Application of Gravity Accelerated Shot Peening at GE Aircraft Engines

Author:  Whalen, Jim
Source:  The Shot Peener Workshop 1992
Doc ID:  1992010
Year of Publication:  1992
Abstract:  
Introduction The shot peening process has been used throughout the automotive and aerospace industries for several decades to impart compressive residual stress into metal components to improve fatigue properties and inhibit stress corrosion cracking. The theory behind applying the peening process is fairly simple and straight forward. Hard spherical particles, typically steel balls or glass beads, are accelerated toward a workpiece. The impact of the hard particles against the softer workpiece causes the workpiece to plastically deform. This deformation produces an internal compressive residual stress which is beneficial to the workpiece peened. The typical peening machine utilizes compressed air to accelerate the peening media toward the workpiece. There are three basic types of pneumatic peening machines; pressure pots, gravity suction, and suction. The difference between these is the method of shot delivery to the blast stream. In the gravity suction and suction shot delivery systems, the shot and the compressed air are fed to the blast nozzle separately, where they are mixed and accelerated. Pressure pot systems add the shot to the compressed air line upstream of the blast nozzle. The shot/compressed air mixture travels to the blast nozzle where it reaches its final velocity and exits to impact the workpiece. No matter which shot delivery system is utilized, the operation of the pneumatic equipment is the same; momemtum is transferred from the compressed air to the media accelerating the media toward the workpiece. In all of the pneumatic peening systems, the blast nozzle positioning is done in several different ways. The most sophisticated and repeatable method is to use NC robots to manipulate either single or multiple nozzles around the workpiece. A less sophisticated but fairly repeatable method is to use nozzle holding fixtures and simple oscillation to move the nozzle fixture over the workpiece. Finally, the least repeatable but most widely used method is to manually position the nozzles around the workpiece. The typical media diameter for the pneumatic peening process is between 0.003" to 0.040". This range of media sizes is used since they are massive enough to plastically deform the part and small enough to provide peening coverage in the shortest period of time. Although the peening process is straight forward for pneumatic equipment, there are a number of significant process parameters which must be monitored and controlled. Listed below are these significant parameters. Media Size Air Pressure Shot Flow Rate Nozzle Diameter Air Jet Size Nozzle Position Nozzle Angle Peen Time The majority of peening machines used in the automotive and aerospace industries are pneumatic equipment. These machines are very versatile and peen a wide range of applications from small gears to large gas turbine rotor disks. However, even though pneumatic peening is widely utilized and some very sophisticated NC peening equipment exists, process control is still difficult and requires a lot of attention. There are many key process parameters in pneumatic peening which must be closely monitored and controlled to assure a consistent peening process. Gravity Accelerated Peening Description: The GASP process overall is a much simpler process than the pneumatic peening process. The equipment is simpler and the process of accelerating the peening media is much more consistent and repeatable. Therefore, there are less key process parameters to control and monitor. Listed below are the GASP key process parameters: Media Size Drop Height Peen Time Part Location/Orientation