Using Finite Element Analysis to predict
temperature profiles in aluminum plate
By Craig Klingler, MS, PE, SECO/WARWICK Corp.,
USA
This article describes how finite element
software has been used to predict through-thickness temperature
profiles of water-quenched aluminum plates. Predictions of
surface reheating are correlated with theoretical
expectations. Using this information, both furnace design
and plate specifications can be adjusted prior to manufacturing
equipment.
Aluminum alloys are the material of choice
for a wide variety of engineering applications.
Aerospace applications in particular demand low weight,
high performance materials.
Certain alloys of aluminum with copper, zinc or other
additives can be heat treated akin to steel, but by aging at
elevated temperatures; these alloys soften after quenching,
aiding greatly in machining.
Aging then hardens the finished product.
Age hardening aluminum alloys are quenched
to produce supersaturated solutions in preparation for near net
shape machining. This
is typically accomplished in a water spray quenching enclosure
using moderately high pressure directed spray nozzles.
Since the solution annealing temperatures are typically
quite close to the temperature at which melting commences, the
rapid temperature change during quench can cause substantial
distortion. This
can limit the plate thickness for a given alloy in a particular
piece of quenching equipment.
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| Solution Heat Treat
Furnace Line |
Cross-Section of Plate
Quench Chamber |
Understanding and predicting the
temperature across the plate during quenching allows more
appropriate design of quenching equipment to avoid distortion.
Equipment utilizing two zones of quench intensity, rapid
and slow, is being developed to ensure adequate hardenability
with minimal distortion. To tailor this equipment to specific aluminum alloys and
sizes, a finite element model was developed to forecast cooling
times and examine the variation in quenching across the length
of the plate.
As plates progress through the quench
enclosure, the lower heat removal rate in the slow cooling zone
allows the plate surface to reheat.
The extent to which this reheating occurs is dependant on
the ratio of heat transfer coefficients in the quench enclosure,
the plate thickness and alloy thermal conductivity and the
length of the rapid quench zone.
This reheating shows how far the edge effects extend into
the plate, and also must not adversely affect the properties of
the alloy in question.
Modeling proceeded using ANSYS 5.7
MultiPhysics. Elements
of SOLID 70 were map meshed with edge sizes constrained to 1
inch (2.54 cm). The plate size
examined was 158" x 48" x 6" (401 x 122 x 15.25 cm) thick. Water bulk temperatures were assumed to be a constant 100°F
(38° C).
Initial plate temperature was applied as 1025° (552°
C).
Rapid quenching was applied on 1.4" ( 3.6 cm) progressive
sections, with the total width of rapid quench being 84" (213
cm), and
a time step of 0.7 seconds, simulating a 7'/minute(213 cm/minute) index
speed through the quench enclosure.
Results
Using a 1" (2.54 cm) square plate
6" (15.25 cm) thick
modeled in similar fashion to the full size, the centerline and
surface temperatures were calculated, Figures 1 and 2.
Maximum reheat surface temperature was 310°F (154° C).
Estimates of the reheating effect made by hand
calculation were 271 to 291°F (133 to 144°C).
Figure
3 shows a cutaway view as the leading edge of the plate passes
into the slow quench zone.
The reheating zone is observed on the plate surface a
short distance in from the edge.
The
indexed plate model produced the quench profiles at surface and
centerline in the approximate center of
the plate, Figure
4.
Shown also are the surface reheat temperatures at
distances from the leading edge of the plate, in Figure
5, using
a time step resolution of 36 seconds.
Maximum surface reheat temperatures are noted within 21
cm of the plate leading edge.
Centerline cooling curves agree well with typical
aluminum solution heat treating quench practice.
Temperature cooling curves in Figure 4 are typical for
surface and centerline across the plate, except for the ends,
where the additional surface area and temperature gradients
along the plate accelerate the cooling.
Conclusion
Temperature variation in relatively thick
aluminum plates during quenching is isolated near the plate
edges, provided the quenching sprays are uniform.
Interior regions experience cooling rates little
different than that expected for an infinite plate under similar
boundary conditions. The
use of analytical models to predict the cooling rates of the
aluminum plates provides reassurance that the material processed
in the furnace will meet or exceed the applicable material
standards.
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