Electric heating – Metal heating – By arc
Reexamination Certificate
1994-12-22
2001-04-10
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121850, C148S565000
Reexamination Certificate
active
06215097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to laser shock peening of gas turbine engine parts and, more particularly, for airfoil leading and trailing edges such as on fan and compressor blades having localized compressive residual stresses imparted by laser shock peening.
2. Description of Related Art
Gas turbine engines and, in particular, aircraft gas turbine engines rotors operate at high rotational speeds that produce high tensile and vibratory stress fields within the blade and make the fan blades susceptible to foreign object damage (FOD). Vibrations may also be caused by vane wakes and inlet pressure distortions as well as other aerodynamic phenomena. This FOD causes nicks and tears and hence stress concentrations in leading and trailing edges of fan blade airfoils. These nicks and tears become the source of high stress concentrations or stress risers and severely limit the life of these blades due to High Cycle Fatigue (HCF) from vibratory stresses. FOD damage may also result in a loss of engine due to a release of a failed blade. It is also expensive to refurbish and/or replace fan blades and, therefore, any means to enhance the rotor capability and, in particular, to extend aircraft engine fan blade life is very desirable. The present solution to the problem of extending the life of fan blades is to design adequate margins by reducing stress levels to account for stress concentration margins on the airfoil edges. This is typically done by increasing thicknesses locally along the airfoil leading edge which adds unwanted weight to the fan blade and adversely affects its aerodynamic performance. Another method is to manage the dynamics of the blade by using blade dampers. Dampers are expensive and may not protect blades from very severe FOD. These designs are expensive and obviously reduce customer satisfaction.
Therefore, it is highly desirable to design and construct longer lasting fan and compressor blades, as well as other gas turbine engine parts, that are better able to resist both low and high cycle fatigue than present day parts. The above referenced U.S. patent applications Ser. No. 08/319,346, entitled “LASER SHOCK PEENED ROTOR COMPONENTS FOR TURBOMACHINERY”, and U.S. patent application Ser. No. 08/373,133, entitled “LASER SHOCK PEENED GAS TURBINE ENGINE FAN BLADE EDGES”, are directed towards this end. The latter, more particularly, provides an airfoil of a fan blade with regions of deep compressive residual stresses imparted by laser shock peening at least a radially extending portion of leading and/or trailing edge surfaces of the fan blade.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and thereby harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled “Method and apparatus for truing or straightening out of true work pieces”. The present invention uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a work piece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477, entitled “Laser shock processing”; and U.S. Pat. No. 5,131,957, entitled “Material Properties”. Laser peening as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled “Laser Peening System and Method”. One issue is manufacturing costs of the laser shock peening process which can be prohibitively expensive. The “on the fly” laser shock peening process of the present invention is designed to provide cost saving methods for laser shock peening.
SUMMARY OF THE INVENTION
A fly method of laser shock peening a gas turbine engine part by continuously moving a metallic gas turbine engine part while continuously firing a stationary laser beam, which repeatably pulses between relatively constant periods, on a portion of the part. The laser beam with sufficient power to vaporize material on the surface of the portion of the part with the pulses around laser beam spots formed by the laser beam on the surface and to form a region having deep compressive residual stresses imparted by the laser shock peening process extending into the part from the laser shock peened surface. The method further includes flowing a curtain of water over the surface upon which the laser beam is firing while moving the part until the laser shock peened surface is completely covered by laser beam spots at least once. The surface may be covered by a paint which is then the material used to produce the plasma or the surface may be unpainted and the metal of the part is used to produce the plasma. The part may be moved linearly to produce at least one row of overlapping circular laser beam spots having generally equally spaced apart linearly aligned center points and the part may be moved and the laser beam fired to produce more than one row of overlapping circular laser beam spots having generally equally spaced apart linearly aligned center points wherein adjacent rows of spots overlap. The laser beam may be fired and the part moved so that the center points of adjacent spots in adjacent rows are also offset from each other a generally equal amount in a direction along a line on which the center points are linearly aligned.
In another embodiment of the present invention, the painted laser shock peened surface is laser shock peened using a set of sequences in which each sequence the surface is painted and then the part is continuously moved while continuously firing a stationary laser beam on the surface such that adjacent laser shock peened circular spots are hit in different ones of the sequences in the set. In a more particular embodiment, the laser beam is fired and the part moved so that the center points of adjacent spots in adjacent rows are offset from each other a generally equal amount in a direction along a line on which the center points are linearly aligned. A more particular embodiment, each spot is hit more than one time using more than one set of the sequences.
Another embodiment of the present invention includes a further a step of removing remelt formed by the laser firing upon the unpainted laser shock peened surface after it is laser shock peened.
A more particular embodiment of the present invention uses the methods above on a gas turbine engine blade such as on the airfoil of a vane or blade of a fan or compressor section having an airfoil with a leading edge and a trailing edge. The invention may be used along a portion of the edge or along the entire edge of the airfoil such that the laser shock peened surface is at least a part of one of the edges extending radially along at least a portion of and chordwise from the edge.
ADVANTAGES
Among the advantages provided by the present invention is a cost efficient method to laser shock peen surfaces of portions of gas turbine engine parts and in particular blades designed to operate in high tensile and vibratory stress fields which can better withstand fatigue failure due to nicks and tears in the leading and trailing edges of the fan blade and have an increased life over conventionally constructed fan blades. Another advantage of the present invention is that fan and compressor blades can be constructed with cost efficient methods to provide commercially acceptable life spans without increasing thicknesses along the leading and trailing edges, as is conventionally done. The present invention can be advantageously used to refurbish existing fan and compres
General Electric Company
Gressel Gerry S.
Heinrich Samuel M.
Hess Andrew C.
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