces 2a, 2b using a machining beam in the form of a laser beam 3. The apparatus 1 has, for generating the laser beam 3, a beam source in the form of a laser source 4, for example in the form of a solid-state laser, e.g., an Nd:YAG laser, a diode laser, or a fiber laser. In the example shown, the laser beam 3 is supplied with the aid of an optical fiber to a machining head 5, which aligns the laser beam 3 with the two workpieces 2a, b. In the example shown, the machining head 5 is moved by means of a movement device 6 relative to the two workpieces 2a,b to be joined, which are arranged stationary in the example shown. The movement device 6 can be, for example, a welding robot or the like, on which the machining head 5 is mounted. In the example shown in FIG. 1, the machining head 5 and therefore the laser beam 3 are moved in a feed direction Y corresponding to the Y direction of an XYZ coordinate system with a feed speed V.sub.W. Alternatively, the machining head 5 may be mounted to a coordinate guide and moved relative to the stationary workpieces 2a, 2b using translatory drives, e.g., linear actuators and/or motors. Alternatively or in addition, the workpieces 2a, 2b may be moved along the Y direction using, e.g., linear actuators and/or motors.
(9) In the example shown in FIG. 1, a weld joint 7 in the form of a so-called fillet joint is formed by means of the laser beam 3 focused on the two workpieces 2a, 2b during the movement in the feed direction Y. It has been shown in the case of such fillet joints, but also in the case of lap joints, that cracks can form along the weld joint 7 with the movement of the laser beam 3 in the (constant) feed direction Y, in particular when the two workpieces 2a,b are formed from materials which are susceptible to hot cracking or when mixed compounds of combinations of certain workpiece materials are to be welded together. To prevent the formation of hot cracks wherever possible, the (in the example shown, linear) movement in the feed direction Y is superimposed with a periodic movement of the laser beam 3, as will be described in more detail below.
(10) In the example shown in FIG. 1, the machining head 5 has a beam deflecting device in the form of a scanner device 8, which comprises a first and second scanner mirror 10a, 10b. By means of associated rotary drives 11a, 11b, the two scanner mirrors 10a, 10b are rotatable about two axes of rotation which, in the example shown, correspond to the X direction and to the Y direction of the XYZ coordinate system. An objective 9 follows the scanner device 8 in the beam path, which objective additionally carries out focusing of the laser beam 3 in order to focus the laser beam 3 deflected by the scanner device 8 in the region of the weld joint 7. During the formation of a weld joint 7 in the form of the fillet joint shown in FIG. 1, the laser beam 3 is aligned in a manner which differs from that illustrated in FIG. 1; generally not perpendicularly, but at an angle to the Z direction in relation to the two workpieces 2a, 2b, for example as described in DE 10 2013 219 220 A1 cited at the outset, which is incorporated by reference in the content of this application in its entirety.
(11) The apparatus 1 also has a control device 13 for controlling the scanner device 8, more precisely the rotary drives 11a, 11b of the two scanner mirrors 10a, 10b, and for controlling the movement device 6. The control device 13 also serves for controlling the laser source 4, in particular for controlling a (current) laser power P.sub.L of the laser source 4.
(12) The control device 13 is designed or programed to control the rotary drives 11a, 11b in such a way that the two scanner mirrors 10a, 10b are rotated about the respective axis of rotation X, Y such that, as a result of the movement of the machining head 5, the movement of the laser beam 3 in the feed direction Y is superimposed with an additional periodic (oscillating) movement. Alternatively, the scanner device 8 may have only a single mirror that can be rotated about the two axes of rotation X, Y.
(13) An example of a periodic movement that progresses along a one-dimensional movement path 12 extending in a transverse direction X perpendicularly to the feed direction Y is illustrated in FIG. 2A. As can be seen in FIG. 2A, the laser beam 3 on the movement path 12 executes an oscillating movement exclusively in the transverse direction X. The closed movement path 12 of the laser beam 3 is generated by the beam deflecting device 8, wherein, in FIG. 2A, the movement in the feed direction Y has not been taken into account. The movement path 12 leads from a first point P1 (shown in FIG. 2A), which represents a first reversal point of the periodic movement in the transverse direction X, via a second point P2, which represents a second reversal point of the periodic movement in the transverse direction X, back to a point P3, the position of which corresponds to the first point P1.
(14) FIG. 2B shows the oscillating speed V.sub.o of the second scanner mirror 10b, which brings about the movement in the transverse direction X perpendicularly to the feed direction Y, wherein the three points in time t.sub.1, t.sub.2, t.sub.3 shown in FIG. 2B correspond to the three points P1, P2, P3, shown in FIG. 2A, along the movement path 12. As can be seen in FIG. 2B, the oscillating speed V.sub.o in the example shown varies between a maximal oscillating speed V.sub.o,MAX and a minimal oscillating speed V.sub.o,MIN=0, which is reached at the respective reversal points P1 to P3. V.sub.W is the feed speed of the relative movement in the feed direction Y that is superimposed with the periodic movement/trajectory as shown in FIG. 2A.
(15) FIGS. 3A and 3B shows an example of a periodic movement, in which the laser beam 3 progresses along a two-dimensional circular movement path 12. In the circular movement path 12, the direction of the (vectorial) oscillating speed V.sub.o changes continuously and depending on the position, whilst the absolute value V.sub.o (illustrated in FIG. 3B) of the oscillating speed V.sub.o remains constant. The power P.sub.L of the laser source 4 also remains constant in the example shown in FIG. 3B and corresponds to the maximal possible laser power (i.e., P.sub.L=100%).
(16) The inventors have recognized that, in terms of the formation of hot cracks along the weld joint 7, optimal results cannot be achieved with the linear or, in the position/time graph, sinusoidal movement path 12 shown respectively in FIGS. 2A and 2B and with the circular movement path 12 shown in FIGS. 3A and 3B.
(17) In contrast, the formation of hot cracks at the fillet joint 7 can be reduced considerably when the superimposed periodic movement of the laser beam 3 takes place along a closed movement path 12 that has at least one stop point H as illustrated, for example, in FIG. 4A. The movement path 12 shown in FIG. 4A progresses symmetrically in the transverse direction X with respect to a center axis M, which contains the stop point H and, in the example shown, corresponds to the Y axis along which the feed motion takes place. The center axis M shown in FIG. 4A corresponds to the joint center N of the weld joint 7 shown in FIG. 1, i.e., the deflection (shown in FIG. 4A) of the movement path 12 takes place in the (positive and negative) X direction or in the transverse direction X, starting from the joint center N of the weld joint 7.
(18) In FIG. 4A, five points P1 to P5 are illustrated, which each form a reversal point of the periodic movement along the two-dimensional movement path 12. Starting from a first point P1, which forms a left reversal point of the periodic movement in the transverse direction X, the movement path 12 progresses in the direction of the center axis M along a first circular arc 14a in the form of a quarter circle with a first circle center point M1. The first circular arc 14a ends on the center axis M at the second point P2, which corresponds to the stop point H. The second point P2 or the stop point H forms a front reversal point, in the feed direction Y, of the periodic movement along the movement path 12.
(19) Starting from the stop point H, the movement path 12 progresses along a second circular arc 14b with a second center point M2, which likewise forms a quarter circle symmetrically with respect to the center axis M, to a third point P3, which forms a right reversal point of the movement path 12 in the transverse direction X. The circle center point M2 of the second circular arc 14b is likewise located on the X axis and the X position thereof corresponds to the X position of the third point P3 along the movement path 12. The radius R of the two circular path sections 14a, 14b can be in the order of magnitude of about 0.1 mm to about 2.5 mm, e.g., about 0.5 mm.
(20) In the case of the movement path 12 shown in FIG. 4A, a movement reversal with a non-continuous change in direction takes place at the stop point H, i.e., the movement path 12 has a kink at the stop point H. Accordingly, the oscillating speed V.sub.O, more precisely the speed component V.sub.O,Y thereof in the feed direction Y, exhibits a discontinuity—during the movement of the laser beam 3 along the first circular-arc-shaped path section 14a towards the stop point H, the movement path 12 has a speed component V.sub.O,Y in the feed direction Y with a positive sign (+V.sub.O,Y). During the path movement of the laser beam 3 starting from the stop point H along the second circular-arc-shaped path section 14b, the speed component in the feed direction Y has a negative sign (but an identical absolute value) (−V.sub.O,Y).
(21) In the case of the movement path 12 shown in FIG. 4A, the stop point H is passed through a second time (from right to left) (point P4), wherein the signs of the speed components V.sub.O,Y in the Y direction reverse accordingly. As can likewise be seen in FIG. 4A, the two circular-arc-shaped path sections 14a, 14b, when reaching the stop point H, exclusively have a speed component −V.sub.O,Y or +V.sub.O,Y in the feed direction Y, but a vanishing speed component V.sub.O,X in the X direction, since the tangent of the two circular-arc-shaped path sections 14a, 14b and therefore the speed vector V.sub.o at the stop point H points in the Y direction.
(22) FIG. 4B shows the time-dependence of the laser power P.sub.L (in % of the maximal laser power) during the movement of the laser beam 3 along the movement path 12 of FIG. 4A. As can be seen in FIG. 4B, the laser power P.sub.L is maximal (100%) in the stop point H, which is, for example, in the order or magnitude of about 5 kW, and minimal in the right reversal point P3, whilst the laser power P.sub.L drops to 80% of the maximal value at the left reversal point P1 or P5. The power profile of the laser power P.sub.L is therefore asymmetrical with respect to the center axis M, although the movement path 12 itself runs symmetrically with respect to the center axis M. A profile of the laser power P.sub.L which is asymmetrical with respect to the center axis M is advantageous, for example, when welding a fillet joint since, in this case, more workpiece material has to be melted on one side of the center axis M than on the other side.
(23) FIG. 5A shows a position graph of a two-dimensional movement path 12, which differs from the movement path 12 shown in FIG. 4A in that the two end points P1, P3 of the circular-arc-shaped path sections 14a, 14b are connected to one another by a semicircular path section 15, which, starting from the end points P1, P3, extends counter to the feed direction Y, i.e., in the negative Y direction, to a rear reversal point P2 of the movement path 12 in the feed direction Y. The radius R of the semicircular path section 15 corresponds to the radius R of the two circular-arc-shaped path sections 14a, 14b of the movement path 12 and, in the example shown, is about R=0.5 mm. It has been shown that, in the case of the movement path 12 shown in FIG. 5A, with certain material combinations, an improved welding result can be achieved when compared to the movement path 12 shown in FIG. 4A. FIG. 1 shows the movement of the laser beam 3 along the weld joint 7, which is composed of the movement in the feed direction (at the feed speed V.sub.w) and the superimposed periodic movement along the movement path 12 shown in FIG. 5A, and the stop point H.
(24) Analogously to FIG. 4B, FIG. 5B shows the time-dependent varying power P.sub.L of the laser beam 3 during the movement along the movement path 12 of FIG. 5A. As can be seen in FIG. 5B, the laser power P.sub.L is maximal (100%) in the stop point H (corresponding to P4 or t.sub.4) and minimal (about 70%) in the rear reversal point P2 in the feed direction Y, whilst the laser power P.sub.L is at 80% of the maximal value P.sub.MAX at the left and at the right reversal point P1 or P3. Between the points in time t.sub.1, t.sub.3 at which the reversal points P1, P3 are reached in the transverse direction X, the power P.sub.L is reduced to the minimal value of the power P.sub.L at the point in time t.sub.2, corresponding to the rear reversal point P2 in the feed direction Y, and increased, linearly in the manner of a ramp. Accordingly, the power P.sub.L is also increased to the maximal power P.sub.MAX, and reduced, in the manner of a ramp between the point in time t.sub.3 and the point in time t.sub.5, which correspond to the two reversal points P3, P5 in the transverse direction X.
(25) FIG. 6A shows a two-dimensional movement path 12 which, unlike the movement paths 12 shown in FIG. 4A and in FIG. 5A, does not run symmetrically with respect to the center axis M, which corresponds to the joint center N of the weld joint 7. The two-dimensional movement path 12 shown in FIG. 6A forms a three-quarter circle, i.e., it has a circular-arc-shaped path section 16 and two linear path sections 17a, 17b, which start from a circle center point P4 and, starting from the circle center point P4, extend in the X direction or in the Y direction to the two end points P1, P3 of the circular-arc-shaped path section 16.
(26) The path curve 12 shown in FIG. 6A has a first stop point H1 and a second stop point H2 along the center axis M. As in the case of the movement paths 12 shown in FIG. 4A and in FIG. 5A, the first stop point H1 forms a front reversal point P5 in the feed direction Y of the periodic movement, wherein a non-continuous change in direction of the two-dimensional movement path 12 takes place at the first stop point H1. A non-continuous change in direction of the movement path 12 in the form of a change in direction through 90° also takes place at the second stop point H2, which is connected to the first stop point H1 via the linear path section 17a extending in the feed direction Y. The speed component V.sub.O,X of the periodic movement in the transverse direction X is zero along the entire path section 17a of the movement path 12, which extends in the feed direction Y and therefore also at the two stop points H1, H2. In the case of the linear path section 17b extending in the transverse direction X, the speed component V.sub.O,Y of the movement path 12 in the feed direction Y is accordingly zero. It has been shown that the movement path 12 shown in FIG. 6A has also been shown to be advantageous with certain material combinations of the workpieces 2a, 2b to be joined, for example for fillet joint welds in aluminum alloys which are susceptible to hot cracking.
(27) FIG. 6B shows the time-dependent power P.sub.L of the laser beam 3, wherein the points in time t.sub.1 to t.sub.6 correspond to the points P1 to P6 along the movement path 12 shown in FIG. 6A. As can likewise be seen in FIG. 6B, the power P.sub.L has a maximal value P.sub.MAX at the two stop points H1, H2 and along the linear path section 17a between the two stop points H1, H2. The power P.sub.L drops to a minimal value of 20% at the rear point P2, in the feed direction Y, of the movement path 12 and has a value of 30% at the two reversal points P1, P3 in the transverse direction X, wherein the power P.sub.L increases linearly or drops linearly between these points (i.e., P1 and P2 or P2 and P3) in each case. The profile of the power P.sub.L shown in FIG. 6B has likewise proven favorable for influencing the welding process.
(28) In summary, a targeted influence of the molten volume can be achieved in the manner described above, in particular when welding fillet joints, but also when welding lap joints or when welding other joint geometries. In particular, an improvement in the connection between the workpieces 2a, 2b forming the partners to be joined can be achieved, because the connection is less susceptible to the formation of hot cracks. The material of the workpieces 2a, 2b can be, for example, aluminum or an aluminum alloy, as is used, for example, for vehicle body components.
Other Embodiments
(29) The methods described herein can be advantageously used not only for the fillet joint 7 shown in FIG. 1, but also for other weld joints, for example for lap joints, for example, when welding is to take place along a weld joint in the region in which the two workpieces 2a, 2b shown in FIG. 1 overlap. The methods described herein can also be used, for example, for improving the connection between two workpieces 2a, 2b made from different materials, which can otherwise generally not be readily connected by a laser welding method, for example copper and aluminum.
(30) Instead of a machining beam in the form of a laser beam, it is optionally possible for another type of high-energy beam, for example, a plasma beam, to be used for carrying out the welding procedure.
