MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu/Suveen Mathaudhu
Office: 308 RBII
Ph: 513-0559
ytzhu@ncsu.edu
Lecture 6: Casting IV
Casting alloys
Department of Materials Science and Engineering
1
NC State University
Melt Fluid Properties
1. Viscosity – Above Tm, most metals behave in a Newtonian manner
(viscosity independent of shear strain rate.), but behavior can change
with alloy composition and phases present; presence of a solid phase
induces non-Newtonian flow
t = hg
For Newtonian flow:
t is the shear stress, h is the viscosity,
is the shear strain rate
g
1. Surface Effects – Problematic when melt flows through small channels or
into corners.
2. Fluidity – the ability of a metal to fill a mold; property of both the metal
and mold
- Fluidity increases with increasing superheat: lowers viscosity and
delays solidification
- Increases with mold temperature: delays solidification
- Depends on type of solidification: dendrites get in the way
- Surface tension and oxide films have an effect
- Mold material heat extraction and wetting phenomena
Department of Materials Science and Engineering
2
NC State University
Casting Alloys: Basics
- Except for metal made by powder metallurgy or electrolytic
methods all alloys are cast as ingots or castings.
- Ingots are worked into final form
- Castings are used “as-cast”
- Ferrous alloys are the most common casting alloys
Department of Materials Science and Engineering
3
NC State University
Casting Alloys: Steel Casting
-Contains < 2% C
-Advantages over wrought steel
products:
- Isotropic properties
- Advantages over other cast alloys:
- Excellent mechanical properties;
heat treatable
- Weldable
- Disadvantages:
- Large solidification shrinkage –
large risers needed
- Good toughness means risers,
gates must be sawed off castings,
not just broken off as in cast iron.
Department of Materials Science and Engineering
4
NC State University
Casting Alloys: Fe-Fe3C Phase Diagram
- Steels have high melting
point, and above 0.15%C and
long freezing range
- Quiz: Answer why does
larger shrinkage occur for cast
steels than for cast iron?
Department of Materials Science and Engineering
5
NC State University
Casting Alloys: Cast Irons
-Contains > 2% C
-Advantages
- Engineering properties: Strength
and hardness, machinability, wear
resistance, abrasion resistance,
corrosion resistance
- Foundry properties: good fluidity,
low solidification shrinkage, ease of
production (why?)
Department of Materials Science and Engineering
6
NC State University
Casting Alloys: Chemical
Composition Effects
- The approximate range in
carbon and silicon
contents of ferrous alloys
- Carbon Equivalent
Si% + P%
C.E.(%) = C% +
3
- Carbide (Fe3C) stablizers
are Cr, Mn and S
Department of Materials Science and Engineering
7
NC State University
Casting Alloys: Gray Iron
- Formed at high C.E. (>4)
- Gray iron is cast iron which
solidifies according to the
eutectic Fe-graphite,
thereby containing graphite
flakes.
- The graphite counters
solidification shrinkage
- The graphite reduces
tensile properties (<1% EL)
- Low cost makes it
preferred where properties
aren’t critical
Department of Materials Science and Engineering
8
NC State University
Casting Alloys: White Cast Iron
- More rapid solidification
and lower C and Si
content(C.E. <3%)
-All C is in the form of
Fe3C
- Very brittle but hard;
excellent wear resistance
- Used for thin section
products
- Virtually unmachinable
Quiz: why unmachinable?
Department of Materials Science and Engineering
9
NC State University
Casting Alloys: Malleable Cast Irons
- Made by heat treatment of white
cast iron structures (10-30 hrs at
850-1000C followed by a slow
cooling to 700C)
- Decomposes Fe3C to Fe + C
- Consists of C aggregates (“temper
nodules”) in ferrite matrix
- Strength and ductility similar to
steel
- Only thin sections possible (?)
- Machinable due to the graphite
- Good for electronic components
Department of Materials Science and Engineering
10
NC State University
Casting Alloys: Nodular/Ductile Iron
- Graphite is made globular during
solidification
- small amount of Mg (0.1%) or Ce
(0.2-0.4%) is added to Fe-C
- Instead of graphite flake,
spheroidal graphite grows
- Combines good castability and
machinability of gray iron with
ductility and toughness of steel
- Can be alloyed or hear treated
- Ductile iron is seeing increasing
use compared to gray iron
Department of Materials Science and Engineering
11
NC State University
Casting Alloys: Aluminum Castings
- Advantages
- Wide range of mechanical
properties
- Relatively corrosion resistant
- Electrical conductivity
- Ease of Machining
- Castable – low melting point
(660 C)
- Lower density (2.7 g/cc)
- Limitations
- Cost per lb > cast iron, steel
- lack of abrasion and wear
resistance
- Low strength compared to
ferrous alloys
Department of Materials Science and Engineering
12
NC State University
Casting Alloys: Al-Si Alloys
- Advantages
- High fluidity (based on Al-Si
eutectic)
- Excellent feeding
- Limitations
- Binary AlSi Not heat
treatable, (but ~0.35 Mg alloys
Mg2Si precipitate to form)
- Two Alloys
- 12% Si used for die casting
(low Tm)
- 5.3% Si contains dentrites of
pure Al in Al+Si eutectic,
0.025% Na refines eutectic
Department of Materials Science and Engineering
13
NC State University
Casting Alloys: Magnesium Alloys
- Advantages
- Lightweight (1.7 g/cc)
- Castable (low melting point –
649 C)
- Limitations
- Very reactive, special
precautions must be taken
(melting under flux)
Department of Materials Science and Engineering
14
NC State University
Casting Alloys: Copper Alloys
- The earliest metallic alloys
made by man from molten
metal
- High Tm (1083 C) makes
steel dies impossible
- Major Alloys:
- Pure copper
- Cu-Zn: brass
- Cu-Sn: bronze
- Cu-Pb: valves
- Cu-Be: gears and
aerospace, precipitation
hardenable
Department of Materials Science and Engineering
15
NC State University
Casting Alloys: Other Alloys
- Tin-based: bearings
- Lead-based: battery
grids, bearings
- Zinc-based: die
castings, automotive
components, trim
- Nickel and cobaltbased: superalloys,
turbine blades
Department of Materials Science and Engineering
16
NC State University
Homework
• Reading Assignment: Chapters 22, 23,
& 24
• Quiz for online students: none
Department of Materials Science and Engineering
17
NC State University