Laser machining method

Abstract: A variety of configurations for convergent-divergent film cooling holes are disclosed, together with the hole drilling methods necessary to achieve the configurations.

Description: The present invention relates to the production of film cooling holes in components subject to the impingement of hot gases, for example in turbomachines, the coolant exiting the components through such holes to film-cool the heated surfaces of the components. In particular, it relates to the production of holes less likely to be blocked by environmental debris entrained in the flow of coolant or hot gas. Typical examples of such components are air-cooled nozzle guide vanes and high pressure turbine rotor blades, which are situated directly downstream of a gas turbine engine's combustion chambers. The film cooling holes are arranged in spanwise rows along the flanks of the aerofoil portions of the blades or vanes so that the streams of cooling air emerging from the holes onto the external surface can collectively protect it from direct contact with the hot gases and carry heat away by merging together to form a more-or-less continuous film of cooling air flowing next to the surface. The process of merging of the individual streams can be aided by flaring the apertures in the external surface so as to encourage the streams of cooling air to fan out towards each other. One problem with operation of engines containing such blades and vanes is that the film cooling holes have been subject to blockage by dust entrained in the cooling air systems, particularly in middle eastern countries. Because of the high temperatures at which these components operate, small dust particles which strike the edges of the holes, due to vorticity of the air flow through or over the entrances and exits of the holes, become slightly plastic and stick to the edges; this accretion process can continue over many hours' service until blockage occurs. Blockage can occur either internally of the aerofoil at the film hole inlets, or on the outside of the aerofoil at their outlets, though it is most serious at their inlets because of the difficulty of cleaning the insides of blades and vanes. Blockage can also occur to a lesser extent part-way through the hole. It is therefore desirable to provide film cooling holes which ease the situation with regard to blockage by dust accretion and can facilitate film cooling of surfaces in which they are provided. Patent US-A-4737613 discloses a method and apparatus for producing air cooling holes in turbine blade aerofoils, using a laser beam to machine the holes. The holes are produced by allowing a spatially stationary high power laser beam to impinge upon the aerofoil surface while the blade is held in a fixture which is rotated simultaneously in two orthogonal axes. The holes so produced can have coolant metering restrictions leading into diffusing (e.g. frustoconical) outlets for the cooling air. They may also have convergent inlets leading into the metering restrictions. Patent US-A-4672727 discloses a method of forming a film cooling hole by first forming a non-penetrating diffusing slot in an aerofoil wall at a highly oblique angle to the outer surface, e.g., by casting it into the aerofoil at the time it is made, or by an electro-discharge machining operation. The film cooling hole is completed by machining a constant diameter metering passage through the wall thickness to connect the bottom of the slot to a cooling air passage within the blade, using, e.g., an electro-discharge or laser hole drilling technique. The surface hole formed by the metering passage is then blocked up, e.g., by welding. The present invention is distinguished from the above prior art in that it enables the production of convergent/ divergent film cooling hole shapes without the use of casting methods or specially shaped electrodes, using a laser machining method which minimise the need to manipulate the article during the machining operation. Accordingly, the present invention provides a method of drilling a film cooling hole in a film-cooled article subject to heating by hot fluid flowing therepast, said article having wall means with one side thereof being a first surface subject to said heating and an opposing side thereof being a second surface subject to cooling by flow of pressurised coolant therepast, said film cooling hole being provided in the wall means to connect said first and second surfaces to allow coolant to flow through said hole onto said first surface for film cooling thereof, said hole having, in coolant flow series, an inlet aperture on said second surface, a convergent inlet portion, a metering restriction, a divergent outlet portion and an outlet aperture on said first surface, said method comprising directing a high-power highly coherent pulsed laser beam onto said first surface in predetermined relationship to an optical axis coincident with a longitudinal axis of a hole to be drilled, said method being characterised by the steps of laterally offsetting the longitudinal axis of said laser beam from said optical axis by a predetermined amount at a predetermined position along said optical axis, directing said laser beam from said predetermined position generally in the direction of said optical axis but intersecting therewith at a point within said wall means, such that said laser beam impinges on said first surface of said wall means at an acute angle determined by said amount of lateral offset of said laser beam from said optical axis at said predetermined position, oscillating said laser beam about said optical axis such that where said laser beam meets said first surface it describes said divergent outlet portion whose apex is said point of intersection with said optical axis, and continuing to drill with said laser beam after said divergent portion has been produced, thereby to produce said metering restriction and said convergent portion. In the case of air-cooled turbine blades or vanes in gas turbine engines, holes produced by the above laser machining method are particularly useful for reducing the above mentioned blockage of the holes by environmental debris entrained in the cooling air, in that the provision of the enlarged entry provides an increased area for egress of cooling air from the interior chamber without substantially increased coolant flow rates out of it, this increased internal hole area therefore taking longer to block up. Furthermore, the resulting reduced entry velocities of the cooling air into the hole produce decreased vorticity at the edges of the entry, so reducing the tendency of particles to be deposited onto the edges from the air stream. Reduced vorticity also results from production of hole edges which make an included angle with the internal surface of the wall means of substantially greater than 90° due to the convergence of the entry. Likelihood of blockage part-way through the hole is also reduced because the length of the hole at the minimum (throttling) diameter is short when produced by the above method. The envelope of oscillation of the laser beam may be a conical surface whose apex is the point of intersection of the laser beam with the optical axis. Alternatively, the envelope of oscillation of the laser beam may be a fan shaped surface whose apex is the point of intersection of the laser beam with the optical axis. Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a perspective view of a known high pressure turbine rotor blade provided with film cooling holes; Figure 2A is a longitudinal cross-section through a prior art film cooling hole; Figure 2B is a plan view on arrow 2B in Figure 2A showing the shape of the prior art film cooling hole's exit aperture; Figure 3A is a perspective view of a cross-section through a film cooling hole configuration in accordance with the invention; Figure 3B is a plan view showing the shape of the film hole's entry aperture; Figure 4A is a cross-sectional side elevation through a film cooling hole according to the invention but having an oblique orientation with respect to the film cooled surface; Figure 4B is a plan view on arrow 4B in Figure 4A showing the shapes of the entry and exit apertures of the film cooling hole; Figure 5A is a cross-sectional side elevation through an alternative form of film cooling hole according to the invention; and Figures 5B is a view on arrow 5B of the hole shown in Figure 5A. Referring first to the complete turbine blade 10 shown in Figure 1, it comprises a root portion 12, having a so-called "fir-tree" sectional shape which locates in a correspondingly shaped slot in the periphery of a turbine rotor disc (not shown); a radially inner platform 14, which abuts the platforms of neighbouring blades to help define a gas passage inner wall for the turbine; an aerofoil 16, which extracts power from the gas flow past it; and an outer shroud portion 18 which again cooperates with its neighbours to help define the outer wall of the turbine's gas passage. Although described here in relation to integrally shrouded blades, the invention is of course equally applicable to unshrouded blades, or indeed other film-cooled components. The interior of the aerofoil 16 contains a chordwise succession of substantially mutually parallel cooling air passages (not shown, but see, e.g., patent specification number GB-A-2228540 for exemplary details), which passages extend spanwise of the aerofoil. One or more of the passages are connected to a cooling air entry port 20 provided in the side face of an upper root shank portion 22 just below the underside of inner platform 14. This receives low pressure cooling air, which cools the aerofoil 16 by taking heat from the internal surface of the aerofoil as it flows through the internal passage and out through holes (not shown) in the shroud 18 and also through the spanwise row of closely spaced small holes 24 in the trailing edge 26 of the aerofoil. Others of the internal passages are connected to another cooling air entry port (not shown) located at the base 27 of the "fir-tree" root portion 12, where high pressure cooling air enters and cools the internal surfaces of the aerofoil 16 by its circulation through the passages and eventual exit through holes (not shown) in the shroud 18. It is also utilised to film-cool the external surface of the flank 28 of the aerofoil 16 by means of spanwise extending rows of film cooling holes 30 to 33. Figure 2 shows a typical cross-section through the wall 34 of the blade 10 in the region of the row of film cooling holes 33, one of the holes 33 being seen in longitudinal cross-section. The hole 33 penetrates the wall thickness obliquely at an angle a of the hole's longitudinal centreline 35 with respect to a normal 36 to the exterior surface 38 of the aerofoil in that region. This measure ensures a less turbulent exit of the stream of cooling air 40 from the hole's exit aperture 42 onto the surface 38, because the stream of cooling air is thereby given a component of velocity in the direction of the flow of hot turbine gases 44 over the surface 38. The film cooling air 40 is as previously mentioned taken from one of the internal passages 46, shown partially bounded by the wall 34 and an internal partition 48. The shape of the exit aperture 42 is of course elliptical. When gas turbine engines are operated in certain arid areas of the world, primarily the Middle East, very fine dust particles, prevalent in the first few tens of metres above ground level and on occasions present at altitudes of thousands of metres, can enter the engine's cooling air system by way of the engine's compressor and pass into the interior of the turbine blades or other cooled blades or vanes. When cooling air flowing along the surface of an internal cooling passage or chamber such as 46 encounters the entry aperture 50 of a hole 33, some of the cooling air flows into the hole and the edges of the entry aperture 50 generate vortices in the flow. It is believed that fine particles are separated from the main flows of air through the passage 46 or through the hole 33 and are deposited in the low velocity regions near the edges, where some of the minerals in the dust particles are heated to temperatures near or at melting point, rendering at least some of the particles tacky or plastically deformable and liable to stick to each other and to the metallic surface. At these points the deposits grow, and the entry aperture 50 slowly becomes blocked. Regarding blockage of the exit aperture 42, the deposits tend to build up on the downstream edge 52 of the hole. Build-up here is more likely to be due to the passing particles in the main turbine gas flow 44 experiencing the edge 52 as a step in spite of the angling of the hole 33 at angle a , the flow therefore becoming detached from the surface at this point and forming a vortex. This is more likely to be the case when the cooling hole is not blowing hard, i.e. when the pressure drop between passage 46 and the external surface 38 of the blade is small. However, for higher pressure drops and consequently greater blowing rates, the hot gas flow 44 meeting cooling air stream 40 will produce a local vortex and this will deposit particles in a similar manner. Either way the deposits grow towards the opposite edge of the exit aperture 42 and eventually block the hole. It is often the internal blockage that is most troublesome to the operator of the engine because it can build up more quickly and also is not easily accessible to abrasive cleaners and the like. Figures 3A and 3B illustrate how this problem can be significantly eased according to the invention by drilling a hole 60 which has an entry aperture 62 leading into a convergent frusto-conical portion 64, a throat 66 and a divergent frusto conical portion 68 leading to an exit aperture 70. The throat 66 connects the narrowest parts of the convergent and divergent portions 64,68 and defines a flow constriction. This is necessary for restricting or controlling the flow rate of film cooling air through the hole 60 according to the difference in pressure between the cooling air chamber or passage within the aerofoil and the exterior surface 38. The longitudinal axis 72 of the hole is in this case normal to the surface 38. The flaring of film cooling holes at entry as well as exit has the following advantages, namely: the length of hole which is at the minimum (air metering) diameter is short, so reducing the likelihood of blockage; the increased hole area and blunter edges of the inlet and outlet apertures leads to reduced vorticity and velocity of air flow past the edges, so that less particles are deposited from the airstream; and the larger apertures take longer to block up. It is important that the air flowing through the film cooling hole 60 does not experience the minimum diameter 66 as a severe vortex-producing step which would tend to cause excessive deposition of particles from the air flow onto the walls of the hole. For this reason it is necessary to ensure that the machining process used to produce the hole is able to attain and maintain good alignment between the two frusto-conical portions 64,68, i.e. they should have a straight longitudinal axis 72 common to both portions. As already pointed out in relation to Figure 2, it can often be advantageous to orient the film cooling hole obliquely with respect to the external surface of the blade so that the cooling air 40 issues from the hole with a velocity component in the same direction as the flow of the hot gases 44 past the hole. Figure 4 shows how this is achieved by inclining the longitudinal axis 80 of the hole 82 at an angle a to the external surface 38 of the blade. The entry and exit portions 84,86 respectively of the hole 82 are still frusto-conical, but the entry and exit apertures 88,90 are no longer circular, now being egg-shaped as shown in Figure 4B. In Figure 4, the entry and exit apertures of the film cooling hole are elongate in the direction of flow 44 over the external surface. However, it can be advantageous to have a configuration like that shown in Figure 5, in which the film cooling hole 100, having entry and exit apertures 102,104 respectively and a throat 106, has its entry and exit portions elongated transversely of the expected direction of hot gas flow 44. After passing through convergent portion 107 and throat 106, the coolant flow 40 diffuses through the divergent portion 108 of the hole to the oval (race-track shaped) exit aperture 104 and forms a broader stream on blade surface 38 than would a similar flow emerging from a circular exit aperture of the same area. Although Figure 5 shows a film cooling hole with its longitudinal centreline extending normal to the external surface 38, it is plainly possible for the hole to be formed with an oblique orientation, e.g. for the purpose of giving cooling air 40 a larger component of velocity in the direction of hot gas flow 44. In laser machining, a pulsed beam of high energy laser light is directed onto the workpiece surface, causing the material to absorb energy until vaporised and removed from the workpiece. Manufacture of convergent-divergent film cooling holes by the method of the present invention involves use of a very thin, highly coherent, laser beam. Holes with diameters upwards of about 0.25mm can be drilled by trepanning with such a beam: the laser beam is passed through an optical system which makes the beam move round the periphery of a cylinder of small diameter related to the size of hole it is desired to drill. In this way the laser beam cuts out the hole around its edge, giving a good surface finish. For a laser drilling process capable of producing all the film cooling holes of Figures 3 to 5, reference should be made to our published patent specification GB-A-2227965, in which shaped holes are produced by laser drilling techniques which utilise an optical system comprising lenses and mirrors to move a laser beam relative to the external surface of a component through paths appropriate to drill out the required shape. The disclosure of GB-A-2227965 is hereby incorporated by reference. In brief, a laser drilling technique appropriate to produce a hole like that of Figure 3 involves using a lens to direct a high-power highly coherent pulsed laser beam onto the surface 38 to be drilled. Before it passes through the lens, the laser beam is reflected from a laterally movable mirror in an optical head positioned above the lens, the mirror being laterally offset from the lens' optical axis by a variable amount, the lens' optical axis defining the longitudinal axis 72 of the hole to be drilled. The optical head is rotatable; thus the mirror is caused to orbit about the optical axis of the lens, so that where the laser beam meets the surface 38 it begins to describe the curved surface of the divergent conical portion 68 whose apex is the point of null movement of the beam within the thickness of the material being drilled. This may be likened to trepanning. The laser beam impinges on the surface at an acute angle determined by the amount of lateral offset of the mirror from the lens' optical axis. The cone angle of the conical portion 68 is of course given by 180° less twice this acute angle of impingement. By appropriately adjusting the relevant machining parameters such as the energy of the laser beam and continuing to drill after conical portion 68 has been produced, the complementary conical portion 64 is also produced, the intersection of the two conical portions forming the throat 66. A hole like that of Figure 4 can be produced in the same way, except that the optical axis of the laser drill must be oriented to coincide with the desired orientation of longitudinal axis 80 with respect to the component's exterior surface 38. The "2-dimensional cone" or "fan" shapes of Figure 5 can be produced by an arrangement in which the optical head is kept stationary (i.e. non-rotating) and the lateral offset of the mirror is varied by oscillating it over a range of movement either side of the lens' optical axis. In this case the laser beam 109 does not perform a trepanning motion but is seen to oscillate through an angle b about the null point 110 where the hole's axis 112 passes through the throat 106. As already explained in connection with Figure 4, it is important from the point of view of minimising deposition of debris from the cooling air onto the walls of the film cooling holes that the throttling restrictions or throats in the various film cooling holes of Figures 3 to 5 do not excite excessive vorticity in the flow of coolant through them. For instance, in Figure 5, the machining process theoretically produces a throat with a sharp angular profile. However, in practice the throat edge 110 would be a radius, and if desired this radius could be made larger, as shown by the dashed lines, by means of a finishing operation using a slightly de-focused laser beam. Alternatively, a laser beam could be trepanned around a slightly larger diameter to produce a throat comprising a short cylindrical passage rather than an edge. These techniques would also be applicable to the other embodiments. Although the above specific embodiments have concentrated on the production of various film cooling hole configurations in the aerofoil portions of stator vanes or rotor blades, such configurations can also be utilised to cool the shrouds or platforms of these devices, or indeed for other surfaces in the engine requiring film cooling.

Claims: A method of drilling a film cooling hole (100) in a film-cooled article subject to heating by hot fluid (44) flowing therepast, said article having wall means (34) with one side thereof being a first surface (38) subject to said heating and an opposing side thereof being a second surface subject to cooling by flow of pressurised coolant therepast, said film cooling hole being provided in the wall means to connect said first and second surfaces to allow coolant (40) to flow through said hole (100) onto said first surface (38) for film cooling thereof, said hole having, in coolant flow series, an inlet aperture (102) on said second surface, a convergent inlet portion (107), a metering restriction (106), a divergent outlet portion (108) and an outlet aperture (104) on said first surface, said method comprising directing a high-power highly coherent pulsed laser beam (109) onto said first surface (38) in predetermined relationship to an optical axis coincident with a longitudinal axis (72) of a hole to be drilled, said method being characterised by the steps of laterally offsetting the longitudinal axis of said laser beam (109) from said optical axis by a predetermined amount at a predetermined position along said optical axis, directing said laser beam from said predetermined position generally in the direction of said optical axis but intersecting therewith at a point (110) within said wall means, such that said laser beam impinges on said first surface (38) of said wall means at an acute angle determined by said amount of lateral offset of said laser beam from said optical axis at said predetermined position, oscillating said laser beam about said optical axis such that where said laser beam (109) meets said first surface (38) it describes said divergent outlet portion (108) whose apex is said point of intersection (110) with said optical axis, and continuing to drill with said laser beam after said divergent portion (108) has been produced, thereby to produce said metering restriction (106) and said convergent portion (107). A method according to claim 1, characterised in that said laser beam has an envelope of oscillation comprising a conical surface whose apex is said point of intersection. A method according to claim 1, characterised in that said laser beam has an envelope of oscillation comprising a fan shaped surface whose apex is said point of intersection (110).

Inventors: COOPER BRIAN GUY, CORFE ARTHUR GEORGE, STROUD DAVID, TOWILL JOHNATHAN PAUL WILLIAM, COOPER, BRIAN GUY, CORFE, ARTHUR GEORGE, STROUD, DAVID, TOWILL, JOHNATHAN PAUL WILLIAM
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1989
Main classification: F01D5
IPC codes: F01D5, B23K26