The Rolling of Aluminum: the Process and the Product

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