Shot peening is one of the original surface modification technologies developed for improving fatigue life via induced compressive stress. The process of shot peening involves accelerating a hard media (metal, ceramics, etc.) toward a part surface at high velocities. Impacts of the media with the part surface induce plastic deformation and resulting compressive residual stresses. Shot peening continues to be a popular surface modification technology; however, the depths and magnitudes of residual stress benefits obtained with shot peening are greatly exceeded by laser peening.
Deep rolling is a surface burnishing process that creates a high degree of cold work. Resulting residual stresses are frequently deeper than shot peening. Like other burnishing processes, deep rolling requires sufficient access to an area and is limited on geometric areas that can be processed. The high degree of cold work also limits the process for elevated temperature applications. Limitations of the burnishing process make laser peening a better option for many applications.
Low Plasticity Burnishing
The low plasticity burnishing (LPB) process is similar to other burnishing processes in which a metal object in contact with the surface of a part is forcefully passed over the desired area. The end result is usually a surface compressive stress and finer surface finish than when the process was initiated. In LPB, the ball is maintained in a spherical socket with fluid supplied to the top of the ball. Fluid pumped to the socket applies the force and helps prevent the ball from contacting the socket directly. A fluid film allows the ball to roll across the surface of a part and helps to reduce the risk of the ball seizing in the socket.
LPB processing provides residual stress results capable of approaching laser peening. The process is constrained by geometry and may require substantial tooling to be implemented. The process is highly dependent on maintaining the fluid supply to the ball and can cause irrecoverable damage to parts if the fluid quality or quantity deviate from specification.
Ultrasonic peening / Ultrasonic Impact
Ultrasonic peening is a surface enhancement process that uses electro-mechanical methods to generate stress waves in a material. Stress waves are induced into the material by vibrating steel pins attached to a calibrated frequency controller. Most ultrasonic peening tools are handheld apparatuses and the technology has been heavily used in the welding industry.Advantages of the process include the ability to be highly transportable, leading to the ability to reach austere environments.
Disadvantages include a need for physical contact with a surface, limiting the process to open geometries and generally flat surfaces. The use of handheld tools also poses difficulties with repeatability and consistency. Ultrasonic treatments roughen the surface finish considerably. In comparison to laser peening, ultrasonic peening has been unable to reach similar compressive stress depths.
Water jet / Cavitation peening
Water jet peening and cavitation peening are terms used to refer to generally the same process. This type of peening uses a jet of high pressure water to impinge the part surface. Upon contact with the surface, the water droplets cavitate and induce stress into the part surface. The advantage of water jet peening over shot peening is due to the limited effect upon surface finish. The water jet peening system also requires only water as a media and has some environmental benefits. Disadvantages of water jet peening include the potential for erosion and cavitation damage, material loss and limited residual stress profiles.