Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1998-08-14
2001-03-20
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C219S121850
Reexamination Certificate
active
06203633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high-powered pulsed lasers, in the shock processing of solid materials, and more particularly, a method for improving properties of solid materials by providing shock waves therein. 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
Older methods for the shock processing of solid materials typically involve the use of high-explosive materials in contact with the solid, or high-explosive materials or high-pressure gases to accelerate a plate that strikes a solid to produce shock waves therein. Such methods have several disadvantages. For example: (a) it is difficult and costly to shock process non-planar surfaces and complicated geometries, (b) storage and handling of the high-explosive materials and high-pressure gases pose a hazard, (c) the processes are difficult to automate and thus, fail to meet some industrial needs, and (d) high-explosive materials and high-pressure gases cannot be used in extreme environments such as high temperatures and high vacuum.
Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces. In shot peening, many small shots or beads are thrown at high-speed against the surface of a material. The shot or beads sometime escape from the treatment equipment and scatter in the surrounding area. Since the shot or beads might get into surrounding machinery and cause damage, shot peening usually cannot be used in a manufacturing line. Ordinarily, shot peening cannot be used on machined surfaces without a likelihood of damaging them. In addition, shot peening has problems maintaining consistency of treatment caused by inherent wear of the shot and the shot peening equipment.
Laser shock processing equipment, however, can be incorporated into manufacturing lines without damage to the surrounding equipment. Shock processing with coherent radiation has several advantages over what has been done previously. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on pre-selected surface areas and the operating mode is easily changed. This allows flexibility in the desired shocking pressure and careful control over the workpiece area to be shocked. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to shock the workpiece repetitively. This is desirable where it is possible to enhance material properties in a step-wise fashion.
Laser peening (here and after referred to as laser shock processing) utilizes two overlays: a transparent overlay (e.g. water) and an opaque layer, (e.g. an oil based or acrylic-based black paint). Processing is typically done with the workpiece at ambient or room temperature. During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque layer, e.g. black paint, causing a rapid vaporization of the paint surface and the generation of a high amplitude shock wave. The shock wave cold-works the surface of the part and creates compressive residual stresses which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part.
Solid materials subject to laser shock processing contain naturally occurring dislocations. These dislocations move through the matrix of the solid material when the solid material is subject to stresses such as bending or pounding. Laser shock processing introduces additional dislocations in the solid material which increase material strength and contribute to residual stress.
One problem with current methods of laser shock processing is that some solid materials, at room temperature, are too brittle to process. When laser processing of workpieces of these materials is done at ambient or room temperature, these material will crack or fracture. An example of a class of solid materials which are brittle at room temperatures, but whose ductility slowly increases with increasing temperatures, are many inter-metallic compounds. Therefore, these materials, as well as others, which may benefit from laser shock processing are prevented from being processed, because of their tendency to crack and break.
An additional problem with current methods of laser shock processing is the inability to modify the amount of compressive residual stresses previously introduced in a solid material by laser shock processing. Once the compressive residual stress is introduced in a solid material, the magnitude or amount of compressive residual stress cannot be altered via the current laser shock processing methods, particularly to reduce the magnitude, if desired.
SUMMARY OF THE INVENTION
The present invention is a method of varying the temperature of a solid material prior to, or subsequent to, laser shock processing. The present invention provides a method of increasing or decreasing the temperature of a solid material followed by laser shock processing. A separate method involves varying the temperature of the solid material subsequent to laser shock processing. In addition, the present invention includes a method of varying the temperature of a solid material both prior to laser shock processing as well as subsequent to laser shock processing. All of the methods of the present invention may be cycled or repeated to achieve the desired compressive residual stress or microstructural changes in the solid material. Furthermore, such methods may be used to create a gradient of stress, hardness, or other associated properties over the laser peened surface.
The invention, in one form thereof, is a method for altering the properties of a solid material by providing shock waves therein. The temperature of the solid material is changed. Laser peening introduces compressive residual stress in the solid material. In one particular embodiment, this process is repeated. In a separate embodiment, the temperature of the solid material is changed following a laser peening.
The invention, in another form thereof, is a method of altering the properties of a solid material by providing shockwaves therein. Laser peening introduces compressive residual stress in the solid material. The temperature of the solid material is changed following laser peening the solid material. In one particular embodiment, the temperature of the solid material is increased following laser peening. In an alternate embodiment, the temperature of the solid material is decreased following laser peening. In yet another embodiment, the process is repeated.
An advantage of the present invention is that the method allows solid material, which would otherwise not be suitable for laser shock processing at room temperature, to be laser shock processed. Some solid material, such as inter-metallic compounds, tend to be brittle at room temperature. Consequently, these metals are subject to cracking during room temperature laser shock processing. When these brittle metals are heated, they become more ductile and malleable. The malleability or ductility of the metal achieved by heating allows these metals to be laser shock processed without cracking.
Another advantage of the present invention is the ability to modify the amount of compressive residual stress introduced in a solid material when laser shock processed. The amount of residual stress introduced in a solid material may be enhanced by lowering the temperature of the solid material prior to laser shock processing. As a general rule, there is an increase in the compressive residual stress introduced in a solid material as the material strength increases. The material strength of a solid material can be increased by decreasing the temperature of that solid material. Since
Clauer Allan H.
Toller Steven M.
Combs Morillo Janelle
Knuth Randall J.
LSP Technologies Inc.
Wyszomierski George
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