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Shot peen forming is a dieless process performed at
room temperature, whereby small round steel shot impact
the surface of the work piece. 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 shot peen forming process is ideal for forming large
panel shapes where the bend radii are reasonably large
and without abrupt changes in contour. Shot peen forming
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 in developing
curvatures than rolling, stretching or twisting of metal.
Saddle-back shapes also are achievable. Because it is
a dieless process, shot peen forming reduces material
allowance from trimming and eliminates costly development
and manufacturing time to fabricate hard dies. The shot
peen forming process also is flexible to design changes,
which may occur after initial design. Metal Improvement
Company can make curvature changes by adjusting the
shot peen forming process.
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 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 shot peen forming
to further improve fatigue and stress corrosion cracking
resistance.
Shot peening of parts that have been cold formed by
other processes overcomes the harmful surface tensile
stresses set up by these other forming processes.
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