Rolled products, i.e. sheet, plate and foil
constitute almost 50 % of all aluminum alloys used, (see Figure 1.0). In
Africa the packaging industry consumes the majority of the sheet and foil for
making beverage cans, foil containers and foil wrapping. Sheet is also used
extensively in building for roofing and siding, in transport for airframes,
road and rail vehicles, in marine applications, including offshore platforms,
and superstructures and hulls of boats. Also, while relatively little is
currently used in the manufacture of high volume production automobiles, it is
expected that the next decade will see sheet used for space frames and body
panels, a market that could easily match the 2 million tons now used for
beverage cans. Plate is used for airframes, military vehicles and bridges,
ships superstructures, cryogenic and chemical vessels and as tooling plate for
the production of plastic products. Foil applications outside packaging include
electrical equipment, insulation for buildings, lithographic plate and foil for
heat exchangers (this lecture deals only with sheet and plate).
This wide range of products demands combinations of
properties that span the whole range available from the aluminum alloy, i.e.
high strength, good corrosion resistance, good formability, good machinability,
high toughness, good ballistic performance etc. (Table 1.0) Also since
in many cases the materials with which the aluminum alloys compete are
relatively low cost, for example tin-plate, paper, wood, mild steel and
plastics, it is essential that the cost of the aluminum products are
themselves as low as possible, consistent with the achievement of the required
properties. Since the cost of smelting aluminum is unlikely to be further
reduced to any degree it is, therefore, essential that semi-fabrication costs
are kept to a minimum.
Figure 1.0
|
K TONNES
|
CANS
|
2013
|
OTHER PACKAGING
|
361
|
ROAD VEHICLES
|
298
|
RESIDENTIAL SIDING
|
190
|
OTHER BUILDING
|
155
|
AIR CONDITIONERS/ APPLIANCES
|
140
|
HOUSEHOLD & FOILS
|
133
|
AIRCRAFT PLANE
|
110
|
AIRCRAFT SHEET
|
96
|
COOKING UTENSILS
|
50
|
LITHOGRAHIC SHEET
|
50
|
MOBILE HOMES
|
36
|
All aluminum alloys can be rolled to sheet but, with
a few notable exceptions mentioned below, the ones utilized are from the 1000,
3000 and 5000 series which are work hardening alloys However, the 2000, 7000
and 8000 heat treatable alloys are used for airframes, 2000 and 6000 series for
automobiles and the 6000 series for some pressure vessels and containers.
Examples exist for the use of plate in all alloys while foil is almost all from
the 1000 series. Table2.0 shows some typical alloys used for specific
end-use applications. (As indicated in other lectures, while there are some 300
different wrought aluminum alloys, probably 80 %of the applications are covered
by perhaps 30 alloys).
Figure 2.0
REQUIRED CHARACTERISTICS
|
ALLOYING ELEMENT
|
PRODUCTS
|
LOWER MELTING POINT
|
Si
|
BRAZING SHEET, FOIL
|
INCREASED CONDUCTIVITY
|
B
|
CONDUCTOR STRIP
|
INCREASED ELASTIC MODULUS
|
Li
|
AEROSPACE SHEET
|
DECREASED DENSITY
|
Li
|
AEROSPACE SHEET
|
STRESS CORROSION RESISTANCE
|
Cr, Zr, Ag
|
AIRCRAFT SHEET
|
SACRIFICIAL CORROSION
|
Zn
|
HEAT EXHANGERS CLAD PRODUCTS
|
VACUUM BRAZING RESPONSE
|
Mg
|
HEAT EXCHANGER
|
RESPONSE TO CHEMICAL OR ELECTROCHEMICAL TREATMENT
|
Si, Cu, Cr
|
DECORATIVE APPLICATIONS
|
The DC ingot is usually cooled after casting to room
temperature and then re-heated to around 500 °C prior to successive passes
through a hot rolling mill where it is reduced in thickness to about 4 - 6 mm.
The temperature of pre-heat of the ingot and the time held at that temperature
is important for some alloys since a process of homogenization takes place
which renders the material in the best condition for rolling and the
achievement of subsequent properties. The strip from the hot rolling mill is
coiled for transport to the cold mill which might be on the same site or
elsewhere. Cold mills, in a wide range of types and sizes are available; some
are single stand, others 3 stand and some 5 stand. Cold rolling speeds vary but
modern mills operate at exit speeds as high as 3000 m per minute. A modern
complex including melting furnaces, DC casting facilities, pre-heat furnaces,
hot mill, cold mill and annealing furnaces involves a very large capital
investment which can only be fully justified on financial grounds if a big
market is assured. Obviously, if such facilities are not fully utilized the
economic viability is threatened. As indicated above, however, hot mill coil
can be obtained for further reduction in cold mills of lower capacity/speed/
complexity. Diagram 1.0
Although
most sheet is produced by conventional hot mills, some considerable effort has
been made by aluminum producers in the United States, Canada and Europe to reduce
both the capital and production costs by the development of continuous strip casting
methods whereby hot metal is poured into some form of strip caster, thus
eliminating the DC casting and hot break down mills. To date, however, only a
limited range of alloy compositions can be produced in this way with end uses
found mainly in foil and building sheet. Final sheet properties for the work
hardening alloys in the 1000, 3000 and 5000 series are achieved either by
temper rolling or temper annealing. In the former case the strip is reduced by
a fixed amount of cold work to achieve the required temper, in the latter it is
rolled to a hard temper and then back annealed to achieve the required
strength. While a given set of mechanical properties, e.g. proof stress,
tensile strength and elongation can be achieved by both routes, other
characteristics, particularly formability, can be influenced by the particular
production route chosen. When 2000, 6000 and 8000 sheet is needed the cold mill
strip is sometimes continuously heat treated to achieve optimum economics, but
the cost of such equipment is high and when relatively small volumes are needed
the sheet is either heat treated individually or in coils. Continuous heat treatment
lines include tension levelers which ensure that the distortion created by water
quenching is removed. For individual sheets stretching is necessary to achieve
the required flatness.
When sheet is rolled its final flatness depends on a
number of characteristics of the starting stock and of mill features. If bad
shape results during rolling i.e. some parts of the strip are longer than
others, buckling occurs and this, in many cases, can be removed by tension
leveling whereby the strip is stretched sufficiently so that short parts are extended
to the point where they have the same length of the long parts and the out of shape
disappears (Diagram 1.1). However, attention to the cross section shapes
of the in-going product, i.e. DC slab or hot mill coil, can greatly improve the
final shape.
Also a most powerful tool is the control of the cold
roll contour. This can be achieved by bending the rolls by means of jacks so
that parts of the roll that want to roll long strip are adjusted and by
alteration of the distribution of the rolling lubricant so that hot parts of
the roll that would roll long are cooled Obviously, the mill cannot make these
adjustments itself and it is necessary to measure the shape of the outgoing
strip and then instruct the mill controls. It is impossible to judge the shape
by slacking off the outgoing tension since this would grossly interfere with
production schedules and result in considerable scrap. In consequence, methods have
been devised whereby the actual shape of the outgoing strip, which, because it
is under tension, can look flat, is assessed by measuring the residual stress
in the strip - long parts have low stresses. Various methods have been the
signal from the shape meter is fed to the mill control jacks and lubricant
sprays and rapid adjustment achieved. In many modern installations concerned
with the high volume production, for instance in the canning industry, the very
stringent flatness requirements are achieved by combinations of mill control
and by the use of tension levelers. Gauge control is achieved in much the same way
as flatness, i.e. by continuously measuring outgoing strip thickness and
adjusting the roll bite accordingly. Without such control it would not be
possible to produce easy opening can-ends!
Properties of Rolled Products
Typical mechanical, physical, chemical and
fabrication properties and characteristics of a range of rolled aluminum
alloys, tempers and heat treatments are given in the following tables extracted
from the aluminum. Note that these data are not guaranteed minimum properties,
but have been harmonized between the various European aluminum producers and
pertain also to other types of wrought products as indicated. There are in addition
a great number of specialty sheet and plate alloys which are being produced for
specific applications, e.g. car-body sheet metal parts. These alloys have
characteristics to suit special demands such as formability. Full details can
be provided by the metal producers. To select the right alloy for a specific
application is often a difficult task
