Electric heating – Metal heating – By arc
Reexamination Certificate
1998-08-13
2001-03-06
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121610, C219S121740
Reexamination Certificate
active
06198069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to laser shock processing, and more specifically, it relates to techniques for relaying the uniform intensity near-field of a laser beam to a work piece such that the intensity profile at the work piece remains uniform and is independent of the effects of phase aberrations and optical diffraction. The invention further relates to phase conjugation as a technique for pulse tailoring to enable the generation of high intensity shocks.
2. Description of Related Art
Using high power lasers to improve material properties is one of the most important industrial applications of lasers. Lasers can transmit controllable beams of high energy radiation for metalworking. Primarily, the laser can generate a high power density that is localized and controllable over a small area. This allows for cost effective and efficient energy utilization, minimizes distortions in surrounding areas, and simplifies material handling. Since the laser pulse involves the application of high power in short time intervals, the process is adaptable to high speed manufacturing. The fact that the beam can be controlled allows parts having complex shapes to be processed. Also accuracy, consistency, and repeatability are inherent to the system.
Improving the strength of metals by cold working undoubtedly was discovered early in civilization, as ancient man hammered out his weapons and tools. Since the 1950s shot peening has been used as a means to improve the fatigue properties of metals. Another method of shock processing involves the use of high explosive materials in contact with the metal surface.
The use of high intensity laser outputs for the generation of mechanical shock waves to treat the surfaces of metals has been well known since the 1970s. The laser shock process can be used to generate compressive stresses in the metal surfaces adding strength and resistance to corrosive failure.
Lasers with pulse outputs of 20 to 100 J and pulse durations of 20 to 100 ns are useful for generating inertially confined plasmas on the surfaces of metals. These plasmas create pressures in the range of 10,000 to 100,000 atmospheres and the resulting shocks pressure can exceed the elastic limit of the metal and thus compressively stress a surface layer as deep or deeper than 1 mm in the metals. Lasers are now becoming available with average power output meaningful for use of the technique at a rate appropriate for industrial production.
In the process of laser shock processing, a metal surface to be treated is painted or otherwise made “black.” The black layer acts as an absorber of the laser energy and protects the surface of the part from laser ablation and from melting due to the high temperature of the plasma. A thin layer of water, typically 1 to 2 mm, is flowed over this black surface. The water acts to inertially confine or, as it is called, tamp the plasma generated as the laser energy is absorbed in the short time pulse duration, typically 30 ns. Other suitable materials which act as a tamper are also possible. A limitation to the usefulness of the process is the ability to deliver the laser energy to the metal surface in a spatially uniform beam. If not uniform, the highest intensity area of the light can cause a breakdown in the water which blocks delivery of meaningful energy to the painted metal surface. A conventional technique to deliver bring the laser light to the surface is to use a simple lens to condense the laser output to a power density of roughly 100 J to 200 J per square centimeter. This condensing technique has the limitation that a true “image” of the laser near-field intensity profile is not obtained at the surface. Rather a field intensity representing something between the near and far fields is generated. Diffraction of the laser beam as it is focused down onto the surface results in very strong spatial modulation and hot spots.
Any phase aberrations generated within the beam, especially those associated with operation of the laser for high average power, can propagate to generate higher intensity areas within the beam. These high peak intensity regions cause breakdown in the water layer, preventing efficient delivery of the laser energy to the surface to be treated. Another potential cause of breakdown in the tamping material is the generation of non-linear effects such as optical breakdown and stimulated scattering. In a normal generation of a 10 ns to 100 ns pulse within a laser, the output slowly builds over a time period exceeding several pulsewidths. This slow, weak intensity helps to seed the non-linear processes which require buildup times of 10 s of nanoseconds. In conventional techniques, the pulse output of the laser is “sliced” by an external means such as a fast rising electro-optical switch or by an exploding foil. These techniques can be expensive and can limit reliability.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide improved techniques for laser shock processing.
A second object of this invention is the control of the stress generating process.
A third object of this invention is to provide pulse tailoring techniques for laser shock processing.
Another object of the invention is to relay an appropriately scaled image of the near field of the laser to the processing surface.
An object of the invention is to use a diffractive optic to tailor the phase of the near field of the beam and with a simple focusing lens (or lenses) generate a spot with tailored spatial shape and intensity profile suitable for specific applications of laser peening.
The invention is a method for formatting the laser spatial shape and for effectively and efficiently delivering the energy to a work surface in the laser shock process. An appropriately formatted pulse helps to eliminate breakdown and generate uniform shocks. The invention uses a high power laser technology capable of meeting the laser requirements for a high throughput process, that is, a laser which can treat many square centimeters of surface area per second. The shock process has a broad range of applications, especially in the aerospace industry, where treating parts to reduce or eliminate corrosion failure is very important. The invention may be used for treating metal components to improve strength and corrosion resistance. The invention has a broad range of applications for parts that are currently shot peened and/or require peening by means other than shot peening. Major applications for the invention are in the automotive and aerospace industries for components such as turbine blades, compressor components, gears, etc.
REFERENCES:
patent: 4734911 (1988-03-01), Bruesselbach
patent: 5103073 (1992-04-01), Danilov et al.
patent: 5127019 (1992-06-01), Epstein et al.
patent: 5239408 (1993-08-01), Hackel et al.
patent: 5880873 (1999-03-01), Dane et al.
patent: 0085278 (1983-09-01), None
Dane, et al., “Laser Peening of Metals—Enabling Laser Technology,” Lawrence Livermore National Laboratory Preprint UCRL-JC-129029, Nov. 1997.
Dane C. Brent
Hackel Lloyd
Heinrich Samuel M.
The Regents of the University of California
Thompson Alan H.
Wooldridge John P.
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