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Aluminum > Chapter 3 part 2: Aluminum Casting Alloys > Flashcards

Chapter 3 part 2: Aluminum Casting Alloys Flashcards

(60 cards)

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1
Q

What percentage of aluminum usage is from aluminum castings?

A

20%

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2
Q

What is the biggest application of aluminum castings?

A

Automotive usage (60%)

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3
Q

Which parts of BEVs will require more aluminum usage?

A

Powertrains
(motor housing, inverters, converters, etc)
Will increase range and efficiency too

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4
Q

Which two characteristics will be required of new aluminum alloys for automobile applications?

A

High yield strength and high conductivity

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5
Q

What is Tesla testing as a replacement for cold metal stamping?

A

Rapid casting (big part casting)

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6
Q

What is solidification?

A

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

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7
Q

How does heat transfer occur in metal casting solidification?

A

Convection: through melt and air gap
Conduction: through solid and mold wall
Radiation: Via ambient surroundings

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8
Q

How can solidification time be changed?

A

By changing the effective heat transfer condition:
- Changing mold material
- Applying pressure
- Changing surface area/volume ratio of mold

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9
Q

What affects the casting solidification grain morphology and size?

A

Solidification conditions
Alloy content

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10
Q

What does the partition coefficient ks determine?

A

ks = Cs/Cl
It is the degree of segregation
It depends on the slope of the liquidus on a phase diagram

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11
Q

What exists in the liquid and solid away from the interface in solute partitioning during solidification?

A

Compositional gradients

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12
Q

Constitutional undercooling

A

When actual melt temperature is above constitutional liquidus, melt is said to be constitutionally undercooled
When melt temperature is below constitutional liquidus, solidification occurs

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13
Q

What are grain growth modes determined by during casting

A

Solute content
Solidification (cooling) rate
Growth velocity

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14
Q

What shape do the grains at the solid/liquid interface tend to be?

A

Columnar or dendritic

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15
Q

Cellular columnar growth
(dilute alloys with low solute content)

A

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

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16
Q

Cellular dendritic growth
(At high growth rates or in solute rich alloys)

A

Growth deviates from heat flow direction to preferred crystallographic directions
Secondary arms appear to more efficiently eliminate solute build-up

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17
Q

Columnar to equiaxed transition

A

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.

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18
Q

Advantages of aluminum for casting

A

Low melting temperatures
Relatively good fluidity
Excellent melt oxidation resistance
Negligible solubility for gases except hydrogen
Relatively good surface finish

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19
Q

Disadvantages of aluminum for casting

A

Shrinkage (volumetric or linear)
Reaction with steel die
Porosity
Hot shrinkage and hot cracking

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20
Q

Volumetric shrinkage

A

Due to density difference between solid and liquid

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21
Q

Linear shrinkage

A

Due to contraction of the lattice

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22
Q

Hydrogen porosity in aluminum

A

Differences in solubility of hydrogen in solid and liquid aluminum
H2 precipitates out and produces porosity
Can be reduced by argon degassing

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23
Q

Pipe shrinkage porosity, columnar solidification

A

Shrinkage-related large voids
Occur in short freezing range metals that exhibit columnar freezing
(eutectics and pure metals)

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24
Q

Microshrinkage porosity

A

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

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25
Hot tearing vs hot cracking ranges
Hot tearing: freezing range (especially long ones) Hot cracking: cooling from solidus temperature in solid state
26
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
27
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
28
How can you determine grain size?
Through proper etching
29
What can grain size help with
Subsequent processing of 1xxx alloys Improves room temp tensile properties and hot-tear resistance
30
At what stage is grain size generally refined?
At the nucleation stage by adjusting critical radius
31
Two ways of refining alpha Aluminum grains
1. Fast cooling rate (chilling) to achieve undercooling 2. Use of grain refiners (reduces surface energy)
32
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.
33
Use of nucleants for grain refinement
Effective nucleants decrease the surface term and hence the critical radius
34
What do effective nucleants have?
A low contact angle A low catalyst-solid interface energy crystallography that is similar to the solidifying liquid
35
Titanium grain refining for aluminum
TiAl3 particles Boron particles added to coat TiAl3 particles and prevent early dissolution
36
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
37
Interdendritic space
Has high solute content and are chemically reactive Second phases also precipitate in interdendritic regions
38
What properties can dendritic microstructure affect?
Corrosion resistance, toughness, and strength Dendrite arm spacing (DAS) is used to control tensile properties
39
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
40
Critical radius for nucleation (r0)
r0 = critical radius below which the particle dissolves Above, the particle is stable
41
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
42
Effect of low temperatures on particle nucleation
Volume free energy is high Low temperature gives low critical radius, higher nucleation rates, and finer precipitates
43
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)
44
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.
45
Age hardening
Controlled decomposition of the supersaturated solid solution (SSS) to form finely dispersed metastable precipitates
46
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
47
GP zones
Supersaturated clusters/disks of solute
48
F temper designation
As cast condition
49
T1 vs. T4 temper designations
T1: quenched from casting temp T4: solution heat-treated then quenched Both then naturally aged at room temp.
50
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
51
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
52
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
53
What do preceding letters for an alloy designation mean?
Differences in minor impurity levels
54
What properties do casting affect?
grain size supersaturation rate second phase type, size, and morphology
55
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
56
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
57
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
58
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
59
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
60
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

Aluminum (4 decks)

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