The fluidity of molten metal of cast alloys (without any addition of fluxes) increases with increase in

The correct option is degree of superheat.

Fluidity in casting refers to the capability of molten metal to flow readily into a mold cavity and fill all its details before solidifying.

The degree of superheat is the difference between the pouring temperature of the molten metal and its liquidus temperature (melting temperature). Mathematically, it is expressed as:

$$\text{Degree of Superheat}=T_{\text{pouring}}-T_{\text{liquidus}}$$

Increasing the degree of superheat provides the molten metal with extra thermal energy. This has two major effects:

1. Delays Solidification: The molten metal takes longer to cool down to its freezing temperature, allowing it to flow a longer distance within the mold before it starts solidifying (crystallizing).
2. Reduces Viscosity: Higher temperatures decrease the dynamic viscosity of the liquid metal, allowing it to flow more freely with less resistance.

Let us briefly analyze why the other options are incorrect:

• Freezing range: A larger freezing range (the temperature difference between liquidus and solidus temperatures) means the alloy solidifies over a wider temperature span, forming dendritic structures that block the flow of molten metal and thus decreasing fluidity.
• Surface tension: Higher surface tension increases the resistance of the liquid metal to flow, especially into thin sections, which decreases fluidity.
• Viscosity: Higher viscosity means greater internal friction and resistance to flow, which directly reduces fluidity.