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Shot peen
forming is a dieless process performed at room temperature,
whereby the surface of the workpiece is impacted by small,
round steel shot. Every piece of shot acts as a tiny peening
hammer, producing elastic stretching of the upper surface
and local plastic deformation that manifests itself as a residual
compressive stress. The combination of elastic stretching
and compressive stress generation causes the material to develop
a compound, convex curvature on the peened side.
The
process is ideal for forming large panel shapes where the
bend radii are reasonably large and without abrupt changes
in contour. It is best suited for forming curvatures where
radii are within the metal's elastic range. Although no dies
are required for shot peen forming, for severe forming applications,
stress peen fixtures are sometimes used. Shot peen forming
is effective on all metals, even honeycomb skins and ISO grid
panels.
Shot peen
forming is often more effective developing curvatures than
rolling, stretching or twisting of metal. Saddle-back shapes
are also possible. Because it is a dieless process, material
allowance from trimming is reduced and costly development
and manufacturing time to fabricate hard dies is eliminated.
The process is also flexible to design changes, which may
occur after initial design. By adjusting the process, curvature
changes can be made.
Parts
formed by shot peen forming exhibit increased resistance to
flexural bending fatigue. Unlike most other forming methods,
all surface stresses generated by shot peen forming are of
a compressive nature. Although shot 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 workpieces should be shot peened
all over prior to or after shot peen forming to further improve
fatigue and stress corrosion cracking resistance. Parts that
have been cold formed by other processes are often shot peened
to overcome the harmful surface tensile stresses set up by
these other forming processes.
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