HORIZONTAL CONTINUOUS CASTING OF Al AND Al-Si ALLOY IN SEMI-INDUSTRIAL CONDITIONS Tomasz WRÓBEL, Jan SZAJNAR Silesian University of Technology, Foundry Department, Towarowa 7, 44-100 Gliwice, Poland, tomasz.wrobel@polsl.pl Abstract In paper is presented idea of construction and influence of selected parts of stand of horizontal continuous casting on quality of aluminum with purity of 99,5% and 99,8% and AlSi2 alloy ingots. The main parts of the made stand belong to induction furnace, which is also tundish, water cooling continuous casting mould, system of recooling, system of continuous ingot drawing and cutting. Mainly was considered influence of horizontal electromagnetic stirrer, which was placed in continous casting mould on refinement of ingots structure. The degree of structure refinement was represented by equiaxed crystals zone content on transverse section of ingot and average area of macro-grain in this zone. Effect of structure refinement obtained by influence of electromagnetic field was compared with refinement obtained by use of traditional inoculation, which consists in introducing of additives i.e. Ti, B and C to metal bath. The results of studies show possibility of effective refinement of Al and AlSi2 alloy primary structure, only with use of horizontal electromagnetic field and without necessity of application of inoculants. This method of inoculation is important, because inoculants decrease the degree of purity and electrical conductivity of pure aluminum and moreover are reason of point cracks formation during rolling of ingots. Keywords: aluminum, alloy, inoculation, continuous casting, electromagnetic field; 1. INTRODUCTION Technology of continuous casting is applying usually in production of ingots of Fe [1, 2], Al [3] or Cu [4] alloys, with high yield and quality. The quality of continuous ingot in comparison with traditional gravity casting with use of ingot mould concerns refinement and uniform of ingot structure, which results from solidification of metal in water cooling continuous casting mould. Moreover the refinement of ingot structure can be increases by forced liquid metal movement in time of its solidification results from influence of electromagnetic field [1 6]. The forced liquid metal movement guarantees elimination of unfavorable (mainly for plastic deformation of ingot) columnar macrograins from ingot primary structure at simultaneously increase amount of favorable equiaxed macrograins [5, 6]. Other method of structure refinement is use of traditional inoculation consists in introducing into metal bath of specified substances, called inoculants [7]. Inoculants increase grains density as result of creation of new particles in consequence of braking of grains growth velocity, decrease of surface tension on interphase boundary of liquid nucleus, decrease of angle of contact between the nucleus and the base and increase of density of bases to heterogeneous nucleation [7 9]. The effectiveness of this type of inoculation depends significantly on crystallographic match between the base and the nucleus of inoculated metal. Therefore active bases to heterogeneous nucleation for aluminum and particles, which have high melting point i.e. titanium borides, titanium carbides, titanium nitirdes, aluminum borides and aluminum titanide [8]. But this effective methods of inoculation in comparison with inoculation with use of electromagnetic field has three faults i.e. inoculants decrease the degree of purity and electrical conductivity of pure aluminum [8] and moreover are reason of point cracks formation during rolling of ingots [10].
2. THE RANGE AND RESULTS OF STUDIES On Fig.1 is presented scheme of the stand of horizontal continuous casting, which was constructed and built in Foundry Departmnet of Silesian Univestity of Technology. The main parts of the made stand belong to induction furnace, which is also tundish, water cooling continuous casting mould, system of recooling, system of continuous ingot drawing and cutting, which realized method of ingot movement defined by combination of forwards feed, stop and backwards performed in specified time and velocity from 100 to 500mm/min set in control system. Whereas on Fig.2 is presented view of example continous casting process of Al made on described stand. Fig. 1 Scheme of stand of horizontal continuous casting: 1 induction furnace (tundish), 2 continuous casting mould, 3 system of recooling, 4 system of ingot drawing, 5 system of ingot cutting Fig. 2 Example view of horizontal continous casting process: 1 continuous casting mould, 2 Al ingot, 3 dummy bar
Important element of presented stand, which strongly influences on structure refinement of continuous ingot is electromagnetic stirrer placed in continuous casting mould (Fig.3), which enables cast of cylindrical ingots with diameter from 20 to 40mm. The principle of operation of stirrer based on construction of induction coil, which generated horizontal electromagnetic filed. Efficiency of influence of electromagnetic field on intensity of liquid metal stirring is depending mainly on value of magnetic induction in inside the induction coil. As show on Fig.4a the value of magnetic induction (B) depends on current intensity (I) supplied to the induction coil. Moreover as show in papers [6 and 8] is a possibility of increasing the force, which creates movement of liquid metal and in result of this the velocity of its rotation in mould, also by increasing the frequency (f) of the current supplied to the induction coil (Fig.4b). Fig.3 Cross-section of continuous casting mould contains electromagnetic stirrer a) b) 70 60 50 40 B [mt] 30 B = 6,26I + 3,02 R 2 = 0,96 20 10 0 0 1 2 3 4 5 6 7 8 9 10 I [A] Fig.4 The influence of the current (I) supplied to the induction coil on magnetic induction (B) (Fig. a) and common influence of magnetic induction and frequency (f) of the current supplied to the induction coil on force value (F), which creates movement of liquid metal (Fig. b)
Forced movement of liquid metal in time of its solidification generated by influence of electromagnetic field results in refinement of ingot primary structure, which was represented by equiaxed crystals zone content (SKR) on transverse section of ingot and average area of macro-grain in this zone (PKR). On Fig.5 7 are presented example macrostructures of ingots of aluminum with purity of 99,5% and 99,8% and alloy AlSi2 respectively in initial state, after traditional inoculation with use of Ti, B and/or C additives and after inoculation with use of horizontal electromagnetic field. a) b) c) Fig. 5 Macrostructure of aluminum with a purity of 99,5% ingots: a - in initial state, b - after inoculation with use of master alloys AlTi3C0.15 + AlTi5B1 (Ti = 25ppm, B = 3ppm, C 0.5ppm) and c - after inocualtion with use of horizontal electromagnetic field (B = 60mT, f = 100Hz) a) b) c) Fig. 6 Macrostructure of aluminum with a purity of 99,8% ingots: a - in initial state, b - after inoculation with use of master alloy AlTi5B1 (Ti = 25ppm, B = 5ppm) and c - after inocualtion with use of horizontal electromagnetic field (B = 60mT, f = 100Hz) a) b) c) Fig. 7 Macrostructure of AlSi2 alloy ingots: a - in initial state, b - after inoculation with use of master alloy AlTi5B1 (Ti = 0,1%, B = 0,02%) and c - after inocualtion with use of horizontal electromagnetic field (B = 60mT, f = 100Hz)
On the basis of obtained results was affirmed that refinement of primary structure of pure Al and AlSi2 alloy obtained with use of horizontal electromagnetic field is larger than in initial state and is comparable with obtained with use of traditional inoculation (Fig.8). Fig.8 Characteristic of primary structure of pure Al and AlSi alloy in initial state, after inoculation by Ti, B and/or C additives and after inoculation by use of horizontal electromagnetic field 3. SUMMARY In conclusion can say, that even endogenous inoculation with small amount of Ti, B and/or C strongly increase on refinement in primary structure of pure aluminum and AlSi2 alloy. It results from reactions, which proceed between inoculating elements and inoculated metal or charge impurities. These reactions lead to formation of active bases to heterogeneous nucleation of aluminum as high melting small particles of type TiB, TiB 2, AlB 2, Al 3 Ti and TiC or TiN, which have analogy in crystal lattice with Al. However on the basis of conducted analysis of the literature and results of authors researches it was affirmed, that the horizontal electromagnetic field generated by stirrer (induction coil) supplied by current with frequency larger than power network, influences liquid metal in time of its solidification in mould simultaneously generates thermal and mechanical erosion of crystallization front, what guarantees
refinement of primary structure of pure Al and AlSi2 alloy without necessity of application of inoculants sort Ti, B and/or C. This method of exogenous inoculation is important, because Ti, B and C decrease the degree of purity and electrical conductivity of pure aluminum. Moreover titanium, boron and carbon are reason of point cracks formation during rolling of ingots. ACKNOWLEDGEMENTS Project financed from means of National Science Centre LITERATURE [1] ADAMCZYK, J. Engineering of metallic materials. Gliwice: Publishers of Silesian University of Technology, 2004, 342 p. ISBN 83-7335-223-6. [2] SZAJNAR, J., STAWARZ, M., WRÓBEL, T., SEBZDA, W., GRZESIK, B., STĘPIEŃ, M. Influence of continuous casting conditions on grey cast iron structure. Archives of Materials Science and Engineering, 2010, vol. 42, no. 1, 45-52 p. ISSN 1897-2764. [3] LEE, D., KANG, S., CHO, D., KIM, K. Effects of casting speed on microstructure and segregation of electromagnetically stirred aluminum alloy in continuous casting process. Rare Metals, 2006, vol. 25, special issue, 118-123 p. ISSN 1001-0521 [4] YAN, Z., JIN, W., LI, T. Effect of rotating magnetic field (RMF) on segregation of solute elements in CuNi10Fe1Mn alloy hollow billet. Journal of Materials Engineering and Performance. 2012, vol. 21, no. 9, 1970-1977 p. ISSN 1059-9495. [5] SZAJNAR, J., WRÓBEL, T. Influence of magnetic field and inoculation on size reduction in pure aluminium structure. International Journal of Materials and Product Technology, 2008, vol. 33, no. 3, 322-334 p. ISSN 0268-1900. [6] WRÓBEL, T. The influence of inoculation type on structure of pure aluminum. From Conference Proceedings METAL 2012: 21st International Conference on Metallurgy and Materials. Brno, Czech Republic: TANGER, 2012, 1114-1120 p. [7] FRAŚ, E. Crystallization of metals. Warsaw: WNT, 2003, 470 p. ISBN 83-204-2787-8. [8] WRÓBEL, T. Structure of pure aluminum after endogenous and exogenous inoculation. Chapter no. 21 from Advances in Crystallization Processes. Edited by MASTAI, Y. Rijeka, Croatia: INTECH, 2012, 539-564 p. ISBN 978-953-51-0581-7. [9] JANERKA, K., BARTOCHA, D., SZAJNAR, J., JEZIERSKI, J. The carburizer influence on the crystallization process and the microstructure of synthetic cast iron. Archives of Metallurgy and Materials, vol. 55, no. 3, 851-859 p. ISSN 1733-3490. [10] KELES, O., DUNDAR, M. Aluminum foil: its typical quality problems and their causes. Journal of Materials Processing Technology, 2007, vol. 186, no.1-3, 125-137 p. ISSN 0924-0136.