aluminum alloys for hobby foundry
DESCRIPTION
TRANSCRIPT
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Aluminum Foundry Alloy Basics
for
BackyardMetalcasting.com
Foundry Joe
Rev 0, May 2010
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Content
Basics of Aluminum Alloys Cast vs. Wrought Primary vs. Secondary
Understand Alloy Element Effects Deliberate Additions
Si, Mg, Cu, Zn, Be Unwanted Additions
Fe, Others
Effects of Gases (H2, O2) Grain Refiners (TiBor) Modifiers (Sr)
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Aluminum Alloy Applications
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Basics of Aluminum Alloys
Wrought alloys Wrought means “worked”
Rolled, Extruded, Forged 6061, 7075 Aircraft Aluminum
3104 Beer cans
Casting alloys are different than wrought Have been specifically designed
to be cast and not worked
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Aluminum Foundry Casting Alloys
In North America the Aluminum Association numbering system is most common
Wrought alloys series have 4 numbers 1000 to 9000 series 6061, 7075 are familiar examples
Foundry alloy series have 3 numbers 100 series to 800 series 319, 356, 380 are the most common foundry alloys
There are more than 60 foundry alloys But just a few make up 80% of tonnage cast
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Wrought vs. Cast AluminumWrought Alloys Cast Alloys
Aluminum Assoc 4 Numbers Aluminum Assoc 3 Numbers
1xxx
99% Aluminum
Low Strength Electrical
wire
1xx Essentially Pure Aluminum
Rotor cages for electrical motors, tricky to cast
2xxx Al-Cu(-Mg) 2xx Al-Cu Hi Strength, Hard to Cast
3xxx Al-Mn(-Mg) Beer can 3xx Al-Si + Cu, Mg
Most Common AlloyEngines, wheels, airplanes
4xxx Al-Si 4xx Al-Si Easiest to Cast, Medium Strength
5xxx Al-Mg 5xx Al-Mg Excellent finish, Marine use, Corrosion resistant
6xxx Al-Mg-Si 6061 Not used
7xxx Al-Zn-Mg(-Cu)
7075 7xx Al-Zn-Mg Molds, tricky to cast
8xxx Other 8xx Al-Sn Bearing Alloy
9xxx Rumored to be Top Secret Military Alloys – Area 51
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Primary vs. Secondary Alloys
Primary Alloy Produced from ore at a smelter Virgin ingot – first time used More expensive
Secondary Alloy Produced from scrap by a recycler Lower cost Does NOT mean low quality!
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Some Foundry Alloy Examples
Alloy Type Si Fe Cu Mn Mg Zn Ti Sn
319 Secondary 5.5-6.5 1.0 3.0-4.0
0.40 0.10 1.0 0.25
356 Secondary 6.5-7.5 0.6 0.25 0.35 0.20-0.45 0.35 0.25
413 Secondary 11.0-13.0 2.0 1.0 0.35 0.10 0.50 0.15
380 Secondary 7.5-9.5 2.0 3.0-4.0
0.50 0.10- 0.50
3.0
712 Secondary 0.30 0.50 0.25 0.10 0.50- 0.65
5.0–6.5
0.15-0.25
Foundry Alloys are mostly aluminum with 2 or 3 deliberate additions of alloying elements to improve properties
They also have some unwanted elements which usually reduce properties
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What do all these Alloying Elements Do?
Deliberate Alloy Additions Si, Mg, Cu, Zn
Unwanted Alloy Additions Fe, Many Others
Other Additives Grain Refiners (TiBor) Modifiers (Sr)
Effects of Gases (H2, O2)
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Deliberate Alloy Additions
These alloy elements are added by design Added on purpose, to create some kind of a
benefit The “Big One” is Silicon
Develops a property called “fluidity” Next biggest is Magnesium
Allows the alloy to respond to heat treat for improved strength
What is Meant by Fluidity?
Fluidity is how far the alloy will flow before freezing If the alloy freezes before it fills the mold you get a
short pour misrun, or cold shut
You could just make the metal hotter (superheat) so it will run more before freezing
But that takes energy and costs money
This pattern is poured to measure fluidity
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How to Improve Fluidity?
Instead of making the metal hotter
we can alloy the Aluminum to lower the melting point, so the alloy will run farther for a given temperature
An eutectic alloy has this property
This is best seen on a phase diagram
A Degree in Metallurgy follows on the next 3 slides
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Solder Phase Diagram
Solder is an Eutectic Alloy
mean lowest melting point
Pure Tin melts at 446°F 230°C
Pure Lead melts at 626°F 330°C
BUT!A 60/40 mix melts at 338°F 170°C
One hundred degrees F cooler!
Less energy required to melt!
Solid
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Aluminum Silicon Phase Diagram
LIQUID
SOLID
SLUSH
Aluminum Silicon is also an Eutectic Alloy
Pure Aluminum melts at 1220°F 660°C
Pure Silicon melts at 2552°F 1400°C
BUT!
An 11.7% mix melts at 1070°F 577°C
One hundred fifty degrees F cooler!
Less energy required to melt!
Pouring temp is temp above melt point required to fill thinnest section before freezing
So by using a eutectic alloy, can reduce the pouring temp
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Silicon Content of Foundry Alloys
319 alloyengine blocks heads, intakes
380 alloytransmission
cases
390 alloyVega engine
block
413 eutectic alloy“perfect” foundry
alloy - lowest shrink
LIQUID
SLUSH
SOLID
SLUSH
Review of Element Effects
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Base Element – Aluminum (Al)
100 Series Alloys are mostly Aluminum
13th element on Periodic Table Aluminum has limited use in pure form
Electrical wire, electric motor rotors Very tricky to cast Usually mixed with other elements
Mix of elements is “an Alloy”
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Effects Of Alloy Elements – Silicon (Si)
300 & 400 Series Alloys have Silicon added
SiO2 is quartz <sand – used to make glass> Added to improve castability
increases fluidity, reduces shrinkage Silicon and Magnesium together
makes alloy heat treatable for strength 356 and 413 alloys are high Silicon alloys
Nice to cast
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Effects Of Alloy Elements – Copper (Cu)
200 Series Alloys have Copper added
Copper increases strength Reduces corrosion resistance Increases shrinkage
makes alloy more difficult to cast
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Effects Of Alloy Elements – Magnesium (Mg)
300 & 500 Series Alloys have Magnesium added
Silicon and Magnesium together makes alloy heat treatable for strength
Increases shrinkage makes alloy more difficult to cast
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Effects Of Alloy Elements – Zinc (Zn)
700 Series Alloys have Zinc added
Zinc and Aluminum has bright surface finish good corrosion resistance good strength
Increases shrinkage makes alloy more difficult to cast
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Unwanted Alloy Additions - Tramps
These alloy elements are not wanted Can’t be helped From smelting process From scrap recycling From contact with iron crucibles, tools
They cause some kind of harm Lower strength Worse castability
Tramp Elements
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Effects Of Alloy Elements – Iron (Fe)
Iron Oxide is rust Iron makes an Alloy more brittle Generally regarded as bad
Try to keep as low as possible Aluminum is an aggressive solvent for iron
Steel crucibles & tools dissolve in Aluminum Use a coating
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Alloy Microstructure
Aluminium is soft and ductileSilicon is hard and brittleBoth = stronger
Think Fiberglas Glass & Resin
SiBrittleDarkColor
AlSoft
Light Color
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Iron in Microstructure
(Fe,Mn)3Si2Al15
Chinese Script (-Fe)
FeSiAl5Needles (-Fe)
SiBrittle
AlSoft
A crack along the brittle Iron phase
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Other Minor Tramp Elements
Chromium, Nickel, Tin Lithium, Calcium, Phosphorous Others
Each and Total
Just control them under the spec limit How?
Add more Aluminum to dilute them down
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Effects of Gases on Alloys
Gases also have a major <bad> effect on Aluminum alloys
Hydrogen
Oxygen
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Effect of Hydrogen Gas - Porosity
Causes “Gas” Porosity Pores reduce strength, ductility Cause leakers Cause machining scrap
Control by “de gassing” Chlorine tablets Inert gas bubbling
Rotary impeller degasser
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Effect of Oxygen Gas - Dross
Aluminum oxidizes immediately Skim your melt and watch it grow Oxides reduce strength
Think crumpled ball of “tin” foil Control by cleaning with salt fluxes
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Aluminum Grain Refiners - TiBor
Create a fine, uniform as-cast grain structure Finer grains increase strength Reduces cracking Combats shrinkage porosity
Grain RefinedNot Grain Refined
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Modifiers (Strontium)
Changes the structure of the AlSi phase from “needle like” to “globular”
Combats shrinkage porosity Improves strength Increases ductility
Sodium was the original modifier but has mostly been replaced by Strontium
less reactive when added to the melt
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Modifiers (Strontium)
Modification of the Silicon phase From long brittle needles to small globules
SiBrittle
AlSoft
Unmodified Modified