Int J Adv Manuf Technol DOI 10.1007/s00170-015-7963-8 ORIGINAL ARTICLE Improvement of AA5052 sheet properties by electromagnetic twin-roll casting J. T. Li1,2 & G. M. Xu1 & H. L. Yu2 & G. Chen1 & H. J. Li2 & C. Lu2 & J. Y. Guo3 Received: 6 April 2015 /Accepted: 11 October 2015 # Springer-Verlag London 2015 Abstract Electromagnetic fields were used in twin-roll cast- 1 Introduction ing (TRC) of aluminum alloy 5052 (AA5052) for improve- ment of the microstructure and mechanical properties. A static Twin-roll casting (TRC) is proposed as an alternative to con- magnetic field induces an inhibiting effect on the melt in the ventional direct chill (DC) casting followed by hot rolling, a cast-rolling area and reduces diffusion of the solutes. It also proven technology for economical production of thin alumi- results in more nucleating opportunities and less segregation, num sheets directly from the melt [1–3]. TRC is a shorter thus enhancing the mechanical properties. However, the static process route that combines casting and dynamic hot defor- magnetic field does not change the orientation of crystal mation in a single step and is suitable for the production of thin growth and columnar crystals still exist in microstructure. sheets (mm gauge). The method involves pouring the melt On the other hand, an oscillating magnetic field can refine into the gap between two rotating and water-cooled cylindrical the suspended particles and induce strong convection. This rollers. The metal solidifies just before reaching the bite of the leads to more uniform distribution of temperature and solute rollers and is then rolled as it passes through the rollers. Com- elements, simultaneously increasing nucleating opportunities pared with conventional DC casting, the TRC technique can and decreasing segregation, thereby enhancing the mechanical lead to reduced capital and operating costs, less energy con- properties. An oscillating magnetic field also inhibits the ori- sumption, and reduced scrap rate. entation of crystal growth and makes finer and equiaxed In the TRC process, several methods have been used for grains. controlling the microstructure and thus the mechanical prop- erties. Berg et al. [4] accomplished gauge reduction from 5 to 1.9 mm. The thicker 5-mm strip did indeed show the typical Keywords AA5052 . Twin-roll casting . Segregation . dual-grain microstructure expected of TRC material in the as- Inhibiting effect . Oscillating effect cast state. However, the 1.9-mm strip exhibited purely equiaxed grains finer in size as compared to the 5-mm strip. No cell structure region was seen within the grains, and solid- * G. M. Xu ification was apparently completed by equiaxed grain growth [email protected] in the mushy zone. Haga et al. [5] combined low superheat * H. L. Yu casting and semisolid casting with an unequal diameter twin [email protected] roll caster with a long solidification length. The macrostruc- ture of the as-cast strip was equiaxed and spherical, not co- lumnar. Haga et al. [6] also carried out semisolid strip casting 1 Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, with a twin roll caster equipped with a cooling slope, leading China to improvement in sheet elongation. Das et al. [7]compared 2 School of Mechanical, Materials and Mechatronic Engineering, the microstructure resulting from TRC with that produced by University of Wollongong, Wollongong, NSW 2500, Australia melt-conditioned TRC (MC-TRC). A uniform, fine, and 3 Hongyanhe Project Department, China Guangdong Nuclear Power equiaxed grain structure was observed in as-cast MC-TRC Engineering Co. Ltd, Dalian 116319, China samples. However, coarse columnar grains with centerline Int J Adv Manuf Technol segregation were observed in the as-cast TRC samples. Kim field. The application of a pulsed electromagnetic field led to et al. [8] investigated the feasibility of producing high-strength an increase in the dendrite fragmentation rate. Yu et al. [20] Al sheets with high solute content using a twin roll strip caster directly applied alternative current (AC) to the melt in the equipped with an asymmetric nozzle. The centerline segrega- mold through the launder during electromagnetic continuous tion and hot tear of the sheets could be reduced, thereby in- casting (EMCC) of copper billets. The imposition of the AC creasing the casting speed. Cheon et al. [9]successfullyfab- current on the melt during EMCC significantly increased the ricated Al alloy sheets using a combination of twin roll strip convection and vibration in the melt, which are very beneficial casting and asymmetric rolling. The Al sheets exhibited ex- for the solidification of the alloy. cellent formability and mechanical properties, far exceeding Electromagnetic fields obviously have an effect on grain those in commercially available Al sheets. Sun et al. [10] refinement and microstructure improvement during melt so- homogenized the AA3105 alloy prior to cold rolling and an- lidification. However, there have been few studies of the effect nealing and found that homogenization reduced the supersat- of an electromagnetic field on TRC. The AA5052 alloy has uration and coarsened the constituent particles, thus reducing been attracting increasing attention in recent years because of the total number of particles. Homogenization also resulted in its desirable attributes for processing, machining, and welding a marked reduction in the recrystallized grain size and faster [21–24], making it suitable for use in the automobile, ship- recrystallization kinetics after cold rolling and annealing. ping, machinery manufacturing, and other industries [25–27]. Birol [11] also investigated the effect of homogenization on However, in the forming process of AA5052 sheets, segrega- TRC-processed thin Al-Mn strips and obtained similar results. tion has always been a critical hindrance to improvement of Imposition of an electromagnetic field can provide more the microstructure and mechanical properties [28, 29]. In the possibilities to control the development of the microstructure. present study, static and oscillating magnetic fields were im- Hicher et al. [12] designed a new mirror furnace crystal posed on the AA5052 melt with a view to abating segregation growth device using a static external electric field. They found and improving the microstructure and mechanical properties. a significant change in the melting temperature and a strong The effects of the electromagnetic field on the microstructure, change in the separation between the liquid and solid states segregation, and mechanical properties such as tensile during growth upon application of an electric field. Ma et al. strength are examined. [13] analyzed the thermal and fluid effects caused by applying alternating electric fields during BiMn/Bi eutectic directional solidification. They found that the microstructure formation depends on the frequency of the applied alternating current 2 Experimental investigation and that it changes spontaneously as the alternating electric field is applied. Li et al. [14, 15] investigated semi-continuous AA5052 sheets were produced by twin-roll casting, both with casting of Al alloy with the application of a static magnetic and without an electromagnetic field. The twin-roll caster has field. They found that the static magnetic field changes the two counter-rotating rollers, which are water-cooled from the microstructure from regular columnar grains to twinned la- inside. The roller shell is made of heat-resistant alloy steel. mellas. Jie et al. [16] used a rotating magnetic field (RMF) The roller gap is adjusted by hydraulic pressure. The twin- during solidification of hypereutectic Al-Si alloy and found roll caster dimensions are listed in Table 1.AnAA5052alloy that it resulted in efficient congregation of the primary Si (composed of 2.8 % Mg, 0.40 % Mn, 0.50 % Cr, 0.25 % Si, phase near the inner wall of the crucible and formation of a 0.10 % Cu, 0.40 % Fe, and 95.5 % Al) was used in the TRC Si-rich layer. This suggested that a forced intense melt flow experiments. An Al ingot was completely melted in an elec- combined with proper cooling conditions can greatly change trical resistance furnace. After the furnace temperature the solidification structure of alloys, which could be beneficial reached 700 °C, Mg, Mn, and Cr were added to the melt. for microstructure control. Zhang et al. [17, 18] studied the The well-stirred melt was then heated up to 800 °C. This influence of a low-frequency electromagnetic field on the mi- was followed by standing and filtration in the temperature crostructure and macrosegregation in continuous casting. In range 720–740 °C. When the temperature reduced to the presence of an electromagnetic field, a substantial reduc- 690 °C, the TRC experiment commenced. Figure 1 presents tion of grain size and macrosegregation of the alloy elements the flow chart of the melting process of AA5052. was achieved. The frequency of the applied electromagnetic The melt was poured into a sodium silicate thermal insula- field was found to play a significant role in grain refinement tion nozzle which was pre-heated to 400 °C. The twin-roll and macrosegregation inhibition. Liotti et al. [19] developed caster with copper coil excitation apparatus on the upper roller an in situ technique for studying the effect of a pulsed electro- was prepared for the AA5052 sheet production and is sche- magnetic field on dendrite fragmentation. They used synchro- matically illustrated in Fig. 2. The experimental rolling speed tron X-ray imaging, involving the passage of an oscillating was 1.4 m/min. The cross section of the AA5052 sheets was current through a foil specimen placed in a static magnetic 5×400 mm. Int J Adv Manuf Technol Table 1 Dimensions of twin-roll caster Diameter of upper roller Diameter of lower roller Minimum rolling speed Maximum rolling speed 500 mm 500 mm 0.5 m/min 7 m/min A static magnetic field was applied to cast-rolling area with 3Results a nominal strength of 0.3 T.