Causes and Solutions for Quality Issues in the Annealing Process of Aluminum Foil

Analysis of quality problems in aluminum foil annealing process

Detection of quality problems in aluminum foil annealing process

Aluminum foil was used as the object of annealing analysis, and B-1 type rolling oil was selected as the commonly used rolling oil. After testing, the rolling oil contained S1 and S2 rolling additives, with S1 additive added to B-1 base oil at a mass fraction of 5%. S1 and S2 rolling additives need to be analyzed for subsequent thermal analysis in the form of simple rolling oil additives. After testing, the basic properties of the rolling oil were: kinetic viscosity at 40°C, flash point at 85°C, and distillation range was from 205 to 245°C. Using the above basic rolling oil and rolling additives, the temperature change value of the oil product was continuously changed, and the natural change reaction of the oil product under heating loss was observed. In addition, the superheat differential analysis method was used to analyze the oxidation, decomposition and polymerization reactions that occurred when the oil was heated. The reason for the formation of cold marks during aluminum foil annealing was understood.

The experimental results show that the corresponding thermal difference analysis curve has two obvious heat absorption peaks with an onset temperature of 55 degrees Celsius, indicating the existence of a certain volatility of the rolling oil. The first stage terminates at a weight loss temperature of about 225.5 degrees Celsius, and the percentage weight loss at this point can reach a maximum of about 58%. After the end of the first stage, the curve starts to change smoothly and gradually enters the second weight loss stage. At this point, oxidation reactions as well as thermal analysis reactions occur, which are manifested in the rolling oil spots that appear during the annealing process. Because the structure of S1 additive is not easily oxidized directly, there is no obvious heat absorption peak. the effect of S2 additive is more obvious in the second stage of weight loss conditions, when the additive produces obvious oil spot phenomenon.

Results of annealing surface contamination analysis

The analysis can determine that the contaminating quality problems generated during the annealing of aluminum foil come from the two-stage weight loss reaction of rolling oil and additives. Because the base oil has fast volatility, low residual and little surface contamination so the lowest contamination rating is basic.

When S1 additive alone is added, because the overall content is low, the amount of annealed surface contamination is also relatively low belonging to the secondary rating. And when S1 and S2 additives are mixed, because of the overall annealing process volatility is slow, serious oxidation reaction will occur, the formation of a variety of oil spots, and S2 is more serious than S1, so in practical applications at most only one additive can be used, otherwise will produce a large number of aluminum foil quality problems.

The reason for the formation of cooling lines is mainly influenced by the thermal conductivity of aluminum foil during annealing. Because the surface density of aluminum foil is smaller and lighter in weight the specific heat capacity is smaller and conversely the heat transfer is higher. In the actual heating process, the thermal expansion properties are larger and the actual elastic stress is higher.

In addition by comparing the surface yield curve of colored aluminum foil, it can be seen that the surface hardening curve of aluminum foil increases rapidly with the increase of ductility, but the overall cooling is faster and easy to produce shrinkage tension, so that the shrinkage tension at the valence position is greater than other positions, plastic deformation occurs first and transverse cooling lines are produced. This is because, by nature, the rate of deformation influenced by heating is the core cause of the appearance of cooling lines.

Preventive measures for annealing quality problems of aluminum foil

Oil spot prevention measures

In the actual aluminum foil annealing process, in order to prevent rolling oil from having a quality impact on aluminum foil, the annealing and de-oiling process should be completed in the first stage of weightlessness as much as possible. The general annealing and de-oiling temperature for aluminum foil is between 220 degrees Celsius and 240 degrees Celsius, a temperature that depends mainly on the final distillation point.

During the annealing process, most of the base oils can be volatilized well. In addition, the incoming material can be controlled with oil. On the one hand, measures can be developed to strengthen the advance inspection of the production process equipment to prevent oil with higher viscosity from dripping onto the foil surface, and on the other hand, the foil surface can be actively cleaned to reduce the amount of surface oil before the actual annealing. When annealing, make sure to ensure a reasonable annealing speed to ensure the full evaporation of oil and prevent carbonization phenomenon.

Preventive measures for cooling lines

Through the analysis of the mechanism of the emergence of cooling lines, the core of the emergence of cooling lines can be prevented lies in the control of the heat on the surface of aluminum foil. Firstly, preheating can be used to improve the uniformity of heat in the annealing process of aluminum foil, reduce the surface elastic stress and improve the temperature diffusion rate, and external heaters can be used if necessary to reduce the cracking tendency. In addition, the surface stability of aluminum foil can be improved by improving the aluminum foil crimping process to reduce the appearance of cooling lines.

Aluminum foil is an aluminum product with more processes, more difficult processing technology and thinner thickness in aluminum processing production. It is usually processed from industrial pure aluminum. The production of aluminum foil involves all rolling processes and heat treatment processes from molten casting, hot rolling, cold rolling to foil rolling. The production level of aluminum foil represents the advanced degree and development level of the aluminum processing industry. Aluminum foil is also a very critical product for daily civilian needs in the national economy. Therefore, it is very important to improve the quality control of the colored aluminum foil annealing process.