Analysis of common problems in the production of high-quality double zero aluminum foil

Analysis of common problems in the production of high-quality double-zero aluminum foil

Aluminum foil is a metal material that is rolled into thin sheets through multiple processes. It is a new type of material with light weight and high strength. Aluminum foil can be used as packaging materials for food, beverages, cigarettes, medicines, photographic plates, household daily necessities, etc. It can be used as an insulating material for buildings, vehicles, ships, houses, etc. It can be used as a battery capacitor material for batteries, flat-screen TVs, laptops, digital cameras and other electronic products. It can also be used as other materials for decorative gold and silver wires, stationery prints and decorative trademarks of light industrial products. Aluminum has a unique metallic luster, strong barrier properties, antibacterial properties, thermal conductivity, and plasticity. It is also non-toxic and odorless, and has surface printing properties. In addition, used aluminum foil materials can be recycled and reused, and can be used for aluminum reprocessing after treatment.

High quality double zero aluminum foil

According to data from the China Nonferrous Metals Processing Industry Association, from 2016 to 2021, my country’s aluminum foil production showed a steady growth trend, with an average annual growth rate of 5.57%. In 2022, my country’s total aluminum foil production will be 5.13 million tons. With the continuous development of society and the continuous advancement of science and technology, people have higher demands for material life. The research and development of aluminum foil for emerging furniture and power batteries, especially the development of aviation, health care, and plant protection aluminum foil products, has further stimulated the demand for aluminum foil.

Characteristics of double-zero aluminum foil

The national standard (GB/T3880-2006) stipulates that aluminum plates with a thickness of less than 0.2 mm are called aluminum foil. Double-zero aluminum foil is aluminum foil with a thickness between 0.001mm and 0.009mm. Aluminum foil with two zeros after the decimal point is usually called double-zero aluminum foil.

• (1) Double-zero aluminum foil has good moisture resistance. The distance between metal ions is smaller than the distance between resin polymer chains, which can prevent water vapor from penetrating. Aluminum foil has much better moisture resistance than resin films of the same thickness. Therefore, even if there are pinholes in the aluminum foil, as long as the diameter of the pinhole is less than 5 μm, oxygen and water vapor still cannot pass through the aluminum foil smoothly.
• (2) Double-zero aluminum foil has good thermal insulation. Aluminum foil has very poor temperature radiation performance and strong reflection ability for sunlight. However, the absorption and emissivity of aluminum foil to radiation energy are particularly small. Since the emissivity of aluminum foil is similar to its absorptivity, it is usually regarded as a gray body in thermal engineering calculations.
• (3) Double zero aluminum foil is a good conductor material. Aluminum is the second best conductor of electricity after gold, silver and copper. The isovolumetric conductivity of aluminum is 57%~62%IACS. This means that when the aluminum foil is wound into a coil or winding, due to its increased surface area, the actual conductivity of the aluminum foil can reach 60%~80%IACS. Therefore, compared with other materials, the use of aluminum foil to make electrolytic capacitors has a higher cost-effectiveness.

In addition, aluminum foil also has certain mechanical properties such as tensile strength, elongation and tear strength. Of course, different aluminum has different mechanical properties due to different thickness, chemical composition and selected rolling methods.

Common problems in the production process of domestic double-zero aluminum foil

The production of double-zero aluminum foil includes the whole process from aluminum foil blank to aluminum foil processing. There are many production processes and the processing is difficult. It mainly involves multiple rolling processes and heat treatment process links such as melting, cold rolling, intermediate annealing, and foil rolling. It is precisely because of the complexity of the production process that there are many factors that affect the quality of double-zero aluminum foil. Equipment model configuration, aluminum foil blank quality, rolling process selection, personnel operation specifications, etc. will affect the production of double-zero aluminum foil.

At present, the common problems in the production process of domestic double-zero aluminum foil are mainly too many pinholes, poor flatness, and oil on the surface of aluminum foil. The pinhole problem is the most serious problem in aluminum foil processing. Pinholes can directly affect the properties of double-zero aluminum foil, especially the moisture resistance, heat insulation, light shielding and antibacterial properties of aluminum foil materials.

The quality of aluminum foil blanks will affect the number of pinholes in aluminum foil. If the blank contains more gas molecules and solid impurities, or even has certain grain unevenness defects, it will affect the number and size of pinholes in aluminum foil processing. The number of pinholes in double aluminum foil produced by imported aluminum foil blanks is generally less than 50/m². The number of pinholes in double aluminum foil with the best processing control can reach about 10/m². Similarly, the number of pinholes in double aluminum foil produced by the processing workshop of the enterprise using domestic materials is as high as 80/m². The chemical composition of aluminum foil blanks is mainly a (FeSiAl) phase, B (FeSiAl) phase, and FeAl phase. The a (FeSiAl) phase is spherical particles, and there are also a small amount of rod-shaped particles. Because the a (FeSiAl) phase has large particles and low hardness, it is easy to crush during the deformation and rolling process. It is an ideal cast compound. FeAl2O3 phase is a needle-like compound with large volume and high hardness. It is not easy to break during the rolling process of aluminum foil, and it is easy to form crack sources, thereby forming pinholes. The compounds in foreign billets are mainly a(FeSiAl2O3) phase and BFeSiAl2O3 phase, while the compounds in domestic billets are mainly a(FeSiAl2O3) phase and FeAl2O3 phase. The number of pinholes in double-zero aluminum foil obtained by rolling domestic raw materials will be more, and the existence of FeAl2O3 phase is also one of the main reasons.

Quality Testing

The casting and rolling process of aluminum foil also has a great influence on the pinhole degree, which is mainly manifested in the convexity of the casting and rolling roller, the roughness of the rolling roller surface, and the rolling oil. The influence of precision and viscosity on the pinhole degree of double-zero aluminum foil. During the casting and rolling process, the size of the roller convexity will affect the number and position distribution of pinholes in the aluminum foil. Only by selecting the appropriate roller convexity can the number of pinholes be better reduced. The greater the roller surface roughness, the more pinholes there are in the aluminum foil after casting and rolling, and the lower the smoothness of the aluminum foil. When the roller roughness is large, the slight depressions on the surface of the aluminum foil will expand, causing pinholes and even easy to break the belt. During the rolling process of aluminum foil, solid particles in the rolling oil or dust in the air will cause defects in the aluminum foil and form pinholes. The particle content in the selected rolling oil should not exceed 0.1%, and the particle size should be less than 10 μm, and also control the dust in the environment during the rolling process to avoid falling on the surface of the aluminum foil and forming new external defects. The viscosity of the rolling oil should not be too large, otherwise it will increase the surface roughness of the aluminum foil and produce new pinholes.

In addition, the rolling speed and unwinding tension will also affect the number of pinholes in the aluminum foil. In the aluminum foil processing process, it was found that when the rolling speed is too fast and the tension is too large, the number of pinholes in the aluminum foil will increase significantly, and too fast may even cause the strip to break. However, if the rolling speed is too slow and the tension is too small, the aluminum foil will be wrinkled and uneven, and the production efficiency of the aluminum foil will also decrease. If the flatness of the double-zero aluminum foil is not good, the tension cannot be evenly added to the lateral width of the aluminum foil during rolling, especially in the production process of wide or ultra-wide double-zero aluminum foil billets, plate defects such as middle waves, side waves and double waves often occur, which will cause severe strip breakage, aluminum folding or wrinkling, and even cause the aluminum foil rolling to be unstable.

3. Analysis of methods to reduce the number of pinholes and improve the quality of double zero aluminum foil

How to better control the pinhole problem of double zero aluminum foil and improve the yield of double zero aluminum foil? This article analyzes the actual production process and can improve the rolling quality of aluminum foil from the following aspects.

3.1 Control the quality and chemical composition of aluminum foil blanks

In the production and processing of double zero aluminum foil, the quality of aluminum blanks is the most important. Controlling each process of aluminum foil blank production, strictly controlling the chemical composition of the blanks, and ensuring that the composition is uniform and stable are the keys to producing high-quality ultra-thin aluminum foil. At present, the blanks for producing double zero aluminum foil are mainly divided into two categories: 1000 series and 8000 series aluminum alloys. Compared with 1 series aluminum alloys, 8 series aluminum alloys have higher content of Mn and Cu. Among these two series of aluminum alloys, 1235 aluminum alloy and 8079 aluminum alloy are mostly used, and 1235 aluminum alloy is widely used in domestic enterprises. Its chemical composition is shown in Table 1

In 1235 aluminum alloy, the impurity elements are mainly Fe and Si. When the Si content is constant and Fe/Si<2, BeSiAl phase is easily formed. After homogenization annealing, B (FeSiAl) phase is difficult to dissolve in the matrix. At the same time, because B (FeSiAl) phase is a rod-shaped crystal, it is easy to reduce the plasticity of aluminum foil blanks, which is not conducive to the next step of processing and production. When Fe/Si>5, the particulate matter in the aluminum foil billet (including larger flake FeAl,) increases, and the mechanical properties of the aluminum foil billet decrease. As the Fe content increases, the processing difficulty of the billet gradually increases, and the production efficiency of aluminum will continue to decrease. Therefore, when using 1235 alloy to produce aluminum foil billets, the content of Fe and Si elements and the Fe/Si ratio can determine the content of the second phase compound structure in the aluminum foil billet, which directly affects the quality of double zero aluminum foil. There are also trace amounts of Cu, Ti, B, etc. in 1235 aluminum alloy. Although the content of these alloy elements is very low, they will also affect the structure and rolling performance of the aluminum foil billet. Therefore, it is necessary to control the chemical composition of the aluminum foil billet.

We can take effective filtering and purification measures to reduce inclusions (A1,0), gas (H), and coarse metal compounds in the billet. At the same time, by adding grain refiners, the grain structure of aluminum billets can be reduced, the grain spacing can be reduced, thereby improving the mechanical properties and processing properties of aluminum foil billets and reducing the generation of pinholes in aluminum foil. At present, there are two main types of billets for domestic enterprises to produce double-zero aluminum foil: the first is to directly roll hot aluminum liquid into cast-rolled coils, and then obtain aluminum foil billets after cold rolling and annealing. The commonly used cast-rolled coils are 7mm. The second is to use semi-continuous casting to generate aluminum ingots, first mill the surface and then hot roll the billets, and finally cold roll them into aluminum foil billets. The hot rolling method can produce billets with uniform organization and fewer defects, which is conducive to the subsequent rolling processing of aluminum foil. The advantage of the cast-rolling method is that it can shorten the production process and reduce the compound size, which is suitable for rolling thinner aluminum foil. A large number of aluminum foil production practices have proved that cast-rolled billets are particularly suitable for the rolling of double-zero aluminum foil products with a thickness of 0.0065 mm, but the surface quality requirements are high. The rolling of double-zero aluminum foil with a thickness of less than 0.006 mm is still mainly produced by hot rolling.

3.2 Choose the right rolling production process

The thinner the aluminum foil is, the fewer pinholes it has, the better the quality is, and the higher the processing yield rate is. This is a reflection of the technical level of a country or an enterprise. At present, only a few countries in the world, such as China, the United States, and Germany, use hot rolling technology to produce 0.0045 mm aluminum foil, and most of my country’s aluminum foil companies can only produce 0.005 mm and above thick aluminum foil. In recent years, a certain company in my country has used domestic aluminum foil billets to roll and produce high-quality double-zero aluminum foil below 0.0045 mm, reducing production costs.

The quality of aluminum foil billets is the key to producing high-quality double-zero aluminum foil, reducing the pinhole rate of aluminum foil, and improving the yield rate and quality of finished products. However, there are many factors that affect the quality of double-zero aluminum foil, and the choice of processing technology is also a very important influencing factor. At present, the processing technology of double-zero aluminum foil mainly focuses on the process links such as aluminum alloy composition control, aluminum melt degassing and slag removal, grain refinement and filtration. Specifically, it is mainly to control the content of Fe and Si and the Fe/Si ratio; control the casting speed and rolling temperature; select the appropriate grain refiner; select the appropriate filtering equipment, etc. These processes are very important for the production of high-quality double-zero aluminum foil, which can ensure that the rolled aluminum foil has good performance and stability.

The production technology requirements of double-zero aluminum foil are high and the processing technology is complex. A rolling process of double-zero aluminum foil is shown in Figure 1. As can be seen from Figure 1, in the entire production process, each link such as rough rolling, intermediate rolling, finishing rolling, heat treatment, slitting, and packaging must be strictly controlled.

1235 aluminum foil

3.3 Choose the appropriate rolling oil

The process of aluminum foil production is to repeatedly roll the aluminum alloy billet to finally form a double-zero aluminum foil that meets the requirements. In the process of repeated rolling, an important procedure is to clean the billet, and the rolling oil is used to clean the billet. The rolling oil forms a lubricating film between the aluminum foil and the roller, which reduces the friction coefficient of the aluminum foil, increases the rolling force, and directly increases the yield rate of the double-zero aluminum foil.

Rolling oil is one of the three major elements of double-zero aluminum foil production. It is a viscous fluid with a certain viscosity. During the rolling process, the rolling oil is firmly adsorbed on the surface of the aluminum foil in the form of a layer of oil film, which plays a role in lubricating the roller and absorbing dust. If the rolling oil itself has a poor filtering effect and there are impurities, the friction coefficient will increase, and too many pinholes will be generated on the surface of the aluminum foil during the finishing rolling process. By reasonably adjusting the rolling oil ratio, controlling the rolling oil temperature, strengthening the filtering measures, and replacing the filter cloth in time, the rolling level of the aluminum foil can be improved and the yield rate of the double-zero aluminum foil can be improved.

3.4 Optimize heat treatment process

In the production process of double zero aluminum foil, heat treatment is very important. The heat treatment of aluminum foil blanks includes three stages: heating, heat preservation and cooling. Through homogenization annealing, intermediate annealing and finished product annealing, the plasticity of aluminum foil blanks can be restored and internal stress can be eliminated. Different heat treatment processes have different functions. In the aluminum rolling process, the heat treatment process should be carefully selected and optimized.

In the ultra-thin double zero aluminum foil blanks produced by continuous casting and rolling, a variety of non-equilibrium phases will appear inside the blanks. The amount is not large, but it has a great impact on the mechanical properties and processing properties of the blanks. Homogenization annealing of aluminum foil blanks at 470~600℃ and heat preservation time of 6~12h can improve the processing performance of aluminum foil, enhance diffusion capacity, reduce deformation energy storage, and reduce dislocation density. Annealing can also promote the precipitation of harmful elements Si and Fe in aluminum alloys.

After proper homogenization annealing, the grain size and second phase of aluminum foil blanks become smaller and the distribution becomes uniform, which is suitable for the next rolling operation. In the homogenization annealing process, selecting the appropriate heating rate, heating temperature, and holding time can make the aluminum billet structure more uniform, the second phase size smaller, and the double zero aluminum foil produced in the end has a small average number of pinholes and a high yield rate.

Intermediate annealing is a partial recrystallization annealing process used to improve the performance of double zero aluminum foil billets after cold rolling deformation. After the cast billet undergoes multiple cold rolling processes, the internal metal lattice is deformed and the grains become longer, which seriously affects the billet rolling. During the intermediate annealing process, as the annealing temperature increases and the time is extended, the grain size of the aluminum foil billet gradually increases and the second phase gradually becomes finer. The intermediate annealing temperature is 320~400 ℃, and the holding time is 4~12h.

The last annealing after the aluminum foil is rolled to the finished thickness is called the finished product annealing. The finished product annealing temperature is 180~260℃, and the holding time is 4~18 h. The rolled coils should be divided and annealed as soon as possible, and the annealing temperature of the finished product should not be too high, so as to remove the rolling oil remaining on the surface of the aluminum foil and ensure the mechanical properties of the aluminum foil. Most domestic companies often use two annealing processes: low-temperature degreasing followed by heating annealing and constant temperature long-term annealing to achieve the effect of degreasing and improving mechanical properties.

3.5 Control the production environment and relative humidity

Humidity also affects the number of pinholes in ultra-thin aluminum foil processing. The higher the relative humidity, the more pinholes there are in the aluminum foil. It will also cause the double-zero aluminum foil to break and increase the scrap rate. Taking 1235 aluminum alloy as an example, when the temperature is constant, the higher the relative humidity during rolling, the greater the hydrogen partial pressure, the higher the hydrogen content in the return material, the more likely it is to produce pores inside the billet, and the number of pinholes in double-zero aluminum increases. In actual production, it is found that when the hydrogen content in the aluminum return material is higher than 0.12 ml/100 gAI, the yield rate of double-zero aluminum foil will be low. The number of pinholes increases. Moreover, under the same production conditions, the yield rate and quality of double-zero aluminum foil produced in the dry autumn period are generally better than those produced in the rainy summer season. The suitable humidity for normal production of double-zero aluminum foil is best controlled below 55%.

The thickness of the double-zero aluminum foil currently produced is mostly around 0.006 mm, while the average diameter of dust in the air is basically between 0.0001 and 0.01 mm, and the particle size of PM10 suspended particles in the air is 0.01 mm. Any particles larger than 0.006 mm will cause pinholes in the aluminum foil. Therefore, it is necessary to pay attention to the environmental sanitation of the production workshop, pay attention to reducing the dust in the workshop, and avoid dust particles in the environment from falling on the rollers and billets. Appropriate dust removal measures such as installing dustproof roofs or using hair dryers can be taken. If conditions permit, a dust-free operation room can be set up for double-zero aluminum foil rolling production to ensure the quality of the aluminum foil. At the same time, it is also necessary to avoid non-rolling losses such as abrasions and scratches during transportation and stacking.

Conclusion

The production of double zero aluminum foil involves multiple processes such as melting, rough rolling, intermediate rolling, finishing rolling, hot rolling, cold rolling, annealing, cleaning, and slitting. To improve the quality of double zero aluminum foil products, it is necessary to strictly control every production link. Not only should high-quality aluminum foil billets be used, aluminum processing technology should be improved, and suitable casting and rolling equipment should be selected, but also the process should be optimized in combination with the problems encountered in the actual production of the enterprise, so as to greatly improve the yield rate and production quality of double zero aluminum foil products.