Chapter 3 part 2: Aluminum Casting Alloys Flashcards
What percentage of aluminum usage is from aluminum castings?
20%
What is the biggest application of aluminum castings?
Automotive usage (60%)
Which parts of BEVs will require more aluminum usage?
Powertrains
(motor housing, inverters, converters, etc)
Will increase range and efficiency too
Which two characteristics will be required of new aluminum alloys for automobile applications?
High yield strength and high conductivity
What is Tesla testing as a replacement for cold metal stamping?
Rapid casting (big part casting)
What is solidification?
Phase change of matter resulting in the transformation of a liquid to solid upon cooling below the freezing temperature
Gibbs free energy is lower and energy is released as latent heat of fusion
How does heat transfer occur in metal casting solidification?
Convection: through melt and air gap
Conduction: through solid and mold wall
Radiation: Via ambient surroundings
How can solidification time be changed?
By changing the effective heat transfer condition:
- Changing mold material
- Applying pressure
- Changing surface area/volume ratio of mold
What affects the casting solidification grain morphology and size?
Solidification conditions
Alloy content
What does the partition coefficient ks determine?
ks = Cs/Cl
It is the degree of segregation
It depends on the slope of the liquidus on a phase diagram
What exists in the liquid and solid away from the interface in solute partitioning during solidification?
Compositional gradients
Constitutional undercooling
When actual melt temperature is above constitutional liquidus, melt is said to be constitutionally undercooled
When melt temperature is below constitutional liquidus, solidification occurs
What are grain growth modes determined by during casting
Solute content
Solidification (cooling) rate
Growth velocity
What shape do the grains at the solid/liquid interface tend to be?
Columnar or dendritic
Cellular columnar growth
(dilute alloys with low solute content)
Differences in solute build up at interface
Perturbations of solute build up at the interface lower the TL, producing liquid buildup zones
Meanwhile, regions where solute content is low have higher TL and grow solids fast, producing columnar cells
Cellular dendritic growth
(At high growth rates or in solute rich alloys)
Growth deviates from heat flow direction to preferred crystallographic directions
Secondary arms appear to more efficiently eliminate solute build-up
Columnar to equiaxed transition
In dilute alloys with short freezing range constitutional undercooling
leads to a narrow central equiaxed zone
In richer alloys with long
freezing range the width of the columnar zone is very narrow,
transition from columnar to equiaxed occurs right away.
Advantages of aluminum for casting
Low melting temperatures
Relatively good fluidity
Excellent melt oxidation resistance
Negligible solubility for gases except hydrogen
Relatively good surface finish
Disadvantages of aluminum for casting
Shrinkage (volumetric or linear)
Reaction with steel die
Porosity
Hot shrinkage and hot cracking
Volumetric shrinkage
Due to density difference between solid and liquid
Linear shrinkage
Due to contraction of the lattice
Hydrogen porosity in aluminum
Differences in solubility of hydrogen in solid and liquid aluminum
H2 precipitates out and produces porosity
Can be reduced by argon degassing
Pipe shrinkage porosity, columnar solidification
Shrinkage-related large voids
Occur in short freezing range metals that exhibit columnar freezing
(eutectics and pure metals)
Microshrinkage porosity
Occurs in equiaxed solidification
Volumetric shrinkage due to solid/liquid phase change
Occurs in long freezing range alloys
which exhibit equiaxed solidification
Produces low ductility & impact resistance
Can be resolved with alloying
Hot tearing vs hot cracking ranges
Hot tearing: freezing range (especially long ones)
Hot cracking: cooling from solidus temperature in solid state
Hot tearing
occurs towards end of semi-solid (solid + liquid) state in equiaxed freezing
solid contracts, tearing the connected dendrite tips
die design, grain refining, hot strength and alloy composition changes can avoid this
Hot cracking
Macroscopic cracks
Occur due to thermal and mechanical
stresses during cooling of the casting
combined with the low “hot-strength” of
aluminum
Die design and hot strength of alloy can prevent this
How can you determine grain size?
Through proper etching
What can grain size help with
Subsequent processing of 1xxx alloys
Improves room temp tensile properties and hot-tear resistance
At what stage is grain size generally refined?
At the nucleation stage by adjusting critical radius
Two ways of refining alpha Aluminum grains
- Fast cooling rate (chilling) to achieve undercooling
- Use of grain refiners (reduces surface energy)
Fast cooling rate to achieve undercooling (grain refinement method)
Fast cooling rate (chilling) increases the volume term
It undercools the liquid→(Tm-T) is large, so the critical nucleus for nucleation is small.
Use of nucleants for grain refinement
Effective nucleants decrease the surface term and hence the
critical radius
What do effective nucleants have?
A low contact angle
A low catalyst-solid interface energy
crystallography that is similar to the solidifying liquid
Titanium grain refining for aluminum
TiAl3 particles
Boron particles added to coat TiAl3 particles and prevent early dissolution
Boron grain refining for aluminum
Produces a-Al and AlB2, both which are effective nucleants for aluminum
Important since Si, Cu and Zn hinder Ti grain refining
Interdendritic space
Has high solute content and are chemically reactive
Second phases also precipitate
in interdendritic regions
What properties can dendritic microstructure affect?
Corrosion resistance, toughness, and strength
Dendrite arm spacing (DAS) is used to control tensile properties
How is dendrite arm spacing (DAS) controlled?
Controlled by cooling rate in semi-solid (mushy) zone
DAS = b(average cooling rate) ^-n
n ~ 1/2 for primary arms
n ~ 1/3 for secondary arms
Critical radius for nucleation (r0)
r0 = critical radius below which the particle dissolves
Above, the particle is stable
Effect of high temperatures on particle nucleation
Volume free energy is small and rate of nucleation is low
Coarse precipitates that nucleate on defects to decrease surface energy
Effect of low temperatures on particle nucleation
Volume free energy is high
Low temperature gives low critical radius, higher nucleation rates, and finer precipitates
Precipitation under controlled conditions
Reheating after slow cooling for a stronger microstructure
Slow-cooled microstructure has coarse precipitates at grain boundaries
By reheating again at lower temperatures, finer, more closely-spaced precipitates can be produced (finer precipitates and higher nucleation rates)
Metastable phase formation
Can form even though they are thermodynamically less stable than the
equilibrium phase for kinetic reasons
Have lower activation energy for nucleation
The alloy can reach equilibrium more quickly through intermediates.
Age hardening
Controlled decomposition of the supersaturated solid solution (SSS) to form finely dispersed metastable precipitates
Stages of age hardening
Guinier Preston (GP) zones
Intermediate precipitates (coherent then semi-coherent)
Equilibrium precipitates
Non-equilibrium (non-coherent) precipitates produce higher strength, at over aging
GP zones
Supersaturated clusters/disks of solute
F temper designation
As cast condition
T1 vs. T4 temper designations
T1: quenched from casting temp
T4: solution heat-treated then quenched
Both then naturally aged at room temp.
T5 vs. T6 vs. T7 temper designations
T5: quenched from casting temp. & artificially aged
T6: Solution heat treated, quenched and artificially aged
T7: Solution heat treated, quenched and artificially overaged
Alloy designation systems for Al
Al - 1xx.x
Cu (aerospace) - 2xx.x
Si, Mg, Cu (transport) - 3xx.x
Si - 4xx.x
Mg (railroad) - 5xx.x
Zn (marine) - 7xx.x
Sn (bearing) - 8xx.x
Digit following decimal for Al alloy designation
0 - chem limits apply to alloy casting
1- chem limits for ingots used to make alloy castings
2- ingot but with somewhat different chemical limits
What do preceding letters for an alloy designation mean?
Differences in minor impurity levels
What properties do casting affect?
grain size
supersaturation rate
second phase type, size, and morphology
Sand casting
disposable sand mold
batch operation - good for large components
slow cooling rate - microstructural mod, grain refinement, heat treatment required for quality
ex: aircraft windshield frame
Permanent mold castings
Metallic mold with coating
High solidification rates
Gravity or low pressure
Good design needed to avoid turbulence
May also need extra treatments for better quality
ex: automotive pistons
Die Casting
Like permanent mold, but metal is forced into water cooled die under high pressure
Rapid casting rates
Fine metastable microstructure and tensile properties
Porosity due to entrapped gases
Can be done at vacuum too
Squeeze casting
Innovative diecasting process
Relatively high solidification rate (not as high as HPDC)
Mould wall contact maintained during volume shrinkage
Sound castings with no or minimum porosity
ex: pistons and wheels, also selective reinforcement with fiber preforms
Investment Casting
Plastic or wax pattern cast
Pattern invested in a ceramic slurry
which hardens, wax is melted out
Cavity is filled with metal
Very precise, detailed castings
ex: Aerospace castings
Lost foam casting
Polystyrene replica of part is covered in sand
Metal is poured in this structure, melting and displacing the foam
Good for parts with intricate shape
ex: automotive engine blocks
