Patent Application
20050194070
1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to methods and articles of manufacture employing laser shock peening a boundary area bordering a laser shock peened surface with a lower fluence oblique laser beam.
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 energy, about 50 joules or more, pulsed laser beams to produce an intense shockwave 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”. The use of low energy laser beams is disclosed in U.S. Pat. No. 5,932,120, entitled “Laser Shock Peening Using Low Energy Laser”, which issued Aug. 3, 1999 and is assigned to the present assignee of this patent. 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. 5,531,570 entitled “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 shock 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 shockwaves 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 shockwave into the component. This compressive shockwave initiated by the laser pulse results in deep plastic compressive strains in the component. These plastic strains produce residual stresses consistent with the dynamic modulus of the material. The many useful benefits of laser shock peened residual compressive stresses in engineered components have been well documented and patented, including the improvement on fatigue capability. These compressive residual stresses are balanced by the residual tensile stresses in the component. The added residual tensile stresses may locally lower fatigue capability of components and, thus, should be reduced and/or minimized. The laser shock peening is performed at selective locations on the component to solve a specific problem. The balancing tensile stresses usually occur at the edge of the laser shock peened area. Small narrow bands or lines of tensile stresses can build up immediately next to the laser shock peened patch or area along the edges of the patch. Extensive finite element analyses are done to determine where these tensile stresses will reside and the LSP patches are designed and dimensioned such that the tensile band(s) end up in an inert portion of the article or component (e.g. not at a high stress line in one of the flex, twist or other vibratory modes). It is desirable to reduce the level of these tensile stresses in the transition area between the laser shock peened and non-laser shock peened areas.
A method for laser shock peening an article including laser shock peening a first area with at least one high fluence normal laser beam at a first surface of the first area and laser shock peening a border area between the first area and a non-laser shock peened area of the article with at least one first low fluence oblique laser beam at the border area. In one particular embodiment of the method, the first low fluence oblique laser beam has a fluence of about 50% of the high fluence normal laser beam and the high fluence normal laser beam may have, for example, a fluence of about 200 J/cm2. In another more particular embodiment of the method, the first low fluence oblique laser beam is used to form only a single row of first low fluence laser shock peened spots in the border area.
Another embodiment of the method further includes laser shock peening a first portion of the border area bordering the first area with the first low fluence oblique laser beam and laser shock peening a second portion of the border area between the first area and the non-laser shock peened area with a second low fluence oblique laser beam wherein the second low fluence oblique laser beam has a lower fluence than the first low fluence oblique laser beam. In a more particular embodiment of the method, the first low fluence oblique laser beam has a fluence of about 50% of the high fluence normal laser beam. The second low fluence oblique laser beam may have a fluence of about 50% of the first low fluence oblique laser beam. The high fluence normal laser beam may have a fluence of about 200 J/cm2 in another more particular embodiment.
Another embodiment of the method further includes laser shock peening the border area with progressively lower fluence oblique laser beams starting with the one first low fluence oblique laser beam wherein the progressively lower fluence oblique laser beams are in order of greatest fluence to least fluence in a direction outwardly from the first area through the border area to the non-laser shock peened area. A more particular embodiment of the method further includes, forming high fluence laser shock peened spots in the first area, forming first low fluence laser shock peened spots in the border area, and operating the high fluence normal and low fluence oblique laser beams at the same power or energy level wherein the first low fluence laser shock peened spots are larger in area than the high fluence laser shock peened spots.
Illustrated in
It is well known to use laser shock peening to counter possible fatigue failure of portions of an article. Typically, one or both sides of the article such as the blade 8 are laser shock peened producing laser shock peened patches or surfaces 54 and pre-stressed regions 56 having deep compressive residual stresses imparted by a laser shock peening (LSP) method extending into the article from the laser shock peened surfaces 54.
The exemplary laser shock peened surfaces 54 illustrated in
In one exemplary embodiment of the method, the first low fluence oblique laser beam 24 has a fluence at the surface 54 of about 50% of the high fluence normal laser beam 16. One particularly useful fluence of the high fluence normal laser beam 16 is about 200 J/cm2. The laser beams may be of the same power and have the same fluence on a surface normal to them but by adjusting either the laser beam or the surface 54 such that the laser beam is oblique to the surface 54 of the article 10. An oblique beam produces an oval laser beam spot while a normal beam produces a circular laser beam spot. If both beams are of equal power, then the fluence across the oval laser beam spot is less than the fluence across the circular laser beam spot. Thus, the same beam may be used to laser shock peen the first area 14 with high fluence laser shock peening and the border area 20 with lower fluence boundary laser shock peening.
High fluence laser shock peened spots 30 formed in the first area 14 are illustrated in
The combination of the energy of the laser and the size of the laser beam provides an energy density or fluence that is usually up to about 200 J/cm2 for the high fluence normal laser beam 16 though somewhat lower fluences may be used. The high fluence laser shock peened spots 30 are illustrated as having a circular shape but may have other shapes such as oval or elliptical (see U.S. Pat. No. 6,541,733, entitled “Laser Shock Peening Integrally Bladed Rotor Blade Edges” by Mannava, et al., issued Apr. 1, 2003). The low fluence laser shock peened spots 31 are illustrated as having an oval shape but may have other shapes such as elliptical. The laser shock peened spots are typically formed in overlapping rows of overlapping spots. Overlaps of about 30% of the diameters between both spots in a row and between spots in adjacent rows is one particular design.
In the embodiment of the method illustrated in
The exemplary embodiments of the lower fluence boundary oblique laser shock peening method disclosed above have been described in terms of the high fluence normal laser beam 16 being used in the first area 14 of high fluence laser shock peening. Alternatively, a high oblique angle laser beam may be used which, though not normal to the surface 54, has a high fluence when compared to the low fluence oblique laser beam or beams 24. The same laser used to produce the high oblique angle laser beam can be used to produce the low fluence oblique laser beam or beams 24. The lower fluence oblique laser beams are angled at significantly smaller oblique angles as compared to a high oblique angle of the high oblique angle laser beam with respect to surface 54.
The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. While there have been described herein, what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.
Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims:
