LPB Provided Much Deeper Compression
Stress Profile of IN718
The Laser Shock Peening process introduces compression with minimal cold working using shock waves to yield the material. Laser peening utilizes high speed, high powered lasers to focus a short duration energy pulse on a coating, usually black tape, that is placed on the surface of the work piece to absorb the energy of the laser beam. A transparent layer, usually flowing water, covers the surface over the tape, and acts as a tamp to direct the shock wave energy into the surface of the material. When the laser is fired periodically, the laser beam passes through the water, explodes the tape, and creates a shock wave sufficient to deform the material to depths typically on the order of a millimeter. The shocking process is repeated in a computer-controlled pattern across the surface, creating a series of slight indentations and regions of residual compression.
Laser shock peening has been applied to a variety of alloys used in aircraft engines, airframes, and other engineering applications. Laser shock peening is now commercially available from at least two suppliers in the United States and is used in Europe. Although limited by cost, quality control and logistics, laser peening has been applied successfully to improve the damage tolerance of several critical compressor blade leading edges and has made major contributions to the field of surface enhancement.
What are the advantages of laser shock peening?
One advantage is that in laser peening, the depth of compression exceeding shot peening produced with low cold work. The subsurface compressive residual stress distribution tends to be maximum at the surface, diminishing nearly linearly with depth. The low cold work provides thermal and mechanical stability similar to LPB®, an advantage in hot section engine applications, or where there may be momentary overload due to impact, etc.
Are there any disadvantages to the laser shock peening process?
Disadvantages associated with laser peening include:
- High capital cost of the laser peening equipment, which may include a clean room environment and million dollar plus installations.
- Laser shock peening requires repeated coating with tape to produce the depth and magnitude of compression achievable in a single pass with LPB®.
- Because the explosive laser shocks remove the tape from adjacent treatment sites, two coating cycles are usually required to achieve one layer of treatment. Multiple layers are generally required to achieve the desired depth of compression.
- Process quality control has slowed the application of laser shock peening due to the difficulty of controlling the processing variables involved. Variation in the thickness and even turbulence of the water layer tamp, as well as debris in the air and on the mirrors affect the compression achieved, even when the laser power is precisely controlled. The laser shock peening process cannot provide the closed-loop control available with LPB®. Internal cracking caused by the superposition of shock waves from opposing faces of blades, or echoing waves from the opposite wall of thicker sections has been found to limit the application. The cost of laser shock peening is the highest of any of the surface enhancement methods, typically 10 to 100 times that of LPB® or shot peening.
For a first-hand look at how Low Plasticity Burnishing can outperform processes such as laser shock peening, contact us by phone at (800) 883-0851 or by email here.