Electric heating – Metal heating – By arc
Reexamination Certificate
1999-07-19
2001-05-08
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121760
Reexamination Certificate
active
06229113
ABSTRACT:
TECHNICAL FIELD
This invention relates to laser drilling and more particularly to a laser drilling method and apparatus that increases the accuracy and repeatability of a hole produced by such means.
BACKGROUND ART
Drilling a hole in a structure often provides certain benefits and advantages. For example, drilling a hole in a structure, such as an airfoil, provides a means for cooling the airfoil. Specifically, airfoils, such as blades and vanes, within gas turbine engines are exposed to high temperature combustion gases, thereby requiring a method for cooling the airfoil. One such cooling method includes creating holes within the airfoil and passing pressurized air therethrough. As the airfoil rotates through the combustion gases, the pressurized air passes through the interior of the airfoil and exits the cooling holes. Depending upon the configuration of the hole, a portion of the pressurized air may pass over the exterior of the airfoil, thereby creating a film of air between the airfoil and the combustion gases. This method is often referred to as film cooling.
Film cooling efficiency is a function of the relative size of the cooling holes. Specifically, film cooling efficiency increases as the size of the holes more closely resemble each other. The manufacturing process used to drill the cooling holes, therefore, must be capable of producing such holes with sufficient accuracy and repeatability. The two methods currently used to manufacture cooling holes include electro-discharge machining (EDM) and laser drilling. EDM is a process wherein an electrode contacts a structure that is typically immersed in a dielectric fluid, thereby causing a spark and erosion at the point of contact. Although the EDM process produces very accurate holes, this process is typically slow and consumes electrodes, thereby increasing set-up time and material costs. Based upon these two characteristics, EDM is often regarded as an expensive method for producing cooling holes.
Laser drilling is typically a less expensive alternative for producing cooling holes and currently includes the use of either an unmodulated pulsed laser beam or a modulated pulsed laser beam. An unmodulated pulsed laser beam (hereinafter referred to as “unmodulated beam”) typically used to laser drill holes has a pulse width of about 0.1 milliseconds (msec) to about 10 msec and a peak intensity on the order of about 1×10
6
W/cm
2
to about 10×10
6
W/cm
2
. A modulated pulsed laser beam (hereinafter referred to as “modulated beam”) used for the same purpose, typically has a pulse width of about 1 nanosecond (nsec) to about 500 nsec and a peak intensity greater than 1×10
8
W/cm
2
. For the purposes of this invention, an unmodulated beam and a modulated beam shall be defined in respect to each other. Specifically, a modulated beam shall be defined as having a shorter pulse width and higher peak intensity in comparison to an unmodulated beam, regardless of the pulse width and peak intensity of the unmodulated beam.
When using a modulated beam, such as a beam having a 100 nsec pulse width and a 1.0×10
9
W/cm
2
intensity, to drill a hole in an airfoil, the modulated beam contacts the airfoil and vaporizes a majority of the material. The modulated beam produces a hole having a typically circular cross section because the material vaporizes rather than boils. Creating the vapor, however, leads to the formation of re-solidified vapor within and/or around the hole. The use of a high intensity beam, such as a modulated beam, also has the potential of creating plasma shielding, which occurs when the intensity of the beam is too high. Upon contacting the surface of the airfoil, the surface ionizes and a plasma layer is created, thereby shielding the internal surface of the hole from additional laser drilling.
When using an unmodulated beam, such as a beam having a 0.5 msec pulse width and a 3.0×10
6
W/cm
2
intensity, to drill a hole in an airfoil, the unmodulated beam contacts the airfoil and melts the material. The molten material escapes the hole primarily in the form of melt droplets but relatively small amounts of material may also exist in the form of vapor. Removing the material in the form of melt droplets, as opposed to vaporizing the material, reduces the amount of re-solidified vapor that forms in the upper portions of the hole and around its entrance. Re-solidified vapor is also referred to as burr. Moreover, when a beam exits a laser, an air stream usually surrounds and/or accompanies the beam so as to prevent any melt or vapor from splashing onto an optical portion of the laser. This air stream, however, often prevents the vapor from escaping the hole, thereby causing re-solidified vapor to form in and around the hole. Hence, utilizing an unmodulated beam minimizes the amount of the material transformed to vapor, thereby preventing re-solidified vapor from forming.
Using an unmodulated beam, however, often creates a boiling reaction between the unmodulated beam and the material. Specifically, the relatively long contact time between the unmodulated beam and the material causes the material to melt and often boil, thereby creating a hole with non-circular cross section. Furthermore, this boiling reaction tends to occur randomly, thereby reducing hole-to-hole uniformity. This melting reaction may also cause the formation of re-cast (i.e., re-melt), which is molten material that re-solidifies and adheres to the internal surface of the hole. Upon adhering to the internal surface, the re-cast behaves mechanically similar to the parent material but has a materially different crystalline structure compared to the parent material. Somewhere during the re-cast formation process, cracks may form, thereby producing undesirable mechanical properties within the airfoil.
What is needed is a method and apparatus that repeatedly produces holes with consistent dimensions in a structure while maintaining its mechanical integrity.
DISCLOSURE OF INVENTION
The method of the present invention uses both an unmodulated and modulated laser beam to create a hole or a cavity, having a circular cross section in a structure, while minimizing the formation of re-solidified vapor in and around the hole or cavity. Minimizing the formation of re-solidified vapor increases the accuracy of the hole or cavity. Therefore, utilizing both an unmodulated and modulated laser beam, in comparison to presently available laser drilling techniques, increases the accuracy of the hole, which in turn increases the hole-to-hole repeatability.
Accordingly, the present invention relates to a method for producing a hole in a structure utilizing an unmodulated beam and a modulated beam. The unmodulated beam first removes a portion of the structural material, thereby forming a guide hole. The modulated beam thereafter enters the guide hole and removes an additional portion of structural material, thereby increasing the cross section of the hole to the size of the effective diameter of the modulated beam. Utilizing both an unmodulated beam and a modulated beam in this order exploits the advantages of each type of beam. Specifically, forming a guide hole utilizing the unmodulated beam quickly removes a significant portion of the structural material because the unmodulated beam removes the material in the form of droplets. The modulated beam thereafter cleans the hole by removing additional structural material by vaporization. Removing a significant portion of the material with the unmodulated beam, before using the modulated beam, reduces the amount of material removed by the modulated beam, thereby reducing the amount of material that will be removed by vaporization. Decreasing the amount of structural material removed by vaporization minimizes the potential that such vapor could potentially re-solidify within or around the hole, thereby increasing the accuracy of the hole.
REFERENCES:
patent: 4092515 (1978-05-01), Joslin et al.
patent: 4870244 (1989-09-01), Copley et al.
patent: 86/02301 (1986-04-01), None
Yeo et al., “A technical review
Cummings Ronald G.
Heinrich Samuel M.
Lefort Brian D.
United Technologies Corporation
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