Materials Science and Engineering A282 (2000) 262 – 269 Strip casting of Al–Pb alloys — a numerical analysis J.Z. Zhao 1 , S. Drees, L. Ratke * Institute for Space Simulatlon, German -Aerospace Center, 51140 Cologne, Germany Received 15 March 1999; received in revised form 20 September 1999 Abstract A numerical model describing the microstructural evolution of strip cast immiscible alloys is presented. The numerical results for Al – Pb-type bearing alloys show that in strip casting a constitutional supercooling region exists in front of the solid/liquid interface where liquid/liquid decomposition takes place. A higher solidification velocity leads to a higher nucleation rate and, therefore, to a higher number density of the minority phase droplets. As a result, the average radius of droplets in the melt at the solid/liquid interface decreases with the solidification velocity. The numerical model is compared with experimental results. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Immiscible; Al – Pb alloy; Strip casting; Microstructure evolution; Numerical modelling www.elsevier.com/locate/msea 1. Introduction Monotectic alloys (especially Al–Pb and Al–Bi al- loys) are potentially more attractive as sliding bearing alloys than conventional Al–Sn or Cu–Sn–Pb alloys, if the soft Pb or Bi phase could be well dispersed in the solid microstructure. Alloys with average compositions above the monotectic one would be especially useful because then the volume fraction of the soft inclusions is high enough to produce sliding bearings with a drastically lower friction coefficient and a very small wear resistance compared to the standard alloys used in current car engines [1]. The application of these alloys, however, has been limited, due to the metallurgical problems associated with the phase diagram. According to the phase diagram (shown in Fig. 1), the components of these alloys are completely miscible only above the binodal line. If such a homogeneous, single-phase liquid is cooled below the binodal line, it transforms into two liquids. Generally, the liquid – liquid decomposition of an initially homogeneous liquid begins with the nucle- ation of the liquid minority phase in the form of droplets. These drops grow by diffusion of solute in the matrix. They can also settle due to gravity or migrate due to a temperature or concentration gradient. Com- monly large compositional and density differences in- variably exist between the two liquid phases that lead to a rapid spatial phase separation or macrosegregation during solidification on earth. Research in the last decades showed that a fine dispersion of minority phase particles in a hypermono- tectic alloy can be obtained by rapid quenching or fast cooling thus passing the miscibility gap within a second or less [2,3]. A special planar flow casting process was invented using the fast cooling of a thin Al–Pb melt film deposited continuously on a fast moving copper or steel substrate [4,5]. Fast cooling rates are also realized in a new vertical strip casting process, as shown sche- matically in Fig. 2, which takes the advantage of the joint action of gravity induced Stokes sedimentation of droplets and the temperature gradient driven Marangoni migration which always is normal to the isotherms. Marangoni or thermocappillary motion is due to the temperature dependence of the interfacial tension. In a temperature gradient a pressure gradient is induced along the interface such that the drop liquid moves from the hot side (higher temperature, lower interfacial tension) to the cold side. Since both liquids are viscous the continuity of the fluid flow induces a fluid motion in the surrounding matrix liquid in the same direction. This leads to a motion of the drop from * Corresponding author. Tel.: +49-2203-6012098; fax: +49-2203- 61768. E-mail address: lorenz.ratke@dlr.de (L. Ratke) 1 On leave from the Harbin Institute of Technology, Harbin, PR China. 0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0921-5093(99)00755-8