Abstract
A stud welding process and a stud welding device for welding a stud to a workpiece are provided, wherein an arc (LB) is generated between the surface of the stud that faces the workpiece and the workpiece by using a pulsed welding current (Is), and the arc (LB) is deflected by means of a magnetic field which is generated by a coil through which a current (I.sub.A) flows. The current (I.sub.A) through the coil for generating the magnetic field for deflecting the arc (LB) is activated synchronously and in anti-phase with the welding current (I.sub.s) by a current (I.sub.A) always being applied to the coil when the welding current (I.sub.s) is at a minimum, and the coil being switched off or the current (I.sub.A) through the coil being reduced to a minimum when the welding current (I.sub.s) is at a maximum.
Claims
1. A stud welding method for welding a stud to a workpiece (2), wherein with an aid of a pulsed welding current (I.sub.s) an arc (LB) is generated between a surface of the stud facing the workpiece and the workpiece, and the arc (LB) is deflected with an aid of a magnetic field generated by a coil through which a current (I.sub.A) flows, the method comprising controlling the current (I.sub.A) synchronously and in opposition to the pulsed welding current (I.sub.S) through the coil for generating the magnetic field for deflecting the arc (LB), by applying a current (I.sub.A) to the coil whenever the pulsed welding current (I.sub.S) is minimal, and the coil is switched off or the current (I.sub.A) through the coil is reduced to a minimum when the welding current (I.sub.s) is maximum.
2. The stud welding method according to claim 1, comprising pulsing the pulsed welding current (I.sub.s) with a pulse frequency (f.sub.p) between 10 Hz and 1000 Hz.
3. The stud welding method according to claim 1, comprising changing the pulsed welding current (I.sub.s) between an upper threshold value (I.sub.s,o) and a lower threshold value (I.sub.s,u) or changing welding power (P.sub.s) between an upper value (P.sub.s,o) and a lower value (P.sub.s,u).
4. The stud welding method according to claim 1, wherein the duty cycle of the pulsed welding current (I.sub.s) is between 10% and 90%.
5. The stud welding method according to claim 1, comprising regulating the pulsed welding current (I.sub.s) for a constant welding power (P.sub.s).
6. The stud welding method according to claim 1, comprising applying the current (I.sub.A) through the coil with a time offset (Δt) with respect to the welding current (I.sub.s).
7. The stud welding method according to claim 1, comprising switching off the current (I.sub.A) through the coil for a predetermined time span (t.sub.a) after starting the pulsed welding current (I.sub.s) and/or for a predetermined time span (t.sub.e) before ending the welding pulsed current (I.sub.s).
8. The stud welding method according to claim 1, comprising pulsing the current (I.sub.A) through the coil with a DC offset (I.sub.A, DC).
9. The stud welding method according to claim 1, comprising changing a rate of rise (t.sub.r) of the current (I.sub.A) through the coil.
10. The stud welding method according to claim 1, comprising inserting a pre-current phase (t.sub.v), before starting the method.
11. The stud welding method according to claim 1, comprising determining a maximum pulse frequency (f.sub.P,max) of the welding current (I.sub.s) from a time profile of the current (I.sub.A) through the coil.
12. A stud welding device for welding a stud to a workpiece, having a welding current source for providing a pulsed welding current (I.sub.s) for generating an arc (LB) between a surface of the stud facing the workpiece and the workpiece, and having a coil for generating a magnetic field for deflecting the arc (LB), comprising a control device for controlling the current (I.sub.A) provided by the coil synchronously and in opposition to the pulsed welding current (I.sub.s), so that the coil can always be supplied with a current (I.sub.A) when the pulsed welding current (Is) is minimal, and so the coil is switched off or the current (I.sub.A) through the coil is reduced to a minimum when the pulsed welding current (I.sub.s) is maximum.
13. The stud welding device according to claim 12, wherein the welding power source is designed to regulate the pulsed welding current (I.sub.s) for a constant welding power (P.sub.s).
14. The stud welding device according to claim 12, wherein a stud holder for receiving the stud to be connected to the workpiece and a lifting device for lifting the stud from the workpiece is provided against a force of a spring.
15. The stud welding device according to claim 14, wherein the control device for determining a maximum pulse frequency (f.sub.P,max) of the welding current (I.sub.s) from the time profile of the current (I.sub.A) through the coil is formed.
16. The method of claim 2, comprising pulsing the pulsed welding current (I.sub.s) between 50 Hz and 150 Hz.
17. The method of claim 4, wherein the duty cycle of the pulsed welding circuit (I.sub.s) is 50%.
18. The method of claim 5, comprising regulating the pulsed welding circuit (I.sub.s) for a constant welding power (P.sub.s), between 2 kW and 10 kW.
19. The method of claim 10, comprising inserting a pre-current phase between 1 ms and 100 ms before starting the method.
20. The stud welding device of claim 13, wherein the welding power source is designed to regulate pulsed welding current (I.sub.s) for a constant welding power (P.sub.s) between 2 kW and 10 kW.
Description
[0026] The invention is explained in more detail with reference to the accompanying drawings. In the drawings:
[0027] FIG. 1 is a block diagram of a stud welding device for welding a stud to a workpiece with a coil for generating a magnetic field for deflecting the arc;
[0028] FIG. 2 shows the time profiles of the pulsed welding current and of the current controlled according to the invention through the coil for generating the magnetic field for deflecting the arc according to a first embodiment;
[0029] FIG. 3 shows the time profiles of the welding current, the welding voltage, the pulsed welding power and the current through the coil for generating the magnetic field for deflecting the arc according to a further embodiment with power control;
[0030] FIG. 4 shows the time profiles of the welding current, the welding voltage, the welding power, and the current through the coil for generating the magnetic field for deflecting the arc according to a further embodiment with a time delay of the coil current;
[0031] FIG. 5 shows three variants of the time profile of the current through the coil for generating the magnetic field for deflecting the arc with three different rates of rise; and
[0032] FIG. 6 shows the time profile of the current through the coil for generating the magnetic field for deflecting the arc to explain the determination of the maximum pulse frequency of the welding current.
[0033] FIG. 1 shows a block diagram of a stud welding device 1 for welding a stud 6 to a workpiece 2 with a coil 3 for generating a magnetic field for deflecting the arc LB. The stud welding device 1 contains a stud holder 8 for receiving the stud 6, which is connected to a corresponding lifting device 9. The lifting device 9 can be formed by a lifting magnet which lifts the stud holder 8 together with the stud 6 against the force of a spring 10 from the workpiece 2. Instead of the spring 10, the lifting device 9 can also be formed by a double lifting magnet which both lifts the stud 9 from the workpiece 2 and presses it against the workpiece 2 (not shown). A welding current I.sub.s is applied to the stud 6 by a welding current source 5, as a result of which an arc LB is ignited between the surface 7 of the stud 6 facing the workpiece 2 and the workpiece 2. The position of the arc LB changes very indefinitely during the welding process, which leads to a different melting of the surface 7 of the stud 6 and the workpiece 2 and, after the stud 6 is pressed against the workpiece 2, a different quality of the welded joint. To influence the position of the arc LB, a coil 3 is arranged about the welding point to generate a magnetic field which is oriented transversely to the arc LB and locally deflects it. In the event of uncontrolled control of the coil current I.sub.A, the position of the arc LB cannot be influenced in a targeted manner. In the present invention, however, the coil 3 is acted upon by a control device 4 (coil current source) with a current I.sub.A, which is controlled synchronously and in opposition to the welding current I.sub.s. This results in a more intensive effect on the position of the arc LB and a more uniform melting of the surface 7 of the stud 6 and of the workpiece 2.
[0034] The welding device 1 is characterized by a relatively simple and inexpensive implementation. In any case, a gas reservoir 11 can be provided which supplies the welding point with a corresponding protective gas G. The welding current I.sub.s and the current I.sub.A through the coil 3 are also generally welding parameters which, according to the invention, are set, controlled, regulated or the like. Of course, the invention can also be carried out if other welding parameters, such as welding power P.sub.s and/or time parameters, are changed.
[0035] FIG. 2 shows the time profiles of the pulsed welding current I.sub.s and the current I.sub.A controlled according to the invention through the coil 3 for generating the magnetic field for deflecting the arc LB according to a first embodiment. The welding current I.sub.s is switched between an upper threshold value for the welding current I.sub.s,o and a lower threshold value for the welding current I.sub.s,u. In the embodiment shown, an upper threshold value for the welding current I.sub.s,o is maintained during the time ton and a lower threshold value for the welding current I.sub.s,u is maintained during the remaining time for the period duration T.sub.P. In the example shown, the switch-on time ton is half the period duration T.sub.P, which is equivalent to a duty cycle of 50%. In this embodiment, a constant welding current I.sub.s is regulated, so that a welding power P.sub.s is established as a function of the welding voltage U.sub.s across the arc LB.
[0036] According to the invention, the current I.sub.A through the coil 3 is controlled synchronously and in opposition to the pulsed welding current I.sub.s. During the time t.sub.on of the upper threshold value of the welding current I.sub.s,o, the coil current I.sub.A is zero or minimal, whereas the coil current I.sub.A is maximum when the welding current I.sub.s is at the lower threshold value I.sub.s,u. As a result, the position of the arc LB is optimally influenced.
[0037] By a slight phase shift of the coil current I.sub.A with respect to the welding current I.sub.s by a time offset Δt, the maximum of the coil current I.sub.A can be reached earlier.
[0038] FIG. 3 shows the time profiles of the welding current I.sub.s, the welding voltage U.sub.s, the pulsed welding power P.sub.s and the current I.sub.A through the coil 3 for generating the magnetic field for deflecting the arc LB according to a further embodiment with power control. In this embodiment, a control is carried out at a constant welding power P.sub.s in which the welding current I.sub.s is changed accordingly, as a function of the welding voltage U.sub.s. When the arc or welding voltage U.sub.s is reduced, the welding current I.sub.s is increased, whereas when the welding voltage U.sub.s increases, the welding current I.sub.s is reduced, resulting in a constant mean welding power P.sub.s. The current I.sub.A through the coil 3 is controlled accordingly in opposition to and synchronously with the welding current I.sub.s. Instead of switching off the coil 3, equivalent to a coil current I.sub.A=0 (see FIG. 2) a certain equal DC component or DC offset I.sub.A,DC to the coil 3 can be created. Before the start of the welding process, a pre-current phase with a duration t.sub.v can be inserted.
[0039] FIG. 4 shows the time profiles of the welding current I.sub.s, the welding voltage U.sub.s, the welding power P.sub.s and the current I.sub.A through the coil 3 for generating the magnetic field for deflecting the arc LB according to a further embodiment. There is no pre-current phase with a duration t.sub.v as shown in FIG. 3, so that the welding process and the inventive synchronous opposite control of the current I.sub.A are started immediately. As shown, however, the control of the current I.sub.A through the coil 3 for generating the magnetic field for deflecting the arc LB started with a time delay and ended a predetermined time period earlier. The coil current I.sub.A can be switched on delayed by a preset time period t.sub.a after the start of the welding process and switched off by a preset time period t.sub.e before the end of the welding process. During the time periods t.sub.a and t.sub.e, the current I.sub.A through the coil 3 can correspond to a certain equal DC component or DC offset I.sub.A,DC, which can be for example 10%-20% of the maximum current I.sub.A. The current I.sub.A through the coil 3 can also be zero during the time periods t.sub.a and t.sub.e. Regardless of the value of the current I.sub.A, the insertion of the time period t.sub.a after the start of the welding process improves the stability of the arc LB. The deflection of the arc LB is therefore started with a delay. A more stable end of the welding method can be achieved by inserting a preset time period t.sub.e before the end of the welding process, in that the deflection of the arc LB is ended early.
[0040] It can be seen from FIGS. 3 and 4 that it is not essential for the invention whether a constant welding current I.sub.s or a constant welding power P.sub.s is regulated, since the relevant pulses of the welding current I.sub.s are also identical in the welding power P.sub.s. It is important that the pulses of the current I.sub.A through the coil 3 are synchronously opposed to both the pulses of the welding current I.sub.s and the pulses of the welding power P.sub.s.
[0041] FIG. 5 shows three variants of the time profile of the current I.sub.A through the coil 3 for generating the magnetic field for deflecting the arc with three different rates of increase t.sub.r. In the top time diagram of FIG. 5, the coil current I.sub.A rises at a very slow rate of increase, resulting in a quasi triangular profile of the coil current. By applying a higher voltage or change of the inductance of the coil 3, a higher rate of rise can be achieved according to the second time diagram. In the last time profile, an almost rectangular profile of the coil current I.sub.A is achieved with a particularly low rate of increase.
[0042] FIG. 6 shows the time profile of the current through the coil for generating the magnetic field for deflecting the arc to explain the determination of the maximum pulse frequency of the welding current. By determining the time constant or rate of rise of the coil current I.sub.A, the maximum achievable pulse frequency f.sub.P can be calculated back. Depending on the amplitude of the coil current I.sub.A, different maximum period times T.sub.P or different maximum pulse frequencies f.sub.P=1/T.sub.P result. In the example with a lower amplitude I.sub.A1, a shorter period T.sub.P1 or a higher maximum pulse frequency f.sub.P1=1/T.sub.P1 results. With a higher amplitude of the current I.sub.A2 through the coil 3, a higher period T.sub.P2 results, which is equivalent to a lower maximum pulse frequency f.sub.P. In the third example with the higher amplitude of the current I.sub.A2, the current I.sub.A is not switched off again immediately after the maximum I.sub.A2 has been reached, but the maximum I.sub.A2 is maintained for a certain time. This results in the period T.sub.P3 or the pulse frequency f.sub.P3=1/T.sub.P3.
