Solidification Of Alloys

1)      Solid solution alloys exhibit different freezing.

2)      Differential freezing promotes growth in a manner other than by the advance of a smooth interface.

3)      Solid solution alloys base upon solid solubility which is the irreversible mixing of two solids into a single phase. Phase may be defined as a physically distinct region of matter having characteristic atomic structure and properties which change continuously with temperature, composition or other thermodynamic variables. The various phases of the system, in principle, can be mechanically separated.

4)      A solid solution binary alloy system is one in which two metals are completely soluble in both solid and liquid state.

5)      A solid solution is the result of metals dissolving in each others crystal lattice.

6)      Copper- nickel and gold- silver are two examples of solid solution alloys.

7)       Following fig. shows the cooling curve of a binary solid solution alloys.

a)      From A to B, the alloy is in liquid state.

b)      Solidification starts at point B and completes at C.

c)      Unlike pure metals, solidification occurs throughout the temperature range (i.e. from Tb to Tc)

d)     Latent heat of fusion is liberated gradually from B to C and it tends to increase the time required for the solidification.

Phase diagram:

1)      If two metals of binary solid solution system are mixed in different proportions and a cooling curve is constructed for each composition, the resulting diagram will be one shown in fig. below which is phase diagram for the alloy system.

1)      A phase diagram shows two different and distinct phases, one is liquid metal solution and other is solid solution.

2)      Within these two phases i.e. liquidus and solidus, the two phases- liquid and solid exit together. These two phages existing together can be mechanically separated by decantation of the liquid phase.

3)      Liquidus is that line (a) above which the alloy is in liquid state and (b) where solidification starts.

4)      Solidus is that line (a) below which the alloy is in solid state and (b) where solidification completes.

5)      If in a phase diagram, for each change of phase, adequate time is allowed for the change to complete so that phage change takes place under equilibrium conditions, the phase diagram will be also known as equilibrium diagram.

6)      Alloy solidification occurring under equilibrium conditions is known as equilibrium solidification.

7)      Equilibrium conditions are not generally attained during the solidification of the castings because the diffusion involved may be extremely sluggish due to fast cooling rate of the castings. Thus most frequently castings solidify under non- equilibrium conditions and the solidification process is known as non-equilibrium solidification.

8)      Non- equilibrium solidification results in porous, columnar (or dendrite), cored material which is usually of very inhomogeneous composition.

9)      Non-equilibrium solidification involves fast cooling rate and does not permit complete diffusion in the solid state and thus result in coring and segregation.  

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.