Solidification Of Pure Metals

1)      Pure metal generally posses:

a)      Excellent thermal and electrical conductivity (e.g. and Al)

b)      Higher ductility, higher melting point, lower yield point and tensile strength; and

c)      Better corrosion resistance, as compared to alloys.

2)      Because of their higher melting points, pure metals exhibit certain difficulties in castings. E.g.

a)      Difficulty during pouring

b)      Occurrence of severe metal mold reaction.

c)      Greater tendency towards cracking

d)     Their mode of solidification, which may produce defective castings

3)      Pure metal melts and solidify at a single temperature which may be termed as melting point or freezing point.

4)      Above freezing point the metal is liquid and below freezing point it is in solid state.

5)      If number of temperature measurements are taken out at different times, while the pure metal is cooled under equilibrium conditions from the molten state till it solidifies, a time-temperature plot will look like below:

a)      Liquid metal cools from A to B.

b)      From B to C, the melt liberates latent heat of fusion, temperature remains constant.

c)      The liquid metal starts solidifying at B and it is partly liquid and partly solid at any point between B and C and at C the metal is purely solid.

d)     From C to D, the solid metal cools and tends to reach room temperature.

e)      The slopes of AB and CD depend upon the specific heats of liquid and solid metals respectively.

1)      If pure metals cool rapidly or even otherwise when it is very pure and does not contain at all any impurities as nucleus to start crystallization, it may cool as per 19-4 b.

a)      Nucleation of solid does not start at point B (i.e. normal solidification temperature) but it does so at B`, i.e. after the liquid metal has super cooled by an amount of delta T. this phenomenon is known as supercooling or undercooling.

b)      Besides pure metals, supercooling may occur in alloys also. E.g. gray cast iron.

2)      When pure metals (and some eutectic alloys) are allowed to solidify in a mold, the portion of molten metal next to the mold wall begins to solidify.

3)      This metal solidifies in the form of a solid skin and then the liquid metal tends to freeze onto it.

4)      The solid skin progresses towards the centre of the mold from all the mold walls. This gives the concept of progressive solidification.

5)      The interface or boundary between the solid metal and melt is a well defined smooth surface.

6)      As the successive layers of molten metal build up in the form of solid skin or as solid metal wall thickness increases, the liquid level in the mold falls, because of solidification shrinkage.

This causes pipe in the solidified ingot or billet and necessitates the provision of risers in order to have castings free from the shrinkage defects.