Heat line formation during roll-casting of aluminium alloys at thin gauges

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1 Heat line formation during roll-casting of aluminium alloys at thin gauges M. Yun, J. Hunt, D. Edmonds To cite this version: M. Yun, J. Hunt, D. Edmonds. Heat line formation during roll-casting of aluminium alloys at thin gauges. Journal de Physique IV Colloque, 1993, 03 (C7), pp.c7-227-c < /jp4: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1993 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE IV Colloque C7, supplkment au Journal de Physique 111, Volume 3, novembre Heat line formation during roll-casting of aluminium alloys at thin gauges M. W N, J.D. HUNT and D.V. EDMONDS Department of Materials, University of Oxford, Parks Road, OxjGord OX1 3PH, UK. ABSTRACT Heat line formation during roll-casting of aluminium alloys at thin gauges has been studied experimentally and numerically. It has been found that the thinner the strip, the less the risk of forming heat lines or the less severe the heat lines are should them form. This behaviour has been discussed in terms of the heat transfer between the strip and the roll within the contact region in the twin-roll casting process. Twin-roll casting combines continuous casting and some hot rolling in a single operation to produce thin sheet directly from the molten metal. Traditionally, heat lines are one of the major defects occurring during the twin-roll casting process for the production of aluminium sheet and can result in low productivity and limited alloy capability [I-31. Furthermore, commercial roll casters normally produce sheet at a thickness of 6 mm or above, and consequently studies of heat line formation have been confined to this condition. This paper reports an experimental study of the factors affecting heat line formation in roll-cast aluminium alloys, especially strip thickness. The work was carried out in Oxford using an experimental twin-roll caster designed and built by the University of Oxford and Davy International, Poole, U.K.. The details of the experimental method and the equipment used have been described previously [4,5]. Various aluminium alloys have been roll-cast under different casting conditions to investigate heat line formation. Among the processing parameters investigated, strip thickness emerged as one of the most important parameters, and is considered in the present paper. In the operation of commercial twin-roll casters, heat lines are found to be semi-continuous longitudinal casting defects, usually only a few centimetres wide, where the cast material leaves the roll bite partially molten [3]. In the present work the nature and patterns of heat lines were found to vary with strip thickness. Fig. 1 shows the effect of strip thickness on heat line formation. The thicker the strip, the more severe the heat lines are. At constant strip thickness increasing casting velocity in the twin-roll casting process increases productivity. The limitation of casting velocity at which stable roll casting cannot continue is the formation of either heat lines or sticking of the strip to the roll. Fig. 2 shows schematically the effect of the casting velocity on the instability of the process in terms of the exit temperature of the strip at constant strip thickness. It can be seen that heat lines may occur when casting velocity exceeds the critical speed V,, in this case stable operation of the caster becomes very difficult. Article published online by EDP Sciences and available at

3 228 JOURNAL DE PHYSIQUE IV It has been found in these experiments that the thinner the strip, the less the risk of forming heat lines or the less severe the heat lines are should they form. Traditional heat lines (longitudinal holes) disappear as the strip thickness gets thinner. In particular, heat line formation was infrequent when casting strip with a thickness less than 3 mm with the twin-roll caster used. Heat line formation has been discussed in terms of the heat transfer between the strip and the roll within the roll bite region in the twin-roll casting process. Bagshaw et a1 [31 suggested that heat lines are the result of two possible steady-state solutions to the heat flow problem, as shown in Fig. 3. The solutions were obtained in a numerical model in conjunction with a doublestep heat transfer coefficient function within the roll bite region. The function is composed of a low heat transfer region followed by a very low heat transfer region then a high heat transfer region. It was proposed that heat lines occurred when the casting velocity was such that part of the strip w'as on the 'upper liquid line' and part of the strip was on the 'lower solid line'. Recent numerical work [6], using a similar model to that of Bagshaw, but with a single step function for the heat transfer region with the contact area has cast doubt on the existence of the double solutions when realistic values of the heat transfer coefficient are used[61. The hysteresis loop only arises when there are very large differences between the low and high heat transfer coefficients. As an alternative mechanism it is suggested that a heat line is produced by an external change in heat transfer coefficient brought about for reasons other than proposed by Bagshaw et al [3]. This might occur because of uneven lubrication or buckling of the semi-solid sheet. Fig. 4 shows an example of the results of calculations for changes in low heat transfer coefficient from 2.3 W ~ m K" ' to ~ 0.03 W cm-2 K" (for three sheet thicknesses). For a local change in heat transfer coefficient of this value it can seen that roll speed should be less than about 6.25 cmlsec to prevent the region being fully liquid in the 2 mm sheet. For 4 and 6 mm sheet this should be less than about 2.62 and 1.62 cmlsec. It can be seen that this approach suggests that higher productivity can be achieved at thinner strip thicknesses before heat lines occur. In conclusion, it is proposed that the strip exit temperature rise due to the change in heat transfer coefficient between the liquid metal and the roll within the contact region is a good measure of heat line susceptibility. ACKNOWLEDGEMENTS The research programme was carried out as part of a Teaching Company Scheme funded by the SERCIDTI and by Davy International, Poole, U.K.. The authors gratefully acknowledge the provision of the aluminium alloy by Alcan International, Banbury. We are also indebted to Dr. R. Cook of Davy International, Poole, U.K., D.J. Monaghan, X. Yang and Dr. J. Jang of the University of Oxford for experimental assistance. REFERENCES 1 ) Lewis D.M., Metallurgical Review, 6 (1 961 ), ) Emley E.F., International Metals Review, 21, June (1976), ) Bagshaw M.J., Hunt J.D. and Jordan R.M., Cast Metals, 1, 1 (19881, ) Edmonds D.V., Hunt J.D., Monaghan D.J., Yang X., Yun M., Browne D.J., Cook R. and Thomas P.M., Proc. of the International Symposium on Extraction, Refining and Fabrication of Light Metals, Ottawa, Ontario, Canada, Aug , (1 991 ), ) Yun M., Monaghan D.J., Yang X., Jang J., Edmonds D.V., Hunt J.D., Cook R. and Thomas P.M., Cast Metals, 4, 2 (1 991), ) M.Yun, D.Phil Thesis, Oxford University, Oxford, U.K, (1 992).

4 (a) An example of large heat lines. AA 1070, 6 mrn thick strip. (b> An example of medium heat lines. AA 1070, 4 mm thick strip. Figure 1 Effect of strip thickness on heat line formation. Heat lines v1 v2 Casting velocity Figure 2 Schematic plot of strip exit temperature against casting velocity at constant strip thickness.

5 JOURNAL DE PHYSIQUE IV - v, Casting speed Figure 3 Sketch of strip exit temperature as a function of casting speed. (after Bagshaw et a1 [3]) Roll speed, cm/sec Figure 4 Effects of low heat transfer coefficient between liquid metal and roll and strip thickness on strip exit temperature. (a), (b), and (c): 6, 4 and 2 rnm thick strips with the heat transfer coefficient of 2.3 w/cm2k; (d), (e) and (0: 6, 4 and 2 mm thick strips with the heat transfer coefficient of w/cm2.~.