Abstract:
A method of manufacturing a through hole in a component between a first surface and a second surface, the through hole including a cavity formed in the first surface and a communicating passage between the second surface and the cavity including the steps of:
   a) laser percussion drilling a plurality of blind holes in the first surface to form the cavity and   b) drilling a communicating passage between the cavity and the second surface.

Description:
Continuation of U.S. application Ser. No. 10/283,146, filed Oct. 30, 2002, now U.S. Pat. No. 6,781,091. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method of forming a shaped hole in a component. In particular the present invention relates to the laser drilling of a cooling hole in a component of a gas turbine engine. 
     2. Description of Related Art 
     Components in a gas turbine engine are often cooled to survive the high temperatures found therein. One method used is thin film cooling by a supply to air, used as a cooling fluid, is provided to a passage within the component, exiting via cooling holes to form a thin film over the external surface of the component. In addition to removing heat from the component by conduction, the thin film of cooling air serves to prevent hot gas within the gas turbine engine impinging upon the component. 
     The cooling air used for thin film cooling must be supplied at a pressure greater than the gas path pressure in which the component is operating. This requires work to be carried out on the cooling air, representing a loss of useful power from the engine. In order to minimise the work required and hence maximise the efficiency of the engine, the cooling holes are shaped to optimise the use of cooling air. The shape most commonly used is a 2D diverging hole exiting at low incidence to the surface of the component to be cooled. This “fan” shaped hole allows the cooling film to form more readily than would a conventional constant-bore hole. 
     At present, such cooling holes are manufactured using EDM spark erosion. This places the component and an electrode in a dielectric such as kerosene or de-ionised water. When a voltage is applied across a small gap between the electrode and the component, material is spark eroded from both. By advancing the electrode into the component a hole is formed. Furthermore, by moving the feed axis of the electrode relative to the component, different shapes can be produced. 
     SUMMARY OF THE INVENTION 
     However, the EDM process has weaknesses. Electrodes tend to bend or skid when used at very shallow angles to the surface. When combined with other problems, such as arcing or damaged electrodes, the repeatability of the process can be poor leading to high levels of scrap. Other problems include lack of flexibility, due to the need for bespoke tooling for each component configuration and a slow process time. 
     Rolls-Royce&#39;s European Patent Application No 89310322.6 describes a laser drilling apparatus for drilling shaped holes in a workpiece. However the holes produced by the apparatus are novel in geometry. The present invention provides the ability to laser drill cooling holes that closely approximate “conventional” cooling holes. This is advantageous as the technology can be used to manufacture existing engine components without necessitating redesign. 
     According to the present invention there is provided a method of forming a through hole in a component between a first surface and a second surface thereof, the through hole comprising a cavity formed in the first surface and a communicating passage between the cavity and the second surface comprising the steps of:
         laser percussion drilling a plurality of blind holes in the first surface to form the cavity,   then forming the communicating passage between the cavity and the second surface.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and apparatus for carrying it out will now be described in more detail with reference to the accompanying drawings in which: 
         FIG. 1  shows a gas turbine engine component, 
         FIG. 2  shows a cross-section through the component illustrating the geometry of a cooling hole, 
         FIG. 3  shows a view on A of  FIG. 2  illustrating the structure of a cooling, 
       FIG.  4 ( a,b,c ) illustrate the conventional laser percussions drilling process for forming a through hole in a component. 
       FIG.  5 ( a,b,c ) illustrate the method for forming the cooling hole according to the present invention, and 
       FIG.  6 ( a,b ) show a cooling hole produced by EDM and the same cooling hole produced according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring now to  FIG. 1  a gas turbine component  2  is shown. The component  2  is a nozzle guide vane used in a gas turbine engine, located in the hot gas path immediately downstream of the engine combustor (not shown). In order to operate in such a hostile environment the component  2  is provided with a number of thin film cooling holes  4 . Cooling air  5 , provided to an internal passage of the component exits through the cooling holes to form a thin cooling film  6  over an external first surface  7  of the component. 
     Referring now to  FIG. 2  a cross section of a film cooling hole  4  is shown to illustrate the general construction of such a feature. The cooling hole  4  comprises a communicating passage  8  and a fan shaped cavity  10 . The communicating passage  8 , typically of constant bore, provides communication between an internal second surface II, bounding an internal passage  12  of the component, and the fan shaped cavity  10 . The passage  8  is also called a controlling hole as it meters the flow of cooling air  5 . The fan shaped cavity  10  diverges from a point  13  located within the bore of the controlling passage  8  hole until it meets the external surface of the component  2 . The fan shaped cavity  10  is so shaped to encourage cooling air emerging from it to fan out, thus forming an effective cooling film  6 . 
       FIG. 3  shows a view along the bore of the cooling hole  4  to provide further illustration of the cavity  10  geometry. The communicating passage  8  has a round bore. The fan shaped cavity  10  diverges from this bore such that a lozenge shaped exit aperture  14  is formed where the cavity  10  intersects the external surface  7  of the component  2 . 
     At present, cooling holes  4  are manufactured using EDM spark erosion. The present invention proposes the formation of the cooling hole by laser drilling, in particular the use of laser percussion drilling to form the fan shaped cavity  10 . 
     Referring now to  FIG. 4  the process of conventional laser percussion drilling is illustrated. 
     Conventionally, laser percussion drilling is used to form a through hole  20  in a component  22 . First, as shown in  FIG. 4   a , a pulsed laser beam  24  is directed at the component  22 . As shown in  FIG. 4   b  the energy of the beam ablates material from the component  22  such that a hole  26  is formed which becomes progressively deeper with each pulse. Such pulses are typically arranged in a pattern, which is repeated until the through hole  20  shown in  FIG. 4   c  is formed. The duration of the pulses, the pauses between pulses and the pauses between patterns are controlled to ensure the optimum conditions for drilling. 
     The diameter  28  of the hole  20  is controlled by defocusing the laser beam  24  to control the area of the component  22  struck by the coherent radiation of the laser  24 . As the hole forms, the laser beam  38  is channelled by the hole  20  itself, which prevents the laser beam  24  from diffusing. This is beneficial as the it allows the same diameter to be maintained throughout the length of the hole without having to alter the position of the focus  32 , however, this channelling effect dissipates energy and percussion drilled holes tend to taper inwards, an effect more pronounced with deeper holes. 
     Conventionally, laser percussion drilling is used to form through holes. Turning now to  FIG. 5 , the present invention utilises laser percussion drilling to form a number of blind holes,  34 , 36 . By controlling the energy of the laser pulses and controlling the number of pulses delivered the holes are drilled to a predetermined depth. 
     First as shown in  FIG. 5   a  a first blind hole  34  is laser percussion drilled. Orienting the axis  38  of the laser beam relative to the component  2  controls the orientation of this hole relative to the component  2 . 
     In a following step, as shown in  FIG. 5   b  a second blind hole  36  is drilled. The second blind hole is drilled coplanar with the first blind hole  34 , oriented such that the holes intersect at a predetermined point  42 . Where the blind holes  34 , 36  intersect one another, the channelling effect, described above, is lost. Hence care must be taken with the orientation of the component relative to the laser to ensure that the required hole is formed. 
       FIG. 5   c  shows a further step wherein the communicating passage  8  is formed. The component  2  is correctly oriented relative to the laser beam and the laser is used to trepan the controlling hole passage. First, a through “pilot” hole is drilled at the centre of the communicating passage. Then, the laser beam orbited relative to the component  2 , thereby trepanning a hole through the component. The diameter of the orbit corresponds to the diameter of the controlling hole  8 . 
     Shown in  FIGS. 5   a  and  5   b  the laser percussion drilling forms a recast layer  40  about the holes  34 , 36 . This recast layer  40  has reduced material properties when compared with the parent metal. It is therefore important to minimise the thickness of the recast layer  40 . By forming the fan shaped cavity  8  from a number of intersecting blind holes  34 , 36  the thickest region of recast  40  are concentrated at the intersection point  42  of the blind holes  34 , 36  which is subsequently removed when the communication passage  8  is drilled between the fan-shaped cavity  8  and the second surface  9 . 
     For comparison,  FIG. 6   a  shows a cooling hole  44  produced by conventional EDM and  FIG. 6   b  shows a fan shaped hole  46  created according to the present invention. Though the holes meet the same design specification, the laser drilled cooling hole  46  can be seen to comprise distinctive “flutes”  48  marking the individual blind holes used to form the fan. These have beneficial effects, promoting laminar flow in the airflow exiting the hole. 
     The hole forming process described above is a relatively simple cooling hole. It will be understood that this is not limiting. For example where a more complex geometry hole is required a greater number of blind holes would be laser percussion drilled to form the cavity. 
     Similarly the communicating passage does not need to be laser trepanned. Instead the hole cold be laser percussion drilled, formed via electro discharge machining or drilled by conventional chip machining.