Metal Based Functionally Graded Materials

This chapter deals with the concept of graded material and main technologies for their production. The technologies for metallic graded materials are divided into two general groups. A sequence of eighth basic production methods is discussed in more details.

This chapter is focused on some specific characteristics of functionally graded materials that need special evaluation.

Sedimentation and flotation seem to be simple physical processes and it should be easy to make use of them. To do this in a real situation one must be very familiar with the processes. Employment of mathematical modeling and numerical simulations is extremely helpful here. A great number of pages have been devoted to this matter [102-108]. Some basic features of these phenomena, which are closely related to the manufacture of graded structures in particle reinforced MMCs, will be given below.

Sedimentation and flotation in the process of MMCs preparation exist because of gravity, and arise as a result of the density difference between particles and the metal matrix melt. Special experiments with a water-SiC suspension will be described on the next few pages in order to give a better understanding of particle sedimentation in MMCs synthesis. A variety of formula to estimate the viscosity of slurries will be presented. Finally, utilization of the phenomena will be discussed.

Centrifugal casting offers greater potential for graded structure manufacture than gravity. The main reason for this is the higher pressure in the melt, which facilitates infiltration in a graded preform. Another reason is that usually the centrifugal force exceeds essentially the gravitational force, and because of this particles in composite slurry move faster than in the case of gravity casting. Moreover, by management of rotation speed one can control the magnitude of the centrifugal force, which means to control particle velocity in liquid composite during solidification. The latter is impossible in gravity casting and gives an important advantage of centrifugal casting for production of a variety of graded structures. A comprehensive mathematical model based on equations for both particle movement and solidification in rotating slurry, which would be capable of describing macrostructure formation in centrifugal casting of particle reinforced MMCs will be presented and short discussion on its application to the manufacture of graded structures will be analyzed.

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Physical processes that govern gasar structure formation include: heat transfer, gas diffusion in melt and solid, gas phase nucleation, and pore evolution. Heat transfer in the ingot (liquid and solid part) defines the solidification velocity, which in turn determinates gas concentration on the solid/liquid interface. This velocity is a component that also determinates the roughness of the interface. Greater roughness means more sites for gas phase nucleation. Gas diffusion in the liquid metal defines gas concentration ahead of the solidification front. Gas flux from the melt into the pores and their size are directly conditioned by gas concentration in the melt. Pore size and pore density (number of pores per unit area in a transversal cross section) depends on nuclei size, which is a function of surface tension σlg, and the number of nucleation sites also depends on quantity of impurity in the melt. All these processes are interconnected and determinate final structure in a complicated way.

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