Continuous casting technology to improve the surface quality of aluminum alloy continuous casting billets

This article systematically reviews various continuous casting technologies to improve the surface quality of aluminum alloys, and analyzes the advantages and disadvantages of various continuous casting methods from different perspectives. The effects of electromagnetic casting, electromagnetic soft contact casting, and air film casting on the surface quality of aluminum alloy billets are discussed in detail.

Keywords: surface quality; segregation; cold shut; air film casting

Ingot casting is one of the important processes in aluminum alloy processing. The quality of ingots greatly affects the processing process and product quality of aluminum alloys. Improving the surface quality of ingots can effectively improve the quality of extruded products and the utilization rate of materials. Ingots with smooth surfaces, fine grains, and uniform structures can be directly extruded into products. With the development of aluminum alloy materials and processing technology, the requirements for the quality of ingots, especially the surface quality, are getting higher and higher.

During the continuous casting process, aluminum alloys are subject to primary cooling by the crystallizer and secondary cooling by cooling water. Due to the cooling shrinkage of the boundary layer, defects such as subcutaneous segregation, cold shut, surface pull marks, segregation tumors, etc. often appear in the ingot. Changing the contact state and heat exchange mode between the surface of the ingot and the inner surface of the continuous casting mold is an important measure to eliminate surface defects and improve the surface quality of the ingot. In order to improve the quality of ingots, ingot casting methods are also constantly improving. Semi-continuous casting technologies mainly include direct cooling casting, hot top casting, and moldless electromagnetic continuous casting technology. On this basis, low-down casting technology, electromagnetic soft contact casting technology, and air film continuous casting technology have been developed.

Direct cooling casting

Traditional direct cooling casting

Traditional direct cooling casting is the earliest method used for continuous casting of aluminum alloys. Its principle is shown in Figure 1a. The effective cooling height of the crystallizer is large, generally 80~120mm. Due to the large effective cooling height of the crystallizer, the crystallizer is made of stainless steel, forged aluminum alloy or copper. Since metal has good thermal conductivity, as long as the liquid metal comes into contact with the inner wall of the crystallizer during the continuous casting process, a primary solidification shell will be formed. After the slab solidifies and shrinks, the solidified part leaves the mold from a few millimeters below the solidification starting point, resulting in a gap between the slab and the inner wall of the mold. Due to the low thermal conductivity of air, the condensation shell will be reheated by the internal melt during the downward movement, and remelt before being sprayed with water for secondary cooling. The metal hydraulic head pushes the solute enriched layer at the solidification front to the surface, forming segregation tumors on the surface of the cast slab, and defects such as wrinkles, stretch marks, and cracks on the surface of the cast slab. This method is now rarely used in continuous casting of aluminum alloys.

Lower your head and cast

Low-down casting is based on traditional direct cooling casting and is designed to reduce the effective cooling height of the crystallizer. Its working principle is shown in Figure 1b. It can be seen from the figure that the metal liquid level in the crystallizer is lower than that of traditional direct cooling casting, about 25~40 mm. The reduction of the metal liquid level reduces the heat transfer of the crystallizer, the liquid cavity is straight, segregation and wrinkles are controlled, and the internal quality and surface quality are effectively improved. In the early days of low-down casting, the metal liquid level was mainly controlled manually, which was difficult to operate and could easily lead to splashing, leakage and other phenomena. It also placed high demands on workers’ operating skills and physical strength, so it was not widely used. However, with the continuous development of laser and computer technology, laser liquid level control systems have been widely used in semi-continuous casting. The metal liquid level fluctuation is small and the ingot surface is smooth.

Figure 1 Schematic diagram of direct cooling casting

Casting with insulation layer on inner wall

Since the control of the liquid level during the low-down casting process places high demands on the workers’ operating skills and physical strength. In order to reduce the dependence of the surface quality of the ingot on labor, an insulation layer is attached to the inner wall of the crystallizer to increase the metal liquid level, as shown in Figure 1c. This greatly reduces the difficulty of workers’ operation and the surface of the ingot is smooth. This method is still used in China, but during the continuous casting process, because the primary solidification shell is formed on the lower edge of the insulation layer, the flexible insulation layer easily falls off under the friction of the primary solidification shell, affecting the stability of the continuous casting process. Therefore, the cast slab produced by this method should not be too long.

Casting with external electromagnetic field (electromagnetic soft contact)

In 1987, Vives proposed the CREM (Casting, Refining, Electromagnetic) process, which arranges induction coils outside the DC casting mold. As shown in Figure 1d, a 50Hz power frequency alternating current is applied to the coil, and the stirring effect produced in the metal melt is used to refine the grains and improve the surface quality.

The alternating current in the coil generates a vertical alternating electromagnetic field inside the melt. The induced current inside the metal melt interacts with the magnetic field, causing the melt to be affected by the Lorentz force. Due to the asymmetry of the geometry of the ingot and the crystallizer in the vertical direction, the magnetic field lines are significantly deflected relative to the center line of the ingot, resulting in the existence of vertical and horizontal components of the Lorentz force inside the melt. The horizontal component of the Lorentz force causes the free surface of the melt to form a convex meniscus, thereby reducing the contact height and contact pressure between the melt and the crystallizer. The so-called soft contact is achieved, and the formation position of the primary solidification shell is lowered, the surface seepage phenomenon is weakened, and the thickness of the surface segregation layer decreases linearly with the increase of input power. The spinning force field formed by the vertical component of the Lorentz force acts as electromagnetic stirring, and the forced convection caused by the electromagnetic force brings the superheated melt in the center area to the edge area of the ingot. Therefore, local overheating in the central area is eliminated and the temperature difference in the entire liquid phase area is reduced. At the same time, the electromagnetic force breaks the primary dendrites to form a new core, refines the grains, increases the melt permeability coefficient, makes it difficult for the melt to penetrate the surface under the static pressure of liquid metal, and inhibits segregation. CREM can effectively improve the surface quality of the ingot, but does not have a significant impact on macrosegregation in a large-scale range on the cross-section of the ingot. When using the CREM process, the conductivity of the crystallizer has an important influence on the distribution of magnetic induction intensity inside the melt. Using a crystallizer with lower conductivity can reduce the eddy current loss of the electromagnetic field and greatly increase the effective power.

The electromagnetic soft contact continuous casting technology effectively reduces the primary cooling intensity. The cast slab can effectively eliminate defects such as subcutaneous reverse segregation and surface segregation tumors. The surface of the cast slab is smooth, which is a better continuous casting process. However, this process requires high electromagnetic field design and high investment costs. At present, it is difficult to find suitable materials with high thermal conductivity, low electrical conductivity, and high temperature resistance to make the crystallizer. Therefore, the development of this technology is restricted by certain conditions.

Hot top casting

Traditional hot top casting

The principle of this method is shown in Figure 2. A refractory material jacket is added to the crystallizer of traditional direct cooling casting to reduce the effective cooling height of the crystallizer and reduce the heat transfer of the crystallizer. Moreover, horizontal pouring is adopted. It is easy to control the aluminum alloy liquid level by installing a liquid level controller on the pouring system. The aluminum alloy flows smoothly, avoiding disturbance and air entrapment during pouring, and the inherent quality of the cast slab is stable. However, there is a hidden cold separation on the surface of the cast slab, and during the pouring process, slag is easy to hang on the lower edge of the hot top, resulting in serious surface pull marks. The emergence of hidden cold shut makes the component segregation thickness of the casting slab exceed 1mm; the crystallizer is inlaid with a graphite ring, which reduces the friction between the solidification shell and the crystallizer through the self-lubricating effect of the graphite ring. In domestic hot top casting, lubricating oil is not used continuously, but a little is used at the corner of the graphite ring and the hot top before pouring starts. This oil is quickly consumed during the casting process, and the draw marks on the rear section of the cast rod are heavier than those on the front section, especially after the graphite ring has been worn for a period of time. Although this method is effective in eliminating hidden cold isolation, it is unstable and cumbersome to operate, seriously affecting its production efficiency.

Fig.2 Schematic diagram of hot top casting

Casting with external electromagnetic field (electromagnetic soft contact)

In recent years, Cui Jianzhong and others developed LFEC (Low Frequency Electromagnetic Casting) technology based on hot top casting and CREM by arranging AC induction coils around the crystallizer and using lower frequencies (less than 50 Hz). Using low-frequency electromagnetic fields, the ability to penetrate the melt is stronger, and more electromagnetic fields act on the aluminum melt. Zhang Beijiang et al. used the low-frequency electromagnetic casting LFEC process to prepare 7075 aluminum alloy ingots with diameters of 100mm and 200mm.

Dong Jie et al. used the low-frequency electromagnetic casting LFEC process to prepare ultra-high-strength Al-Zn-Mg-Cu series B96 and 7A60 aluminum alloy ingots with diameters of 100mm, 200mm and 270mm. The results show that LFEC can not only refine the structure and improve the surface quality, but more importantly, it can also increase the relative content of intragranular alloy elements. It can also enhance the crack resistance of the ingot and reduce macrosegregation.

Air film casting

In the late 1970s, metallurgical researchers in the United States, Germany, and Japan developed a new casting method-air film casting based on hot top casting. The crystallizer in the United States uses a porous graphite ring. The oil and gas mixture enters the interior of the crystallizer through the porous graphite ring under the action of pressure, reducing the friction between the initial condensation shell and the inner wall of the crystallizer. Crystallizers in Germany and Japan use annular air gaps. Oil and gas enter the crystallizer through the annular air gaps, and the liquid metal solidifies and forms under the constraints of the oil and gas mixture. Although the crystallizer structures in the United States, Germany, and Japan are different, their casting principles are the same. The casting principle is shown in Figure 3: Gas and lubricating oil are introduced into the casting mold. During the pouring process, a gas film is formed between the inner wall of the casting mold and the molten metal. The molten metal moves downward under the constraints of the gas film, and the outside solidifies. Remove the mold and spray water for secondary cooling to form an ingot.

Fig.3 Schematic diagram of air film casting

The characteristics of gas film continuous casting are: ① Liquid metal solidifies and forms under the constraints of gas, and the surface of the ingot is relatively smooth. ② The solidification heat transfer mode is changed. Since there is a gas film between the crystallizer and the molten metal, the thermal conductivity of the gas film is greatly reduced. Therefore, the air film reduces the effective cooling height of the crystallizer and reduces the cooling intensity of the crystallizer. Most of the heat released by solidification is taken away by the cooling water, while the crystallizer takes away very little heat. ③The shape of the solid-liquid interface is changed. Since the heat released by solidification is mainly taken away by the secondary cooling water, the curvature of the solid-liquid interface and the depth of the liquid phase cavity are reduced. ④ The intrinsic quality of the cast slab is improved. Due to the relatively enhanced secondary cooling capacity, the curvature of the solid-liquid interface is reduced, the internal grain size of the cast slab is reduced, the consistency of the grain size is improved, the dendrite spacing is reduced, and the problem of Macrosegregation. The thermal stress generated by solidification is reduced, reducing the tendency of cracks inside the slab.

The air film continuous casting technology effectively shortens the effective cooling intensity of the crystallizer. Due to the existence of the air film, the lateral heat dissipation is greatly reduced, the depth of the molten pool is shallow, the surface of the resulting cast slab is smooth, and the subcutaneous segregation layer is thin. Figure 4 shows a comparison of the surface quality of the slabs obtained by traditional hot top casting and air film casting. It can be seen from the figure that the slabs obtained by air film casting have no obvious cold insulation and surface segregation, and the surface of the slab is smooth.

(a) Hot top casting

(b) Air film casting

Fig.4 Surface quality of the ingot

In recent years, air film continuous casting technology has realized fully automatic computer control of the pouring process, and several companies such as the United States and Switzerland are still producing and selling complete sets of continuous casting equipment. However, the gas film continuous casting technology has obvious effects on small-sized slabs. For large-sized slabs, the gas pressure is difficult to control and the surface quality is uneven.

Electromagnetic casting

Electromagnetic Casting (EMC) technology is a moldless semi-continuous casting technology successfully developed by former Soviet scholar Getselev24 in the early 1960s. The principle is shown in Figure 5. When a medium frequency current is passed through the induction coil, the electromagnetic pressure on the liquid metal, the additional pressure caused by the metallostatic pressure and surface tension are balanced by the three forces, achieving contactless casting. In order to obtain the constraint required to support the melt, a sufficient magnetic induction intensity gradient must be formed on the melt surface. The electromagnetic field frequency used in EMC is usually 2000 Hz~3000Hz. After the emergence of electromagnetic casting, it was quickly applied in Czechoslovakia, Hungary, Germany and other countries. Mitsubishi Chemical Corporation of Japan introduced this technology in October 1972. Companies such as Kaiser, Alusuisse, Alcoa, Reynolds and Pechiney in the United States have successively introduced this patent. Among them, two companies, Kaiser and Alusuisse, based on the introduction of patents from the former Soviet Union, invested heavily and worked hard for several years to achieve multi-block (4~5 blocks) and large-section (500mm×1300mm) electromagnetic casting. Currently, approximately 1.2 million tons of aluminum alloys are produced annually in the United States and Europe using Alusuisse’s electromagnetic casting technology.

Fig.5 Schematic diagram of EMC

My country began research on electromagnetic casting of industrial devices in 1974. Northeast Light Alloy Processing Plant successfully completed the development of electromagnetic casting aluminum alloy ingots. They used electromagnetic casting technology to successfully cast aluminum alloy ingots with a diameter of 320mm. The results show that the surface quality of the electromagnetic casting billet is significantly improved, the dendrite spacing and grains are significantly refined, the segregation of chemical components is reduced, and the mechanical properties are significantly improved. In 1986, under the organization of the Nonferrous Metal Corporation, Southwest Aluminum Processing Plant, North China University of Technology, Northeast Light Alloy Processing Plant and Dalian University of Technology and other units carried out research on billet electromagnetic casting. Southwest Aluminum Processing Plant introduces German electromagnetic casting technology to cast ingots of various alloys. Dalian University of Technology has conducted fruitful research on EMC and achieved good results, preparing high-quality ingots of different specifications.

However, EMC aims to improve the surface of the ingot and uses medium and high-frequency electromagnetic fields to achieve moldless casting. It has high requirements on the equipment control system and is very prone to leakage. At the same time, due to the high frequency, the EMC process has limited impact on the flow field and temperature field of the melt inside the ingot. In the production process of large-size ingots of duralumin and superduralumin alloys, which are difficult to cast, it is difficult to obtain uniform casting. structure and eliminate internal stress in the ingot.

Conclusion

In DC casting, because the cooling length of the crystallizer is too long, the primary solidification shell is prone to secondary remelting during the continuous casting process, resulting in poor surface quality of the cast slab. Low-down casting, casting with an insulation layer on the inner wall of the crystallizer, external electromagnetic field casting, hot top casting, air film casting, etc. can effectively shorten the cooling length of the crystallizer and improve the surface quality. However, there are still many defects and deficiencies, so continuous casting technology needs to be further developed and improved.

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