shot peen forming process
Introduction to shot peen forming
Shot peen forming is a partially effective forming process derived from the shot peening surface treatment. There are two basic areas of peen forming, conventional peen forming and stress peen forming. The overall forming of the component is reached through a great number of statistically distributed shot impacts. The shot is usually made up of steel balls which are accelerated to a specific velocity and directed in a desired pattern. During impact of the shot, a certain amount of its kinetic energy is transformed into plastification of the component. When shot velocity is fairly low only a thin layer of the component's material is elongated resulting in a convex curvature. If shot velocity is increased, concave curvatures are produced. (see below)
(click for larger image)Due to the phenomenon shown above, shot peen forming allows for a greater accuracy in forming materials than traditional forming methods. All essential parameters of shot peen forming must be measured and controlled when peening is in progress. Parameters include:
- Shot Velocity
- Mass Flow
- Traverse Rate
- Mass per Area
- Coverag
The above must be closely monitored to achieve small tolerances given the variations and fluctuations in material characteristics and the shot peen forming process itself. When peening convex curved component parts, tolerances down to 0.2 mm are possible.
Conventional peen forming
In conventional shot peen forming, the material is simply secured with no pre-stressing or aid of any kind in the deformation. Basically, a pure compressive forming operation. The impact of the shot forms an axisymmetric zone of deformation around the center of the indentation. The deformations caused essentially depend on the component geometry. For example, length-width ratio, thickness, shape, direction of rolling, the shot size and velocity, and material properties. By adjusting certain shot peening variables to adjust for a convex or concave effect, a wide variety of shapes are possible. For example, line-by-line coverage of the component enables certain uni- or multi-axial curvatures to be generated. The extremely flexible shot peen forming process enables difficult geometries to be produced which would require high machine effort and energy consumption if processed by conventional methods.
Stress peen forming
As opposed to conventional methods, in stressed shot peen forming part of the component is pre-stressed, either by bending or stretching. The stressed area of the component is then shot peened. Combined with shot peening, the pre-stressing forces required to achieve the desired deformation are very low as opposed to bending or stretching alone. Many times a simple milled plate is all that is needed for a contouring tool.
(click for larger image)In conventional bending processes, the material must in part be bent well beyond the final profile due to spring back. Since this sets up tensile stresses on the convex side, damage to the material may result. For these reasons, simple bending is unsuitable for large and/or complicated parts. When shot peen forming is combined with a superimposed bending stress, the part is elastically pre-stressed in the desired direction of curvature. The cost for bending forms when combined with shot peening are much lower due to the much smaller stresses required than pure bending.
In components made from stretching material, such as stamping, the material must flow from the thickness into the length. Extreme forces, and sometimes other factors, are required to achieve complete plasticization through the component cross section and is necessary in a pure stretching process. In addition, tooling, such as dies, can greatly increase investment cost. In shot peen forming with elastic stretching pre-stresses, costs associated with tooling, equipment, and time can be lowered substantially. Formed sheet metal many times will want to take on a spherical shape. During stressed shot peen forming, the curvature radius decreases in the applied stressed direction and increases in the perpendicular direction.
Parts formed by shot peen forming exhibit increased resistance to flexural-bending fatigue. Another distinct advantage with shot peen forming, unlike most other forming methods is that all surface stresses generated are of a compressive nature. Although peen-formed pieces usually require shot peening on one side only, the final result causes both sides to have compressive stress. These compressive stresses serve to inhibit stress corrosion cracking and to improve fatigue resistance. Some work pieces should be shot peened all over prior to or after peen forming to further improve fatigue and stress corrosion characteristics. Parts which have been cold formed by other processes often undergo shot peening to overcome the harmful surface tensile stresses set up by these other forming processes.
The benefits of shot peen forming, either by conventional or stressed, in many cases, provide the best manufacturing solution for large and/or complex parts. For more information on shot peen forming, feel free to contact us at any time. For details on our specific capabilities, please visit the shot peen forming page.
