Metal controlled freezing process

The viscosity of the molten metal controls the penetration between the fiber interstices and although increasing the temperature will lower the viscosity, it will also increase oxidation of the carbon fiber. The requisite viscosity of molten metals should be about the same as the viscosity of H2O at 20°C (1 cP) (Figure 15.9). Freeze choking must be avoided during the infiltration process caused when the infiltration velocity drops to a level where the fibers ahead of the infiltration front extract sufficient heat to solidify the metal melt [118]. 

Ethylene glycol provides the best freezing-point and heat-transfer characteristics. However, the quality of the water it is mixed with and the high temperatures involved cause many problems that chemists must control. Water usually contains Ca Mg, Cl, S04, and dissolved O2. The engine and radiator are made of many metals, all of which can corrode. Corrosion is an oxidation process, so O2 must be removed. Acids produced also tend to dissolve away the metal. This corrosion scale reduces the heat transfer and, if the scale falls away, then these particles can bombard other sections of the metal and either remove more scale or "sand blast" away any inhibitor film and expose more metal to corrosion. These particles eventually plug up the radiator or wear out the water pump. 

During the solidification process, control of the direction of movement of the solid-liquid growth front can be achieved by the control of heat flow in the mould. Solidification can be initiated by the use of chills these are metal inserts which conduct heat away more rapidly than the mould material. On the other hand, ty using insulating materials or exothermic compounds, heat flow conditions can be controlled to delay freezing in a particular part of the mould. 

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