Most people in the metalworking industries are familiar with shot-peening technology (not to be confused with the shot-blasting process) where the momentum of spherical media is used to induce residual compressive stress into metal surfaces. When manufacturing fatigue-critical components, it is common to perform shot peening after either the heat-treating or final-machining process.

In part locations that are susceptible to fatigue failure, the residual compressive stress from peening will offset the high tensile stresses that occur during service loading that could potentially initiate fatigue failure. Shot peening has been in use for over 50 years and there are process controls and industry standards guiding its implementation. The aerospace industry is currently one of the heaviest users of shot-peening technology.

Similar to shot peening decades ago, the aerospace industry was one of the first to implement laser peening. Preventing fatigue-crack initiation is of utmost concern on highly loaded components where failure can result in the loss of human life. Metal Improvement Company, LLC (MIC) is currently developing laser-peening applications for almost all industries currently served by shot peening.

Fig. 1. Shot peening compared to laser peening

Laser vs. Shot Peening

Laser peening is the newest peening technology and has been actively processing parts for industry for over four years. Laser peening also has its own SAE/AMS specification (AMS 2456) and has been ISO9001 and FAA certified. Figure 1 shows a visual comparison of shot peening and laser peening.

Shot peening uses the shot-stream energy (which consists of the shot mass and velocity) to impart a residual compressive stress into the surface of a metal part. Laser peening directs an intense beam of light to the critical surface. This creates high-pressure plasma that generates a shock wave, driving the compressive stress deep into the surface.

Fig. 1. Shot peening compared to laser peening

As shown in Figure 1, the laser-peening process has unique aspects when compared to shot peening. The first aspect is the surface to be peened is under a laminar flow of water. The water layer is commonly called a tamping layer. Its primary purpose is to act as an inertial stop when the high-pressure plasma is formed. The plasma is formed in nanoseconds and the mass of the water prevents it from expanding, thus driving the energy into the workpiece surface. The second aspect is the use of an ablative layer. Unlike a mask used in shot peening (to prevent surfaces from being hit with shot-peening dimples), an ablative layer is applied in the locations requiring laser peening. The ablative layer acts as a sacrificial layer, preventing a slight burning of the surface that would occur without it.

Shot peening is a random, spray-type process where the surface is showered with a stream of shot media. Laser peening is a CNC-controlled, single-spot process where relatively large spots are placed alongside each other with a slight overlap. Laser spot sizes are typically 3 x 3 mm up to 5 x 5 mm. Laser spots are typically applied at rates of 3-6 Hz depending on component application.

The primary differences on the workpiece from the peening processes are the depth of the residual compressive layer and amount of cold work. Figure 2 shows a comparison of peening-process depths.

Fig. 2. Residual compressive layer depths from peening processes

Benefits of Laser Peening

Laser peening typically achieves a residual-compressive layer that is four to five times deeper than shot peening. Many tensile-related failures benefit from a deeper compressive layer as a fatigue crack grows during each tensile load cycle. A deeper compressive layer offers more resistance to crack growth for a greater period of time than does a shallower layer.

A failure mode that benefits significantly from a deeper compressive layer is FOD (foreign object damage). FOD consists of surfaces that are damaged during operation, thus creating an initiation site for fatigue failure. If you assume a 0.010” surface flaw (after peening) in Figure 2, the compressive layer from shot peening has essentially been pierced through its depth. The laser-peening layer still offers approximately 100 ksi (at 0.010” depth) of residual compressive stress to resist crack initiation and growth.

When imparting compressive stress, it is generally desirable to impart as much compressive stress with as little cold work as possible. More cold work results in a more aggressive surface finish along with less desirable compressive surface stress. One of the drawbacks of shot peening is that when a deep compressive layer is desired, the user generally has to strike the surface with a large shot at high velocity, which increases the amount of cold work at the surface. With laser peening, there is no direct contact with the surface. There is a pressure wave front but no physical contact, which produces an excellent surface finish relative to the energy of the laser peening. It is not recommended to shot peen at a 0.010” Almen C intensity as large cold work is imparted to the material’s surface. Laser peening at 0.010” C on thick section components is common for MIC as the cold work is minimal and surface finish is quite good.

Fig. 3. Robotic spot pattern

Laser Peening Process

Metal Improvement Company’s laser-peening system uses a laser energy source that is integrated with a multi-axis robot. These systems communicate to perform highly repeatable and self-correcting laser-peen spot patterns in the areas of concern.

Figure 3 shows a part geometry with the spot pattern that has been optimized for that component. Controls are built into the system such that the energy output for each individual spot is digitally stored according to the part serial number. This allows for a highly traceable quality-control system.

When smaller components are processed, they are laser peened with a stationary-beam system. The robot picks up the component and with each shot of the laser, the part is indexed 3-5 mm (depending on spot size) to the next laser-spot location. A secondary robotic arm supplies the flow of water for the tamping layer.

Fig. 4. Moveable-beam system examples

When larger components are processed, they are laser peened with a moveable-beam system. This system uses hardware for tracking and alignment of a laser beam that moves with the robot. Essentially, a stationary laser beam is converted to a moveable laser beam using this configuration. Since the laser beam moves with the robot, moving a large part is not required. Figure 4 shows a moveable-beam system.

For components that are not practical for shipping, MIC has manufactured a transportable laser-peening system where laser peening can be performed in-situ. A semi-trailer has been custom manufactured to contain all necessary systems to run MIC’s laser-peening technology. The only hook-up required is an electrical supply. Robots are brought to the worksite and the laser energy source is supplied from the laser-peening system located inside the semi-trailer.

Fig. 5. Portable laser peening

Metal Improvement Company is currently approved and is laser peening commercial aerospace components. As of this publication, over 22,000 wide-chord fan blades have been laser peened for jet engines used on commercial, wide-body aircraft. In addition, over 500 fan hubs have been laser peened for use in commercial aerospace. These components are being processed on both stationary beam and moveable beam laser-peening systems.

Laser peening is not a replacement for traditional shot peening, which has solved fatigue failures for 50 years in a cost-effective manner. Laser peening fills an important void by serving fatigue applications that go beyond the current limits of shot peening. IH

For more information: Contact David Breuer, Regional Sales Director – Laser Peening, Metal Improvement Co. LLC, Metal Improvement Company, 10 Forest Avenue, Paramus, NJ 07652; tel: 201-843-7800; fax: 201-843-3460; e-mail: Dave_Breuer@metalimprovement.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: shot peen, shot blast, laser peen, residual compressive stress, fatigue failure