Laser peening is a mechanical surface enhancement process rather than a thermal one. Using a high-energy pulsed laser beam, shock waves are generated that propagate through the target material and produce compressive residual stresses. The shockwave mechanics are responsible for inducing coldwork in the microstructure, which improves the durability and performance of the material.
The laser peening process requires three elements to be performed efficiently: laser beam, target material, confining media.
Laser Beam Characteristics
- Neodymium glass (Nd) laser (1054 nm wavelength)
- 8 - 40 ns laser pulse length
- 1 - 50 J energy per shot
- 1 - 6mm spot diameter
- 4 - 10 GW/cm^2 power density
The target for the laser pulse is typically an opaque overlay that is applied to the part surface. The opaque overlay fucntions as a sacrificial layer which absorbs the laser energy and protects the part surface from thermal effects. The performance of different opaque overlays is nominally the same, and they are selected primarily based on part geometry and processing conditions. The most common overlays include:
- RapidCoater (LSPT Proprietary technology)
Upon interacting with the target, the laser beam vaporizes a small amount of the target material. The vaporized material absorbs energy through the duration of the laser pulse, and is superheated into a rapidly expanding plasma. This plasma burst generates a pressure pulse at the surface of the material, which propagates into the part as a compressive stress wave.
The confining media acts as a tamping layer to confine the expanding plasma - amplifying the pressure pulse and subsequent stress wave. Without a confining media, the plasma formation quickly dissipates, releasing most of its energy into the free space around the target. With a confining layer surrounding the plasma burst, the stress wave is amplified and directed into the component surface, enhancing the effectiveness of the laser peening process. The confining media must be transparent to the laser beam, and conform readily to the part surface. Water is the most typical confining media used in industrial processing.
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