Abstract:
A process using a single millisecond laser is presented. The process is traversed over the surface of a component a number of times. The tilt angle of the laser beam with respect to a drilling axis during a subsequent traverse is different from the tilt angle with respect to the drilling axis during the first traverse of the surface. By using the single millisecond laser which although producing rougher surfaces than other millisecond lasers has higher material-removal rates, repeated traverses using different process parameters allow millisecond lasers to be successfully used to produce smooth surfaces.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 11154361.EP filed Feb. 14, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a process for laser drilling, wherein a laser is passed over the surface to be machined a number of times and in the process various angles are set. 
         [0003]    In many components, in particular cast components, areas of removed material, such as recesses or through-holes, have to be produced retrospectively. In particular in the case of turbine components which have film-cooling holes for cooling purposes, holes are introduced retrospectively after the component has been produced. Such turbine components, or indeed components for high-temperature applications in general, often also have layer coatings, such as for example a metallic layer and/or a ceramic outer layer. The film-cooling holes then have to be produced through the layers of the substrate (casting). Equally, such coated components are refurbished after use and provided with new layers, during which stage the interior of the through-holes is also coated (coat down), and this material then has to be removed again. This involves considerable work with expensive equipment and complex processes. 
       SUMMARY OF INVENTION 
       [0004]    Therefore, it is an object of the invention to provide a process allowing the process to be carried out quickly and inexpensively. 
         [0005]    EP 1 681 128 A1 shows a laser drilling process in which a diffuser of a film-cooling hole is produced in meandering form. 
         [0006]    The object is achieved by a process as claimed in the claims. 
         [0007]    The dependent claims list further advantageous measures which can be combined with one another as desired, in order to obtain further advantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the drawings: 
           [0009]      FIGS. 1 ,  2 ,  3  show views of a film-cooling hole in various perspectives; 
           [0010]      FIGS. 4-11  show process steps of a laser drilling process; 
           [0011]      FIG. 12  shows a turbine blade or vane; 
           [0012]      FIG. 13  shows a combustion chamber; 
           [0013]      FIG. 14  shows a gas turbine; 
           [0014]      FIG. 15  shows a list of superalloys. 
       
    
    
       [0015]    The figures and the description represent only examples of the invention. 
       DETAILED DESCRIPTION OF INVENTION 
       [0016]      FIG. 1  shows a component  1  with a hole  7 . 
         [0017]    The component  1  comprises a substrate  4  (for example a casting or DS or SX component). 
         [0018]    The substrate  4  may be metallic and/or ceramic. In particular in the case of turbine components, such as for example turbine rotor blades  120  or guide vanes  130  ( FIG. 12 ), heat shield elements  155  ( FIG. 13 ) and other casing parts of a steam or gas turbine  100  ( FIG. 14 ), but also of an aircraft turbine, the substrate  4  consists of a nickel-, cobalt- or iron-based superalloy ( FIG. 15 ). In the case of turbine blades or vanes for aircraft, the substrate  4  consists for example of titanium or a titanium-base alloy. 
         [0019]    The substrate  4  has a hole  7 , for example a through-hole. However, it may also be a blind hole. 
         [0020]    The hole  7  comprises a lower region  10 , which starts from an inner side of the component  1  and is for example symmetrical (for example circular, oval or rectangular) in form, and an upper region  13 , which if appropriate is formed as a diffuser  13  at an outer surface  14  of the substrate  4 . The diffuser  13  constitutes a widening of the cross section compared to the lower region  10  of the hole  7 . 
         [0021]    The hole  7  is for example a film-cooling hole. In particular the inner surface  12  of the diffuser  30 , i.e. in the upper region of the hole  7 , is supposed to be smooth, because unevenness causes undesirable turbulence and diversions, in order to allow optimum flow of a medium, in particular a cooling medium, out of the hole  7 . Much lower demands are placed on the quality of the hole surface in the lower region  10  of the hole  7 , since this has much less effect on the flow characteristics. 
         [0022]      FIG. 2  shows a component  1  which is designed as a layer system. 
         [0023]    At least one layer  16  is present on the substrate  4 . This may for example be a metal alloy of the MCrAlX type, where M stands for at least one element selected from the group consisting of iron, cobalt or nickel. X stands for yttrium and/or at least one rare earth element. 
         [0024]    The layer  16  may also be ceramic. 
         [0025]    A further layer (not shown), for example a ceramic layer, in particular a thermal barrier coating, may also be present on the MCrAlX layer. 
         [0026]    The thermal barrier coating is for example a fully or partially stabilized zirconium oxide layer, in particular an EB-PVD layer or a plasma-sprayed (APS, LPPS, VPS), HVOF or CGS (cold gas spraying) layer. 
         [0027]    A hole  7  comprising the lower region  10  and the diffuser  13  is also introduced into this layer system  1 . 
         [0028]    The statements made above in connection with the production of the hole  7  are applicable to substrates  4  with or without layer  16  or layers  16 . 
         [0029]      FIG. 3  shows a plan view of a hole  7 . 
         [0030]    The lower region  10  could be produced by a material-removing manufacturing process. By contrast, this would be impossible, or at least only possible at considerable outlay, for the diffuser  13 . 
         [0031]    The hole  7  may also run at an acute angle to the surface  14  of the component  1 . 
         [0032]      FIG. 4  shows the start of the material-removal process (machining) of a surface  29 . 
         [0033]    The surface  29  that is to be machined has a trailing edge  32  and an opposite end side  35  (Y direction) with two sides  23 ,  26  (X direction). Similar conditions apply analogously for triangular, polygonal or round surfaces. 
         [0034]    At the start of the material-removal process, the laser beam  20  is preferably centered preferably on the middle of the trailing edge  32  of the surface  29  that is to be machined and is preferably defocused above the material-removal plane. 
         [0035]    When first starting up, the laser beam  20  is preferably tilted with respect to the drilling axis  17 , very particularly preferably by 2°. The drilling axis  17  is preferably an axis of symmetry of the hole  7  or of a symmetrical region of the hole  7 . In the case of a hole as shown in  FIGS. 2 ,  3 , this is the axis of symmetry of the lower region  10 . The laser beam  20  is preferably tilted with respect to an end side  32 ,  35 , i.e. preferably perpendicular to the direction of traverse. 
         [0036]    Then, the laser (OFF) is moved in the X direction to a first side  23 . 
         [0037]    Then, the laser material-removal process starts (laser ON) from the first side  23  toward a second side  26 . 
         [0038]    When the side  26  of the surface  29  that is to be machined has been reached, the laser is switched off. 
         [0039]    In the next step as shown in  FIG. 5 , the laser is moved with the control system, in particular a CNC machine, in the X direction in the direction of the first side  23 , preferably to the center of the hole, for preference to the same point ( FIG. 4  left) as at the start of the process, and the laser is shifted one line width in the Y direction (toward the end side  35 ). 
         [0040]    Then, as shown in  FIG. 4  left, the laser is moved back in the X direction toward the first side  23  (laser OFF), so that from there (laser ON) laser beams once again machine the surface  29  from the side  23  to the other side  26 . 
         [0041]    Depending on the size of the surface  29  that is to be machined, the traverse, i.e.  FIGS. 4 ,  5 , is repeated until the other end side  35  (the opposite side from the trailing edge  32 ) has been reached through displacement of the Y direction of the surface  29  that is to be machined, as shown in  FIG. 6 . 
         [0042]      FIG. 10  shows the beam path of the laser beam  20  on the surface  29 . 
         [0043]    The laser beam  20  is preferably always moved only from one side  23  in the X direction toward the other side  26 , i.e. the laser is switched off when the laser position is being returned to the side  23 . 
         [0044]    This represents a further invention, independent of the tilting of the laser beam  20 , but is preferably combined with that of the tilting of the laser beam  20 . 
         [0045]    In  FIGS. 7 to 9 , the same surface is machined again, starting in the same way as in  FIGS. 4 and 5 ,  6 , but preferably with an increased tilt angle with respect to the drilling axis  17 . 
         [0046]    Preferably, during the second traverse over the surface  29  the laser beam  20  is always moved over the surface  29  from the other, second side  26  (laser) and not from the first side  23  as in the first traverse of the surface ( FIG. 11 ). 
         [0047]    This may even be repeated a third time, in which case the tilting angle is preferably increased further and the laser beam  20  is preferably always moved from the other side  23  again. 
         [0048]    The multiple traverse over the surface  29  that is to be machined results in a smoothing if an excessively rough surface or melted zones were present during the first pass. This applies in particular for the removal of material from a hole. 
         [0049]    In particular, low laser energies are used here, in particular of two joules. 
         [0050]    Another advantage is that only one laser, in particular a single laser, is used, and in particular this is a millisecond laser, in particular with a pulse duration of 0.25 ms, to machine the component  1 , and no further lasers are required. 
         [0051]    If the process is used to remove “coat down”, it is preferable for a hole to be introduced in the “coat down” in the hole  7  that has already been produced prior to the machining of the surface  29 . 
         [0052]      FIG. 12  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0053]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0054]    The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  and a blade or vane tip  415 . 
         [0055]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0056]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . 
         [0057]    The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. 
         [0058]    The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
         [0059]    In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
         [0060]    Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0061]    The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0062]    Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
         [0063]    Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
         [0064]    In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
         [0065]    Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
         [0066]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0067]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (HI)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0068]    The density is preferably 95% of the theoretical density. 
         [0069]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
         [0070]    The layer preferably has a composition Co-30 Ni-28 Cr-8 Al-0.6 Y-0.7 Si or Co-28 Ni-24 Cr-10 Al-0.6 Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10 Cr-12 Al-0.6 Y-3 Re or Ni-12 Co-21 Cr-11 Al-0.4 Y-2 Re or Ni-25 Co-17 Cr-10 Al-0.4 Y-1.5 Re. 
         [0071]    It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX. 
         [0072]    The thermal barrier coating covers the entire MCrAlX layer. 
         [0073]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0074]    Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0075]    Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused. 
         [0076]    The blade or vane  120 ,  130  may be hollow or solid in form. 
         [0077]    If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines). 
         [0078]      FIG. 13  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which generate flames  156 , arranged circumferentially around an axis of rotation  102  open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0079]    To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
         [0080]    On the working medium side, each heat shield element  155  made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
         [0081]    These protective layers may be similar to the turbine blades or vanes, i.e. for example MCrAlX: M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element or hafnium (Hf). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0082]    It is also possible for a, for example, ceramic thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0083]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0084]    Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. 
         [0085]    Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element  155  are also repaired. This is followed by recoating of the heat shield elements  155 , after which the heat shield elements  155  can be reused. 
         [0086]    Moreover, a cooling system may be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then, for example, hollow and may also have cooling holes (not shown) opening out into the combustion chamber space  154 . 
         [0087]      FIG. 14  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0088]    In the interior, the gas turbine  100  has a rotor  103  with a shaft  101  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. 
         [0089]    An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
         [0090]    The annular combustion chamber  110  is in communication with a for example annular hot gas duct  111 . There, for example four series-connected turbine stages  112  form the turbine  108 . 
         [0091]    Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot gas duct  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
         [0092]    The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . 
         [0093]    A generator (not shown) is coupled to the rotor  103 . 
         [0094]    While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot gas duct  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  expands at the rotor blades  120 , imparting its momentum, so that the rotor blades  120  drive the rotor  103  and the latter drives the generator coupled to it. 
         [0095]    While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0096]    To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
         [0097]    Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
         [0098]    By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . 
         [0099]    Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0100]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0101]    It is also possible for a thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0102]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0103]    The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 .