Interfacial tension between a liquid (melt) and a gaseous phase (air, mold cavity). It therefore refers to forces acting between two phases which are in contact with each other.
The surface of a melt is subject to a tension acting to contract this surface within itself. Thus, surface tension is defined as the force with which a liquid surface having a width of 1cm attempts to contract itself. The dimension of surface tension in SI units is kg/s2, equivalent to N/m.
Surface tension depends on the composition of the relevant melt and its temperature; as the temperature increases, the surface tension decreases, except with copper and aluminum melts. In addition, the surface tension is significantly influenced by the C content. At 2.2% C and a temperature of 1420°C, the surface tension is 1500N/m. In comparison, at 3.9% C and a temperature of 1300°C, the surface tension is only 1150N/m.
Surface tension has a considerable influence on the melts ability to completely fill the mold cavity. Cast alloys having a high surface tension fill the mold cavity less well than those of a lower surface tension. Moreover, the temperture dependence also results in the casting properties (see Castability) deteriorating as the casting temperature decreases. Therefore, cold shuts are not necessarily the results of premature solidification due to a low casting temperature, but the metal flow may also come to a standstill due to excessive surface tension.