Casting Flashcards
Why is solidification time important?
Affects production rate and hence economics
Affects resulting microstructure and hence properties
Chvorinov’s rule states that
Solidifaction time of a section is proportional to (volume / surface area) ^2
What matters is not total time of solidification but Local Velocty of solidification interface
WHat are the main forms of casting defects?
Porosity and turbulence
FLuidity
Shrinkage
What is porosity and turbulence?
Air may be trapped, leading to large scale porosity (blow-holes) or smaller pores
Pressure die casting always results in turbulence and porosity becuase the metal enters the mould really fast
This can be improved by putting the metal under a vacuum to degass it
In sand casting, rapid metal flow can cause damage to the mould, which therefore relaeases sand into the casting, and causes loss of accuracy in dimension
Oxide covering of liquid metals can become entrapped in casting with turbulent flow
Solution to porosity
Well desgined gating systems
Avoid changes in thickness which are sudden
Porosity arising from dissolved gasses can be reduced by degassing
Fluidity
Misruns and cold shuts
CAn be fixed by redesign of running and gating systems
Raising pouring temp and pre heating mould
Explain Shrinkage
Moulds must be designed so that the semi-solid casting can shrink safely. Semi solid has little strength
WHen section changes cannot be avoided, solidification patter may be altered by use of chills, to cause early solidification in regions of vulnerability
Exaplin stable growth of metal
If molten metal is poured into a mould to cast an Ingot, Solidification will start due to heterogenous nucleation on the mould walls
Latent heat will be released at the Solid Liquid Interface as solid forms
Heat is removed from the melt by conduction through the mold wall, and the solidification front moves in towards the centre of the ingot
This means the solification rate is proportional to rate of removal of latent heat
Growth rate is determined by rate at which heat is lost from the system
Why do metals grow more easilly compared to non- metals?
Metals grow easily as the interface between solids and liquids is rough on an atomic scale
This leads to blobby shapes
Non metals are smooth on an atomic scale, so grow more slowly. Growth takes place by steps growing across solid in an orderly way.
Leads to angular particles, e.g needels crystals and snow flakes
Explain solidification of liquid
Nucelation always requires super cooling of the liquid, so First growth of nuclei is into liquid which is below the equillbirium solidification temp.
This growing nuclei throws out latent heat which warms liquid immediately around them.
The whole of the liquid soon warms up to the solidifaction temp
This means
Temp of interface is still Tm
Temp in liquid initally fals below Tm to allow nucelation
Fast dentritic growth takes place in the first few seconds. Growth occurs in particular crystallographic directions, leading to distinctive crystal shapes
Growth is unstable, i.e rate not determined by heat loss
Explain solidication of alloys
Liquid and solid have different compositions
Initially may have nucleation, so often have fast unstable dentritic growth for a short time only
Temp gradient in liquid, positive after initial period
Solidifcation takes place over a range of temperatures, and produces solid that has a range of chemical compositions
In all cases last liquid to solidify is most impure
Exaplin the sodification sequence for pure and impure materials
Liquid is supercooled. Solidification begins when sufficient undercooling is present to allow nucleation (most probably heterogeneously on microscopic particles of impurity or sides of vessel). First solid and surrounding water warmed up locally to melting point, but bulk of water still supercooled. Thin “thermal” dendrites grow into supercooled water, with arms in fast crystallographic growth directions.
B: Latent heat has now warmed up all of the remaining liquid so that it is all at 0ºC. There is no supercooling, so no more dendritic growth (and no more nucleation either – remember that this requires undercooling).
C: Existing dendrites fatten.
D: Dendrites join up: no liquid remains. Each dendrite forms one crystal (grain), and the interfaces between them (where they impinge on one another) are now grain boundaries. Air bubbles forming from gas rejected by liquid as it solidifies will be found in the last liquid regions to solidify: between dendrite arms and at grain boundaries.
Exaplin the structure of castings and weld pools
Heterogenous nucleation occurs on coll mould wallss
Dense array of small grains forms the chill zone
Nuclei in chill zone are randomly orientated, only those with fast growth normal to wall survive due to competitive growth.
Favourably orientated nuclei grow out into metal to form a columnar zone
Central part may be occupied by an equiaxed zone heterogenously nucleated by inclusions in the melt
Why do we want a 100% equiaxed structure?
Easier gran size control
Reducing extent of columnar zone reduces the chemical inhomogeneity of the casting.
A fully columnar casting can suffer centre-line failure due to impurity segregation
Interfaces between columnar grains contain high proportions of impurities and are the last regions to solidify
Feeding liquid metal into into these regions is difficult, so fully columnar castings can contain interconnected porosity
Where can nuclei for grains in equiaxed zone come from?
Oxide or solid metal nuclei formed on surface of melt during pouring
Inoculants. Chosen to have low wetting angle
Turbulence in melt displacing dentrite arms
Turbulence can be increased by vibrating mould
How can you make single crystal castings?
Need to make sure that only one crystal ‘Seed’ can grow
Crystals from the chill zone grow competitively, this amount of competition can be increased by making the crystal grow through a ‘maze’.
Genrally a pigtail is used. The pigtail is ahelical cavity which only selects one crystal from the chill zone
