Abstract:
Methods and systems are implemented for piercing a metal workpiece by means of a laser beam and a process gas The methods and systems form a hole in the workpiece using the laser beam and using an inert gas as the process gas, such that the formed hole extends only partially through the workpiece, widen the upper part of the hole into a trough that surrounds the hole on the top side of the workpiece using the laser beam and using oxygen as the process gas, and fully pierce the hole using the laser beam and using oxygen as the process gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority under 35 U.S.C. §120 to PCT Application No. PCT/EP2014/001533 filed on Jun. 5, 2014, which claimed priority to German Application No. DE 102013210857.5, filed on Jun. 11, 2013. The contents of both of these priority applications are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to systems and methods for piercing a workpiece by a laser beam and a process gas. 
       BACKGROUND 
       [0003]    During laser-cutting, bulges of melted and resolidifying metal or metal oxide are formed in the area surrounding the piercing hole when piercing thick workpieces, the dimensions of said bulges increasing significantly with the thickness of the sheet metal. These adhesions around the piercing hole are an impediment, particularly with inner cutting geometries and small components, where the cutting contour necessarily runs along very close to the piercing site. Moreover, the distance sensor system of the laser machining head detects the bulges as an interfering contour and controls the movement of the laser machining head away from the workpiece, giving rise to processing problems. In order to avoid these problems, it is necessary to reduce or prevent bulges adhering to the workpiece. 
         [0004]    Japanese patent publication JP05057469A discloses a piercing method in which a wide, shallow trough is produced in the surface or the workpiece in a first processing step with a large distance between the process gas nozzle on the machining head and the workpiece surface. A pierced hole is then produced with a smaller distance between the nozzle and the workpiece surface, a lower laser power and lower process gas pressure. 
         [0005]    German patent publication DE102012014323A1 discloses producing, during piercing, a wide cylindrical hole in the upper half of the workpiece in a first process step with a large distance between the process gas nozzle and the workpiece surface. In a second process step disclosed in DE102012014323A1, the laser beam is then focused on the bottom surface of the wide hole, in order to produce a fully pierced hole. During this second step, the process gas is only directed into the wide hole and not at the area surrounding the hole. 
       SUMMARY 
       [0006]    Various embodiments of the invention disclosed herein provide a quick piercing method that involves a small heat input into the workpiece during piercing. A low heat input during piercing is particularly advantageous for a defect-free contour cut in the area surrounding the piercing point. 
         [0007]    In one aspect, the invention features a method for piercing a metal workpiece by a laser beam and a process gas comprising the following steps: 
         [0008]    a) forming of a pierced hole in the workpiece that does not go all the way through by piercing the workpiece by means of the laser beam using an inert gas, in particular nitrogen, as the process gas; 
         [0009]    b) widening of the upper part of the pierced hole into a trough that surrounds the pierced hole on the top side of the workpiece, in particular concentrically, by means of the laser beam which is set to a larger beam diameter on the workpiece than in step a) and using oxygen as the process gas; and 
         [0010]    c) full piercing of the pierced hole by means of the laser beam and using oxygen as the process gas. 
         [0011]    In particular embodiments, in the two steps a) and b) a funnel-shaped partial piercing (piercing funnel) is formed on the top side of the workpiece which makes expelling of the melt easier. In step c) piercing then takes place down to the underside of the sheet metal, wherein the laser beam acts particularly well deep in the material due to the previously formed piercing funnel, which results in quick processing progress. 
         [0012]    In the first process step a) of the piercing method according to the invention, the workpiece is pierced using a pulsed or continuous laser beam and an inert gas, preferably nitrogen, and a pierced hole is formed which does not extend right through the workpiece thickness. The pierced hole advantageously extends only into the upper half of the workpiece. The use of an inert gas as the process gas means that there is no uncontrolled burn-off of the workpiece material. Moreover, the energy input into the workpiece is kept low, which has a positive effect on machining accuracy during subsequent cutting of the contour. 
         [0013]    The workpiece material which melts during piercing accumulates about the pierced hole and forms a bulge. In the second consecutive process step b) of the piercing method according to particular embodiments of the invention, the upper part of the pierced hole is widened due to the fact that as the beam diameter of the laser beam on the workpiece is greater than in step a) and oxygen is used as the process gas with pulsed or continuous laser radiation, a trough is produced in the top side of the workpiece, preferably concentrically to the pierced hole in particular embodiments. 
         [0014]    In this second process step b), the bulge from the first step along with further material from the workpiece surface is oxidized by the oxygen and the oxidic slag thereby created is blown off by the process gas flow. A shallow trough is formed, from the bottom area whereof the pierced hole produced in the first step a) extends further into the inside of the workpiece. The trough therefore extends only fractionally as far into the workpiece as the pierced hole. In this way, a funnel-shaped hole is produced in the top side of the workpiece overall. The two-stage embodiment of the piercing funnel according to the invention has the advantage that there is no longer any workpiece material adhering to the workpiece surface about this hole and there is only a minimal heat input into the workpiece. A piercing funnel with a sharply defined edge and clean surrounding area is thereby produced. 
         [0015]    In particular embodiments, the focal point of the laser beam in the first process step a) lies above the workpiece surface, so that a relatively wide pierced hole is produced as a subsequent “shaft” of the piercing funnel. The focal point of the laser beam in the second process step b) is arranged even further above the workpiece surface in particular embodiments and, in addition, the diameter of the laser beam in the focus, i.e. the focus diameter, is greater than in the first process step a). In this way, the power density of the laser beam on the workpiece surface is reduced and the beam diameter on the workpiece is enlarged, so that a shallow trough with a large diameter is created in the workpiece surface during irradiation of the workpiece. 
         [0016]    The displacement of the laser focal point in a vertical direction may, for example, be achieved by displacing one or a plurality of lenses in the focusing optical system in the machining head or by adaptive mirrors in a free-beam optical system. In the case of fiber-guided laser radiation, the enlargement of the focus diameter can be achieved by switching between a fiber core and fiber cladding in a double-clad fiber, for example, between the laser and the machining head, as described in DE102010003750A1. 
         [0017]    The distance of the machining head from the workpiece surface, i.e. the distance between the process gas nozzle and the workpiece surface, is also changed at least in addition in certain embodiments. Even during the first process step a), a comparatively large distance should be set between the process gas nozzle and the workpiece surface (of approx. 20 mm, for example), so that any contamination of the process gas nozzle by melted metal can be avoided. In the second process step b), the distance between the process gas nozzle and the workpiece surface is enlarged still further (e.g. to approx. 80 mm), so that the radiation surface on the workpiece surface is enlarged in particular embodiments. 
         [0018]    Full piercing of the pierced hole right through to the underside of the workpiece takes place in the third process step c), in which the laser beam is positioned centrally above the pierced hole and, using oxygen as the process gas and with pulsed or continuous laser radiation, starting from the bottom of the piercing funnel the workpiece is pierced through, wherein the focal point of the laser beam may lie deeper than the funnel base in the workpiece on account of the funnel-shaped preliminary piercing. This has crucial advantages for efficient energy input during full piercing, as the laser beam can be focused deep in the material without shadows. 
         [0019]    In addition, it has emerged that the funnel-shaped contour of the pierced hole produced in process steps a) and b) is particularly well suited to conveying the slag produced during further piercing (full piercing) far away from the pierced hole. This cools the melt before it returns to the workpiece surface, so that it does not adhere to said workpiece surface. 
         [0020]    The method according to the invention can be implemented both on a solid-state laser cutting machine and also on a laser cutting machine with CO2 laser and is advantageous, particularly when piercing plate-shaped workpieces made of structural steel. 
         [0021]    The distance between the process gas nozzle and the workpiece surface during full piercing is smaller (e.g. approx. 5 mm) than in the two process steps a) and b), in particular embodiments, so that the process gas can be effectively coupled in the pierced hole. Full piercing can take place either in an uncontrolled manner with continuously increasing laser power or in a controlled manner with the help of a piercing sensor system known in the art. The pierced hole produced in the first process step a) at the bottom of the trough produced in the second step is used in this third process step c) as a light channel for the laser beam. In this way, the laser radiation reaches the workpiece more effectively and a better depth action of the laser radiation is achieved. The fully pierced hole therefore has a significantly smaller diameter and there is a smaller heat input into the component than with customary piercing methods. In this way, less slag is created which could accumulate on the workpiece and a defect-free contour cut is made possible in the area surrounding the piercing point. 
         [0022]    The three process steps a) to c) can be carried out discretely in succession. In this case, the laser beam is switched off following completion of the first process step a) and the machining head is moved upwards away from the workpiece vertically. During or after this, the process gas is changed from nitrogen to oxygen. The laser beam is then ignited and the second process step b) is executed. The positioning movement of the machining head between the second and third process step b), c) likewise takes place with the laser beam switched off. Alternatively, the three process steps a) to c) may follow one another continuously, wherein the laser beam remains switched on. The change in beam diameter on the workpiece surface takes place by moving the machining head vertically to the workpiece surface, for example. The gas changeover between the first and second process steps a), b) is carried out during movement of the machining head. 
         [0023]    Following the full piercing of the workpiece, in an optional fourth process step d) the pierced hole can be enlarged before the actual cutting process begins. This is necessary if the focus diameter of the laser beam during the cutting of the workpiece is greater than during piercing. A wide kerf is then produced during cutting, wherein a large amount of melt is created. If the pierced hole is too narrow, the melt created at the start of cutting cannot be expelled from the workpiece downwards through the pierced hole quickly enough and a cutting tear can result. 
         [0024]    In order to enlarge the pierced hole, the piercing point in the fourth processing step d) is irradiated with the laser beam set to a larger focus diameter than in the third process step c). The gas pressure of the process gas is set so high that the melt created is reliably expelled downwards out of the workpiece. The focal point of the laser beam is disposed beneath the workpiece surface, just as in the third process step c). The distance between the process gas nozzle and the workpiece surface corresponds to the distance in the third process step c). In particular embodiments, the focus diameter of the laser beam is increased gradually or continuously during the widening of the fully pierced hole. This increase in diameter may be achieved by moving the lenses in the machining head, for example, as described in W02011131541A1. Alternatively or in addition, with a fiber-guided laser beam the beam path can be switched between the core and the cladding of a double-clad fiber, as described in DE102010003750A1. A continuous increase in the focus diameter up to the diameter for the subsequent cutting process increases the process reliability in a particular way when enlarging the fully pierced hole. 
         [0025]    In particular embodiments, before step a), the top side of the workpiece is oiled, e.g. sprayed with oil, at the piercing point, in order to reduce the adhesion of any spatters that may be created in step a) and step b). 
         [0026]    In a further aspect, the invention also relates to a laser processing machine comprising a laser beam generator, a movable laser machining head with a process gas nozzle from which the laser beam emerges along with the process gas, a device for selecting the process gas and a control system which is programmed to control the movement of the laser machining head and the selected process gas of the device according to the piercing method according to the invention. 
         [0027]    The invention furthermore also relates to a computer program product which comprises code means adapted to execute all steps in the piercing method according to the invention when the program runs on a control system of a laser processing machine. 
         [0028]    Further advantages and advantageous embodiments of the subject matter of the invention can be inferred from the description, the drawing and the claims. 
         [0029]    Likewise, the features referred to above and those listed further down can each be used individually or jointly in any combinations. The embodiments shown and described should not be regarded as a definitive list, but instead they are there to exemplify the description of the invention. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0030]      FIG. 1  shows a laser cutting machine suitable for executing the piercing method according to the invention; 
           [0031]      FIGS. 2 a -2 d    show the individual process steps of the piercing method according to the invention; 
           [0032]      FIGS. 3 a , 3 b    each show a photo of the workpiece top with a fully pierced hole which has been produced by a conventional piercing method ( FIG. 3 a   ) and by the piercing method according to the invention ( FIG. 3 b   ); and 
           [0033]      FIGS. 4 a , 4 b    each show a cross-sectional photo of a fully pierced hole which was produced by a conventional piercing method ( FIG. 4 a   ) and by the piercing method according to the invention ( FIG. 4 b   ). 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  is a perspective view of a processing machine  1  which shows the structure of a laser cutting machine as an exemplary embodiment of a laser processing machine. Further exemplary embodiments are, for example, a laser welding machine or a combined punching/laser cutting machine. This processing machine  10  has, for example, a CO2 laser, diode laser or solid-state laser as laser beam generator  2 , a movable laser machining head  3  and a workpiece support  4 . A laser beam  5  is produced in the laser  2 , which laser beam is guided by means of a fiber optic cable (not shown) or tilted mirrors (not shown) from the laser  2  to the machining head  3 . A workpiece  6  is arranged on the workpiece support  4 . The laser beam  5  is directed onto the workpiece  6  by means of a focusing optical system arranged in the machining head  3 . The processing machine  1  is, moreover, supplied with process gases  7 , for example oxygen or nitrogen. Alternatively or in addition, compressed air or application-specific gases can also be provided. The use of individual gases depends on the material of the workpiece  6  being machined and the quality requirements in relation to the cutting edges. Furthermore, a suction device  8  is provided which is connected to a suction channel  9  located beneath the workpiece support  4 . The process gas  7  is fed to a process gas nozzle  10  of the processing head  3 , from which it emerges along with the laser beam  5 . 
         [0035]    When cutting the workpiece  6  using oxygen as the process gas, the material of the workpiece  6  is melted and in large part oxidized. When using inert gases such as nitrogen or argon, for example, the material is only melted. The melted particles created are then blown out, possibly along with the iron oxides, and drawn out along with the cutting gas via the suction channel  9  by the suction device  8 . 
         [0036]    The individual process steps of the piercing method according to the invention are shown in a steel workpiece  6  in  FIGS. 2 a    to  2   d.    
         [0037]    In the first process step a) shown in  FIG. 2 a   , the workpiece  6  is pierced by means of the pulsed or continuous laser beam  5  and using nitrogen as the process gas  7  and a pierced hole  11  is formed which does not extend all the way through the workpiece. The pierced hole  11  advantageously extends only into the upper half of the workpiece  6 . Using nitrogen as the process gas  7  means that there is no uncontrolled burn-off of the workpiece material. Moreover, the energy input into the workpiece  6  is kept low, something which has a beneficial effect on the machining accuracy during the subsequent cutting of the contour. 
         [0038]    The workpiece material which melts during piercing accumulates around the pierced hole  11  and forms a bulge  13  on the top side of the workpiece  12 . 
         [0039]    In the subsequent second process step b) shown in  FIG. 2 b   , the upper part of the pierced hole  11  is widened in that by means of the laser beam  5  which is set to a larger beam diameter on the workpiece  6  than in step a) and using oxygen as the process gas  7  with pulsed or continuous laser irradiation, a trough  14  is produced in the top side of the workpiece  12  concentrically to the pierced hole  11 . 
         [0040]    In this second process step b) the bulge  13  from the first step a) along with further material from the workpiece surface is oxidized by the oxygen and the oxidic slag thereby created is blown off by the process gas flow emerging from the process gas nozzle  10 . A shallow trough  14  is formed, from the bottom area whereof the pierced hole  11  produced in the first step a) extends further into the workpiece interior. The trough  14  therefore extends only fractionally as far into the workpiece  6  as the pierced hole  11 . In this way, a funnel-shaped partial piercing (piercing funnel)  15  with a sharply defined edge and clean surrounding area is formed overall in the top side of the workpiece  12 . The two-stage formation of the piercing funnel  15  has the advantage that there is no longer any workpiece material adhering to the top side of the workpiece  12  around this piercing funnel  15  and there is only a minimal heat input into the workpiece  6 . Moreover, the piercing funnel  15  makes it easier for the melt to be expelled during the further full piercing of the workpiece  6 . 
         [0041]    The focal point of the laser beam  5  during the first process step a) preferably lies above the top side of the workpiece  12 , so that a relatively wide pierced hole  11  is created as a subsequent “shaft” for the piercing funnel  15 . It is particularly preferable for the focal point of the laser beam  5  to be arranged even further above the top side of the workpiece  12  in the second process step b) and, in addition, for the diameter of the laser beam  5  in the focus, i.e. the focus diameter, to be greater than in the first process step a). In this way, the power density of the laser beam  5  on the top side of the workpiece  12  is reduced and the beam diameter on the workpiece  6  enlarged, so that a flat trough  14  with a large diameter is formed during radiation of the workpiece  6  in the top side of the workpiece  12 . 
         [0042]    Even during the first process step a), a comparatively large distance d1 should be set between the process gas nozzle  10  and the top side of the workpiece  12  of approx. 20 mm, for example, so that contamination of the process gas nozzle  10  by the melted metal is avoided. In the second process step b), the distance d2 between the process gas nozzle  10  and the top side of the workpiece  12  is increased still further (d2&gt;d1), e.g. to approx. 80 mm, so that the radiation surface on the top side of the workpiece  12  is enlarged. 
         [0043]    Full piercing of the pierced hole  11  right through to the underside of the workpiece  6  takes place in the third process step c) shown in  FIG. 2 c   , in which the laser beam  5  is positioned centrally above the pierced hole  11  and, using oxygen as the process gas  7  and with pulsed or continuous laser radiation, starting from the bottom of the piercing funnel  15  the workpiece  6  is pierced through, wherein the focal point of the laser beam  5  may lie deeper than the funnel base in the workpiece  6  on account of the previously formed piercing funnel  15 . This has crucial advantages for efficient energy input during full piercing, as the laser beam  5  can be focused deep in the material without shadows, which results in quick progress with the process. 
         [0044]    In addition, it has emerged that the piercing funnel  15  produced in process steps a) and b) is particularly well suited to transporting the slag produced during further piercing (full piercing) in process step c) far away from the pierced hole  11 , as indicated by the arrows  17  in  FIG. 2 c   . This causes the melted material to cool before it returns to the workpiece surface, so that it does not adhere to the top side  12  of the workpiece. 
         [0045]    The distance d3 between the process gas nozzle  10  and the top side of the workpiece during full piercing may be smaller (e.g. approx. 5 mm) than the distance d1, d2 in the two process steps a) and b), so that the process gas can be effectively coupled in the pierced hole  11  (d3&lt;d1,d2). The pressure of the process gas can then be set lower than in process steps a) and b). Full piercing can take place either in an uncontrolled manner with continuously increasing laser power or in a controlled manner with the help of a piercing sensor system known in the art. 
         [0046]    The pierced hole  11  produced in the first process step a) at the bottom of the trough  14  produced in the second process step b) is used in this third process step c) as a light channel for the laser beam  5 . In this way, the laser radiation enters in the workpiece more effectively and a better depth action of the laser radiation is achieved. The full pierced hole  16  therefore has a significantly smaller diameter than the original pierced hole  11  and there is a smaller heat input into the workpiece  12  than with hitherto customary piercing methods. 
         [0047]      FIG. 3 a    shows a photo of the top side of the workpiece  12  with a fully pierced hole  16  which was produced by a conventional piercing method and  FIG. 3 b    a photo of the top side of the workpiece  12  with a pierced hole  16  which has been produced by the piercing method according to the invention. While the workpiece top  12  shown in  FIG. 3 a    is contaminated with slag bulge  13  around the fully pierced hole  16 , significantly less slag has accumulated on the top side  12  of the workpiece in  FIG. 3 b   , as a result of which a defect-free contour cut is made possible in the area surrounding the fully pierced hole  16 . 
         [0048]      FIG. 4 a    shows a cross-sectional photo of a fully pierced hole  16  which was produced using a conventional piercing method and  FIG. 4 b    shows a cross-sectional photo of a fully pierced hole  16  which was produced using the piercing method according to the invention. While the fully pierced hole  16  shown in  FIG. 4 a    has an irregular, wide hole diameter over the entire workpiece thickness, the hole diameter of the fully pierced hole  16  shown in  FIG. 4 b    widens out in controlled fashion over the workpiece thickness more evenly, narrowly and only on the top of the workpiece on account of the piercing funnel  15 . 
         [0049]    After the full piercing of the workpiece  6 , as shown in  FIG. 2 d   , in an optional fourth process step d), the fully pierced hole  16  can be enlarged before the actual cutting process begins. This is necessary if the focus diameter of the laser beam  5  during the cutting of the workpiece  6  is greater than during piercing. A wide kerf is then produced during cutting and a large amount of melt created. If the fully pierced hole  16  is too narrow, the melt created at the start of cutting cannot be expelled from the workpiece  6  downwards through the fully pierced hole  16  quickly enough and a cutting tear can result. 
         [0050]    In order to enlarge the fully pierced hole  16 , the piercing point in the fourth process step d) is irradiated with the laser beam  5  set to a larger focus diameter than in the third process step c). The gas pressure of the process gas  7  is set so high that the melt created is reliably expelled downwards out of the workpiece  6 . The focal point of the laser beam  5  is disposed beneath the workpiece top, just as in the third process step c). The distance d4 between the process gas nozzle  10  and the top side  12  of the workpiece corresponds to the distance d3 in the third process step c), in other words, d4=d3. 
         [0051]    As shown in  FIG. 1 , the laser processing machine  1  further comprises a device (e.g. a changeover valve)  18  for selecting the process gas fed to the process gas nozzle  10  and a control system  19  which is programmed to control the traversing movement of the laser machining head  3  along with its process gas nozzle  10  and also the selected process gas  7  of the device  18  according to the piercing method described above. 
         [0052]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.