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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