Page 443 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 16.11 Specialized Formmg Processes 23

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FIGURE l6.50 (a) Schematic illustration of the magnetic-pulse-forming process used to
form a tube over a plug. lb) Aluminum tube collapsed over a hexagonal plug by the magnetic-
pulse-forming process.

tube over the inner piece. The higher the electrical conductivity of the workpiece,
the higher the magnetic forces. It is not necessary for the workpiece material to have
magnetic properties.
It has been shown that the basic advantages of this process is that the forma-
bility of the material is increased, dimensional accuracy is improved, and springback
and wrinkling are reduced. Magnetic coil design is an important consideration for
the success of the operation. Plat magnetic coils also can be made for use in opera-
tions such as embossing and shallow drawing of sheet metals. The process has been
found to be particularly effective for aluminum alloys. First used in the 1960s, elec-
tromagnetically assisted forming is now applied to (a) collapsing thin-walled tubes
over rods, cables, and plugs; (b) compression-crimp sealing of automotive oil filter
canisters; (c) specialized sheet-forming operations; (d) bulging and flaring opera-
tions; and (e) swaging end fittings onto torque tubes for the Boeing 777 aircraft.

Peen Forming. Peen forming is used to produce curvatures on thin sheet metals by
shot peening (see Section 342) one surface of the sheet. As a result, the surface of
the sheet is subjected to compressive stresses, which tend to expand the surface
layer. Because the material below the peened surface remains rigid, the surface ex-
pansion causes the sheet to develop a curvature. The process also induces compres-
sive surface residual stresses, which improve the fatigue strength of the sheet.
Peening is done with cast-iron or steel shot discharged either from a rotating
wheel or by an air blast from a nozzle. Peen forming is used by the aircraft industry to
generate smooth and complex curvatures on aircraft wing skins. Cast-steel shot about
2.5 mm in diameter, traveling at speeds of 60 m/s, have been used to form wing pan-
els 25 m long. For heavy sections, shot diameters as large as 6 mm may be used. The
peen-forming process also is used for straightening twisted or bent parts, including
out-of-round rings to make them round.

Laser Forming. This process involves the application of laser beams as a heat
source in specific regions of the sheet metal. The steep thermal gradients developed
through the thickness of the sheet produce thermal stresses, which are sufficiently
high to cause localized plastic deformation of the sheet. With this method, a sheet,
for example, can be bent permanently without using dies. In laser-assisted forming,