Laser and Shot Peening – What is it and what does it accomplish?
David Breuer - Metal Improvement Co., Inc.The first portion of David's presentation is essentially a repeat of his Oct. 2003 presentation to the Indianapolis Chapter ASM. on shot peening. It will not be repeated here.
This year the presentation continued into the subject of Laser Peening.
In the past year Laser Peening has become a production process. Metal Inprovement Co. has two production facilities for the process, one located in Livermore, CA, and the other in the United Kingdom.
Laser peening is similar in some benefits to shot peening, but a "shock wave" set up by reaction of a surface coating, and water layer to a high energy focussed laser pulse is the source of the surface deformation. As practiced by Metal Improvement, the coating may either be a type of paint or tape. This cuts down greatly of the reflective losses of most metal surfaces to laser light. At the time the laser strikes, a uniform layer of water is applied to the outside of the coating, which intensifies the impact of the energy to the surface by setting up the shock wave. Both of these components are critical to the formation of a uniform crater which gives the same residual compressive to the surface and to an extended depth below the surface, which is critical to reducing the onset of fatique cracks during the lifetime of the treated part. A major benefit of the laser peening process is that the depth (below the surface) of the material zone under compression is up to five times greater than can be achived with shot peening. Residual compression can be found to depths of .5 mm (0.020") and beyond.
Laser peening is applied one crater at a time, so it is a relative slow process and many times more expensive than shot peening. Because of this limitation, laser peening is generally reserved for the most critical of fatigue problems, and it is only applied to the most critical sites on a part (unlike shot peening which is generally applied to entire parts, unless masked where the peening in not desired). As practiced by Metal Improvement the size of each laser generated crater is a 4mm x 4mm square on the part surface, and these are layed down in continuous rows and columns over the fatigue critical area at a rate of about 5 spots / sec. (by contrast, shot peening processes produces thousands of craters per sec.) The square spot of Metal Improvement has and advantage over some competitive designs, in that is is relative easy to ensure complete coverage of an area by having the straight spot edges touch each other.
The actual process is currently totally under computer control. A CAD model of the part is used to develop robotic motions to manipulate the exact portions of the part to be peened into the stationary path of the laser indexing by 4 mm for each strike, while another robot is manipulated to supply the required stream of water to the target spot. The exact location and energy associated with each spot is measured and recorded by the computer as the program executes. If an individual 4mm x 4mm spot failed to reach the desired energy, it may be re-shot using the data which is available.
Currently most laser peening is applied to millitary aircraft applications in critical notch sensitive areas to increase the performance (maximize permitted service stresses) of the part before the onset of fatigue cracking.
Currently Metal Improvement is working on two development projects for the laser peening process. They are working on a "moving beam" process (one which would not require the part to be so extensively and precisely manipulated by robots), and also a "portable system" (to enable treatment of parts which are impossible to bring to the production facilities.
An excellent book "Shot Peening Applications" is available from Metal Improvement Company, Inc. in soft cover or CD-ROM. The e-mail is: METALIMP@ix.netcom.com and web site is www.metalimprovement.com.