2 Casting Flashcards
What is poisoning
atoms of an alloy addition which halt the growth of steps, leading to smaller more rounded particles of the brittle second phase.
Why are the mechanical properties of pressure die-castings generally poor
Metal forced into the mould at high velocity, so there is turbulent flow which entraps air. This leads to high porosity, and more trapped oxide, both giving lower strength and toughness.
Why Sand castings cannot be used for thin sections.
For thin sections, high melt pressure is needed. This would lead to turbulence and high flow velocity, which would erode mould walls. Also, tolerance of sand castings is poor, so integrity of thin sections could be compromised.
Why strength of casting increases by adding powder.
Acts as an inoculant – heterogeneous nucleation is promoted on the particles, giving a fine grain size, and lower impurity segregation, enhancing strength.
Why Carbon steels routinely contain around 1wt% Mn, and aluminium powder may be added shortly before casting.
Both additions deal with macrosegregation of dissolved impurities in the steel, forming solid inclusions throughout the casting, instead of concentrating the impurities into the final region of the casting to solidify. Mn reacts with S to form MnS (to avoid brittle FeS on the final grain boundaries); Al reacts with O to form Al2O3 (‘killing’ the steel), otherwise gas is released on solidification, leading to macroporosity in the centre of the casting.
Why Gravity die casting is not suitable for polymer castings
Polymer viscosities are high (due to the molecular structure) and their densities are low, while metals have low viscosity and high density. Gravity is sufficient in many casting processes to carry metals into a mould, but polymer casting needs high externally applied pressure to force the polymer into the cavity. High temperatures would reduce polymer viscosity, but would cause degradation.
What shape is austenite
FCC
Gamma phase
What shape is ferrite
BCC
Alpha phase
What is Cementite
Iron Carbide Fe3C
What is Pearlite
alternate plates of α Ferrite and Fe3C Cementite
Minimises carbon diffusion distance
Formed on slow cooling of eutectoid steel
What is Bainite
finer mixture of α (ferrite) and Fe3, formed at faster cooling rates and lower temperatures than pearlite.
Plates don’t have time to form
What forms on slow cooling of hypo eutectoid steel
α ferrite
forms on prior austenite grain boundaries (rejecting C into the remaining austenite). At
723°C, the remaining austenite, now containing 0.8wt% C, transforms to pearlite.
What’s the driving force for a phase transformation
The driving force for transformation is a reduction in
Gibbs free energy, ∆G
What’s hardenability
Hardenability is the ability of a steel to form martensite on quenching
What is the critical diameter
we define the critical diameter Do for a steel as the diameter of bar,
quenched in a given medium, which forms 50% martensite at its centre.
What is tempering
Martensite is too brittle for use as a bulk microstructure in a component, so it is
softened and toughened by tempering. Tempering is reheating to a temperature
below Austentite transformation
to allow the supersaturated solution of carbon to precipitate as spheroidal
Fe3C precipitates in a matrix of ferrite.
What is the carbon equivalent
A simple empirical measure of both hardenability and weldability
The higher the CE the higher the hardenability, hence CE provides a warning for loss of weldability.
What is the equivalent diameter
the equivalent diameter of a component is the diameter of an infinitely
long circular cylinder which, if subjected to the same cooling conditions as the component, would have a cooling rate on its axis equal to that at the position of slowest cooling in the component.
Reasons for alloying
Prevention of porosity (Al to r move Oxygen)
Prevention of brittle products (Mn to react w sulphur to stop brittle FeS)
Increase hardenability
Increase weldability
Increase strength and toughness
Increase corrosion resistance (Cr to form Cr oxide instead of Fe oxide)
What is fettling
cutting off the
sprue, runners and risers from a casting
Advantages of sand casting
Versatile, low material and equipment costs, OK for large simple parts; internal detail
and re-entrant features possible.
Disadvantages of sand casting
Poor dimensional accuracy and surface finish; not suitable for thin sections;
relatively high labour costs; “dirty” process.
Types of permanent pattern, expendable mould casting
Sand casting
Investment casting - A hybrid process: here “permanent pattern” actually means a permanent mould is used to make an
expendable pattern. This pattern is covered in an expandable ceramic/ refractory shell, in which the
casting itself is produced.
Evaporative mould casting - Closely related variant, using a polystyrene foam pattern.
Types of permanent mould casting
Pressure die casting - Externally applied pressure permits use of higher viscosity fluid, thinner sections, and minimises waste
from runners, risers etc). Susceptible to entrapped bubbles due to turbulence, which can be detrimental
to properties.
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’s chvorinov’s rule for solidification time
solidification time of a section is proportional to [Volume/Surface area]2
ts ∝ (V/A)2
4 factors that dictate the formation of casting defects
- alloy composition
- design of the mould and gating system
- melt pressure and temperature
- shape details of the casting.
Physical origins : fluidity, turbulence, and shrinkage.
Types of fluidity defects and how to fix them
Misruns and Cold shuts (streams of metal bring too cold to fuse)
Solutions:
Redesign running and gating systems (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).
Types of turbulence defects
Porosity and mould damage (sand casting)
How to avoid shrinkage defects
Feeder design: (V/A) FEEDER
> (V/A) CASTING
So solidification of feeder is slower
Use eutectic composition, solidifies at one temp
Use a uniform cross section, changes in cross section size are difficult to deal with
Smooth changes in section size to avoid stress concentration. Chills can be used to onset cooling and thus solidification early at vulnerable regions
Gibbs free energy equation
G = H – TS (enthalpy H;
absolute temperature T; entropy S).
Partition coefficient eqn
Cs
/
Cl
= k
Cs = equilibrium conc of solidus line
What’s macrosegregation and why is it bad
segregation on the scale of the whole casting
may lead to poor mechanical properties (e.g. high concentrations of weak second phases in the central grain boundaries). And if the segregating solute is a gas,
this may lead to the formation of macroporosity in the centre of the casting.
What’s constitutional supercooling
Constitutional supercooling, which occurs during solidification, is due to compositional solid changes, and results in cooling a liquid below the freezing point ahead of the solid–liquid interface.
greater driving force for solidification ahead
of the solid-liquid interface than at the interface itself.
What structure can form due to constitutional supercooling
Dendrites
if a small part of the solid interface spontaneously 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.
What are the 3 regions of grain structure in a casting
Chill zone, columnar zone,
Equiaxed zone
In general, heterogeneous nucleation occurs first on the cool mould walls, with a dense array of small grains forming the chill zone. As cooling continues, favourably-oriented nuclei grow inwards to form a columnar zone. The central part may be occupied by an equiaxed zone, heterogeneously nucleated within the melt. The proportions of the three structures depend on the alloy and the cooling
conditions.
What problem can fully columnar castings have
Interconnected porosity
What can act as nuclei for the heterogeneous nucleation of equiaxed grains
Oxide particles entrapped during pouring
INOCULANTS, small amounts of specific solid (fine powder) added just
before pouring. Inoculants must have a very low contact angle to promote easy
nucleation.
Method to reduce macrosegregation, bad as impurities can create plane of weakness
Add alloying elements to trap impurities in harmless form throughout casting instead of on boundaries
Eg all C steels contain sulphur as an impurity; the addition of Mn leads to the formation of a dispersion of MnS particles thoughout the casting, rather than letting the S segregate to the grain boundaries, forming brittle FeS.
A second solution is to promote the formation of the equiaxed zone, trapping impurities
in more dilute concentrations over a large area of grain boundary: grain-scale segregation.
What is the rule of thumb diffusion equation
x2 = Dt
where diffusion of an element takes place over a characteristic distance x in time t, with a diffusion rate D = Do exp (-Q/RT) (cf. Materials Databook).
Method to reduce porosity, dissolved gas coming out of solution
during solidification.
Vacuum degassing - though expensive, improves casting quality – the metal is
held under vacuum before casting.
Alloying - dissolved gas
can also be rendered harmless by converting it into solid particles (e.g. oxides)
during solidification.
Example: ‘killing’ a steel means adding Al powder (or another oxide former), to
particles, removing much of the oxygen from solution.
What is homogenisation
Homogenisation is a prolonged “soak” of a casting at high temperature in the single-phase field
Methods for removing microporosity
Microporosity in ingots is largely removed by hot rolling – but this is not a solution for near-net shape castings. Hot isostatic pressing (HIPing) can be used to remove microporosity
2 benefits of using close to eutectic comp for casting
Low melting temp- cheaper
Low solidification range- high fluidity, easier to feed
Why metals solidify easier than non-metals
because the surface energy of the solid-liquid interface is relatively low, so the surface can be atomically
“rough” and there are many sites
available for atoms to attach
themselves to the new solid.
Non-metals (e.g. graphite, silicon) have a higher surface energy with the liquid, so prefer to form an atomically smooth interface. Overall this is slower than forming a solid metal phase, and leads to elongated, angular particle shapes, such as needles or flakes.
What’s poisoning
atoms of an alloy
addition which halt the growth of steps, leading to smaller more rounded particles of
the brittle second phase.
Improves strength, toughness and ductility
Eg adding Mg in spheroidal graphite/modular cast iron
What is grey cast iron
Microstructure: ferrite + graphite (+ iron carbide)
Graphite plates are weak therefore poor tensile properties but good for damping mechanical vibration.
Graphite is also v low density so it’s formation means grey cast iron shows very low shrinkage on solidification. useful for producing castings directly to final size and shape.
