Electric heating – Metal heating – By arc
Reexamination Certificate
1999-05-07
2003-05-20
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121850
Reexamination Certificate
active
06566629
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods for improving properties of solid materials by providing shock waves therein where the laser beam impacts the solid material on a hidden surface. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Known methods for shock processing of solid materials, particularly laser shock processing solid materials, typically using coherent energy as from a laser, orient the laser beam normal, i.e., perpendicular to the workpiece.
When particular constraints of processing are created, based on the shape of the material or other geometric factors such as when attempting to laser shock harden integrally bladed rotors (IBR's), blind bores, slots, or dovetail sections, the laser beam may not have a direct, line of sight access to the area to be shock processed.
Laser shock processing techniques and equipment can be found in the U.S. Pat. No. 5,131,957 to Epstein, along with that of U.S. patent application Ser. No. 08/547,012 entitled LASER PEENING PROCESS AND APPARATUS, assigned to the assignee of the present invention and hereby incorporated by reference.
Known laser shock processing systems tend to form a relatively small, in cross sectional area, laser beam impacting on the surface of the workpiece. This is because a sufficient laser energy must be applied over a particular area to sufficiently work a surface of the workpiece. The smaller the area with the same amount of energy leads to a greater energy per unit area application. The more energy per unit area applied, the deeper the residual compressive stresses are applied to the workpiece.
Laser shock processing of hidden surfaces would benefit particular types and areas of workpieces if such could be accomplished.
One disadvantage in the art of laser shock peening is the failure to teach laser shock processing of hidden surfaces. The current state of the art of laser shock processing contains limitations which prevent effective hidden surface laser shock peening. One limitation which has prevented effective laser shock peening of hidden surfaces is that a pulse of coherent energy may damage a reflective member used to direct a pulse of coherent energy to a hidden surface. The energy density of a pulse of coherent energy necessary to effectively process a workpiece in order to impart deep compressive residual stresses will normally exceed the damage tolerances of a reflective member. This is because the reflective member must necessarily be placed near the target surface due to space constraints near a hidden surface. As a result, prior to this invention, attempting to use a reflective member to redirect a beam of coherent energy to a hidden surface for laser shock peening resulted in damage or destruction of the reflective member due to the converging beam's decreasing cross-section as it nears the workpiece surface.
A second limitation in the art of laser shock peening which prevents hidden surface laser shock peening is that a non-uniform energy density may be applied to the hidden surface of a workpiece. This is especially an issue when the hidden surface is contoured. For example, the hidden surface could be contoured such that part of the workpiece surface is closer to the last focusing optic of a laser source. The energy density of a beam of coherent energy varies as the distance from the last laser focusing optic increases. This is because the laser beam is converging after the last focusing optic. The shape and design of the last focusing optic affect how the laser beam energy density will vary as a function of distance from the last laser focusing optic. Using current optics in the art of laser shock processing results in the portion of the workpiece contour surface which is closer to the last focusing optic, to be impacted with a pulse of coherent energy having a different energy density than the contour section which is further away from the last laser focusing optic. Consequently, the energy density applied to a hidden surface would be non-uniform.
Another limitation in the art which prevents effective laser shock peening of hidden workpiece surfaces is inconsistently imparted compressive residual stresses over the area processed. For example, when the energy applied to a workpiece surface varies in energy density, the result is non-uniform or inconsistent compressive residual stresses imparted to the workpiece. A consequence of non-uniform compressive residual stresses imparted in a workpiece is the need to reprocess the workpiece with more overlapping laser shock processing spots. As a consequence of either having to reprocess the same spot or providing more overlap of spots, there is an increase in processing time and cost. Moreover, the result of applying a non-uniform energy density across the spot to be processed results in creation of an unpredictable compressive residual stress profile. In addition, non-uniform energy density application to a workpiece results in a less effective and less efficient use of the pulse of energy.
What is needed in the art is a way to modify the laser beam to consistently and uniformly work hidden areas of the workpiece.
SUMMARY OF THE INVENTION
The present invention provides a method of laser shock peening that can be used in a production environment to apply laser shock processing treatment to hidden surfaces once thought not applicable for treatment.
The present invention includes use of a reflective member inserted into a recess of the workpiece. The reflective member is created to reflect an inbound laser beam to the hidden surface within the workpiece. Different geometries and forms of the reflective member are given, some dependent on the shape of the recess.
The term recess as used in this application is that of an opening, port, hole, channel, or other space within the workpiece. The term hidden surface as used in this application is an interior surface of the workpiece, not normally available for direct line-of-sight laser processing. Typical recesses with hidden surfaces include, the interior surfaces that define holes and blind bores, the interior roof of dovetail slots as can be found in aircraft gas turbine disks, and other similar openings and ports in workpieces.
The invention, in one form thereof, is a laser peening method for processing a hidden surface of a workpiece. The hidden surface is disposed within a recess having an opening. The laser peening method comprises the steps of inserting a reflective member into the recess. Means are provided for preventing damage to the reflective member. A pulse of coherent energy is directed to reflect off of the reflective member and impact the hidden surface of the workpiece to create a shock wave.
In one embodiment, the reflective member is a focusing mirror. In an alternate embodiment, the means for preventing damage to the reflective member comprises providing a gap between a transparent overlay applied to the workpiece and the reflective member. In an alternate embodiment, means for preventing damage to the reflective member comprises providing the pulse of coherent energy at a minimum energy density when reaching the reflective member and an operational energy density when the pulse impacts the workpiece.
In one specific further embodiment, the reflective member is a flow of fluid flowing along the length of a recess having an opening, such as a dovetail shape. The laser processing beam would be incident on the flow of fluid and would be reflected to the workpiece hidden surface by refraction and total internal reflection in the fluid flow.
The invention, in another form thereof, is a laser peening method for processing a hidden surface of a workpiece. The hidden surface is disposed within a recess having an opening. The laser peening method comprises the steps of
Clauer Allan H.
Dulaney Jeffrey L.
Toller Steven M.
Walters Craig T.
Evans Geoffrey S.
Knuth Randall J.
LSP Technologies Inc.
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