DESCRIPTION OF INVENTION

Field of Technology

Addition of alloying elements; addition of zirconium; recrystallization threshold; thermal treatment of aluminium containers, slugs.

Technical Problem

Aluminium or aluminium alloy containers are manufactured from aluminium slugs using counter-rotating extrusion. The manufacture of containers is composed of several stages, such as the forming of container body, hot colouring/lacquering, printing, forming of dome and drying at high temperatures. Draying of internal coat of the container is performed at temperatures around 280 °C.

The problem that arises is the decrease of mechanical properties in lacquering and drying of the container for 10-15%. The process of recrystallization, i.e. the growth of crystal grains, starts at such high temperatures, which causes the decrease of mechanical properties.

State of the Art

Aluminium containers are manufactured from slugs which are previously stamped from a narrow aluminium strip of 99.5-99.7% Al quality or aluminium alloy of the AIMn type. Containers are manufactured by counter-rotating extrusion and are lacquered on the inside and outside after the forming. Colouring is performed in several stages. First, containers are internal coated and polymerized at around 280 °C. In the next step, containers are external painted with the ground colour and dried between 140-200°C. This is followed by the printing of external surface of the container and drying of lacquer of the container at 170 °C.

Description of new invention

The subject of the invention is the addition of alloying element to reduce the decrease of mechanical properties in drying of coloured aluminium containers and solves the technical problems of:

- increasing the temperatures at which the recrystallization process starts;

- reducing the decrease of mechanical properties after thermal treatment.

The manufacture of containers is composed of several stages, such as the forming of container body, hot colouring/lacquering, printing, forming of dome and drying at high temperatures. Draying of the internal coat of the container are coloured at temperatures around 280 °C. Colouring at such high temperature results in the decrease of mechanical properties by 10-15%, since the process of recrystallization, i.e. the growth of crystal grains, already starts at this temperature, which results in the decrease of mechanical properties. The temperature at which the recrystallization process starts may be increased with the proper selection of alloying elements, thus retaining the mechanical properties of the basic material before colouring and drying. By adding Zr or similar alloying elements, the decrease of mechanical properties of the material may be reduced or the same mechanical properties as those of the basic material of aluminium slugs may be retained. The addition of Zr or similar alloying elements results in the increase of the recrystallization temperature and thus prevents the decrease of mechanical properties of the material or consequently leads to higher mechanical properties of the material.

The correct selection and quantity of added alloying element makes it possible to raise the recrystallization threshold of material in the manufacture of aluminium containers above 300 °C, thus retaining the properties of the material or reducing the decrease of mechanical properties after colouring. In this way, aluminium containers with thinner walls and higher burst pressure may be manufactured, and less material may be used in the manufacture of aluminium container.

Aluminium containers for cosmetics and food industry are manufactured from aluminium slugs using counter-rotating extrusion. Aluminium slugs are semi-manufactured products stamped from a narrow aluminium strip.

Aluminium strip is usually casted by the horizontal casting system which enables casting of wide spectre of aluminium alloys. Narrow aluminium strip, which is casted using "Rotary strip caster" system, is limited to the casting of the aluminium alloys series 1XXX and 3XXX.

With this system, the melt is poured into the groove of the copper or steel casting wheel surrounded by a continuous steel belt. The belt closes the groove and prevents the outflow of melt.

The aluminium or aluminium alloy melt is produced in a melting furnace which is fed with process waste from stamping, with blocks of T-form electrolytic aluminium or with electrolytic aluminium. When liquid aluminium, heated to approximately 720 °C, is formed in the furnace, slag is removed from the melt and poured into the holding furnace, where alloying of aluminium melt is performed.

In accordance with the required chemical composition of the alloy, alloying elements, such as manganese (Mn), iron (Fe), silicon (Si) and titanium (Ti) are added. The majority of alloying elements are added into the aluminium melt in the form of tablets, small blocks or wire. Master alloys AlTi75% and AlFe75% are added in the form of tablets, while manganese as the master alloy AlMn25% is added in the form of blocks or loaves. The number and the % sign next to the reference of the master alloy indicate the mass fraction of added alloying element to aluminium. For example, the indication AlFe75% means that the master alloy contains approximately a 75% mass fraction of iron. Silicon is added into the melt in the form of metallurgical pure silicon. Master alloy AlTi5Bl (5% titanium; 1% boron (B)) is added in the form of wire, which also serves to refine the microstructure. Additions of zirconium (Zr) are added into the aluminium alloy in the form of master alloy, for example in the form of master alloy AlZr20%, which, however, does not limit the substance of the invention. To obtain the optimal effect of Zr, it is added within the limits 0.08 to 0.2 of mass fraction. By adding Zr, the addition of Ti into the melt is also corrected, since the addition of Zr reduces the effect of grain refinement of Ti. In case of alloying with manganese, iron or zirconium, care must be taken to ensure that they are alloyed at least in 15-minute intervals, since complex intermetallic phases may be formed between them.

Gases must be removed from the melt prepared in this way. The most common procedure for removing gases is purging of melt with inert gas. Argon filter, fitted between the casting furnace and casting machine, is used in the manufacturing process. It is installed in a tank consisting of mixing and flow chamber. Argon (Ar) is injected into the melt through a graphite stirrer. It is dispersed in the melt the form of small gas bubbles diffused with hydrogen (H). This process continues until the partial pressures of hydrogen and gas bubble are equal. As a result of buoyancy, bubbles rise to the surface, where hydrogen combusts. When hydrogen and other undesired impurities are removed from the melt, it travels from the flow filter through the ceramic filter, used to remove inclusions of oxides, slag residues and impurities from the melting furnace.

Purified melt then travels to the casting machine, consisting of a casting wheel and steel belt. Melt flows from the casting channel into the area between the continuous steel belt and water-cooled wheel. The casting wheel is manufactured from copper or steel alloy. The aluminium strip may reach a temperature of 530 °C at the casting wheel outlet. It first leads to the hot rolling mill and then to the cold rolling mill through a roller track.

The process of strip rolling is performed by the reduction of input narrow strip with minimal transverse deformation. Longitudinal rolling is a continuous forming operations that reduces the cross-section of the material between the counter rotating rollers. Reduction in the hot rolling mill is 40-70% of strip thickness, while in the cold rolling mill it reaches 30-50%.

Rolled narrow aluminium alloy strip then travels to the stamping line, where slugs are stamped using a stamping machine. Stamping machines usually allow from 60 to 625 strokes per minute.

Stamped slugs fall on the conveyor belt below the stamping machine. From here, they are led into annealing containers and into annealing furnaces, where slugs are softened and the oil remaining from stamping is burned off.

After the annealing, the slugs are surface-treated by sandblasting, vibrating or tumbling (slugs rubbing against each other), since a certain degree of roughness is required for counter-rotating extrusion in order to retain oil on the surface of slugs during the counter- rotating extrusion process.

Slugs are then transported to packaging manufacturers, where aluminium alloy containers are manufactured by counter-rotation extrusion.

The essence of the invention is further explained below with the description of the embodiment:

Exemplary embodiment:

The manufacture of aluminium alloy containers is composed of several stages, such as the forming of container body, hot colouring/lacquering, printing, forming of dome and drying at high temperatures. Draying of internal coat of the container is performed at a temperature of 280 °C. By adding Zr, the decrease of mechanical properties of the material may be reduced or the same mechanical properties as those of the basic material may be retained. The addition of Zr results in an increase of the recrystallization temperature and thus prevents the decrease of mechanical properties of the material or consequently leads to improved mechanical properties of the material. The correct quantity of added Zr raises the recrystallization threshold of material in the manufacture of containers above 300 °C, thus retaining the properties of the material or reducing the decrease of mechanical properties after colouring. This makes it possible to manufacture aluminium containers with thinner wall and higher burst pressure, and ensures that less material is in the manufacture of aluminium alloy containers.

Aluminium containers for cosmetics and food industry are manufactured from aluminium slugs using counter-rotating extrusion. Aluminium slugs are semi-manufactured products stamped from a narrow aluminium strip.

The aluminium alloy melt is produced in a melting furnace which is fed with electrolytic aluminium. When liquid aluminium, heated to approximately 720 °C, is formed in the furnace, slag is removed from the melt and poured into the holding furnace, where alloying of aluminium melt is performed.

In accordance with the required chemical composition of the alloy, alloying elements are added. Additions of zirconium (Zr) are added into the aluminium alloy in the form of master alloy AlZr20%.

The amount of zirconium added is 0.12 mass fraction. By adding Zr, the addition of Ti into the melt is also corrected, since the addition of Zr reduces the effect of grain refinement of Ti.

Gases must be removed from the melt prepared in this way by purging of melt with inert gas. Purified melt then travels to the casting machine. Melt flows from the casting channel into the area between the continuous steel belt and water-cooled wheel made of steel alloy. The aluminium strip may reach a temperature of 520 °C at the casting wheel outlet. It first leads to the hot rolling mill and then to the cold rolling mill through a roller track. The process of strip rolling is performed by the reduction of input narrow strip with minimal transverse deformation. Reduction in the hot rolling mill is 60% of strip thickness, while in the cold rolling mill it reaches 40%.

Rolled narrow aluminium strip then travels to the stamping line, where slugs are stamped using a stamping machine. Stamped slugs fall on the conveyor belt below the stamping machine. From here, they are led into annealing containers and into annealing furnaces, where slugs are softened and the oil remaining from stamping is burned off. After the annealing, the slugs are surface-treated by sandblasting. Slugs are then transported to packaging manufacturers, where aluminium containers are manufactured by counter- rotation extrusion.

It is self-evident that the above described invention can be also used in other particular form not changing the substance of the invention.