Process and Product Comparison of Cast-Rolling Method and Hot-rolling Method for Producing Air-Conditioning Aluminum Foil

Abstract: The mechanical properties and structure of aluminum foil produced by the casting-rolling method and the hot-rolling method have their own characteristics, and the air-conditioning foils are suitable for the requirements of the corresponding fin forming method. Compared with the hot-rolling method, the casting-rolling method has the advantages of short working procedure, low energy consumption and low cost. Through technological innovation, cast-rolled billets can be used to produce air-conditioning foil products with a wide range of adaptability. Considering both technical and economic aspects, the production of air-conditioning foil by the casting-rolling method has obvious advantages.

Huge aluminum coils

At present, there are two production methods of aluminum foil for cooling fins of air conditioners in my country (hereinafter referred to as air conditioning foil): one is the hot-rolled billet method, and the other is the cast-rolled billet method. The traditional hot rolling method is to cast aluminum melt DC into ingots with a thickness of several hundred millimeters. After homogenization, finishing, heating, hot rough rolling, hot finishing rolling, etc., it is rolled to a thickness of several millimeters, and then it is cold rolled. , Aluminum foil rolling and other processes; while the casting-rolling method is to directly process the aluminum melt into a cast-rolled coil with a thickness of 6-8mm with a double-roll casting-rolling machine. Compared with the hot rolling method, the cast rolling method has the advantages of less investment, high efficiency and low cost. Since it was used in industrial production, it has been continuously promoted, improved and perfected.

Air-conditioning foil is used to make heat-dissipating fins of air-conditioners. The heat-dissipating fins can be formed by stretching, thinning and stretching. After years of research and development at home and abroad, the hot-rolling method is becoming more and more perfect. The hot-rolling method is generally preferred for the production of aluminum foil for stamping products. However, what are the characteristics of the air-conditioning foil produced by the hot rolling method? Which fin forming method is more suitable for it? There are not many discussions. The casting-rolling method is an aluminum processing method that is gradually being improved. Whether it can produce qualified or even high-quality air-conditioning foil is still controversial. This article attempts to analyze the billet characteristics and deformation characteristics of the hot rolling method and the cast-rolling method from the technical and economic point of view, and combine the field practice and scientific research results of the cast-rolling method to produce air-conditioning foils. The affirmative answer.

1  process flow

The process flow of the cast-rolling method for producing air-conditioning foil: smelting-cast-rolling-cold rolling-aluminum foil rolling; the process flow of the hot-rolling method for producing air-conditioning foil: melting-casting-saw-cutting-milling-face homogenization, heating-hot-rolling-a Cold rolling-aluminum foil rolling. Compared with the two methods, the hot rolling method has a long and complicated process, at least 4 more processes than the cast-rolling method. The casting-rolling method provides us with a shortcut to directly produce thin slabs suitable for cold rolling from liquid metal. It greatly simplifies the production process of plate, strip and foil products and significantly reduces the cost, thereby making its products more competitive . The energy consumption analysis of the American Hunter Engineering Company on the casting-rolling method shows that the Hunter casting-rolling method can save 48.6% energy than the hot-rolling method, and its initial investment is only 10%-15% of the hot-rolling method. The investment is only 40%-46% of the hot rolling method. In view of the characteristics of the casting-rolling method, the level of industrial technology and the allocation of resources, the twin-roll casting-rolling process has first received the attention of developing countries. Its huge market demand has further stimulated the interest of developed countries in equipment research and development and technological progress.

There are more than 110 twin-roll casting and rolling mills in our country, and their capacity to produce aluminum billet exceeds 935kt/a[, becoming the world’s third largest twin-roll aluminum strip casting after Pechiney Aluminum Group and FataHunter Group. A large rolling country, it is the country with the largest number of cast-rolling mills in the world, and its casting-rolling aluminum billet capacity accounts for 33.31% of the cold-rolling aluminum billet capacity. According to expert predictions, based on the rapid progress of cast-rolling technology, the characteristics of my country’s economic development and historical reasons, the proportion of cast-rolled aluminum billets in my country should undoubtedly be larger than that of developed countries, reaching 50%, which is more appropriate to be equal to hot-rolled aluminum billets. . Therefore, the casting-rolling method has a broad market in my country’s aluminum processing industry and is facing opportunities and challenges of technological progress.

2 Product organization and performance
2.1 Organizational characteristics

The hot rolling method is usually DC cast into 300-600mm thick aluminum ingots, hot rolled to 2.5-12mm thick hot rolled aluminum coils, after cooling to room temperature, further cold rolling, annealing and aluminum foil rolling; continuous casting The rolling method directly produces 6-8mm thick cast-rolled strip and coil from the aluminum melt, and then performs subsequent processing such as cold rolling. The cooling rate of traditional DC casting is generally 2-3 degrees Celsius per second, and the ingots produced by it generally have three organizational areas, namely, the outer chill area, the middle columnar crystal area, and the central equiaxed crystal area; while the double-roll casting cooling The speed is about 300 degrees Celsius per second. The cast-rolled crystallization has the characteristics of rapid solidification and directional crystallization. The structure of the produced strip is generally only columnar crystals and equiaxed crystals, which are usually formed in the center of the thickness of the cast-rolled strip. Equiaxed crystal regions, and columnar crystals are formed on the surface area. Figure 1 is a physical photo of the solidification and deformation process of aluminum melt from the nozzle to the roll gap of the casting roll.

The high cooling rate of twin-roll casting greatly refines the internal structure of the cast-rolled strip, promotes the formation of metastable phases, and causes supersaturation of alloy elements such as Mn, Fe, Cr and Zr, and the particle size of intermetallic compounds Smaller, only about 1/5 of the hot rolled billet. Figure 2 shows the distribution of dendrite network sizes in the thickness direction of cast-rolled aluminum strip. Under normal circumstances, the dendritic network spacing from the surface to the center of the cast-rolled strip is 4-6um, with a comprehensive average of 5um, and the intermetallic compound particle size is 1um. In ultra-thin and rapid casting and rolling, smaller dendrite spacing (1.5um) and higher cooling rate are also reported. Table 1 lists the phase composition observed in Al-Fe-Si alloy DC ingots and continuous cast-rolled strips. It can be seen from Table 1 that the phase composition of DC ingots and continuous cast-rolled strips are different due to different cooling rates and element contents. When the Fe/Si ratio is large, Al-Fe binary intermetallic compounds are formed; when the Fe/Si ratio is equal to or less than 1, Al-Fe-Si ternary intermetallic compounds are formed; as the cooling rate increases, the meta-stable The probability of phase formation increases.

Figure 1 The actual photo of aluminum melt solidification in the casting-rolling zone

Figure 1 The actual photo of aluminum melt solidification in the casting-rolling zone

Figure 2 The change of dendritic network size in thickness direction of cast-rolled Al-Fe-Si strip

Figure 2 The change of dendritic network size in thickness direction of cast-rolled Al-Fe-Si strip

Table 1 Phase composition observed in Al-Fe-Si alloy DC ingot and continuous cast-rolled strip

Table 1 Phase composition observed in Al-Fe-Si alloy DC ingot and continuous cast-rolled strip

2.2 Performance characteristics
The initial hot-rolled billet is generally a deformed structure, while the cast-rolled strip is a cast structure with 10%-25% cold deformation. Air-conditioning foils are usually produced with Al-Fe-Si or Al-Fe-Mn 1XXX and 8XXX aluminum alloys. For cast-rolled strips, due to the strong supersaturated solid solution and intragranular segregation of alloy elements Mn, Fe, and Si, and the columnar crystals in the oriented arrangement of cast-rolled strips tend to be more inclined to the longitudinal elongated arrangement of parallel plates during cold deformation. Not broken, as a result, after cold rolling and annealing, the recrystallization process of cast-rolled aluminum sheet is significantly lagging behind that of hot-rolled aluminum sheet, the temperature at the end of recrystallization is much higher, and the recrystallization temperature range is also significantly expanded. Some documents point out that recrystallization The temperature can be increased up to 140 degrees Celsius, as shown in Table 2. This feature determines that when producing air-conditioning foils, cast-rolled billets are superior to hot-rolled billets for certain varieties.

Table 2 The recrystallization temperature of cold-rolled aluminum plates produced by the casting-rolling method and the hot-rolling method

Table 2 The recrystallization temperature of cold-rolled aluminum plates produced by the casting-rolling method and the hot-rolling method

For the 1050 alloy, if the homogenization treatment is not carried out, after cold rolling, the aluminum plate produced by the casting-rolling method has a strength 10%-15% higher than that of the aluminum plate produced by the hot-rolling method within a comparable thickness range after cold rolling. 10% to 20% higher; and after homogenization treatment and then cold rolling, the strength of the cast-rolled plate is not lower than that of the hot-rolled plate, and its elongation at break is also similar [6]. The performance comparison of the 1235 aluminum alloy foil produced by the hot rolling method and the cast-rolling method is shown in Table 3. The cast-rolling method adopts the company’s 7MM cast-rolled strip after cold rolling and intermediate annealing, and then supplies the aluminum foil for rolling in the H14 state; The hot-rolled blank for comparison is 1235-H14 aluminum foil wool imported from South Korea. The alloy, state and thickness of the two blanks are comparable. It can be seen from the data in Table 3 that due to the greater degree of supersaturation of the alloying elements, the strength of the aluminum foil after the cast-rolled billet is rolled is higher than that of the aluminum foil rolled by the hot-rolled billet, and the elongation is lower. At the same time, the work hardening speed of the cast-rolled strip during cold working is relatively high.

Table 3 Properties of 1235 aluminum alloy foil by hot rolling method and cast rolling method

Table 3 Properties of 1235 aluminum alloy foil by hot rolling method and cast rolling method

3 Adaptability of casting-rolling method and hot-rolling method to produce air-conditioning foil
3.1 Fin forming method
The cooling fins of the air conditioner are mainly formed by stretching and thinning and stretching, and there are also dual-use or improved methods on this basis. The fin structure also has the shape of flat fin, corrugated fin, and longitudinal slit fin. These are all in order to adapt to the requirements of heat exchangers in the direction of high performance, miniaturization and light weight. Figure 3 is a schematic diagram of the two main fin forming methods. Among them, stretch forming is an earlier forming method. Its forming speed is slow (200 times/MIN). The height of the fin flange is generally not less than 1.8mm, and the height can reach 3.2mm. It is suitable for 1XXX series. 8. Air conditioning foils of 8XXX series alloys that require large deep drawing capabilities.

Due to the low tensile strength, the thickness of the aluminum foil is generally above 0.11mm; the thinning stretch forming is developed for the purpose of high speed and thin wall A forming method that matured around 1980 and has now become the main forming method for heat sink fins. The forming speed can reach 350 times/min or more. The flange height of the fin is generally below 2.0 mm. The air conditioner foil used The thickness is usually not greater than 0.11mm, suitable for industrial pure aluminum, Al-Fe-si or AI-Fe-Mn aluminum alloy air conditioning foil containing Mn, Ti, Zr and other trace elements. The aluminum foil is required to have a certain degree of plasticity. In the case, maintain a higher intensity. Some improved methods for thinning and stretching require that the air-conditioning foil has high strength, high plasticity and high deep drawability at the same time, so as to adapt to the thinning of the fin and achieve the purpose of reducing costs.

Figure 3 Schematic diagram of fin forming process

Figure 3 Schematic diagram of fin forming process

3.2 Alloy composition of air conditioning foil
At present, the alloy composition of air conditioning foil produced by hot rolling in my country is concentrated on 1050, 1100, 1200, 3102 aluminum alloy varieties, while the casting-rolling method mostly adopts 8xxx series or adds trace elements such as Mn, Ti, Zr on the basis of lxxx series. Of aluminum alloy. In order to meet the requirements of thinning, anti-corrosion and high performance of fin aluminum foil, 1xxx series air-conditioning foils with poor corrosion resistance, formability and process performance have gradually faded out of the market. Table 4 shows the commonly used aluminum alloy grades for air-conditioning foils in my country.

Table 4 Air-conditioning foil aluminum alloy grades and element content (mass fraction/%)

Table 4 Air-conditioning foil aluminum alloy grades and element content (mass fraction/%)

3.3 Adaptability of air conditioning foil performance
Although there are differences in the structure and properties of aluminum foil produced by the casting-rolling method and the hot-rolling method, this does not indicate which of the hot-rolling method and the casting-rolling method is more suitable for the production of air-conditioning foils, because there are more varieties of air-conditioning foils and forming properties. , The forming methods are very different, and the performance requirements have their own focuses. For example, some require high strength, some require high deep drawing performance, and some require both. The following combines the forming characteristics of the air conditioner fins to discuss the adaptability of the air conditioner foil produced by the hot rolling method and the cast rolling method to different forming processes.

3.3.1 Thinning stretch forming From the comparison between the hot rolling method and the cast-rolling method described above, it can be seen that the cast-rolled billet itself has some characteristics, such as the directional rapid crystallization of the cast-rolled strip, strong texture, and work hardening speed Fast, high product strength, poor plasticity, high recrystallization temperature, large interval, etc., resulting in high ear-making rate of finished foil stamping products and poor deep drawing performance. However, with the development and popularization of air-conditioning foils characterized by high-strength indicators, especially as the price competition of air-conditioners intensifies, the requirements for cost reduction are becoming more urgent. Therefore, the thickness reduction of air-conditioning foil has become a development trend. In this way, the high strength characteristics of cast-rolling aluminum foil may become an advantage of the cast-rolling method for producing air-conditioning foil. The HL01 air conditioning foil developed by our company on the basis of 1050 alloy has the characteristics required for thinning and stretching air conditioning foil.

Figure 4 is the softening curve of HL01 air conditioning foil. It can be seen from the figure that the strength of the HL01 air conditioning foil produced by the cast-rolled billet is basically stable within the range of 250-300 degrees Celsius. The softening curve and metallographic analysis show that the recrystallization start temperature of aluminum foil is about 270 degrees Celsius. Since the performance of the air conditioning foil used in the fin thinning and stretching method is basically at the stage of metal recovery and partial recrystallization, the emphasis is on the stability and improvement of the strength of the aluminum foil. Therefore, compared with the hot-rolled aluminum foil, the cast-rolled aluminum foil has higher strength and larger recrystallization interval, and is more suitable for the production of thinning-stretch forming air-conditioning foil.

Figure 5 is a photo of the metallographic structure of the thinned and stretched air-conditioning foil. It can be seen from the figure that the structure is a partially recrystallized deformed microstructure, while the cast-rolling air-conditioning foil has a more uniform structure and a finer dispersion of compound particles. A higher strength and better thinning and tensile properties.

Figure 4 HL01 air conditioning foil softening curve (annealed at each temperature for 1h)

Figure 4 HL01 air conditioning foil softening curve (annealed at each temperature for 1h)

Figure 5 Metallographic structure of two air-conditioning foils(a), (b) a HLol air-conditioning foil; (c), (d) a Japanese MF03 air-conditioning foil (used Hot rolled billet production)

Figure 5 Metallographic structure of two air-conditioning foils(a), (b) a HLol air-conditioning foil; (c), (d) a Japanese MF03 air-conditioning foil (used Hot rolled billet production)

3.3.2 Stretch forming Generally, the hot-rolled aluminum foil has higher elongation and lower strength, which is more suitable for the requirements of the fin stretch forming method. The cast-rolling air-conditioning foil has higher strength and poor plasticity, which is not conducive to deep drawing deformation. In order to overcome the shortcomings of low plasticity of cast-rolling air-conditioning foil and meet the requirements of high deep drawing and high-flange fin forming, HL02 air-conditioning foil with Fe and Mn added to industrial pure aluminum and its casting-rolling production process were developed. Industrial production shows that the height of the flange of the HL02 air-conditioning foil fin can reach 3.2 mm, showing excellent deep drawing performance. Figure 6 shows the changes in the tensile strength, elongation and cupping properties of HD32-O air conditioning foil with the thickness of the aluminum foil, and Figure 7 is the graph of the HI/) 2-0 air conditioning.

As shown in the figure, when the thickness of the air conditioning foil is reduced to 0.076 mm, its elongation still reaches 24%, the strength is 127 MPa, and the cupping value is as high as 7.0 mm. It can be seen that the HL02 air-conditioning foil developed with new technology has obtained extremely high plasticity and deep drawing performance, and at the same time has quite high tensile strength. It not only fills the gap in the production of soft (0) state and high deep-drawing performance air-conditioning foil by the casting-rolling method, but also has good thinning and versatility due to high strength. Through the use of different production processes, HL02 air conditioning foil can not only meet the requirements of stretch forming, but also meet the requirements of thinning stretch forming, especially for the fin forming method that requires high strength and high plasticity, HL02 air conditioning foil Have good adaptability.

Figure 6 Mechanical properties of HL02-O air-conditioning foil produced by the cast-rolling method

Figure 6 Mechanical properties of HL02-O air-conditioning foil produced by the cast-rolling method

Figure 7 Load-deformation curve of HL02-O air-conditioning foil produced by the cast-rolling method

Figure 7 Load-deformation curve of HL02-O air-conditioning foil produced by the cast-rolling method

4 Unification of blank characteristics and product performance requirements
The hot rolling method and the cast-rolling method have their own characteristics. The literature [6] studied the processing characteristics of 1050 aluminum alloy hot-rolled billets and cast-rolled billets, as shown in Figure 8. Choose 4 kinds of blanks, namely 10MM and 4.3MM hot-rolled blanks and 8mm and 6mm cast-rolled blanks. After the cast-rolled billet and the hot-rolled billet are rolled to the same thickness, they are homogenized at 550 degrees Celsius for 1 hour, then cold rolled to 0.5 mm, and then annealed at 200-450 degrees Celsius. The results show that the tensile properties of the 10mm hot-rolled blank samples are better than those produced by other blanks. But when the annealing temperature is higher than 350 degrees Celsius, the cupping values of all the produced materials are almost the same, and other performance indicators such as: elongation at break, elastic modulus, etc. also have the same changing law after high temperature annealing.

Figure 8 Cupping value of 0.5MM aluminum plate produced by hot rolling method and cast rolling method

Therefore, even if the same blank is used, when the process of producing the foil is different, different performance results will be produced; and the foil with different blanks and technological innovation may achieve the same performance. According to the previous discussion, the hot rolling method is more suitable for the production of air-conditioning foils with high deep-drawing performance, while the cast-rolling method is more suitable for the production of high-strength air-conditioning foils. Moreover, after process improvements, the casting-rolling method can produce A universal air-conditioning foil product with a fin forming method.

5 Concluding remarks Currently, cast-rolled aluminum billets in my country account for one-third of cold-rolled aluminum billets
1. The ratio in developed countries is only about 17%, and they have sufficient hot-rolled billet supply capacity. Due to historical and practical reasons, there is little or no need for developed countries to conduct process research on the production of air-conditioning foils and other stamping products from cast-rolled aluminum billets. Inherent concepts lead people to doubts about the production of deep-drawn aluminum products by the casting-rolling method. This research work shows that the cast-rolled aluminum billet can produce high-quality air-conditioning aluminum foil, and, from the perspective of economy, the cast-rolling method is more advantageous than the hot-rolling method. The new development trends of air-conditioning foils are: First, the trend of thinning. In order to reduce costs, the thickness of domestic high-quality air-conditioning foils has been reduced to 0.095mm, and there are reports as thin as 0.08mm in foreign countries; the second is the trend of high flanges, commercial air conditioners The flange height of the fins in the H26 state of the air conditioner is close to 2mm, while the flange height of the fins in the H22 and 0 state of the central air conditioner is up to 3.2mm. Foreign companies have put forward orders for air-conditioning foil products with a flange height of 6.4mm. Requirements, this can reduce the number of layers of air-conditioning fins and reduce costs; the third is the trend of low flanges, the flange height is generally less than 1.3mm, which can meet the requirements of miniaturization and individualization of air conditioners. All the above development trends require the technological progress of air-conditioning foil to develop in the direction of high strength, high plasticity and high deep drawing performance. Therefore, to expand the air conditioning foil market, companies should vigorously develop new alloys and research new processes.

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