Electric heating – Metal heating – By arc
Reexamination Certificate
2000-05-02
2002-11-19
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
C072S053000
Reexamination Certificate
active
06483076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser shock processing operation, and, more particularly, to a method and apparatus for accurately and automatically tracking the position of a workpiece, such as an integrally bladed rotor, by dynamically adjusting the position of the workpiece in order to compensate for the presence of distortion or manufacturing variations.
2. Description of the Related Art
Laser shock processing, or laser shock peening, or laser peening, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser pulses directed onto the surface area of a workpiece. Laser shock processing is an effective method of increasing fatigue resistance in metals by treating fatigue critical regions. For a more thorough background in the prior history of laser shock processing, a reference can be made to U.S. Pat. Nos. 5,131,957 and 5,741,559, such patents are explicitly hereby incorporated by reference.
Laser shock processing, as understood in the art and used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a workpiece by producing an explosive force by instantaneous ablation or vaporization of a painted, coated, or un-coated surface. Laser peening has been utilized to create a compressively stressed layer in the subsurface of a workpiece, thereby considerably increasing the resistance of the workpiece to fatigue failure. Laser shock processing typically utilizes two overlays: a transparent overlay (usually water) and an opaque overlay, typically an oil-based, acrylic-based, or water-based paint or tape. Laser shock processing can also utilize only a transparent overlay or bare surface. During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque overlay or bare surface, causing vaporization of a portion of the opaque overlay or bare surface, which results in rapid plasma formation and the generation of a high amplitude shock wave. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece may be processed by producing a matrix of overlapping spots that cover the fatigue-critical zone of the part.
Laser shock processing is being used for many applications within gas turbine engines, such as leading and trailing edges of fan and compressor airfoils. These laser peening applications, as well as others, are in need of improved positioning techniques to reduce setup time and improve the quality and consistency of the processed part. The quality of laser peening depends, in part, upon the accurate and repeatable positioning of the laser beam on the part.
Current laser-beam-positioning methods for laser peening parts are accomplished by moving the workpiece to a definite, hard-coded position in space and then firing the laser. The index of this point in space for purposes of identifying the target area can be any convenient feature of the workpiece or part manipulator (reference point), e.g. a corner of the platform of an airfoil, not the coordinates of the point where the laser hits the part, i.e., target area. Consequently, as the reference point of the part (or subsequent similar parts) is moved to the same location, small deformations, distortion, and variations within dimensional tolerances in each individual part (specifically, at or near the target area) will change the exact point where the laser hits the part.
When laser peening thin sections, such gas turbine engine blades, it is usually desirable to use two-sided processing methods and maintain the symmetry of the shockwaves in order to most efficiently and effectively laser peen the part. Typically, matching the shockwaves generated on opposite sides of a thin section is accomplished by maintaining a substantially identical laser spot size and shape on opposite sides of the part for each laser pulse within the spot pattern being processed. If the angles at which the laser beams are presented to the part are maintained as congruent, and the reference point for the part (which can be many inches away from the target area where processing is to occur) is held as constant, small deformations due to previous processing or dissimilarities between parts can cause the laser to hit the part in an asymmetric manner causing improper processing.
Because of the compressive residual stresses imparted by laser peening, small distortions in the part can occur, especially in thin airfoils. Under circumstances where a first series of laser-peened spots causes a slight deformation in the part, the application of a second series of spots over substantially the same target area may result in the beams no longer being disposed substantially opposite one another, depending upon the amount of distortion and the angle on incidence of the laser beams. If the part-positioning program is hard-coded, the operator may never be aware that subsequent series of laser-peened spots were misaligned. These types of in-process misalignment problems can lead to significant variation in the quality and performance of the laser-peened parts, without the operator even realizing the source of the variability.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for processing a workpiece whereby the laser peening system (or associated hardware or software) automatically detects, and then automatically compensates for, deviations from the ideal positioning of a part. If all parts were identical, the part manipulator could be preprogrammed for the ideal part and each part would then be processed identically, using the same program. However, deviations from the ideal part occur.
By way of background, such deviations or departures from an ideal construction may stem from normal manufacturing tolerances, tolerance or repeatability problems associated with the part fixture that holds the part in the manipulator, distortions caused by earlier laser peening steps on the part, or any other effect that would cause the part to not be positioned in an ideal or pre-determined location relative the laser beam.
Dissimilarities can exist between the workpiece being processed and the template or test workpiece that was used to derive the part processing program. Due to a lack of exact reproducibility in the manufacturing process, the same manufacturing operation can produce a series of workpieces of the same type that have dimensional variations relative to the ideal workpiece, yet still be within manufacturing tolerances. The problem that arises relates to the fact that the part program for controlling the positional movements of the workpieces during laser shock processing is based upon the ideal part construction or a test part that was used during programming; accordingly, any dimensional or structural deviations can result in the laser beam impinging upon the production workpiece in a manner or place different from that contemplated in regard to the ideal workpiece.
Another issue that adversely affects the reliability and repeatability of laser peening arises from the fixturing of the part within the part manipulator. Slight misalignments that may occur during mounting of the part, which may be caused by normal tolerance problems or human errors, can lead to significant misalignment of the laser beam on the part.
Still another issue that adversely affects the reliability and repeatability of laser peening relates to distortion effects that can arise during laser peening because of the compressive residual stresses imparted by the process. Consequently, it is possible that the alignment for subsequent laser peening sequences on a part may be adversely affected by preceding laser-peening sequences. Even if the distortion of the part remains within manufacturing tolerances, the subsequent laser-peening sequences may be ineffectual or even deleterious, if processing continues after a misalignment ha
Dulaney Jeff L.
O'Loughlin Mark E.
Toller Steven M.
Dunn Tom
Johnson Jonathan
Knuth Randall J.
LSP Technologies, INC
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