DAY 18: FERROUS ALLOYS, ALUMINUM AND MORE

Cast Iron Aluminum Titanium Copper Goals for tomorrow: Whatever we miss

today + Zn, Mg, other metals. Review. Material selection discussion.

TYPES OF CAST IRON

1. Grey 2. White3. Malleable4. Ductile5. Compact Graphite

The form in which the carbon appears in the iron matrix is very important, and determines the use and properties.

CAST IRON

steel Cast iron

Notes:

1.Significance of Eutectic (ledeburite is the name of the eutectic mix of and Fe3C.)2.Silicon is often an important 3rd component

4

FE-C TRUE EQUILIBRIUM DIAGRAM

Graphite formation promoted by

• Si > 1 wt%

• slow cooling

Adapted from Fig. 11.2,Callister 7e. (Fig. 11.2 adapted from Binary Alloy Phase Diagrams, 2nd ed.,Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

1600

1400

1200

1000

800

600

4000 1 2 3 4 90

L

+L

+ Graphite

Liquid +Graphite

(Fe) Co , wt% C

0.65

740°C

T(°C)

+ Graphite

100

1153°CAustenite 4.2 wt% C

GRAY CAST IRON

By far the cheapest and most common. Presence of Silicon + slow cooling rate causes

graphite to form instead of cementite, Fe3C. The graphite is in flake form. See micrograph.

What we have:1.Graphite flakes. Look like cornflakes2.Pearlite matrix

http://www.nikonsmallworld.com/gallery/search/descriptions/cast+iron/1

MORE ON GRAY IRON

The presence of the sharp-cornered flakes creates stress risers which make gray iron very brittle.

However, there are good results as well

1. Low shrinkage – very castable!2. Excellent machinability3. Excellent damping

This material is very useful in situations where no tensile strength is required.

PROPERTIES AND USES

About 3.5% C, 2% Si. UTS about 40 Ksi Ductility and yield strength not given, because

it’s really brittle.

How do the properties of cast iron make it such an ideal material for this application?

WHITE CAST IRON

We cool fast enough and with less Silicon so that graphite flakes do not form.

The result is a material with lots of cementite, Fe3C. Extremely hard and brittle. Result:

1. Not easily machined2. Often you have white iron on the surface and

grey iron in the interior of the casting. This is a chilled casting and is desirable.

3. Uses: where very hard, wear resistant surface

WHITE CAST IRON

Massive cementite – white + Pearlite

DUCTILE CAST IRON

Nodular iron or Spherical. We take away as much S as possible and innoculate with a Mg alloy. This causes graphite to form in spherical clumps instead of flakes. This has a tremendous effect on mechanical properties.

Yield: 276 MPA, 40 Ksi Ultimate: 414 MPA, 60 Ksi Ductile: 18% EL Please note that this stuff is castable. This is

very important – has steel like properties in a casting!

NODULAR / SPHERICAL CAST IRON

USES OF DUCTILE IRON

Complex geometries + need for tensile strength.

Valve bodyShafts which take tensionPump housingsGearsEngine blocks

Ductile Iron can be really strengthened by austempering. That way the spherical graphite lives in a bainite matrix.

http://www.ductile.org/didata/section4/4intro.htm

MALLEABLE IRON - BLACKHEART

This has similar properties to the previous iron. But to get it, we start with white cast iron and reheat it for an extended period of time (hours).

The cementite decomposes into graphite in clumps. (Recall that grey iron has sharp flakes).

Consequently, we have ductility. This approach is not used as frequently now as

it was in the past. White cast iron is hard to get in thick sections,

therefore the same is true for malleable iron.

COMPACT GRAPHITE IRON

A relatively new approach – get somewhere in between gray and ductile as far as properties go.

I’ll let you do the research. This one will not be on the next quiz.

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TYPES OF CAST IRONGray iron graphite flakes weak & brittle under tension stronger under compression excellent vibrational

dampening wear resistant

Ductile iron add Mg or Ce graphite in nodules not flakes matrix often pearlite - better

ductility

Adapted from Fig. 11.3(a) & (b), Callister 7e.

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TYPES OF CAST IRONWhite iron <1wt% Si so harder but

brittle more cementite

Malleable iron heat treat at 800-900ºC graphite in rosettes more ductile

Adapted from Fig. 11.3(c) & (d), Callister 7e.

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PRODUCTION OF CAST IRON

Adapted from Fig.11.5, Callister 7e.

ALUMINUM FACTS

Light metal: 2.7 g/cm3 vs 7.9 g/cm3 for steel. Can be very strong: UTS up to 80 ksi.

Competitive with steel at a fraction of the weight.

Very corrosion resistant. Forms a tenacious passive oxide layer on the surface.

Extremely plentiful in earth’s crust, yet very hard to extract from its ore. Hall-Heroult process requires extensive electricity for electrolysis.

Very recyclable. Not good at high temperatures.

PRECIPITATION HARDENING IN ALUMINUM

Super-saturated

Ageing- get coherent precips and get max strength

Over-aging. Precipitates no longer coherent.

AGEING CURVESUTS Ductility

A precipitation hardened Al alloy will be sensitive to exposure to high temperatures over extended time, will be difficult to weld and will be more prone to corrode.

TYPES OF ALUMINUM ALLOYS

Wrought, no heat treatment, but may be strengthed with cold work.

Wrought, heat-treatable and cold worked Cast, Heat treatable. Aluminum is very

castable. Lots of die casting is done. Due to low mp, about 660C.

Fact: Al has a modulus of elasticity about 1/3 that of steel. Al structures will be far less stiff than steel for the same geometry.

WROUGHT ALLOY

1100 series alloy. Basically commercially pure Al. According to Matweb

Yield: 103 Mpa, 15 Ksi. UTS: 110 Mpa, 16 Ksi. Ductility: 25%EL. (in 50mm) Widely used in cheap aluminum products.

Not for strength.

WROUGHT – HEAT TREATABLE

The strongest Al is 7075. Alloys are Zn, Mg, and a little Cu. This alloy depends of precipitation hardening.

Yield: 462 Mpa, 67 Ksi. (Competitive with steel.)

UTS: 524 Mpa, 76 Ksi. Ductility: 11%EL. (in 50mm) Widely used in aerospace applications where

a superior strength to weight ratio is needed. Aircraft structural parts.

HOW IS ALUMINUM USED IN CARS

http://www.autoaluminum.org/main/aluminum-processes-and-materials/aluminum-processes-and-materials

http://www.autoaluminum.org/main/news-releases/automotive-aluminum-use-reaches-all-time-high-in

TITANIUM

Light metal: 4.5 g/cm3 vs 7.9 g/cm3 for steel. Can be very strong: UTS up to 200 ksi.

Competitive with steel at a fraction of the weight, but you have to pay the price.

Very corrosion resistant. Forms a tenacious passive oxide layer on the surface.

Very reactive at high temperatures. Has to be produced in special process. Expensive.

MORE ON TITANIUM

Wikipedia says up to 77 tons will be used in each A-380.

Extremely corrosion resistant.

Now also seeing commodity use. Be careful, it might be a fake. I.e. golf clubs!

Komsomolets: Soviet era attack sub had Titanium hull.

OR DID IT?

COPPER

Heavier than Fe, about 9.0 gm/cc. One of the earliest metals. MP 1100C. In unalloyed form, not very strong, but

extremely ductile. Yield: 5 Ksi, UTS: 30.5 Ksi, %EL = 60%.

Electical conductivity is key to its use in all kinds of electrical equipment and conductors.

Copper is also the base metal for two families of alloys:

1. Brasses (with Zn)2. Bronzes (with Sn and other metals)

Property Industry/Type of Application

Aesthetics Architecture, sculpture, jewelry, clocks, cutlery.

BactericideDoor hardware, marine internal combustion engines, crop treatments.

Corrosion resistance

Plumbing tubes and fittings, roofing, general and marine engineering, shipbuilding; chemical engineering, industrial processes including pickling, etching and distilling; domestic plumbing, architecture, desalination, textiles, papermaking.

Electrical conductivity

Electrical power generation, transmission and distribution, communications, resistance welding, electronics.

Low temperatureproperties

Cryogenics, liquid gas handling, superconductors.

Mechanical strength/ductility

General engineering, marine engineering, defense, aerospace.

Non-magneticInstrumentation, geological survey equipment, minesweepers, offshore drilling.

Non-sparking Mining and other safety tools, oxygen distribution.

ElasticityElectrical springs and contacts, safety pins, instrument bellows, electronic packaging.

Thermal conductivity

Heat exchangers and air-conditioning/refrigeration equipment, automotive radiators, internal combustion engines, mining.

Copper

Copper

BRASS

Very attractive metal. Excellent corrosion resistance. Excellent ductility

Cartridge brass: 30%Zn

Yield 63 KsiUTS 76 KsiDuctility %EL = 8%

(After cold working.)

BRONZE

Numerous alloys based on Sn, Ni, Aluminum, etc. Tin bronze: UTS is 45 ksi, Yield is 22 ksi, %EL =

25%. (Cast alloy) Used for bearings, bushings, fittings and gears.