Chapter 11: Metal Alloys Applications 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?
Taxonomy of Metals
Metal Alloys
Ferrous
Steels
Steels
<1.4wt%C
<1.4
wt% C
Nonferrous
Cast Irons
Cast
Irons
3-4.5
wt%C
3-4.5 wt% C
Cu
Al
1600
d
1200
L
g
austenite
g+L
a800
ferrite
600
400
0
(Fe)
L+Fe3C
1148°C
4.30
1000
727°C
Eutectoid:
0.76
1
2
Eutectic:
g+Fe3C
Fe3C
cementite
a+Fe3C
3
Ti
microstructure:
ferrite, graphite
cementite
T(°C)
1400
Mg
4
5
6
Co , wt% C
6.7
Steels
High Alloy
Low Alloy
low carbon Med carbon
<0.25 wt% C 0.25-0.6wt% C
high carbon
0.6-1.4wt% C
heat
plain
treatable
Cr,V
Cr, Ni
Additions none
none
none
Ni, Mo
Mo
Example 1010 4310
1040
4340 1095
Hardenability 0
+
+
++
++
TS
0
+
++
+
EL
+
+
0
Name
plain
Uses
auto
struc.
sheet
HSLA
bridges
towers
press.
vessels
plain
crank
shafts
bolts
hammers
blades
pistons
gears
wear
applic.
wear
applic.
tool
Cr, V,
Mo, W
4190
+++
++
-drills
saws
dies
increasing strength, cost, decreasing ductility
austenitic
stainless
Cr, Ni, Mo
304
0
0
++
high T
applic.
turbines
furnaces
V. corros.
resistant
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 Steel
Low carbon
• Good
formability and
weldability
• Strengthening
by coldwork
• Structure
usually
pearlite and
ferrite
Medium carbon
High carbon
• Can be
quenched to
form martensite
or bainite
• Compromising
structure
between ductility
and strength
• Low toughness
and formability
• Good hardness
and wear
resistance
• Can form
martensite by
quenching but
risk of cracking
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 Steels
Free-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 Steels
Maraging 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
Bake hardenable steel
Dual Phase Steel
Tool Steels
Water hardening (W)
Cold Work
O – Oil hardening
A – Air hardening
D – High C high Cr
Shock resistance (S)
High speed
T – W base, M – Mo base
Hot work
H1-H19 – Cr base
H20-H39 – W base
H40-H59 – Mo base
Plastic mold (P)
Special purpose
L – Low alloy
F – carbon-tungsten
• High carbon,
high strength
ferrous alloy
• Balance of
toughness,
strength and
wear resistance
Tool Steels
Stainless Steels
Series
200
300
400
500
Alloys
Cr, Ni, Mn or Ni
Cr and Ni
Cr, (C)
Low Cr (<12%) and (C)
Structure
Austenitic
Austenitic
Ferritic or martensitic
Martensitic
• 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
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 (a) + C (graphite)
– generally a slow process
Fe-C True Equilibrium Diagram
T(°C)
Graphite formation
promoted by
1600
• Si > 1 wt%
1400
• slow cooling
L
g
Austenite
1200
Liquid +
Graphite
g +L
1153°C
4.2 wt% C
1000
•Ductile or Nodular iron
•White iron
•Malleable iron
•Compacted graphite iron
800
740°C
0.65
•Gray cast iron
g + Graphite
a+g
600
400
(Fe)
a + Graphite
0
1
2
3
4
90
Co , wt% C
100
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
• Cu Alloys
• Al Alloys
-lower r: 2.7g/cm3
Brass: Zn is subst. impurity
(costume jewelry, coins,
-Cu, Mg, Si, Mn, Zn additions
corrosion resistant)
-solid sol. or precip.
Bronze : Sn, Al, Si, Ni are
strengthened (struct.
subst. impurity
aircraft parts
(bushings, landing
& packaging)
gear)
• Mg Alloys
NonFerrous
Cu-Be:
-very low r: 1.7g/cm3
Alloys
precip. hardened
-ignites easily
for strength
-aircraft, missiles
• Ti Alloys
• Refractory metals
-lower r: 4.5g/cm3
vs 7.9 for steel
-reactive at high T
-space applic.
• Noble metals
-Ag, Au, Pt
-oxid./corr. resistant
-high melting T
-Nb, Mo, W, Ta
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 nonferrous 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
• T high enough for
recrystallization
• Larger deformations
vs.
Cold working
• well below Tm
• work hardening
• smaller deformations
Metal Fabrication Methods - I
FORMING
CASTING
JOINING
• Forging (Hammering; Stamping) • Rolling (Hot or Cold Rolling)
(wrenches, crankshafts)
force
(I-beams, rails, sheet & plate)
roll
die
A o blank
A d often at
elev. T
• Drawing
force
Ao
Ad
roll
• Extrusion
(rods, wire, tubing)
die
Ao
Ad
(rods, tubing)
Ao
tensile
force
die
die must be well lubricated & clean
force
container
ram
billet
die holder
extrusion
die
ductile metals, e.g. Cu, Al (hot)
container
Ad
Metal Fabrication Methods - II
FORMING
CASTING
JOINING
• 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
Metal Fabrication Methods - II
FORMING
CASTING
JOINING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
• trying to hold something that is hot
• what will withstand >1600ºC?
Sand
Sand
molten metal
• cheap - easy to mold => sand!!!
• pack sand around form (pattern) of
desired shape
Metal Fabrication Methods - II
FORMING
CASTING
JOINING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
Investment Casting
• pattern is made from paraffin.
Sand
Sand
molten metal
• Investment Casting
(low volume, complex shapes
e.g., jewelry, turbine blades)
plaster
die formed
around wax
prototype
wax
• mold made by encasing in
plaster of paris
• melt the wax & the hollow mold
is left
• pour in metal
Metal Fabrication Methods - II
FORMING
CASTING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
Sand
• Die Casting
(high volume, low T alloys)
Sand
molten metal
• Investment Casting
(low volume, complex shapes
e.g., jewelry, turbine blades)
plaster
die formed
around wax
prototype
JOINING
• Continuous Casting
(simple slab shapes)
molten
solidified
wax
Continuous casting
Metal Fabrication Methods - III
FORMING
CASTING
• Powder Metallurgy
(materials w/low ductility)
• Welding
(when one large part is
impractical)
pressure
filler metal (melted)
base metal (melted)
fused base metal
heat
area
contact
JOINING
unaffected
piece 1
heat affected zone
unaffected
piece 2
densify
• Heat affected zone:
point contact
at low T
densification
by diffusion at
higher T
(region in which the
microstructure has been
changed).
Thermal Processing of Metals
Annealing: Heat to Tanneal, Soaking, then cool slowly.
• Stress Relief: Reduce
stress caused by:
-plastic deformation
-nonuniform cooling
-phase transform.
• Spheroidize (steels):
Make very soft steels for
good machining. Heat just
below TE & hold for
15-25 h.
Types of
Annealing
• Process Anneal:
Negate effect of
cold working by
(recovery/
recrystallization)
• Full Anneal (steels):
Make soft steels for
good forming by heating
to get g, then cool in
furnace to get coarse P.
• Normalize (steels):
Deform steel with large
grains, then normalize
to make grains small. (air cool)
Fe-Fe3C diagram
Heat Treatments
800
a) Annealing
Austenite (stable)
T(°C)
TE
A
b) Quenching
P
600
c) Tempered
Martensite
B
A
400
0%
M+A
200
50%
M+A
90%
a)
b)
10
-1
10
10
time (s)
3
10
5
c)
Hardenability--Steels
• Ability to form martensite
• Jominy end quench test to measure hardenability.
specimen
(heated to g
phase field)
24°C water
flat ground
Rockwell C
hardness tests
Hardness, HRC
• Hardness versus distance from the quenched end.
Distance from quenched end
Why Hardness Changes with Position
Hardness, HRC
• The cooling rate varies with position.
60
40
20
0
1
2
3
distance from quenched end (in)
T(°C)
0%
100%
600
400
200
M(start)
AM
0 M(finish)
0.1
1
10
100
1000
Time (s)
Hardenability vs Alloy Composition
• "Alloy Steels"
(4140, 4340, 5140,
8640)
--contain Ni, Cr, Mo
(0.2 to 2wt%)
--these elements
shift the "nose".
--martensite is
easier to form.
100
Hardness, HRC
• Jominy end quench
results, C = 0.4 wt%
C
10
3
60
2 Cooling rate (°C/s)
100
4340
80 %M
50
40
4140
8640
20
5140
0 10 20 30 40 50
Distance from quenched end (mm)
800
T(°C)
600
A
400
200
0 -1
10
10
B
TE
shift from
A to B due
to alloying
M(start)
M(90%)
103 105 Time (s)
Equivalent distance and Bar diameter
(Quenched in water)
(Quenched in oil)
Radial hardness profile
(Quenched in water)
(Quenched in oil)
Quenching Medium & Geometry
• Effect of quenching medium:
Medium
air
oil
water
Severity of Quench
low
moderate
high
Hardness
low
moderate
high
• Effect of geometry:
When surface-to-volume ratio increases:
--cooling rate increases
--hardness increases
Position
center
surface
Cooling rate
low
high
Hardness
low
high
Precipitation Hardening
• Particles impede dislocations.
700
• Ex: Al-Cu system
T(°C)
• Procedure:
600
--Pt A: solution heat treat
(get a solid solution)
--Pt B: quench to room temp.
--Pt C: reheat to nucleate
small q crystals within
a crystals.
• Other precipitation
systems:
• Cu-Be
• Cu-Sn
• Mg-Al
500
400
a
a+L
q+L
A
20
30
40
50
wt% Cu
composition range
needed for precipitation hardening
Temp.
Pt A (sol’n heat treat)
Pt C (precipitate q)
Pt B
q
a+q
C
300
0 B 10
(Al)
CuAl2
L
Time
Precipitate Effect on TS, %EL
• 2014 Al Alloy:
400
300
200
100
149°C
204°C
1min
1h 1day 1mo 1yr
precipitation heat treat time
• %EL reaches minimum
with precipitation time.
%EL (2 in sample)
tensile strength (MPa)
• TS peaks with
precipitation time.
• Increasing T accelerates
process.
30
20
10
0
204°C
149°C
1min
1h 1day 1mo 1yr
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
Metal Alloy Crystal Structure
• 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
Summary
• 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.