Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

Fig. 3: Experimental setup for extrusion with subsequent electromagnetic compression
tween <strong>the</strong> workpiece and <strong>the</strong> tool coil during
<strong>the</strong> compression process can be neglected. For
example, with an exiting speed <strong>of</strong> 50 m/min,
and 100 μs for <strong>the</strong> electromagnetic forming
process, a relative movement between <strong>the</strong>
extrudate pr<strong>of</strong>ile and <strong>the</strong> tool coil <strong>of</strong> about
160 μm results, which meets <strong>the</strong> geometrical
tolerance field <strong>of</strong> macroscopic forming processes.
Therefore, <strong>the</strong> application <strong>of</strong> electromagnetic
compression subsequent to extrusion
is possible without a compensation <strong>of</strong> <strong>the</strong>
relative speed between <strong>the</strong> workpiece and <strong>the</strong>
tooling.
To integrate both processes, a tool coil for
compression was positioned behind <strong>the</strong> die
exit and coaxial to <strong>the</strong> extrudate in order to
reduce <strong>the</strong> workpiece cross section locally
(Fig. 1). Additionally, a counter die in <strong>the</strong> shape
<strong>of</strong> a mandrel can be mounted to <strong>the</strong> mandrel
<strong>of</strong> a porthole extrusion die, which extended
into <strong>the</strong> tool coil (Jäger et al., 2009). By this,
besides achieving a more defined geometry in
comparison to a free forming operation, an
increase <strong>of</strong> <strong>the</strong> geometrical complexity <strong>of</strong> locally
compressed areas can be achieved.
For processing open sections, like L-, T
and double-T-shaped pr<strong>of</strong>iles, a rolling pro-
cess was developed. By executing <strong>the</strong> subsequent
forming operation as a rolling process,
<strong>the</strong> continuous run out <strong>of</strong> <strong>the</strong> pr<strong>of</strong>ile can be
taken into account.
Fig. 4: Dimensions <strong>of</strong> <strong>the</strong> mandrel for electromagnetic forming
A special rolling stand was developed and
mounted behind <strong>the</strong> extrusion platen <strong>of</strong> an
extrusion press, allowing <strong>the</strong> continuous forming
<strong>of</strong> <strong>the</strong> outgoing extrusion pr<strong>of</strong>iles. The
rollers mesh toge<strong>the</strong>r with sinusoidal teeth,
creating a corrugated contour on <strong>the</strong> web <strong>of</strong>
a continuously fed I-beam-shaped pr<strong>of</strong>ile. The
contour <strong>of</strong> <strong>the</strong> forming dies stores <strong>the</strong> desired
geometry <strong>of</strong> <strong>the</strong> workpiece. As <strong>the</strong> exit speed <strong>of</strong>
<strong>the</strong> extrudate varies, compensation strategies
have to be provided. For this, <strong>the</strong> rolling stand
was swivel-mounted and <strong>the</strong> rotational speed
<strong>of</strong> <strong>the</strong> rollers was adapted permanently (Fig. 2).
Extrusion and electromagnetic forming
A test rig for evaluating <strong>the</strong> concept <strong>of</strong> <strong>the</strong>
integration <strong>of</strong> an electromagnetic compression
step subsequent to extrusion was built (Fig. 3).
A tool coil was positioned behind <strong>the</strong> die exit
in order to reduce <strong>the</strong> workpiece cross section
locally. To adapt <strong>the</strong> tool coil and <strong>the</strong> extrudate
geometry, a field shaper made out <strong>of</strong> a
CuCrZr-alloy, electrically insulated with a
polyimide foil, is used. Beyond shaping and
concentrating <strong>of</strong> <strong>the</strong> electromagnetic field, <strong>the</strong>
field shaper also prevents an overheating <strong>of</strong>
<strong>the</strong> tool coil by <strong>the</strong>rmal radiation <strong>of</strong> <strong>the</strong> processed
hot extrudate. To assure a uniform air
gap between <strong>the</strong> field shaper and <strong>the</strong> workpiece
guiding rollers made out <strong>of</strong> brass are ar-
EXTRUSION
ranged pairwise at <strong>the</strong> inlet and run-out side
<strong>of</strong> <strong>the</strong> tool coil. For heat treating <strong>of</strong> <strong>the</strong> extruded
and subsequently hot deformed workpieces
made <strong>of</strong> heat treatable alloys, an additional
quenching setup was positioned behind
<strong>the</strong> tool coil. Atomizing nozzles are located
around <strong>the</strong> press axis providing air atomized
water mist streams. To use <strong>the</strong> press heat <strong>of</strong>
extrusion for <strong>the</strong> subsequent <strong>the</strong>rmo-mechanical
processing steps efficiently, <strong>the</strong> whole
setup is designed compactly. From <strong>the</strong> die exit
to <strong>the</strong> quenching, <strong>the</strong> whole setup has a length
<strong>of</strong> less than one meter.
Experimental trials were performed using
a 250-ton direct extrusion press (Collin
PLA250t) and a pulse power generator (Maxwell-Magneform
series 7000). Solenoid coils
fabricated from copper and embedded in a
fibre-reinforced resin were used as tool coils
(Poynting GmbH). Using an EN AW-6082 <strong>aluminium</strong>
alloy, a porthole extrusion die and a
squared field shaper, a squared hollow pr<strong>of</strong>ile
(30 x 30 x 1.8 mm) was extruded with an exiting
speed <strong>of</strong> about 16 mm/s while compression
by electromagnetic forming was applied
periodically in intervals <strong>of</strong> approximately
10s. Semi-finished products with local bulges
arranged around <strong>the</strong> circumference <strong>of</strong> <strong>the</strong>
squared tube can be produced.
In order to expand <strong>the</strong> range <strong>of</strong> geometrical
shapes with cross-sections o<strong>the</strong>r than that
<strong>of</strong> <strong>the</strong> extruded <strong>aluminium</strong>, a mandrel can be
used to define <strong>the</strong> cross section geometry. A
squared mandrel for <strong>the</strong> processing <strong>of</strong> a round
tube was chosen and specially designed to prevent
force fit between <strong>the</strong> workpiece and <strong>the</strong>
die (Fig. 4). By using a squared field shaper,
with an aperture <strong>of</strong> 43.4 x 43.4 mm and a
length <strong>of</strong> 27 mm <strong>of</strong> <strong>the</strong> concentrator, in combination
with <strong>the</strong> extrusion <strong>of</strong> a round tube
(Ø 40 x 2 mm) fur<strong>the</strong>r trials were performed.
In this combination <strong>of</strong> field shaper, workpiece
and mandrel an adapted air gap and
pressure distribution along <strong>the</strong> workpiece
circumference can be achieved which helps
to prevent a force fit between <strong>the</strong> extrudate
and <strong>the</strong> mandrel. In order to character-
ize <strong>the</strong> forming results when a mandrel was
used as a form defining tool, an experimental
study was performed at charging energies
<strong>of</strong> 6.4 kJ, 8 kJ and 8.8 kJ. To measure <strong>the</strong>
high speed current pulse in <strong>the</strong> tool coil a
Rogowski coil was used. The best forming result/geometrical
accuracy could be achieved at
a charging energy <strong>of</strong> 8 kJ. For this a maximum
coil current amplitude <strong>of</strong> 62 kA and frequency
<strong>of</strong> <strong>the</strong> discharging current <strong>of</strong> 4.5 kHz were detected.
To prevent damage by overheating <strong>of</strong>
<strong>the</strong> tool coil and <strong>the</strong> electrical isolation, <strong>the</strong>
field shaper was cooled by compressed air
ALUMINIUM · EAC CONGRESS 2011 41