Metal Alloys Application and Processing

ISSUES TO ADDRESS...

• How are metal alloys classified and how are they used?

• What are some of the common fabrication techniques?

• How do properties vary throughout a piece of material that has been quenched, for example?

• How can properties be modified by post heat treatment?

Chapter 11: Metal Alloys - Applications and Processing

Taxonomy of MetalsMetal Alloys

Steels

Ferrous Nonferrous

Cast Irons Cu Al Mg Ti<1.4wt%C 3-4.5wt%CSteels

<1.4 wt% CCast Irons3-4.5 wt% C

Fe3C

cementite

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

austenite

+L

+Fe3Cferrite

+Fe3C

+

L+Fe3C

(Fe) Co , wt% C

Eutectic:

Eutectoid:0.76

4.30

727°C

1148°C

T(°C) microstructure: ferrite, graphite cementite

Steels

Low Alloy High Alloy

low carbon <0.25 wt% C

Med carbon0.25-0.6 wt% C

high carbon 0.6-1.4 wt% C

Uses auto struc. sheet

bridges towers press. vessels

crank shafts bolts hammers blades

pistons gears wear applic.

wear applic.

drills saws dies

high T applic. turbines furnaces V. corros. resistant

Example 1010 4310 1040 4340 1095 4190 304

Additions noneCr,V Ni, Mo

noneCr, Ni Mo

noneCr, V, Mo, W

Cr, Ni, Mo

plain HSLA plainheat

treatableplain tool

austenitic stainless

Name

Hardenability 0 + + ++ ++ +++ 0TS - 0 + ++ + ++ 0EL + + 0 - - -- ++

increasing strength, cost, decreasing ductility

Steels

Nomenclature AISI & SAE

10xx Plain Carbon Steels

11xx Plain Carbon Steels (resulfurized for machinability)

15xx Mn (10 ~ 20%)

40xx Mo (0.20 ~ 0.30%)

43xx Ni (1.65 - 2.00%), Cr (0.4 - 0.90%), Mo (0.2 - 0.3%)

44xx Mo (0.5%)

where xx is wt% C x 100

example: 1060 steel – plain carbon steel with 0.60 wt% C

Stainless Steel -- >11% Cr

System and Composition of Plain Carbon Steel and Alloy Steel

Low carbon

Medium, High carbon

Plain Carbon SteelLow carbon• Good

formability and weldability

• Strengthening by coldwork

• Structure usually pearlite and ferrite

High carbon• Low toughness

and formability• Good hardness

and wear resistance

• Can form martensite by quenching but risk of cracking

Medium carbon• Can be

quenched to form martensite or bainite

• Compromising structure between ductility and strength

Compare to other engineering materials• High strength and stiffness, reasonable toughness, easy to

recycle and low cost• Rust easily, require surface protection

Effect of alloy elements• Bi, Pb - improve machinability• B 0.001-0.003% - powerful hardenability agent• Cr 0.5-2% - increase hardenability, 4-18% - corr. resist.• Cu 0.1-0.4% corrosion resistance• Mn 0.25-0.4% - combine with S to prevent brittleness• Mo 0.2-5% - stable carbides• Ni 2-5% - toughener, 12-20% - corrosion resistance• Si 0.2-0.7% - strength, 2% - spring, higher% - magnetic p.• S 0.08-0.15% - free machining• Ti - fix C in inert particles, reduce mart. hardn. in Cr steels• W - hardness at high temperature• V - stable carbide, inc. str. with remain ductility, fine grain

Alloy Steels

HSLA• Large applications• High yield (nearly twice

of plain C steel), good weldability and acceptable corrosion resistance

• Limited ductility and hardenability

• Resist to form martensite in weld zone

Dual-Phase Steel• Quench from temp.

above A1 but below A3 to form structure of ferrite and martensite

• Strength comparable to HSLA while improve formability with no loss of weldability

• Automotive structure and body application

Alloy SteelsFree-machining steels• S, Pb, Bi, Se, Te or P • Making chip-breaking

discontinuity in structure and a build-in lubrication

• Higher cost may compensated with higher speed and lower wear of cutting tools

• Additives may reduce concerned properties such as strength, ductility

• cold working also improve machinability

Bake-Hardenable steel sheet

• Significant in automotive steel sheet

• Low carbon steel• Good formability and

increase strength after forming with heat exposure in paint-baking process

• Good spot weldability, crash energy, low cost easy recycle

Alloy SteelsMaraging Steels• Super high strength alloy• Typical composition is

0.03% C, 8.5% Ni, 7.5% Co, 0.1% Al, 0.003% B, 0.1% Si, 4.8% Mo, 0.4%Ti, 0.01% Zr, 0.1% Mn, 0.01%S and 0.01%P

• Can be hot worked to get soft, tough, low martensite and easy to machine

• Can be cold worked and aging with a yield of 1725 MPa and %EL 11%

• Weldability

Steel for HighTemp.• Good strength, corrosion

resistance, creep resistance

• Plain C steel – 250 C• Conventional alloy – 350 C• High temp. ferrous alloy

tend to has low carbon (less than 0.1%)

• Can be used at higher than 550 C

Dual Phase Steel

Bake hardenable steel

Tool SteelsWater hardening (W)Cold Work

O – Oil hardeningA – Air hardening

D – High C high CrShock resistance (S)High speed

T – W base, M – Mo baseHot work

H1-H19 – Cr baseH20-H39 – W base

H40-H59 – Mo basePlastic mold (P)Special purpose

L – Low alloyF – carbon-tungsten

• High carbon, high strength ferrous alloy

• Balance of toughness, strength and wear resistance

Tool Steels

Stainless Steels

• Oxide of additive elements is tough, adherent, corrosion resistance and heals itself

Ferritic stainless steel• Normally contain >12% Cr (Cr is ferrite stabilizer)• Corrosion resistance• Limited ductility or formability but weldable (no martensite

can form in weld zone)• The cheapest stainless steel

Series Alloys Structure

200

300

400

500

Cr, Ni, Mn or Ni

Cr and Ni

Cr, (C)

Low Cr (<12%) and (C)

Austenitic

Austenitic

Ferritic or martensitic

Martensitic

Stainless Steels

Martensitic Stainless• The lower content of Cr

lead to more stable of austenite at high temp.

• Slow cool may allow carbide of Cr (loss of chromium oxide film)

• Higher cost than ferritic stainless steel due to the heat treatment (austenitization, quench, stress relief and temper

Austenitic Stainless• Ni is austenite stabilizer• The most expensive stainless

due to Ni cost• Mn and N are used as

stabilizer instead of Ni to reduce cost but lower quality

• Non-magnetic, highly corrosion resistance except HCl and other helide acid/salt

• Outstanding formability(FCC)• 304 alloy (18-8) is popular

one, high response to CW

Popular stainless steels

Stainless Steel (1)

Stainless Steel (2)

Precipitation hardenable stainless steel is the special class• Martensitic or austenitic type, modified by addition of

alloying elements like Al to form hard intermetallic compound during temper

Cast Iron

• Ferrous alloys with > 2.1 wt% C– more commonly 3 - 4.5 wt%C

• low melting (also brittle) so easiest to cast

• Cementite decomposes to ferrite + graphite

Fe3C 3 Fe () + C (graphite)

– generally a slow process

Fe-C True Equilibrium Diagram

Graphite formation promoted by

• Si > 1 wt%

• slow cooling

•Gray cast iron

•Ductile or Nodular iron

•White iron

•Malleable iron

•Compacted graphite iron

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

Production of Cast Iron

Types of Cast Iron

Gray 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

Types of Cast Iron

White iron• <1wt% Si so harder but brittle• more cementite

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

Types of Cast Iron

Compacted Graphite Iron• Mg/Ce and others are added• Worm-like shape graphite• Microstructure is between

gray cast iron and ductile iron• Sharp edge of graphite

should be avoided• High thermal conductivity• Better resistance to thermal

shock, fracture and fatigue• Lower oxidation at elevated

Temp.

Limitations of Ferrous Alloys

1) Relatively high density

2) Relatively low conductivity

3) Poor corrosion resistance

Nonferrous Alloy

Nonferrous Alloys

NonFerrous Alloys

• Al Alloys-lower : 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct.

aircraft parts & packaging)

• Mg Alloys-very low : 1.7g/cm3 -ignites easily -aircraft, missiles

• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals

-Ag, Au, Pt -oxid./corr. resistant

• Ti Alloys-lower : 4.5g/cm3

vs 7.9 for steel -reactive at high T -space applic.

• Cu AlloysBrass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant)Bronze : Sn, Al, Si, Ni are subst. impurity (bushings, landing gear)Cu-Be: precip. hardened for strength

Non-Ferrous Alloys

• Cast Alloy – Forming or shaping by appreciable deformation is not possible, ordinarily by casting. So, brittle.

• Wrought Alloy – amenable to mechanical deformation

Sometimes the heat treatability of an alloy is frequently mentioned as “heat treatable”

Cu and its alloys• 3 important properties are

high electrical and thermal conductivity, useful strength with high ductility and corrosion resistance

• Heavily than iron• Pure Cu – wire, cable• Cu-Zn – brass – popular

alpha brass - ductile, form.

beta brass – Zn rich, brittle• Cu-Ni – high thermal

conductivity, high strength at high temperature

• Cu-Sn - bronze

Al and its alloys• The most important of non-

ferrous metal• Light weight, corrosion

resist., good elec./thermal cond., workability, recycle

• Serious weakness is low modulus of elesticity

• Pure Al – soft, ductile• Alloy for mechanical appl.

strength as HSLA level • Alloy for corrosion resist.

difficult to weld• Al-Li – high strength, great

stiffness, lighter weight

Mg and its alloys• Lightest of commerc. Metal• Pure Mg – weak • Alloy – poor ductility, wear,

creep and fatigue• Modulus less than Al• In positive side, high

strength/weight ratio, high energy absorption, good damping of noise/vibration

• Higher purity alloy – good corrosion resistance

• Formability – at high temp.• Good machinability/weldab.• Fire hazards

Ti and its alloys• Strong, light weight,

corrosion resistance• Good mechanical

properties up to 535 C• High cost, fabrication

difficulty, high energy content and high reactivity at elevated temperature

• Fabrication can be by casting, forging, rolling, extrusion or welding

Refractory metal• Extremely high melting T• Nb (2468 C), Mo, W (3410

C), Ta• Ta-Mo to improve corrosion

resistance

Super alloys• Use in aircraft turbine

component• Difficult to form and

machine• Special methods are used,

EDM, electrochemical, ultrasonic m/c

Noble metals• Au, Ag, Pt, Pd, Rh, Ru, Ir

and Os• Expensive

Miscellaneous nonferrous• Ni (coating)• Pb• Sn• Alkaline

Metal Fabrication

• How do we fabricate metals?– Blacksmith - hammer (forged)– Molding - cast

• Forming Operations – Rough stock formed to final shape

Hot working vs. Cold working• T high enough for • well below Tm

recrystallization • work hardening

• Larger deformations • smaller deformations

FORMING

roll

AoAd

roll

• Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate)

Ao Ad

force

dieblank

force

• Forging (Hammering; Stamping) (wrenches, crankshafts)

often atelev. T

Metal Fabrication Methods - I

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

• Extrusion (rods, tubing)

ductile metals, e.g. Cu, Al (hot)

tensile force

AoAddie

die

• Drawing (rods, wire, tubing)

die must be well lubricated & clean

CASTING JOINING

FORMING CASTING JOINING

Metal Fabrication Methods - II

• Casting- mold is filled with metal– metal melted in furnace, perhaps alloying

elements added. Then cast in a mold – most common, cheapest method– gives good production of shapes– weaker products, internal defects– good option for brittle materials

• Sand Casting (large parts, e.g., auto engine blocks)

Metal Fabrication Methods - II

• trying to hold something that is hot

• what will withstand >1600ºC?

• cheap - easy to mold => sand!!!

• pack sand around form (pattern) of desired shape

Sand Sand

molten metal

FORMING CASTING JOINING

plasterdie formedaround waxprototype

• Sand Casting (large parts, e.g., auto engine blocks)

• Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades)

Metal Fabrication Methods - II

Investment Casting

• pattern is made from paraffin.

• mold made by encasing in plaster of paris

• melt the wax & the hollow mold is left

• pour in metal

wax

FORMING CASTING JOINING

Sand Sand

molten metal

plasterdie formedaround waxprototype

• Sand Casting (large parts, e.g., auto engine blocks)

• Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades)

Metal Fabrication Methods - II

wax

• Die Casting (high volume, low T alloys)

• Continuous Casting (simple slab shapes)

molten

solidified

FORMING CASTING JOINING

Sand Sand

molten metal

Continuous casting

CASTING JOINING

Metal Fabrication Methods - III

• Powder Metallurgy (materials w/low ductility)

pressure

heat

point contact at low T

densification by diffusion at higher T

area contact

densify

• Welding (when one large part is impractical)

• Heat affected zone: (region in which the microstructure has been changed).

piece 1 piece 2

fused base metal

filler metal (melted)base metal (melted)

unaffectedunaffectedheat affected zone

FORMING

Annealing: Heat to Tanneal, Soaking, then cool slowly.

Thermal Processing of Metals

Types of Annealing

• Process Anneal: Negate effect of cold working by (recovery/ recrystallization)

• Stress Relief: Reduce stress caused by:

-plastic deformation -nonuniform cooling -phase transform.

• Normalize (steels): Deform steel with large grains, then normalize to make grains small. (air cool)

• Full Anneal (steels): Make soft steels for good forming by heating to get , then cool in furnace to get coarse P.

• Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for

15-25 h.

Fe-Fe3C diagram

a) Annealing

b) Quenching

Heat Treatments

c)

c) Tempered Martensite

time (s)10 10 3 10 510 -1

400

600

800

T(°C)

Austenite (stable)

200

P

B

TE

0%

100%50%

A

A

M + A

M + A

0%

50%

90%

a)b)

Hardenability--Steels• Ability to form martensite• Jominy end quench test to measure hardenability.

• Hardness versus distance from the quenched end.

24°C water

specimen (heated to phase field)

flat ground

Rockwell Chardness tests

Har

dnes

s, H

RC

Distance from quenched end

• The cooling rate varies with position.

Why Hardness Changes with Position

distance from quenched end (in)Ha

rdn

ess

, H

RC

20

40

60

0 1 2 3

600

400

200A M

A

P

0.1 1 10 100 1000

T(°C)

M(start)

Time (s)

0

0%100%

M(finish) Martensite

Martensite + Pearlite

Fine Pearlite

Pearlite

Hardenability vs Alloy Composition• Jominy end quench

results, C = 0.4 wt% C

• "Alloy Steels" (4140, 4340, 5140,

8640) --contain Ni, Cr, Mo (0.2 to 2wt%) --these elements

shift the "nose". --martensite is

easier to form.

Cooling rate (°C/s)

Har

dne

ss, H

RC

20

40

60

100 20 30 40 50Distance from quenched end (mm)

210100 3

4140

8640

5140

1040

50

80

100

%M4340

T(°C)

10-1 10 103 1050

200

400

600

800

Time (s)

M(start)M(90%)

shift from A to B due to alloying

BA

TE

Equivalent distance and Bar diameter

(Quenched in water) (Quenched in oil)

Radial hardness profile

(Quenched in water) (Quenched in oil)

• Effect of quenching medium:

Mediumairoil

water

Severity of Quenchlow

moderatehigh

Hardnesslow

moderatehigh

• Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increases

Positioncentersurface

Cooling ratelowhigh

Hardnesslowhigh

Quenching Medium & Geometry

0 10 20 30 40 50wt% Cu

L+L

+L

300

400

500

600

700

(Al)

T(°C)

composition range needed for precipitation hardening

CuAl2

A

Precipitation Hardening• Particles impede dislocations.• Ex: Al-Cu system• Procedure:

--Pt B: quench to room temp.--Pt C: reheat to nucleate small crystals within crystals.

• Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al

Temp.

Time

--Pt A: solution heat treat (get solid solution)

Pt A (sol’n heat treat)

B

Pt B

C

Pt C (precipitate

• 2014 Al Alloy:

• TS peaks with precipitation time.• Increasing T accelerates process.

Precipitate Effect on TS, %EL

precipitation heat treat time

tens

ile s

tren

gth

(MP

a)

200

300

400

1001min 1h 1day 1mo 1yr

204°C

non-

equi

l. so

lid s

olut

ion

man

y sm

all

prec

ipita

tes

“age

d”

few

er la

rge

prec

ipita

tes

“ove

rage

d”149°C

• %EL reaches minimum with precipitation time.

%E

L (2

in s

ampl

e)10

20

30

0 1min 1h 1day 1mo 1yr

204°C 149°C

precipitation heat treat time

Metal Alloy Crystal Structure

Alloys• substitutional alloys

– can be ordered or disordered– disordered solid solution– ordered - periodic substitution

example: CuAu FCC

Cu

Au

• Interstitial alloys (compounds) – one metal much larger than the other – smaller metal goes in ordered way into

interstitial “holes” in the structure of larger metal

– Ex: Cementite – Fe3C

Metal Alloy Crystal Structure

• Steels: increase TS, Hardness (and cost) by adding --C (low alloy steels) --Cr, V, Ni, Mo, W (high alloy steels) --ductility usually decreases w/additions.• Non-ferrous: --Cu, Al, Ti, Mg, Refractory, and noble metals.• Fabrication techniques: --forming, casting, joining.• Hardenability --increases with alloy content.• Precipitation hardening --effective means to increase strength in Al, Cu, and Mg alloys.

Summary