Electric heating – Metal heating – By arc
Reexamination Certificate
2001-08-31
2003-05-27
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121850
Reexamination Certificate
active
06570125
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 at oblique angles to the surfaces and to articles having simultaneously laser shock peened spots with offset centers on opposite sides of an article.
2. Background 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. 5,531,570 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 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.
U.S. Pat. No. 5,492,447 discloses laser shock peening an interior annular region by orbiting a laser beam at an oblique angle to the interior surface of the interior annular region. U.S. Pat. No. 5,911,890 teaches controlling the incident angle of the laser beam applied to the workpiece and controlling the shape of the beam with lenses, polarizers, and particular transparent overlay geometries. The apparatus and methods disclosed includes use of structure for controlling the position and incident angle of the laser beam and controlling the polarization and/or the shape of the incident impact area, based on such incident angle. The patent teaches that an oblique incident angle laser beam having a circular cross-section causes the shape of the impact spot to be elliptical and that the consequences of such a change of the incident spot shape necessarily changes the energy density applied to the workpiece. The patent further teaches that the energy density per unit area compared to other areas on the same surface creates a possibility of non-uniformly working the material, thereby, possibly losing some of the benefits of laser shock peening. This non-uniformity of energy application to a workpiece may cause severe problems, particularly, when hitting a workpiece from opposite sides at the same time, as used with a split beam laser system. Such opposing hits are sometimes needed on workpieces of thin cross-section, such as disks, blades, and other workpieces of different geometries. In conventional split beam processing, there is a possible effect of not having the laser processed portions on the opposite sides of the workpiece worked identically, and at the same time when elliptical spots are utilized. Such non-uniform working of the workpiece may cause over or under working of the material or distortion of the workpieces, thereby, not achieving the goals of laser shock processing. Furthermore, based upon the oblique angle along with the particular transparent overlay material utilized, polarization issues regarding the reflection of the laser beam from the surface of the transparent overlay layer can possibly degrade and reduce the energy applied to the workpiece. Thus, in general, the patent concludes not to use elliptical laser spots but rather a more complicated apparatus and method to significantly reduce the non-uniformity of the applied energy to a workpiece by modifying the shape of the applied laser energy pulse. The patent teaches to pass the pulse of energy through a lens to reform the shape of the incident area on the workpiece, to counteract geometric effects created by the workpiece surface orientation on the incident area shape. Oblique laser beams are very useful for laser shock peening blades edges of an integrally formed bladed rotor sections of what is often referred to as an integrally bladed rotor (IBR) or an integrally bladed disk (BLISK) having two or more spaced apart rows of blades integrally mounted or formed on a drum rotor or disk.
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 using oblique laser beams and eliminate the mid-plane cracks by lowering the combined tensile stress of the reflected waves just below the maximum or allowable tensi
Mannava Seetharamaiah
Rockstroh Todd Jay
Suh Ui Won
Rosen Steven J.
VG Ramaswamy
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