Mould
Filling PhenomenonLet us review two major characteristics of molten metal related to mould filling - fluidity and turbulence, and see how they are related to flow related defects.
Fluidity is not a physical property. It is a technological characteristic. It indicates the ability of liquid metal to flow through a given mould passage - even as it is solidifying - and fill the cavity to reproduce the design details. It is quantified in terms of the solidified length of a standard spiral casting.
The fluidity as defined by the foundry community is different that defined by physicists (as the reciprocal of viscosity). The casting fluidity is driven by metallostatic pressure and hindered by: viscosity and surface tension of molten metal, heat diffusivity of mould, back pressure of air in mould cavity and friction between the metal-mould pair.
Metallostatic head: The metallostatic pressure is given by ? g h where ? is the metal density and h is the height of liquid metal column above the filling point. A higher metallostatic pressure gives higher velocity of molten metal, and thereby higher fluidity.
Surface tension: For a flat plate of thickness t, the relation between head, thickness and surface tension is given by: ? g h = ? / t, where ? is the surface tension. At the pouring temperature, the surface tension of aluminum and iron is 0.5 and 0.9 N/m respectively; similar to mercury at room temperature (0.46 N/m), but higher than water (0.07 N/m).
Heat diffusivity: Moulds with high heat diffusivity transfer heat faster from the molten metal, causing it to freeze earlier and stop flowing. It is given by v(Km ?m Cm), where Km is thermal conductivity, ?m is density and Cm is specific heat of the mould material.
Back Pressure: As molten metal advances in the mould, the back pressure of air that is being compressed in the cavity ahead effectively reduces the metallostatic pressure, and thus hinders filling. The back pressure depends on the cavity volume, mould permeability and the velocity of the advancing front. Venting helps.
Friction: The rough surface of sand mould hinders metal flow. Thus mould coating (usually water based, containing silica flour and graphite) reduces the friction between the metal and mould, contributing to higher fluidity. In general, fluidity of pure metals is higher than alloys. Within alloys, eutectics have higher fluidity than non-eutectics. The fluidity of grey iron ranges between 0.5-1.0 m, and can be estimated by the empirical equation:
Turbulence implies irregular, fluctuating flow with disturbances. It is observed when: (1) inertia forces (which make the fluid continue in the same direction), are much higher than the drag forces (which tend to stop the fluid motion), and (2) there are obstructions in the path of flow, such as a sharp corner or a change of section thickness. The drag forces include those caused by viscosity and surface tension. The viscous forces mainly operate in the bulk of the liquid metal, whereas surface tension forces operate near the mould wall. Thus we have two types of turbulence: bulk and surface.
Bulk turbulence is quantified by Reynolds number Re, which is the ratio of inertia to viscous pressure in a fluid. It is given by ? V d / µ where ? is the density, µ is the viscosity and V is the velocity of the liquid; d is a characteristic dimension of the flow path. If Re is more than 2000, then the flow is usually turbulent.
Surface turbulence is quantified by the Weber number We, which is the ratio of inertia to surface tension pressure in a fluid. It is given by ? V2 r / ? where r is the radius of curvature of the free liquid surface. For We is less than 1, surface turbulence is absent. When it is 100 or more, surface turbulence is prominent, leading to violent mixing of surface layers with the bulk of the molten metal.
