Forms of cast structure

3.1.3 Forms of cast structure

Because of the interplay of a variety of physical and chemical factors during freezing, the as-cast grain structure is usually not as uniform and straightforward

Figure 3.5 Chill-cast ingot structure .

Structural phases: their formation and transitions 45 dendrites, the rapid growth directions are h1 0 0i for

fcc and bcc crystals and lie along the direction of heat flow. Sideways growth is progressively hindered so that the crystals develop a preferred orientation and a characteristic columnar form. They therefore introduce directionality into the bulk structure; this effect will be most pronounced if the metal itself is strongly anisotropic (e.g. cph zinc). The preferred growth directions for cph crystals are h1 0 1 0i. The growth form of the interface between the columnar crystals and the liquid varies from planar to dendritic, depending upon the particular metal (or alloy) and thermal conditions.

As the columnar zone thickens, the temperatures within the liquid become more shallow, undercooling more prominent and the presence of kindred nuclei from dendritic multiplication more likely. Under these conditions, independent nucleation (Section 3.1.1) is favoured and a central zone of equiaxed, randomly- oriented crystals can develop (Figure 3.5). Other fac- tors such as a low pouring temperature (low superheat), moulds of low thermal conductivity and the presence of alloying elements also favour the development of this equiaxed zone. There is a related size effect, with the tendency for columnar crystals to form decreasing as the cross-section of the mould cavity decreases. However, in the absence of these influences, growth predominates over nucleation, and columnar zone may extend to the centre of the ingot (e.g. pure metals). The balance between the relative proportions of outer columnar crystals and inner equiaxed crystals is impor- tant and demands careful control. For some purposes,

a completely fine-grained stucture is preferred, being stronger and more ductile. Furthermore, it will not contain the planes of weakness, shown in Figure 3.5, which form when columnar crystals impinge upon each other obliquely. (In certain specialized alloys, how- ever, such as those for high-power magnets and creep- resistant alloys, a coarse grain size is prescribed.)

The addition of various ‘foreign’ nucleating agents, known as inoculants, is a common and effective method for providing centres for heterogeneous nucle- ation within the melt, inhibiting undercooling and producing a uniform fine-grained structure. Refining the grain structure disperses impurity elements over a greater area of grain boundary surface and generally benefits mechanical and founding properties (e.g. duc- tility, resistance to hot-tearing). However, the need for grain refinement during casting operations is often less crucial if the cast structure can be subsequently worked and/or heat-treated. Nucleating agents must remain finely dispersed, must survive and must be wetted by the superheated liquid. Examples of inoculants are tita- nium and/or boron (for aluminium alloys), zirconium or rare earth metals (for magnesium alloys) and alu- minium (for steel). Zirconium is an extremely effective grain-refiner for magnesium and its alloys. The close similarity in lattice parameters between zirconium and magnesium suggests that the oriented overgrowth (epi- taxy) of magnesium upon zirconium is an important

factor; however, inoculants have largely been devel- oped empirically.