GB2349106A - Laser drilling - Google Patents

Abstract

The adherence of spatter to the surface of a metal workpiece during laser percussion drilling is avoided by applying to the surface of the workpiece a coating of a composition comprising a particulate material distributed in a polymeric matrix. The particulate material may be silicon carbide and the polymeric matrix may comprise a high modulus silicone sealant.

Description

LASER DRILLING This invention relates to laser drilling and particularly, although not exclusively, to laser percussion drilling.

Laser percussion drilling is a process in which short pulses of laser energy are directed at a workpiece. The laser beam heats the material of the workpiece (for example, metal) ; the material vaporises, and is ejected in both liquid and vapour phases from the site at which the laser beam strikes the workpiece, so forming a hole. The laser beam is pulsed, so that the hole becomes progressively deeper with each pulse.

Spatter is a problem commonly associated with holes produced using a laser percussion drilling process. Spatter arises when some of the ejected molten or vaporised material is not completely expelled, but resolidifies and adheres to the workpiece around the periphery of the hole. Figure 1 is a scanning electron microscope micrograph of a hole 2 formed using a laser percussion drilling process, the hole being surrounded by spatter 4.

Laser percussion drilling is used, for example, to form effusion cooling holes in turbine engines. Spatter is undesirable for such applications, since the flow and efficiency of the cooling air is crucially dependent on the characteristics of the hole and the surface of the workpiece surrounding it. Consequently, the spatter needs to be removed in a subsequent finishing process.

Spatter removal can be a particular problem if an inert coaxial assist gas (for example argon or nitrogen) is used during the drilling process. Inert gases are commonly used for this purpose to provide an inert environment for the drilling process, to prevent impingement of debris on the focus lens of the laser, and to control molten and vaporised material during drilling. It has been found the use of such gases can make the resulting spatter very difficult to remove. Complete removal, for example by abrasive bead blasting, may not be possible without causing undesired surface modification of the workpiece and the hole geometry.

Various attempts have been made to reduce the adherence to workpieces of spatter generated during laser percussion drilling.

For example, Otsat et al ["Method for Machining with Laser Beam", US Patent 3,440,388, April 1969] masked the material surface with paraffin wax or silicon grease, so that the material expelled from the workpiece was deposited onto the coating.

Sharp et al ["A Novel Anti-Spatter Technique for Laser Drilling : Applications to Surface Texturing", Proceedings of ICALEO 97, San Diego, CA, USA, v83 Part 1, pp 41-50, 1997] employed a surfactant fluid to prevent the wetting of molten material onto the workpiece surface when drilling blind holes (~1. 6mm deep) in titanium. Similarly, Kamalu ["Laser Drilling of Mild Steel Using an Anti-Spatter Coating", Poster Presentation (P15) in ICALEO 98, Fl, USA, 1998] used an oil-based surface-active medium in an attempt to reduce spatter accumulation.

A problem with the use of surfactants or other coating materials in fluid form is that they tend to be blown away from the surface when an assist gas is used and will simply flow off the workpiece if the surface to be drilled is incline to the horizontal.

According to a first aspect of the present invention there is provided a composition for application to the surface of a workpiece during laser drilling, the composition comprising a particulate material distributed in a polymeric matrix.

The material of the polymeric matrix may include cross-linkable polymeric chains so that the material is curable after application to the workpiece to provide a substantially solid coating. The material of the polymeric matrix may, for example, comprise a high modulus silicone sealant.

The particulate material may be a ceramic or refractory material, such as silicon carbide. The particulate material preferably consists predominantly of particles which have a particle size which is not less than 501lm and not more than 1 1 011m.

The particulate material and the polymeric matrix may be present in the composition in substantially equal proportions by weight. The components of the composition, and their relative proportions, are preferably such as to render the composition spreadable before any curing is accomplished.

According to a second aspect of the present invention, there is provided a method of laser drilling a workpiece comprising applying a coating of a composition in accordance with the first aspect of the present invention to the surface to be drille, and subsequently laser drilling into the surface through the coating.

If the composition includes a curable polymeric matrix, curing is effected before laser drilling takes place.

Preferably the composition is applied at a thickness not greater than 1 mm, and more preferably not greater than 0.5mm, and not less than 0.1mm and more preferably not less than 0.25mm.

The process preferably also includes the step of removing the composition after the laser drilling operation is complete. Depending on the nature of the composition, removal may be achieved by heating the workpiece, for example by the application of steam to the composition, or by use of an appropriate solvent.

The laser drilling operation may take place in the presence of a coaxial assist gas, and the laser beam may be directed normally with respect to the surface to be drille, or at any other angle.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1, referred to above, is a scanning electron microscope micrograph showing a hole formed by laser percussion drilling in a conventional process; Figure 2 is a diagrammatic sectional view of the hole shown in Figure 1; Figure 3 shows, in schematic form, a laser percussion drilling process in accordance with the present invention, with a coating applied to the workpiece; Figure 4 shows the surface of the coated workpiece after laser percussion drilling ; Figures 5 and 6 are micrographs of arrays of laser percussion drilled holes formed using a conventional process under different operating conditions; Figures 7 and 8 are micrographs of arrays of laser percussion drilled holes formed using a process in accordance with the present invention under different operating conditions.

With reference to Figure 3, a coating 6 is applied to a workpiece 1 in a region at which an array of holes is to be formed using a laser percussion drilling process. Metal removal in the drilling process is effected by a laser beam 8. The laser beam 8 passes through a nozzle 10 through which a blast of gas is directed at the workpiece coaxially with the laser beam 8. The gas may, for example, be oxygen, argon, air or nitrogen.

The coating 6 is a composite material comprising particles of silicon carbide in a matrix of a high modulus black silicone sealant, for example the sealant available from Bostik Limited under the designation 100HMA (HMA represents High Modulus Acetoxy).

Such a sealant has a high optical absorption and has high heat stability and temperature resistance. It also exhibits low moisture absorption and good adhesion to most surfaces.

The silicon carbide particles are in the form of granules having a particle size not less than 50pm and not more than 11 0pm. The particles are distributed evenly through the matrix. Silicon carbide has a low coefficient of thermal expansion and a high dissociation temperature of 2700 C. It is extremely hard, and produces a vigorous exothermic reaction when added to molten iron.

The composition is applied to the surface of the workpiece 1 at a thickness t which is not less than 0.3mm and not more than 0.45mm. It has been found that, if the coating thickness is less than 0.3mm, the coating tends to be unstable during the drilling process. If the coating has a thickness greater than 0.45mm, an increase in the laser energy is required to remove the coating where each hole is to be formed. Also, a thicker coating increases heat transfer parallel to the surface of the workpiece, which can cause undesirable effects.

The silicon carbide and the silicone sealant are mixed together in equal proportions by weight to provide a spreadable composition which can be applied to the workpiece using a knife edge. After application, the coating composition is allowed to cure. The time required for curing to be completed will depend on the nature of the matrix material and the ambient conditions. For example, if the matrix material is 1 OOHMA, curing will be completed in approximately twelve hours at room temperature, or in about thirty minutes or less at approximately 250 C. Curing at an elevated temperature is preferred, since this results in improved adhesion of the coating to the workpiece surface.

Laser percussion drilling is performed after the coating has cured. The laser percussion drilling process initially removes the coating and subsequently removes the metal of the workpiece. However, during the metal removal stage, the coating composition prevents any molten metal from contracting, and adhering to, the metal of the workpiece surrounding the drilled hole.

It has been found that the presence of the silicon carbide in the composition results in the formation in the coating of a smaller hole than occurs when the silicone sealant is used alone, and also reduces any tendency for the coating to be peeled away at the first laser pulse.

Figure 4 is a micrograph showing the surface of the coating composition following laser percussion drilling. It will be appreciated that the coating composition around the periphery of each drilled hole is substantially unaffected by the drilling process and consequently the composition provides an effective mask for the surface of the workpiece. In tests, no spatter is deposited on the coating, suggesting that the coating assists in the full ejection of molten and vaporised metal from the workpiece.

The coating has been found to be capable of resisting the heat generated during laser drilling without decomposition, and to restrict heat propagation transversely of the direction of the laser beam. These characteristics avoid excess removal of the coating during laser drilling, so that the hole formed in the coating is not much larger in diameter than that formed in the workpiece. This minimises the exposed surface of the workpiece and so minimises the area of the workpiece on which spatter can accumulate.

When the drilling process has been completed, the coating composition can be removed. For example, short exposure to steam (for two to three minutes) will soften the composition and enable it to be peeled away. Any remaining residues can be removed by use of a proprietary sealant remover or other solvent. Alternatively, such a sealant remover or other solvent could be used to remove the coating composition instead of steam. Suitable silicone sealant removers are available from Dow Corning, and from Mykal Industries.

Removal of the coating composition leaves the surface of the workpiece surrounding the holes in its original condition, without any traces of spatter.

Figures 5 to 8 provide a comparative indication of the effectiveness of the coating composition. Figures 5 and 6 show arrays of holes drilled using a conventional laser percussion drilling process without the use of any coating composition. In Figure 5 the holes are drilled in a direction normal to the surface of the workpiece, while in Figure 6 the holes are drilled at an angle of 45 to the surface of the workpiece. In aloi of Figures 5 to 8 the workpiece is made from NIMONIC 263 alloy. In Figures 5 and 6, coaxial assist gas has been used. This gas is oxygen in micrographs (a), argon in micrographs (b), air in micrographs (c) and nitrogen in micrographs (d). The micrographs on the left of each Figure show the entire array of holes, which the micrograph on the right is centred on one of the holes in the array.

It will be appreciated that, in the examples of Figures 5 and 6, a significant amount of spatter has occurred, which would have to be removed in a subsequent finishing operation. By contrast, in Figures 7 and 8 which broadly correspond to Figures 5 and 6 but show workpiece in which laser percussion drilling was performed after a coating composition had been applied to the workpiece, the coating composition having been removed after drilling is complete. It will be appreciated that the workpiece surfaces are left with substantially no spatter.

Furthermore, it is significant that the same coating composition substantially eliminates spatter under a wider variety of spatter properties and drilling conditions. In particular, as shown by Figures 7 and 8 the coating is effective in spatter prevention over a wide range of assist gases, including reactive gases (oxygen and air) and inert gases (argon and nitrogen).

Claims (1)

1. CLAIMS A composition for application to the surface of a workpiece to prevent spatter adherence to the workpiece during laser drilling, the composition comprising a particulate material distributed in a polymeric matrix.

   A composition as claimed in claim 1 in which the composition comprises substantially equal proportions by weight of the particulate material and the polymeric matrix.

   A composition as claimed in claim 1 or 2, in which the polymeric matrix is a curable material.

   A composition as claimed in any one of the preceding claims in which the material of the polymeric matrix is a high modulus silicone sealant.

   A composition as claimed in any one of the preceding claims in which the material of the polymeric matrix is black.

   A composition as claimed in any one of the preceding claims in which the particulate material is a ceramic material.

   A composition as claimed in claim 6 in which the particulate material is silicon carbide.

   A composition as claimed in any one of the preceding claims in which the particulate material consists predominantly of particles having a particle size which is not less than 50m and not more than 110pm.

   A composition as claimed in any one of the preceding claims which is spreadable.

   A composition for application to the surface of a workpiece to prevent spatter during laser drilling, the composition being in accordance with claim 1 and substantially as descried herein.

   A method of laser drilling a workpiece, comprising applying a coating of a composition in accordance with any one of the preceding claims to the surface to be drille, and subsequently laser drilling into the surface through the coating.

   A method as claimed in claim 11 in which the composition is cured after application to the surface and before laser drilling.

   A method as claimed in claim 11 or 12 in which the thickness of the coating is not greater than 1 mm.

   A method as claimed in claim 13 in which the thickness of the coating is not greater than 0.5mm.

   A method as claimed in any one of claim 11 to 14 in which the thickness of the coating is not less than 0.1 mm A method as claimed in claim 15 in which the thickness of the coating is not less than 0.25mm.

   A method as claimed in any one of claims 11 to 16 in which the coating is removed after laser drilling.

   A method as claimed in claim 17 in which the coating is exposed to steam prior to removal.

   A method as claimed in claim 17 or 18 in which the coating is removed by use of a solvent.

   A method as claimed in anyone of claims 11 to 19 in which the laser drilling is accomplished in the presence of a coaxial assist gas.

   A method of laser drilling a workpiece as claimed in claim 11 and substantially as described herein.