Aluminium rolling
Aluminum sheets – thin and thick, as well as foil are made by rolling a fairly thick aluminum billet – an ingot or an intermediate semi-finished product – a rolled product – between rolling rollers in order to reduce the thickness of the workpiece. The length of the workpiece increases. This is somewhat similar to rolling out dough when rolling out layers for a layer pie. But, of course, rolling aluminum is much more difficult.
Sheet products
The aluminum slab passes through several rolling stands until it reaches a specified thickness. The rolling rolls press on the slab and intermediate sheet from above and below. Depending on the specified thickness, the finished rolled sheet falls into one of three categories:
- Aluminum plate (thick sheet): thickness from 6.3 mm or more. Used in the aerospace industry. For example, airplane wings and other structures.
- Aluminum sheet: thickness from 0.2 mm to 6.3 mm. Beverage cans, road signs, automotive structures.
- Foil: thickness from 0.2 mm to 0.2 mm. Product packaging, building insulation and much more.
Hot and cold rolling of aluminum
The process begins when the rolling mill receives the aluminum slab. Depending on what mechanical properties the finished aluminum sheet should have, technologists decide whether the ingot needs to be heated or not.
Aluminum rolled ingots [2]
If the ingot is not heated before rolling, then the exit from the rolling mill will be a cold-rolled sheet. Cold rolling makes aluminum harder and stronger by changing its microstructure. However, as a result of cold rolling, the metal becomes more brittle.
If the slab is heated before rolling, the rolling process is called hot rolling. The heating temperature depends on the chemical composition of the aluminum alloy. Hot rolling prevents unfavorable hardening of the metal and allows aluminum to remain ductile.
Hot rolling
Aluminium rolled 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 (Figures 2 and 3). The strip from the hot rolling mill is coiled for transport to the cold mill.
Figure 2 – Single Stand Hot Mill [3]
Figure 3 – Breakdown Mill /Hot Tandem Mill [3]
Cold rolling
Cold mills, in a wide range of types and sizes are available; some are single stand, others 3 stand and some 5 stand (Figures 4 and 5). 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.
Figure 4 – Single Stand Reversing Cold Mill [3]
Figure 5 – Multi Stand Reversing Cold Mill [3]
Other rolling operations
The passage of aluminum through a rolling mill is only one step – although the most important one – in a sequential chain of technological stages in the production of rolled aluminum. Any aluminum flat product obtains its final properties not only during rolling itself, but in close interaction with the preparatory technological stages:
- preparing aluminum melt;
- casting aluminum into ingots;
- scalping the bullion;
- heating the ingot for rolling;
- intermediate annealing,
as well as post-rolling operations, such as:
- heat treatment or final annealing;
- stretch editing;
- straightening with rollers;
- edge trimming;
- aging.
Modern aluminum rolling mills perform most or all of these operations to produce sheets and foils with specific dimensions, properties and other characteristics specified by the customer.
How aluminum rolling developed
By the time aluminum appeared on the industrial scene, humanity already had thousands of years of experience in casting and forging metals, as well as several hundred years of familiarity with rolling them into sheets and even foil. Metal rolling was mastered in developed countries back in the 16th century. All of these well-known metal processing methods were adapted and tested on aluminum.
- The first aluminum frying pan was stamped in the 1890s, and until the beginning of the 20th century, almost all aluminum was used to make such kitchen utensils.
- The first aluminum boats using aluminum sheets were built in the 1890s, including the racing yacht Defender in 1895.
- By 1903, about 3 thousand tons of aluminum were already consumed annually in the world, about a third of which was in sheet form.
- In the same year, the first aluminum foil was rolled in France. In the USA, the production of aluminum foil began in 1913 – for candy wrappers and chewing gum.
- Thin aluminum skins for aircraft appeared in 1917.
- The industrial method of producing high-strength aluminum sheets coated with various aluminum alloys (Alclad) to improve corrosion resistance originated in the USA in 1926.
- Hot and cold continuous rolling mills were introduced in the USA in 1926. The maximum rolling speed of the first such rolling mill was about 60 m per minute. Modern rolling mills roll aluminum sheets 35 times faster.
- Standard two-roll and four-roll rolling mill configurations were invented at the beginning of the last century, but still remain the mainstay of the rolling industry.
- In recent decades, various “cluster” mill designs have emerged that use work roller support by two or more support rollers.
Sources:
3. TALAT Lecture 1301 – The Rolling of Aluminium: the Process and the Product / Roy Woodward, Aluminium Federation, Birmingham
