Fluid reaction surfaces (i.e. – impellers) – Specific blade structure – Coating – specific composition or characteristic
Reexamination Certificate
1999-11-12
2001-10-02
Verdier, Christopher (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Specific blade structure
Coating, specific composition or characteristic
C219S121850, C219S121680, C148S525000
Reexamination Certificate
active
06296448
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to methods of simultaneously laser shock peening opposite sides of an article using offset laser beams and to articles having simultaneously laser shock peened spots with offset centers on opposite sides of an article.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high power pulsed lasers to produce an intense shock wave at the surface of an article 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 shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No.: 5,756,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No.5,591,009 entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,531,570entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447 entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328 entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled “Laser Peening System and Method”. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'D, to create the beneficial compressive residual stresses.
The pressure pulse from the rapidly expanding plasma imparts a traveling shock wave into the component. This compressive shock wave caused by the laser pulse results in deep plastic compressive strains in the component. These plastic strains produce residual stresses consistent with the dynamic modules of the material. Dual sided simultaneous laser shock peening includes simultaneously striking both sides of an article by two laser beams in order to increase the compressive residual stress in the material. The laser beams are typically balanced in order to minimize material distortion. The initial compressive waves pass through the material from each of the sides and are reflected back from the interface of the two initial compressive waves. The reflected waves turn into a tension wave. The combined tensile stress of the reflected waves, when the reflected tension waves from the both sides meet at mid point in the same axial direction, can be greater than the strength that the material can handle and a crack can be initiated at the mid plane where the two shock waves meet.
Another characteristic of LSP that limits its engineering effectiveness is the formation of deleterious release waves that create tensile strains. The released waves may form spontaneously following the compressive front or may result from reflection at a surface with impedance mismatch such as at the outer surface of a component being laser shock peened. When multiple release waves are simultaneously propagating in a component, they may add in a manner termed superposition. This superposition of tensile waves may reduce the effectiveness of the beneficial compressive strains or may even cause tensile fracture within the component. This superposition of the two spatially concentric waves thus reduces the beneficial effects which may be measured by HCF testing.
Thus, it is highly desirable to have a process for and to produce an article that is simultaneously laser shock peened on two opposite sides and eliminate the mid-plane cracks by lowering the combined tensile stress of the reflected waves just below the tensile stress of the material. It is also highly desirable to be able to eliminate or reduce loss of HCF benefits or effectiveness of the beneficial compressive strains from laser shock peening caused by the superposition of tensile waves.
SUMMARY OF THE INVENTION
A method for laser shock peening an article includes aiming and then simultaneously firing first and second laser beams with sufficient power to vaporize material on longitudinally spaced apart first and second surface portions of the article to form first and second regions having deep compressive residual stresses extending into the article from the first and second laser shock peened surface portions, respectfully. In one embodiment, the first and second laser beams are aimed such that first and second centerlines of the first and second laser beams impinge the first and second surface portions at first and second laser beam center points through which pass parallel first and second axes that are substantially normal to the first and second surface portions at the first and second laser beam center points, respectfully, and such that the first and second axes that are offset. In a first more particular embodiment of the present invention, the first and second laser beams are aimed such that the first and second centerlines intersect and are angled with respect to each other. In a second more particular embodiment of the present invention, the first and second laser beams and the first and second centerlines are parallel and offset with respect to each other.
Another more particular embodiment of the present invention, the laser beams are aimed and fired in a manner to produce first and second patterns on the first and second surface portions of the article having overlapping adjacent rows of overlapping adjacent one of the first and second spots, respectively. The patterns are formed by continuously moving the article, while holding stationary and continuously firing the laser beams with repeatable pulses with relatively constant periods between the pulses, wherein the surface portions are laser shock peened using sets of sequences, and wherein each sequence includes continuously firing the laser beams on the surfaces such that on each of the surface portions adjacent ones of the laser shock peened spots are hit in different ones of the sequences in the sets. A more particular embodiment includes coating the surface portions with an ablative coating before and in between the sequences in the set.
In one more embodiment of the present invention, the article is a gas turbine engine airfoil and the first and second surface portions are on pressure and suction sides, respectively, of the airfoil along a leading edge of the airfoil.
The present invention includes a laser shock peened article having laser shock peened first and second surface portions with first and second regions having deep compressive residual stresses extending into the article from the first and second laser shock peened surface portions, respectfully, wherein the first and second surface portions com
Risbeck James D.
Suh Ui W.
General Electric Company
Gressel Gerry S.
Hess Andrew C.
Verdier Christopher
Woo Richard
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