The manufacture of aluminum sheets involves the transformation of aluminum and its alloys through the stages of melting, casting, pre-rolling preparation, flat rolling, heat treatment and finishing. This creates single sheets or laminated materials with a rectangular cross section.
Plate thickness can reach up to 200 mm, with categories including thin plates, thick plates (5-80 mm) and extra thick plates. Sheet width generally ranges from 1 to 5 m, with lengths ranging from 2 to 10 m.
Strips are normally no more than 2mm thick and no more than 600mm wide and are supplied in rolls.
Aluminum and aluminum alloy sheets and strips are supplied in hot-rolled states, annealed states, various degrees of soft states, and various heat-treated states.
There are two methods of producing aluminum sheets and aluminum alloys: block method and strip method.
The block method involves cutting the hot-rolled plate into multiple blocks and then cold rolling them individually into finished products. The strip method involves rolling the board to a certain thickness and length and then rolling it as it is rolled.
Once it reaches its final thickness, it is cut into single sheets of aluminum. This method presents greater productivity and produces better quality products.
The production process of aluminum alloy sheets and strips can be divided into steps such as pre-rolling preparation, hot rolling, cold rolling, heat treatment and finishing.
Preheating preparation mainly includes casting quality inspection, equal heating, sawing, milling, aluminum packaging and heating. Using a quality foundry is a prerequisite to guarantee the quality of the finished product.
Most castings used in modern aluminum alloy sheet production are produced using the semi-continuous casting method. These castings are large, with finely structured internal dendrites.
During semi-continuous casting, the cooling rate is very high, making the solid phase diffusion process challenging and causing irregular chemical composition and microstructure, such as segregation within the crystal, reducing plasticity.
Therefore, some aluminum alloys, especially hard aluminum alloy castings, require homogenization treatment to eliminate or reduce uneven composition and structure while relieving casting stresses.
The homogenization temperature for aluminum alloys should be 10-15°C below the eutectic temperature of the alloy's lowest melting point, and maintaining it for 12-24 hours can essentially eliminate irregularities in composition and structure.
For hard aluminum alloy castings, the homogenization temperature is 480-495°C, maintained for 12-15 hours. For Al-Zn-Mg-Cu alloys, the temperature is 450-465°C, maintained for 24 hours.
When the surface of the ingot presents defects such as segregation precipitates, inclusions, scars and cracks, grinding must be carried out. This is a crucial factor in ensuring the good surface quality of the finished product. The amount of milling depends on the depth of the defects, generally 4-10 mm.
Coating is a unique process in the production of aluminum alloy sheets and strips. It involves placing liner sheets on the top and bottom of the casting and combining them into one body through hot rolling.
The purpose of the coating is to increase the corrosion resistance of aluminum alloy sheets and strips, protect the base metal from corrosion and improve technological performance. The coating sheet must have adequate chemical composition and adequate thickness.
For hard aluminum-based sheets, pure aluminum with a copper and zinc content of less than 1% is used as the covering sheet. When super-hard aluminum is the base, an Al-Zn alloy with a zinc content of 1-3% is used as the facing sheet.
Depending on the sheet thickness and application, the coating layer of the finished sheet is 2%, 4% and 8% of the sheet thickness.
Coating to improve processability is called process coating, intended to prevent surface cracking when the casting is opened. The coating layer is 0.5-1.5% of the finished sheet thickness.
The purpose of heating the casting is to increase its plasticity and reduce resistance to deformation, facilitating hot rolling.
The heating temperature of the casting is determined based on the alloy phase diagram and plasticity graph. The heating temperature must allow hot rolling to begin at the highest allowable temperature.
For pure aluminum and low-alloy aluminum alloy ingots, the heating temperature is 500 ℃ or higher; for hard aluminum alloy ingots it is 390-430℃; and for superhard aluminum alloys, it is 370-410 ℃.
The heating time aims to achieve a uniform temperature across the entire cross-section of the casting. Too long a heating time results in a very thick oxide layer on the surface of the casting, which is not conducive to the combination of the coating sheet and ingot. The casting is heated in a continuous heating furnace with air circulation.
Hot rolling of aluminum alloy castings is to supply billets for cold rolling or to directly produce hot rolled thick sheets.
Depending on the production scale, there are three methods of hot rolling aluminum alloy castings:
(1) Single frame hot rolling
Which completes the entire process from the start of the billet to the completion of hot rolling in a hot rolling machine.
Large castings are used to improve production efficiency, and reversible rolling mills are used. Four-roll mills are used to increase the plate width and improve the plate shape. With single-structure hot rolling, the temperature drop of the rolled part is large, the final rolling thickness is large (6-8mm), the weight of the coil is relatively light, and the quality and production efficiency of the rolled part are not ideal.
(2) Double Stand Hot Lamination
This process begins with a reversible mill performing initial billet forming and hot rough rolling of the ingot, before the part is transferred to a second four-high reversible mill for hot finish rolling. As rough rolling and finishing rolling are now separate tasks, not only are production capacity and efficiency improved, but the quality of rolled products also improves. The final lamination thickness can reach 2 mm.
(3) Semi-continuous hot rolling
This involves 1-2 reversible rolling mills for billet forming and hot rough rolling, before the part is moved to 3-6 four-high tandem rolling mills for hot finishing rolling, with each stand performing a single pass. As large ingots are rolled at high speed, not only the production scale is large, but the rolling interval time is also short, therefore the finishing rolling temperature is high, producing better quality annealed coils.
The hot rolling process system includes parameters such as pass reduction, rolling temperature, rolling speed, and lubrication and cooling. Greater pass reduction is beneficial for deformation to penetrate the workpiece, reducing the likelihood of edge cracking and roll curl. However, the pass reduction is limited by roll-bit conditions.
Furthermore, during the billet forming stage, both the pass reduction and the rolling speed should not be excessive to ensure a smooth transition from the cast ingot structure to the deformed structure.
When rolling aluminum coated ingots, to ensure a good bond between the coating plate and the ingot, the reduction of the first pass must be controlled within the range of 2% to 4%. Edge rolling can improve the stress state at the edge of the workpiece, reducing edge cracks.
Implementing coating on the side of the ingot and rolling the edges can eliminate edge cracks during hot rolling of aluminum alloys. In the later stages of rolling, as the length of the part increases, the rolling speed must also be increased proportionally.
To achieve flat and smooth rolling quality and reduce rolling force, adequate lubrication is crucial during hot rolling. Lubrication for hot rolling of aluminum alloys generally employs water-based emulsions.
The emulsion is made from a mixture of emulsifying agent and water, with a concentration of % to %, slightly lower in the rolling of hard alloys. The emulsifying agent consists of transformer oil, oleic acid and triethanolamine.
To obtain good flatness in hot-rolled coils, it is essential to control the roll gap shape to 546 degrees, achieved through the use of hydraulic roll bending, cooling of the sectional roll body and control of the original roll convexity (see control roll shape), along with appropriate adjustment of laminating schedule and speed.
The emulsion sprayed on the roller, in addition to lubricating, also has a cooling function. The pressure at the spray nozzle must be around , with a flow rate of 56L/(cm•s).
Cold rolling allows the production of coils with superior flatness, smoother surface, thinner and more uniform thickness and better structure and properties compared to hot rolled coils.
Cold rolling can be carried out on a single stand rolling mill or a tandem rolling mill. Currently, four-high, single-stand non-reversible rolling mills are most commonly used, with rolling speeds of 520 m/s, or up to 2,540 m/s for tandem rolling.
Comprehensive automatic control is implemented through computer systems such as automatic flatness control (AFC), automatic gauge control (AGC), automatic tension control (ATC) and automatic speed regulation (ASR), thus producing high-quality products. quality with thickness deviations reduced to ±3~5μm and flatness less than 10 I units.
Under conditions where equipment capabilities permit, lubrication and cooling are effective, and the workpiece does not crack at the edges and can achieve a good surface, cold rolling should aim for high pass reduction.
For pure aluminum and soft alloys, the allowable pass reduction is 50% to 70%, generally 40% to 50%; for hard alloys, it is around 40%, generally below 30%. Reducing the pass should make the rolling force basically the same in each pass, ensuring that the rolled coils have uniform thickness and good flatness.
Under conditions where edge cracks do not occur, the total reduction rate of cold rolling for pure aluminum and soft alloys can reach more than 95%, and hard alloys can reach 90% to 92%.
To avoid edge cracks and web breaks, alloys with low plasticity require pre-annealing with hot web rolling, and 1 to 2 intermediate annealings are performed during cold rolling.
The thickness of the last intermediate annealing, or the total reduction rate of the last cold rolling pass, has a crucial role and impact on the performance of the final product.
Stress on rolling parts during rolling affects their thickness, flatness and uniformity. The stress must be lower than the yield strength of the rolls, and its magnitude depends on the plasticity and tendency to cracks at the edges of the rolls.
During the acceleration, constant speed and deceleration stages of the bearing, fluctuations in voltage must be minimized.
The function of process lubrication during cold rolling (see Cold Rolling Process Lubrication) and cooling is to reduce friction, decrease rolling pressure, improve the surface quality of rolling parts, cool the rolls and parts rolling mill and control the roll profile (see Roll Profile Control). ). Cooling lubricants must have lubricating, flushing and cooling properties simultaneously.
For rolling speeds below 5 m/s, a water-based emulsion with a concentration of 2% to 8% can be used; For high-speed rolling, rolling oil composed of base oil and additives is used, known as complete oil lubrication.
Whether emulsion or full oil, both need to be filtered during the recycling process to remove aluminum and alumina ash washed from parts and rolls.
In the filtered cooling lubricant, the impurities should be less than 0.5 g/L and the particle size of the impurities should be less than ~μm.
Heat Treatment – In addition to hot-rolled and cold-hardened products, aluminum alloy plates and strips must undergo separate annealing or quenching and aging treatment as required (see Heat Treatment of Non-Ferrous Alloy Material).
Finishing refers to the processing and arrangement of plates and strips before delivery, after rolling and final heat treatment, including final cutting, straightening, polishing, packaging, etc. These steps can be completed on a production line or performed separately.
Final cutting includes transverse cutting and longitudinal cutting. Cross cutting cuts the rolled strip into sheets, while longitudinal cutting divides the wide rolled strip into several narrow coils.
Trimming also removes front and back edges with size discrepancies and parts with substandard surface quality.
Straightening can reduce or eliminate internal stresses in plates and strips after lamination or heat treatment and the unevenness caused by this. Straightening includes roller straightening, tension straightening and polishing.
Tension straightening is divided into plate tension straightening and strip tension straightening. Using repeated bending, stretching or thinning, the plates and strips are subjected to plastic deformation of 1% to 2% to achieve the objective of straightening.
