A basic model of the transport phenomena occurring during solidification of multicomponent mixtures is presented. The model is based on a two-phase approach, in which each phase is treated separately and interactions between the phases are considered explicitly. The macroscopic transport equations for each phase are derived using the technique of volumetric averaging. The basic forms of the constitutive relations are developed. These relations link the macroscopic transport phenomena to microscopic processes such as microstructure development, interfacial stresses, and interfacial heat and mass transfer. Thermodynamic relations are presented, and it is shown that nonequilibrium effects can be addressed within the framework of the present model. Various simplifications of the model are examined, and future modeling needs are discussed.

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A volume-averaged two-phase model for transport phenomena during solidification

In this paper processes for fabrication of aluminium-alloy composites containing paniculate non-metals, the net shape forming of these composites, their microstructures, their friction and wear behaviours and their mechanical properties are described. Composites of two wrought (2014 and 2024) and one cast (201) aluminium alloys containing 2 to 30 wt% of Al2O3 and SiC particles in the size range of 1 to 142μm were prepared. The non-metallic particles were added to a partially-solid vigorously-agitated matrix alloy. The particles were then retained in the matrix until interface interaction, for example, the formation of MgAl2O4 spinel in the case of Al2O3 particles, were faciliated. These composites were solidified and subsequently reheated to above their liquidus temperature and formed under high pressure in a closed-die forging type of apparatus. Composites with particulate additions of size larger than 5μm possessed homogeneous structures; particles of size 1μm, however, tended to cluster. The wear behaviour of the composites was studied using a pin-on-disc type machine. It was shown that composites containing large amounts of non-metals, ≈ 20 wt%, exhibit excellent wear resistance whilst those with small to moderate amounts of non-metals possess tensile properties comparable to the matrix alloy. Increasing the amount of particulate additions results in reduced ductility. Finally, a method was investigated of producing components with high weight-fractions of non-metals near their surface.

Composites of aluminium alloys: fabrication and wear behaviour

The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites. The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of “in situ hybridising” and “in situ alloying” are described.

Reinforcement coatings and interfaces in aluminium metal matrix composites

Solidification synthesis of cast metal-ceramic particle composite materials by dispersing hard or soft ceramic particles (including microballoons) and short fibres in molten alloys before solidification is described. Microstructures synthesized using a variety of casting techniques such as gravity or pressure die casting, centrifugal casting, and squeeze casting are discussed. Techniques to obtain a selected distribution of dispersed particles in cast alloy matrices by controlling solidification parameters and inducing melt particle wettability are reviewed. These cast metal-ceramic particle composites represent low cost, high performance, tailor made substitute materials for a variety of automotive and electromechanical applications such as pistons, cylinder liners, bearings, and current collectors, resulting in savings of material and energy.

Solidification, structures, and properties of cast metal-ceramic particle composites

A general micro/macroscopic model of solidification for 2-D or 3-D castings, valid for both dendritic and eutectic equiaxed alloys, is presented. At the macroscopic level, the heat diffusion equation is solved with an enthalpy formulation using a standard FEM implicit scheme. However, instead of using a unique relationship between temperature and enthalpy (i.e., a unique solidification path), the specific heat and latent heat contributions, whose sum equals the variation of enthalpy at a given node, are calculated using a microscopic model of solidification. This model takes into account nucleation of new grains within the undercooled melt, the kinetics of the dendrite tips or of the eutectic front, and a solute balance at the scale of the grain in the case of dendritic alloys. The coupling between macroscopic and microscopic aspects is carried out using two time-steps, one at the macroscopic level for the implicit calculation of heat flow, and the other, much finer, for the microscopic calculations of nucleation and growth. This micro/macroscopic approach has been applied to one-dimensional and axisymmetric castings of Al-7 pct Si alloys. The calculated recalescences and grain sizes are compared with values measured for one-dimensional ingots cast under well-controlled conditions. Furthermore, the influence of casting conditions on temperature field, undercooling, grain size, and microstructural spacings is shown to be predicted correctly from axisymmetric calculations with regard to the expected experimental behavior.

Modeling of equiaxed microstructure formation in casting

The microstructures of electron transparent submicron aluminum alloy powders produced by an electrohydrodynamic process are described. The observations are coupled with thermodynamic, kinetic, and heat flow concepts to deduce the thermal history and solidification mode of the powders. The range of microstructures observed includes: homogeneous plane-front solidified single crystals; cellular crystals; and powders containing blocky segregates, multiple grains, and twins. In general, a decrease in the size of the submicron particles decreases the amount of segregation. This observation appears to be caused by an increase in undercooling prior to nucleation which leads to higher liquid/solid interface velocities. On the other hand, the same trend promotes multiple nucleation and increases the incidence of twin crystals. Liquid/solid interface breakdown from planar to cellular in the middle of the powders is correlated with solute “trapping” during recalescence and rejection of solute ahead of the solidification front after recalescence. The latter manifests itself as a solute depleted zone in the plane-front to cellular transition region. Finally, some of the microstructural observations lend credence to the recent extension of the morphological stability theory to rapid solidification.

Microstructures of rapidly solidified aluminum alloy submicron powders

A combined theoretical and experimental study of steady-state fluid flow, heat flow and segregation in axi-symmetric ingot production is presented, with specific applications in continuous casting and ESR. The fluid flow-segregation model involves the coupling of convective heat and fluid flow in the fully liquid metal pool ahead of the liquidus isotherm to the interdendritic fluid flow responsible for macrosegregation in the “mushy” zone of ingots solidifying under axi-symmetric conditions. Experiments on low-temperature Sn-Pb alloys verify the solidification model.

Effect of fluid flow on macrosegregation in axi-symmetric ingots

Auger and electron diffraction studies of Al2O3 isolated from a composite prepared by introducing fibers into a vigorously agitated Al−Mg melt indicated the presence of MgAl2O4 on the fiber surface. The evidence suggested that the spinel was present as discrete crystals. The thickness and coverage of the spinel is likely to vary with processing conditions.

The interface phase in Al−Mg/Al2O3 composites

Rapid melting and solidification of a semi-infinite substrate subjected to a high intensity heat flux over a circular region on its bounding surface moving with a constant velocity is considered. General expressions are developed for the coefficients in the finite difference equation governing the heat transfer in moving orthogonal curvilinear coordinate systems. These expressions are reduced to their specific forms in terms of dimensionless nodal temperature and enthalpy for a moving oblate spheroidal coordinate system. Quasisteady state conditions are assumed and the thermal properties of the substrate in the liquid and solid phase are considered constant and equal. It is also assumed that the substrate melts and solidifies at a single temperature. Temperature distributions in the molten region and the adjacent heat affected zone are computed along with the liquid-solid interface shape, its velocity and other important solidification variables. Both uniform and Gaussian heat flux distributions within the circular region are considered. The results are presented in their most general form—in terms of dimensionless numbers when possible. Specific criteria for the melting of the substrate are established. It is shown that the three variables, absorbed heat fluxq, the radius of the circular regiona and the velocity of the moving fluxU, could be combined into two independent variables. That is, the dimensionless temperature distribution in the metal pool and the solid substrate remain the same as long as the productsqa andUa orU/q are kept constant. The effect of these variables on cooling rate in the liquid and the ratio of temperature gradient to growth rate at the solid-liquid interface are discussed using an aluminum substrate as an example.

Rapid melting and solidification of a surface due to a moving heat flux

This book contains papers divided among the following sections: process monitor and control in welding; plasma processing and refining; strip casting; modelling of welding processes; CAD/CAM in casting; investment and die casting; ingot, continuous and other shape casting; and rapid solidification and microstructural evolution

R. Mehrabian

Papers

Microstructures of rapidly solidified aluminum alloy submicron powders

Effect of fluid flow on macrosegregation in axi-symmetric ingots

The interface phase in Al−Mg/Al2O3 composites

Rapid melting and solidification of a surface due to a moving heat flux

Modeling and control of casting and welding processes