Handout 3: Casting of metals Flashcards
Give the process and steps of sand casting
- A solid re-usable pattern (often wooden) is made of the component.
- Sand with a small amount of resin binder is packed around the pattern in a box called a drag.
- The drag is inverted and the pattern is lifted out, leaving a cavity. In-gates and runners may be carved or moulded into the sand.
- Interior detail may be produced by inserting a core (also molded out of sand) into the cavity. The upper part of the mould( the cope ) is formed from sand, incorporating a pouring basin, a sprue, vents, runners and risers(either moulded from patterns - e.g. runner pin and riser pin shown.
- Mould bolted together, metal poured in. Once the casting has solidified, the sand mould and any cores are broken up and brushed out.
What are the different classifications of casting processes?
Ingot or Continous casting:
- Permanent mould
Shaped casting
- Permanent mould
- Permanent pattern
Describe ingot or continous casting
- Continous casting for most high-volume steel; “direct chill” (DC) casting for wrought aluminium alloys.
- Ingot casting (permanent mould) used for lower volume alloys.
- Post-processing:
- Homogenisation + Thermomechanical processing, e.g. hot/cold rolling, forging, extrusion + heat treatment
Describe shaped casting
Permanent mould
- Simple shapes: need easy removal of mould, and multiple use.
- Moulds expensive(tool steel; may make 103-106 castings).
- Typically used for large numbers of small parts
- Examples:
- pressure die casting
- gravity die casting
- centrifugal casting
Permanent pattern
- More intricate shapes: mould is created around a pattern and destroyed to remove the casting.
- Low setup costs and production rates.
- Used for larger parts, or when small production runs.
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Examples:
- Sand casting
- Investment casting
- Evaporative mould casting
Post processing:
- “Fettling” (trim solidified feeder channels)
- Machine/grind critical areas (improve tolerances and surface finish around joints, seals, contact surfaces)
- Drilling
- Some casting heat-treated to improve properties.
Give the advantages of the different permanent pattern moulding methods.
Sand casting
- Advantages: Versatile, low material and equipment costs, OK for large simple parts, internal details and re-entrant features possible.
- Disadvantages: Poor dimensional accuracy and surface finish, not suitable for thin sections; relatively high labour costs; “dirty” process.
Investment casting
- A hybrid process: here “permanent pattern” actually means a permanent mould is used to make an expandable pattern. This pattern is covered in an expandable ceramic/ refractory shell, in which the casting itself is produced.
- Advantages: Excellent accuracy and surface finish
- Disadvantages: Limited to small parts; labour intensive; more expensive
Evaporative mould casting
- Closely related variant, using a polystyrene foam pattern.
- Advantages: High accuracy and surface finish (especially with small-bead polystyrene); lighter patters than wax, so suitable for large parts.
- Disadvantages: Labour again relatively high
Give descriptions of the different types of permanent mould casting.
Pressure die casting
- Externally applied pressure permits use of higher viscosity fluid, thinner sections, and minmises waste from runners, risers etc. Susceptible to entrapped bubbles due to turbulance, which can be detrimental to properties.
- Limited to low-melting point alloys (because the dies must not distort or wear whilst making many thousands of castings)
Gravity die casting
- Variant process using gravity feed (as in sand casting) but with permanent mould in two separable parts, as in pressure die casting.
Centrifugal casting
- Used for axisymmetric hollow parts (e.g. pipes)
What are the differences in alloy compositions between casting alloys and wrought alloys.
Casting uses dedicated alloy compositions that provide castability(i.e the ability to fill moulds, without major defects). This is controlled by the melt viscosity, the freezing range between liquidus/solidus and the desirability of a lower melting point.
Compositions for thermomechanical (wrought) processes are dominated by solid-state formability and microstructural control for properties (e.g. by heat treatment). For example:
- wrought carbon steels (Fe + 0.1-0.8wt% C): hot/cold formed.
- cast iron (Fe + 4wt% C): only cast
- wrought Al alloys (Al + 1-5wt% Mg, Cu, Zn or Si): hot/cold rolled, extruded
- cast Al alloys (Al + 12% Si): only cast
Castings generally have poorer mechanical properties than wrought counter parts, largely because of porosity, and high content of second phases (brittle). But casting plus heat treatment can lead to high-quality componentes (e.g. jet engine nickel alloy turbine blades)
What is Chvorinov’s rule?
Solidification time of a section is proportional to [Volume/Surface area]2
ts = k(V/A)2
Physical basis:
- k(V/A) = shortest length scale
- ts = k(heat flow distance)2
What are the three main physical origins of defects?
fluidity
turbulance
shrinkage
(Porosity is also caused by dissolved gas being released during solidification)
What are Misruns and cold shuts, what is the solution?
Misrun: caused by lack of fluidity
Cold shut: caused by streams of metal bring too cold to fuse.
Solutions:
Redesign running and gating system (position, size and number of ingates and vents). Increase fluidity by raising pouring temperature or preheating mould, or by changing alloy composition (lower freezing range).
What turbulance defects in moulds ?
Turbulance may arise close to where metal is poured into mould, or within the mould if there are abrupt changes in section.
- air may be trapped, leading to the formation of large-scale porosity. The surface of the liquid metal is often oxide covered: with turbulent flow, more oxide is formed and entrapped in the casting.
- Pressure die-casting always results in turbulance and porosity.
- In sand casting, the mould may also be damaged by rapid metal flow at ingates, leading to sand particle defects in the casting, and causing loss of dimensional accuracy of the casting.
Describe what shrinkage defects are in moulds.
Metals shrink considerably during casting, both due to thermal contraction (in liquid and solid state), and due to crystallisation on solidification.
Moulds are designed to hold a reserve of molten metal (the “risers”) to allow metal to be fed in during solidification. Hence the metal in the feeder head must solidify last, otherwise parts of the casting may be starved, leading to porosity.
What are “mushy freezers” and what are their associated problems?
Feeding the mould to accommodate shrinkage is more difficult in “mushy freezers” i.e. alloys with a large freezing range (between liquidus and solidus). Liquid percolates more slowly through the semi-solid region, giving a greater risk of shrinkage defects. This is one advantage of using eutectic compositions.
How are issues such as stress buildups (leading to hot tearing) or shrinkage cavaties avoided?
- Maintaining a more uniform mould section
- Making changes in section more gradual rather than sharp corners.
- When section changes are unavoidable, the solidification patern may be altered by the use of chills, to cause early solidification (and thus strengthening) in vulnerable regions (e.g. embeddding metal inserts in the sand mould.)
What is Gibbs free energy? and explain how it becomes the driving for solidification. (Draw a G vs T graph)
- G = H - TS (H enthalpy, absolute temperature T, entropy S)
- On cooling a liquid below melting temperature, the system can reduce its free energy by transforming from liquid to solid. The free energy difference G between the two states is the driving force (approximately proportional to the undercooling change in T)
Explain homogeneous nucleation.
Solid crystals first nucleate: i.e. form spontaneously within the liquid when cooled below its melting temperature. Nuclei have a critical radius above which they are stavle and subsequently grow - the greater the driving force, the smaller the critical radius.
Homogenous nucleation - involves the formation of isolated solid spheres within the melt.
Explain heterogeneous nucleation
Involves the formation of a spherical cap of solid on a substrate in contact with the melt (i.e the mould wall or a solid particle within the liquid)
The undercoolig for hetergeneous nucleation is typically a few degrees - much smaller than for homogeneous - so nucleation from surfaces and particles dominates in casting.
Heterogeneous nucleation is characterized by the contact angle, which depends on the balance between three surface energies: γSL,γNL,γNS.
Draw and explain the initial transient of 1D solidification of liquid with initial concentration Co.
- Solidification starts at temperature T0.
- For typical solidification rates, there is essentially no diffusion in the solid, hence the depleted concentration gradient in the solid is “locked in”, with the excess solute being pushed into the liquid ahead of the solid-liquid interface: this is segregation. Solute diffusion in the liquid extends some distance ahead of the interface, but the concentration profile as maintained as the interface also moves to the right.
- With each incremental advance of the interface, the solute level in the solid stays below average for the alloy. Hence the peak solute concentratio i the liquid at the interface rises, with that in the corresponding solid maintaining the partition coefficient ratio, such that Cs= kCL.
Draw and explain the steady state of 1D solidification of liquid with initial concentration Co.
As the interface advances, Cs rises asymptotically towards Co, at which point steady state is reached.
The solidification temperature = T1, with Cs=Co and Cl=Co/k.
The “bow-wave” of elevated solute advances at the same speed as the interface.
What is constitutional supercooling of alloys?
Consider again the steady-state distribution of composition. From the phase diagram, the higher the concentration of the liquid, the lower its solidification temperature. This temperature therefore has essentially the same form of the concentration curve, but inverted.
Now superimpose the actual temperature gradient in the casting (decreasing from right to left, the direction of head conduction). This temperature is equal to the local solidification temperature at the interface, but there are then two scenarious, depending on the gradient in the temperature (dT/dx) compared to the gradient in the local solidification temperature (dTm/dx).
- case (i): (dT/dx) > or = (dTm/dx): T in liquid everywhere above its local melting point; solidification is stable at the interface (leading to a planar solidification front or “cellular growth of a few grains”.
- case (ii): (dT/dx) < (dTm/dx): in a region ahead of the interface, T in the liquid is below its (local) melting point: this is known as constitutional supercooling, since it is a gradient in composition (coupled with a thermal gradient) that leads to the supercooling. Here solidification is unstable, leading to dendrite formation.
What are dendrites?
In constitutional supercooling, there is a greater driving force for solidification ahead of the solid-liquid interface than at the interface itself. Unstable growth occurs as follows: if a small part of the solid interface spontaenously extends into the surrounding liquid, it can continue growing rapidly into the melt, leading to long thin crystals. The same behaviour may then be repeated on the sides of these crystals, giving secondary side arms.
- The distinctive crystal shape formed during solidification is known as a dendrite.
- Once the arms of a given dendrite meet, they form a single grain, and their characteristic structure is not visible - the final structure only reveals the grain boundaries where the crystal orientation changes.
Give a brief description of continous casting and how it avoids porosity.
Over 90% of steel is produced by continous casting. the steel is vacuum degassed before casting, but macroporosity problems are avoided: most released gas escapes upwards through the liquid “sump” into which liquid steel is fed continously.
Draw the diagram for the as-cast and homogenised version of Al-4% Cu. Explain the differences.
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As-cast:
- Dark regions are inter-dendritic regions and grain boundaries.
- The contrast arises from etching of impurities, and from porosity.
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Homogenised:
- Dendritic structure within grains no longer visible: impurities have redistributed.
- Dark spots in the grain are porosity trapped between dendrite arms.
- Note that microporosity remains after homogenisation: the pores are filled with gas mostly in the form of molecules rather than atoms, and the diffusion rate for molecules is very small (due to their size).
Explain the weaknesses of Al-13% Si eutectic are avoided.
Al-13% Si eutectic
- In unmodified Al-Si, the silicon forms a fine dispersion of hard, brittle needles in the soft Al matrix. Considerably improved strength, toughness and ductility are achieved by poisoning : adding just 0.01% sodium immediately before casting modifies the growth to give finer, more rounded particles of silicon.
What are the compositions required to produce ‘nodular’ cast iron.
Improved mechanical properties can again be achieved by poisoning: changing the growth mechanism of the (non-metallic) graphite. In this case, the addition of approx. 0.5wt% of magnesium or cerium causes the graphite to form as rounded nodules, significantly improving the strength and toughness.
What are the dimensional advantages on solidification of cast iron.
A further benefit of grey cast irons is that they show very low shrinkage. Graphite is a low density phase, so the formation of graphite counteracts the contraction of the iron on solidification. For near-eutectic compositions of grey cast iron there is effectively zero volume change on solidification - useful for producing casting directly to final size and shape.
