ME 206 Manufacturing Processes

Asim Tewariasim.tewari@iitb.ac.in

Room ME Bld 321Ph: 7521

Case: I (Equilibrium Solidification)

Unidirectional solidification of alloy Xo

(a) A planar S/L interface and axial heat flow. (b) Corresponding composition profile at T2 assuming complete equilibrium.

Conservation of solute requires the two shaded area to be equal.

Case :II (Solidification with No Diffusion in Solid, Perfect Mixing in Liquid)

Planar front solidification of alloy Xo, assuming no diffusion in the solid, but complete mixing in the liquid. The figure shows the mean composition of the solid as dotted line.

Composition profile just under T1

Composition profile at T2

Composition profile at the eutectic temperature and below.

Case :II (Solidification with No Diffusion in Solid, Perfect Mixing in Liquid)

Case :II (Solidification with No Diffusion in Solid, Perfect Mixing in Liquid)

This is known as non-equilibrium lever rule or the Scheil - equations.

Ignoring the difference in molar volume between the solid and liquid, we get

Case :III (No Diffusion in Solid, Diffusional Mixing in Liquid)

Planar front solidification of alloy Xo assuming no diffusion in the solid and no stirring in the liquid.

Composition profile when S/L interface temperature is between T2 and T3 . Steady-state solidification at T3. The composition solidifying equals the composition of the liquid far ahead of the solid(X0)

Composition profile at TE and below, showing the final transient

Liquid to solid formation

Nucleation Homogeneous Heterogeneous (Second phase particles, mould

boundaries, etc.)

Growth

Nucleation: Homogeneous

Nucleation: Heterogeneous

Grain refiners

Grain refiners fine particles added to provide heterogeneous nucleation sites.

This results in large number of nuclei leading to finer grain size.

Finer grain size results in higher yield strength alloys.

Al-Si alloys system has Ti, TiB, TiB2, TiC as as grain refiners

Modifiers Strontium (Sr) Strontium is a slightly weaker modifier and

amounts ranging from 80ppm to 200ppm may be required depending on the alloy and

the cooling rate. Generally available in two master alloys: Al-

10%Sr and Sr-10%Al. The first is the most popular with foundries as it is inert and

easy to handle while the second is reactive and flammable. Sr is known to cause a reduction in the

feeding distance and a greater tendency towards porosity. Sr is more popular with many foundries as it

fades much more slowly: hours rather than minutes.

Sodium (Na) One of the first chemical modifiers

discovered. Na is the strongest of the chemical modifiers

in terms of amount required to modify. 30 ppm will completely modify most Al-Si

alloys. The major disadvantage is the fade rate. Na

is lost from the melt by volatilisation in as little as 20 minutes. Na can be added elementally via Na metal.

Packed in sealed cans or foil packs, the master alloy must be kept dry and handled

carefully as it is a hazardous material. Na can be added by fluxes as well.

Modifiers/refiner Antimony (Sb) Sb refines the Si phase as opposed to modifiying it.

I.e. the microstructure remains acicular but becomes much finer; thermal

modification then renders it indistinguishable from modified material. Sb does not fade but is incompatible with Sr or Na. Phosphorous (P) Phosphorous poisons the other modifiers/refiners

where the Al-Si eutectic is concerned. P is added to hypereutectic alloys to refine primary

Si for wear resistance.

ME 206 Manufacturing Processes Case: I (Equilibrium Solidification)Case :II (Solidification with No Diffusion in Solid, Perfect Mixing in Liquid)Case :II (Solidification with No Diffusion in Solid, Perfect Mixing in Liquid)Case :II (Solidification with No Diffusion in Solid, Perfect Mixing in Liquid)Case :III (No Diffusion in Solid, Diffusional Mixing in Liquid)Liquid to solid formationNucleation: HomogeneousNucleation: HeterogeneousGrain refinersModifiersModifiers/refiner