Aluminum foil: technology

The thickness of the aluminum foil

Aluminium foil - is a type of flat rolled aluminum. aluminum foil thickness is from 5 to 150 micrometers (from 0,005 to 0,15 millimeters). Flat aluminum thick rolled over 0,15 mm - this is a tape, sheets and plates.

How to make the aluminum foil

Aluminum foil is made in four main steps:

• hot rolling of aluminum strip;
• cold rolling of aluminum strip;
• cold rolling of foil;
• and cutting the foil winding into rolls for consumers.

Hot rolling aluminum strip from slabs

Conventional technology of production of flat rolled aluminum - sheets, strips and foil - starts with the casting of molten aluminum into large rectangular ingots of aluminum in the shape of a trapezium - aluminum slabs - by weight 10 to 25 tons and a length of 10 m. This occurs in specific vertical semi-continuous casting machines.

The slabs were subjected to a preliminary annealing at a temperature 350-450 ° C and then hot rolled into strips in rolling mills - single stand or tandem - the thickness of the strip 6 to 2,5 mm and wound into rolls at a temperature of about 300 ºС (figure 1).

Figure 1 - Scheme aluminum strip production technologies of slabs

Continuous casting and rolling aluminum strip

A more economical in energy consumption is the technology of direct continuous casting of molten aluminum in the ribbon thickness from 12 to 20 mm and hot rolled on a continuous tandem rolling mill. However, application of this process is limited to a small number of aluminum alloys, which are not subject to segregation of alloying elements during rapid cooling, which occurs in the continuous casting. These alloys primarily include all brands of unalloyed aluminum.

Third - the most economical - the tape casting method is a strip thickness at the outlet from 6 to 3 mm directly from the aluminum melt between two rolls of twin-roll casting units (figure 2). The disadvantage of this method - a rather low productivity.

Figure 2 - the twin-roll casting machine
for the production of aluminum strip.

This molding method is also suitable only for pure aluminum or aluminum alloys with a low content of alloying elements.

All three of these methods of production of hot-rolled aluminum strip have their advantages and disadvantages with respect to quality of the material strips, productivity, Energy consumption and labor requirements. Therefore, the volume of investment and maintenance costs of equipment are key factors in choosing the optimal solution for each individual producer.

Cold rolling of aluminum strip

After finishing hot rolling step of cold-rolling step begins tape.

The process of cold rolling aluminum strips may be performed on different types of rolling mills. For small rolls weighing up 5 tons, single-stand reversible rolling mills are often used (Figure 3).

Figure 3 - reversing mill for rolling aluminum strip

To work with rolls weighing 10 to 15 tons, irreversible single rolling stands are usually used (Figure 4).

Figure 4 - Irreversible mill for rolling aluminum strip

 

For large rolls - over 25 tons - and for large production volumes, multi-stand tandem rolling mills are used (Figure 5).

Figure 5 - Tandem mill for rolling aluminum strip

Although the bands rolling at this stage is called a "cold", during each pass, the tape is heated to approximately 100 oC. Therefore it is required to apply for a large number of cooling rolls water-oil emulsion, to maintain the thermal balance of the process. After every three or four passes of rolls cooled to room temperature over several hours.

Strain hardening material

During each pass through the mill material is aluminum strip is subjected to significant plastic strain and therefore strain hardening receives - hardening, also known as work hardening. In the process of cold rolling comprise one or two annealing to recrystallize the grain structure of the material tape. The degree of cold-rolling effect on the grain structure of the material depends on the chemical composition of the used aluminum or an aluminum alloy grade.

Rolling thin aluminum strip

Final rolling operation for almost all kinds of flat rolled aluminum processing is irreversible on single stand rolling mills. At this stage, a thin ribbon, which is not only for the further production of foil, but also, in particular, for manufacturing aluminum cans for beverages, including, aluminum beer cans. This thin lentv require tight tolerances on thickness and flatness. Besides beer cans made of thin aluminum strip is produced as a lithographic plate, that, require a higher quality of its surface.

Rolling of aluminum foil

aluminum foil rolling technology requires special accuracy exhibiting gaps between the rolls and the foil tension parameters. All rolling mills for the production of foil provided with special devices, to ensure the strict requirements for its quality.

On the last pass rolled foil in two layers. To do this, the penultimate pass it folded in half, and after the last pass again separated and wound into two separate bays. That's why one side of the finished foil, exterior during rolling, brilliant, and the other, internal satined.

Cutting and rewinding aluminum foil

The last stage of the production of foil - cutting wide and large rolls of foil to rewind them into rolls of various sizes and volumes - such, needed industrial processors or final consumers.

Alloys for aluminum foil

The main alloys for the production of aluminum foil are:

• technical aluminum grades: 1100, 1145, 1050, 1235;
• low alloy Al-Mn series: 3003 and 3102;
• Al-Si-Fe alloys with a high iron content: 8006, 8011, 8011A, 8111, 8079.

Seem to be, only 7xxx series aluminum alloy, from which it is made is an alloy foil 7072. In this alloy, the zinc content is nominal only 1,0 % when the copper content is not more than 0,10 %. This foil is used for the manufacture of automotive radiators. The fins of the alloy 7072 mounted on the tube, for example, alloy 3003, through which coolant passes, and provide her a cathodic corrosion protection.

Iron in aluminum foil

Thin aluminum foil (6 microns) is the lightest "absolute barrier" for packaging liquid products, when it is mounted on a paper or plastic-based. At the beginning of the commercial application of foil it was pure primary aluminum Al 99,5 (1050), which comes directly from the aluminum electrolysis, as well as technical grade alumina 1200 with a slightly higher content of iron. Foil alloys with a high content of iron 8xxx series have been introduced to increase the strength of the foil in the finished product.

Traditional aluminum foil "alloys" – aluminum brand 1050 and 1200 contain iron and silicon, which are their major impurities in an amount of 0,1 to 0,4 %. These technical grade alumina have relatively low strength properties compared to other aluminum alloys thermally neuprochnyaemymi, for example, alloyed with magnesium and manganese. Only 0,05 % iron may dissolve in aluminum near the melting temperature and is much smaller - in the solid solution at room temperature. The influence of iron is based on the following [2]:

• interaction with dislocations, which gives an increase in strength properties;
• suppression of grain growth and the creation of centers of recrystallization.

The figure 6 shows a reinforcing effect on the strength properties of iron , as well as a significant increase in formability (elongation). The figure 7 It shows the grain structure after final annealing alloys with different contents of iron.

Figure 6 - Influence of the iron content in the annealed aluminum technical
on its strength properties and elongation [2]
(foil 50 m, annealing at bay 350 ºS)

Figure 7 - Influence of the iron content in the annealed aluminum technical
on the size of the recrystallized grain [2]

With increasing iron content occurs grain refinement, which explains the favorable effect of iron on the foil forming ability. High iron content in the foil several complicating its subsequent meltdown - recycling. Increased iron content in the aluminum foil may be the cause of pollution other aluminum iron alloys, in which its content is limited.

Sources:

1. Materials of the company Achenbach (Germany)
2. Aluminium Alloys: The Physical and Mechanical Properties /ed. Jürgen Hirsch, Birgit Skrotzki, Günter Gottstein – 2002