Abstract
A method for piercing a workpiece by means of a laser beam includes radiating a pulsed laser beam onto a workpiece to form a piercing breakthrough, wherein a radiated mean pulse power (P.sub.mittel) of the pulsed laser beam is reduced during piercing.
Claims
1. A method for piercing a workpiece by a laser beam, comprising: radiating a pulsed laser beam onto a workpiece to form a piercing breakthrough; wherein a radiated mean pulse power of the pulsed laser beam is reduced during piercing.
2. The method according to claim 1, wherein the radiated mean pulse power is reduced discretely and/or in steps and/or continuously.
3. The method according to claim 1, wherein a line of best fit of the radiated mean pulse power has a negative slope during piercing.
4. The method according to claim 1, wherein the radiated mean pulse power and/or a pulse frequency is reduced in steps and, during each step, lies within a band with a predetermined minimum value and a predetermined maximum value.
5. The method according to claim 1, wherein the radiated mean pulse power is reduced by at least one of the following changes in pulse parameters of the pulsed laser beam: lengthening a pulse off-time, shortening a pulse on-time, reducing a pulse frequency, and reducing a relative pulse duty cycle.
6. The method according to claim 1, wherein the radiated mean pulse power is reduced by lengthening a pulse off-time and keeping a pulse peak power and/or a pulse on-time and/or a pulse energy constant during piercing.
7. The method according to claim 1, wherein a first pulse frequency in a first piercing step at a start of piercing is greater than or equal to a predetermined limit pulse frequency.
8. The method according to claim 1, wherein the radiated mean pulse power is reduced in at least two steps, wherein a first pulse frequency in a first piercing step is greater than or equal to a predetermined limit pulse frequency and a second pulse frequency in a second piercing step is less than the first pulse frequency or than the limit pulse frequency.
9. The method according to claim 7, wherein the predetermined limit pulse frequency is based on a thickness and/or a material of the workpiece, and/or wherein the predetermined limit pulse frequency specifies a pulse frequency from which on a piercing stop occurs.
10. The method according to claim 1, wherein at least one of pulse parameters of the pulsed laser beam, selected from the group comprising a mean pulse power radiated, a pulse off-time, a pulse on-time, a pulse frequency, a relative pulse duty cycle and a pulse peak power, is adjusted based on a material and/or a thickness of a workpiece and/or on a current piercing time and/or on a current piercing depth.
11. The method according to claim 1, wherein a pulse off-time and/or a pulse on-time of the pulsed laser beam is in a range between 0.01 ms and 100 ms or between 0.1 ms and 10 ms.
12. The method according to claim 1, wherein a pulse frequency of the pulsed laser beam is in a range between 200 Hz and 3000 Hz or between 400 Hz and 2000 Hz.
13. The method according to claim 1, wherein, when piercing a workpiece with a thickness of more than 20 mm, the radiated mean pulse power is changed in at least two steps.
14. The method according to claim 1, wherein the radiated mean pulse power is controlled as a function of at least one of the following parameters: thickness of the workpiece, material of the workpiece, current piercing time, current piercing depth, type of process gas, process gas pressure, imaging ratio of an optical system of a laser machining head, focal position, focal diameter, intensity distribution of the laser beam, nozzle type, nozzle diameter, and nozzle distance from the workpiece.
15. The method according to claim 1, wherein a metallic workpiece is pierced.
16. A method of laser cutting, comprising: a method for piercing according to claim 1; and cutting by means of the laser beam starting from the formed piercing breakthrough.
17. A device for laser material machining of a workpiece, comprising: a laser source for generating a laser beam; a laser machining head for radiating the laser beam onto said workpiece; and a control device configured to control said device to perform the method for piercing according to claim 1.
18. The method according to claim 15, wherein the metallic workpiece is a metallic sheet.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Exemplary embodiments of the disclosure are shown in the figures and are described in more detail below. In the figures:
[0034] FIG. 1 shows a schematic structure of a device for laser material machining according to embodiments of the present disclosure;
[0035] FIG. 2 shows a schematic diagram of the pulse parameters;
[0036] FIG. 3A shows a graph of measured values of the breakthrough time as a function of the pulse frequency for different workpiece thicknesses;
[0037] FIG. 3B shows micrographs of a piercing process in a stainless steel sheet having 30 mm thickness at different pulse frequencies;
[0038] FIG. 4A shows a schematic diagram of a piercing process with a limit pulse frequency at a first point in time;
[0039] FIG. 4B shows a schematic diagram of the piercing process of FIG. 4A at a second point in time after a piercing stop has occurred;
[0040] FIG. 6 shows a schematic diagram of the radiated laser power or the radiated pulse sequence as a function of the piercing time during a method for piercing with stepwise decrease of the pulse frequency at a constant pulse on-time according to embodiments of the present disclosure;
[0041] FIGS. 7A to D show schematic diagrams of the relationship between pulse frequency and piercing depth during the piercing method of FIG. 6;
[0042] FIG. 8 shows a schematic diagram of the pulse frequency as a function of the piercing time during a method for piercing with stepwise decrease of pulse frequency bands according to embodiments of the present disclosure; and
[0043] FIG. 9 shows a schematic diagram of the pulse frequency as a function of the piercing time during a method for piercing with decrease of a line of best fit of the pulse frequency according to embodiments of the present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0044] Unless otherwise stated, the same reference symbols are used in the following for the same elements and elements with equivalent effect.
[0045] FIG. 1 shows a schematic diagram of a device for laser material machining according to embodiments of the present disclosure. The device for laser material machining may include a laser machining head 100, in particular a laser cutting head, for radiating a laser beam 10 onto a workpiece 1, a laser source 200 for generating the laser beam 10, and a control device 300. The control device 300 is set up or configured to control the device, in particular to control the device according to a method according to one of the embodiments described in this disclosure.
[0046] The laser source 200 emits a laser beam 10, also called a machining beam, which is guided and focused onto the workpiece 1 by machining optics. The machining optics and/or the laser source 200 are connected to the control device 300. In addition to the control function, the control device 300 may also have an evaluation and/or calculation function. The machining optics may have transmitting and/or reflecting optical elements for beam guidance and beam shaping. Furthermore, the device for laser material machining may include a gas supply for supplying a process gas into a machining zone on the workpiece 1.
[0047] During piercing, the laser beam 10 is directed onto the workpiece 1 in a pulsed manner. A schematic overview of the pulse parameters is shown in FIG. 2. The pulse period or pulse period T results from the pulse on-time t.sub.an and the pulse off-time t.sub.aus:
T=t.sub.an+t.sub.aus
[0048] The pulse on-time t.sub.an denotes the time period of the laser pulse during which energy is radiated onto the workpiece 1. Correspondingly, the pulse off-time t.sub.aus denotes the time span during which no or virtually no energy is radiated onto the workpiece 1. The reciprocal of the pulse period T is called the pulse frequency: f=1/T. The pulse frequency is therefore a function of both the pulse on-time and the pulse off-time. The relationship between pulse on-time and pulse period is referred to as the relative pulse duty cycle R: R=t.sub.an/T. The pulse peak power P.sub.peak may correspond to the maximum laser power provided by the laser source 200, hereinafter maximum laser power, P.sub.max. The radiated energy during a pulse, i.e. the so-called pulse energy, is calculated from the product of pulse peak power and pulse on-time:
E.sub.Puls=P.sub.peak×t.sub.an.
[0049] Accordingly, the radiated mean pulse power is calculated from the product of the relative duty cycle and the pulse peak power or from the product of the pulse peak power and the pulse on-time and pulse frequency:
P.sub.mittel=P.sub.peak×R=P.sub.peak×t.sub.an×f=P.sub.peak×(1−t.sub.aus/(t.sub.an+t.sub.aus)).
[0050] For the purposes of the present disclosure, not only the time of the pulse period during which no energy is radiated may be regarded as a pulse off-time, but also a time of the pulse period during which the radiated energy or the radiated power remains below a threshold value. Such a threshold value for the radiated power is shown in FIG. 2 as P.sub.SW,aus. This means that, when the current power radiated onto the workpiece is below said threshold value P.sub.SW,aus, there is a pulse off-time. The threshold value may be, for example, 30% of the maximum laser power (P.sub.SW,aus=0.3×P.sub.max) or 20% of the maximum laser power or even 10%× of the maximum laser power.
[0051] According to the present disclosure, the control device 300 is configured to control an energy input into the machining zone or onto the workpiece 1 as a function of one or more of the following process parameters and boundary conditions: material thickness or workpiece thickness, material of the workpiece, current piercing time, current piercing depth, process gas type, process gas pressure, focal position, imaging ratio of the optical system, focus diameter and nozzle distance to the top of the workpiece. Based on at least one of the parameters mentioned, the energy input may be controlled by the pulse on-time (t.sub.an), pulse off-time (t.sub.aus) and/or the pulse peak power (P.sub.peak) as a function of the current piercing time or piercing depth. By knowing the causal relationships in the process zone, it is possible to adjust the process parameters in a targeted manner and thus to increase process efficiency.
[0052] As a rule, the piercing rate (change in piercing depth over time) decreases with increasing piercing depth, and the breakthrough time, i.e. the time from the first radiation of the laser beam onto the workpiece until the breakthrough, increases with increasing piercing depth s. FIG. 3A shows the breakthrough time (in seconds) as a function of a selected pulse frequency (in Hertz) for different material thicknesses or workpiece thicknesses (in millimeters). Here, the pulse frequency is constant during the entire piercing process. With decreasing pulse frequency, the breakthrough time increases disproportionately. The cause is, inter alia, the increasing resolidification of the melt on the wall of the piercing hole. As a result, the piercing rate decreases and the process efficiency drops. Furthermore, an area of saturation, in which depth subtraction is stopped, is hatched, and a so-called limit pulse frequency is drawn as a function of the material thickness. The limit pulse frequency describes a pulse frequency threshold above which reliable piercing is no longer possible under the given process parameters and boundary conditions. The limit pulse frequency shifts towards lower pulse frequencies as the material thickness increases.
[0053] FIG. 3B shows micrographs of piercing breakthroughs in a workpiece of 30 mm thickness made of stainless steel at a peak pulse power of 6 kW (cf. top curve in FIG. 3A). At a pulse frequency of 350 Hz (constant), the breakthrough time is 3.6 s (first image from the left in FIG. 3B), at a pulse frequency of 400 Hz (constant), the breakthrough time is 2.5 s (second image from the left in FIG. 3B), and at a pulse frequency of 500 Hz (constant), the breakthrough time is 1.7 s (third image from the left in FIG. 3B). At a pulse frequency of 588 Hz (constant), there is a piercing stop (no breakthrough, to the right in FIG. 3B).
[0054] In FIGS. 4A and 4B, the piercing hole 3 is shown schematically for two different points in time when piercing at a limit pulse frequency. From a certain piercing depth s, the time between the individual pulses (pulse off-time) is too short or the pulse frequency is too high to expel the molten material from the piercing hole 3 in sufficient quantity, so that a piercing stop occurs. A further reason for the piercing stop is the decreasing introduced energy of the laser beam 10 at the piercing base 5 with increasing piercing depth s and the increasing distance that the melted material has to travel before exiting the piercing hole 3 (see FIG. 4A). As the piercing depth increases, the radiated energy also heats the sides of the piercing hole 3 so that cave-like melting may occur. In the case of high material thicknesses, e.g. greater than 20 mm, the piercing process may go into saturation (see FIG. 4B), at which point the piercing rate approaches 0 (piercing stop), when the pulse frequency is arbitrarily large or greater than a respective limit pulse frequency. The residual melt in the piercing hole 3 and the subsequent pulses lead to an ever increasing heating of the machining zone from pulse to pulse. This results in heat accumulation in the puncture hole (pulse-to-pulse mechanism), which leads to a specific area of material transversely to the direction of piercing being melted and filling the piercing base 5 with material again and again. The previously achieved piercing depth s decreases and, as a result, a limit piercing depth s.sub.grenz is reached (cf. FIG. 4B and FIG. 3B on the right). A reliable breakthrough is not possible (piercing stop in piercing direction).
[0055] According to the invention, in order to avoid a piercing stop while shortening the breakthrough time, the mean pulse power radiated onto the workpiece during piercing is reduced. In this way, the piercing rate can be maximized along the piercing depth and, at the same time, a breakthrough can be ensured. Several embodiments of the method for piercing according to the present disclosure are described below.
[0056] In the following, according to embodiments, a method for piercing a workpiece and a device for laser material machining with a control device configured to carry out this method are provided. According to embodiments, the mean pulse power radiated is reduced by lowering the pulse frequency during piercing. However, the present invention is not limited thereto. Alternatively, the mean pulse power radiated may be reduced, for example, by lengthening the pulse off-time and/or shortening the pulse on-time and/or reducing a relative duty cycle.
[0057] In the embodiment illustrated in FIG. 5, the pulse frequency is reduced in stages or steps or discretely. For example, the pulse frequency is lowered at least once, preferably at least twice, during the breakthrough time. In this case, the method for piercing may include at least a first piercing step, step 1, at a first pulse frequency f.sub.1 and a second piercing step, step 2, at a second pulse frequency f.sub.2, the first pulse frequency f.sub.1 being greater than the second pulse frequency f.sub.2. The first piercing step, step 1, therefore takes place at the start of piercing and the second piercing step, step 2, takes place after the first piercing step, step 1. However, the method may also include any number, e.g. n, piercing steps, the pulse frequency of each piercing step being lower than that of a preceding piercing step. The pulse frequency may be constant during a piercing step. Alternatively, as described below for FIG. 9, the pulse frequency during a piercing step may be in a range of 80% and 120%, preferably 90% and 110%, of the mean pulse frequency f.sub.n of this nth piercing step (i.e. f.sub.n±0.2×f.sub.n or f.sub.n±0.1×fn). The change in pulse frequency between the piercing steps, i.e. Δf.sub.1,2, may be the same or different. Preferably a first change, i.e. Δf.sub.1,2, is greater than a second change, i.e. Δf.sub.2,3. Likewise, the duration of the individual piercing steps may be the same or different. A duration of the first piercing step is preferably the longest.
[0058] As an alternative to a stepwise or discrete decrease, the pulse frequency may be reduced in any combination of discrete and continuous decreases during the breakthrough time (see also FIG. 8).
[0059] With regard to FIG. 5, at least one change or decrease in the pulse frequency (f<f.sub.1) is described. The pulse frequency at the start of piercing or at the beginning of the breakthrough time f.sub.1 and/or the pulse frequencies f.sub.2 to f.sub.n of the subsequent piercing steps may be adjusted as a function of the thickness of the workpiece and/or the current piercing time and/or the current piercing depth.
[0060] In one embodiment, piercing at variable pulse frequency may occur with two changes in pulse frequency. The pulse frequency at the start of piercing and/or in the first piercing step is preferably greater than or equal to the limit pulse frequency for this workpiece, which may depend on the workpiece thickness and the workpiece material.
[0061] A further embodiment is shown schematically in FIG. 6. As in FIG. 5, the pulse frequency is reduced in n piercing steps until breakthrough. In addition, in the embodiment shown in FIG. 6, the pulse on-time t.sub.an remains constant or quasi-constant throughout the entire piercing process. In the latter case, the pulse on-time may deviate by ±20% or ±10% of the mean pulse on-time during the breakthrough time. In other words, a quasi-constant pulse on-time may be within a band from 0.8×t.sub.an to 1.2×t.sub.an or from 0.9×t.sub.an to 1.1×t.sub.an.
[0062] The value for the pulse on-time t.sub.an is preferably selected as a function of the workpiece thickness. In this case, the mean pulse power or the pulse frequency is reduced by increasing the pulse off-time t.sub.aus, for example as a function of the piercing time (FIG. 6) or the piercing depth (FIG. 7). This may ensure that, as the piercing depth increases, the melt has sufficient time between the pulses to be expelled from the piercing hole, for example by the process gas blown in.
[0063] In the embodiment shown in FIG. 6, instead of a constant pulse on-time, the radiated pulse energy E.sub.Puls, i.e. the product of pulse peak power and pulse on-time, may be kept constant, while the other pulse parameters may be selected to be the same as in the embodiment shown in FIG. 6. Optionally, the pulse energy may be set to a material- and thickness-dependent minimum value.
[0064] In a further embodiment illustrated in FIG. 7, the workpiece thickness is at least 20 mm and the pulse peak power is at least 4 kW. The method for piercing includes at least three piercing steps with different pulse frequencies. The piercing process is started at a first pulse frequency that is greater than or equal to a limit pulse frequency that is dependent on the workpiece thickness (FIG. 7A: f.sub.grenz≤f.sub.1). This may correspond to step 1 of FIG. 5. After a predetermined piercing time and/or piercing depth, the pulse frequency is reduced to a second pulse frequency f.sub.2, which is lower than the first pulse frequency or the limit pulse frequency (FIG. 7B: f.sub.2<f.sub.1 or f.sub.2<f.sub.grenz≤f.sub.1). This may correspond to step 2 of FIG. 5. After a further predetermined piercing time and/or piercing depth, the pulse frequency is reduced to a third pulse frequency f.sub.3, which is lower than the second pulse frequency (FIG. 7C: f.sub.3<f.sub.2) and maintained until breakthrough 9 (cf. 7D). In the example of piercing a workpiece made of stainless steel of 30 mm thickness at a peak pulse power of 6 kW from FIG. 3, the breakthrough time could be reduced by more than 50%, namely from 1.7 s at a constant pulse frequency of 500 Hz to 0.8 s, by this method. The pulse frequency was changed twice during piercing: from a first pulse frequency at the start of piercing of f.sub.1=588 Hz to a second pulse frequency of f.sub.2=400 Hz and then to a third pulse frequency of f.sub.3=350 Hz. The first pulse frequency f.sub.1 of 588 Hz was higher than the limit pulse frequency (cf. FIG. 3A). This resulted in an optimized piercing process in terms of a short piercing time and reliable breakthrough.
[0065] As mentioned above, the mean pulse power or the pulse frequency does not have to be decreased monotonically or strictly monotonically. Instead, the mean pulse power or the pulse frequency may be decreased in steps or stages, wherein the mean pulse power or the pulse frequency may move within a power band or pulse frequency band in each step. That is, during each step the mean pulse power or the pulse frequency lies within a band with a specified minimum value and a specified maximum value. The power band may be defined by a deviation of ±20%, in particular ±10%, of the average mean pulse power during the piercing step, step n. Similarly, the pulse frequency band may be defined by a deviation of ±20%, in particular ±10%, of the average pulse frequency f.sub.n during the piercing step, step n. FIG. 8 shows examples of pulse frequency bands in which the pulse frequency may vary during the individual steps 1, . . . n. The pulse frequency bands shift to smaller values with each step. For example, the pulse frequency bands may decrease in steps or stages. In FIG. 8, the pulse frequency bands are defined as follows: f.sub.1,min=0.8×f.sub.1 (or 0.9×f.sub.1); f.sub.1,max=1.2×f.sub.1 (or 1.1×f.sub.1); f.sub.2,min=0.8×f.sub.2 (or 0.9×f.sub.2); f.sub.2,max=1.2×f.sub.2 (or 1.1×f.sub.2); f.sub.n,min=0.8×f.sub.n (or 0.9×f.sub.n); and f.sub.n,max=1.2×f.sub.n (or 1.1×f.sub.n). In other words, slight increases in the pulse frequency may be neglected as long as the pulse frequency is lowered on average over time until breakthrough. If the mean pulse power is reduced by changing another pulse parameter, for example by lengthening the pulse off-time, this change may also be made in steps, wherein said pulse parameter, in each step, may lie in a band of ±20%, in particular ±10%, of the mean value of said pulse parameter in this step.
[0066] Likewise, the mean pulse power or the pulse frequency may be reduced continuously or quasi-continuously. It may be sufficient that the mean pulse power or the pulse frequency vary in a band which, on average, decreases over time (cf. FIG. 9). That is, the mean pulse power or the pulse frequency decrease over time. In other words, the mean pulse power or the pulse frequency may be considered to be decreased when a corresponding line of best fit or regression (dashed line in FIG. 9) has a negative gradient during the breakthrough time, i.e. from the start of piercing to breakthrough. This is because a very brief increase in the mean pulse power or the pulse frequency can be neglected for the piercing method according to this disclosure. When the mean pulse power is decreased by changing another pulse parameter, for example by lengthening the pulse off-time, this change may also be continuous or quasi-continuous, as long as a line of best fit or regression for this pulse parameter has a negative slope during the breakthrough time.
[0067] According to the embodiments of this disclosure, a method and a device for piercing a workpiece by means of a laser beam are provided, which minimize a breakthrough time or maximize a piercing rate while ensuring reliable piercing and preventing a piercing stop.
