1. MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu/Suveen Mathaudhu
Office: 308 RBII
Ph:
Lecture 6: Casting IV
Text book,
Office hour, by appointment
Casting alloys
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2. Melt Fluid Properties
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
For Newtonian flow:
τ is the shear stress, η is the viscosity, is the shear strain rate
Surface Effects – Problematic when melt flows through small channels or into corners.
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
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3. 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
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4. 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.
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5. Casting Alloys: Fe-Fe3C Phase Diagram
Steels have high melting point, and above 2.0%C.
Quiz: Answer why does larger shrinkage occur for cast steels than for cast iron?
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6. 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?)
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7. Casting Alloys: Chemical Composition Effects
The approximate range in carbon and silicon contents of ferrous alloys
Carbon Equivalent
Carbide (Fe3C) stablizers are Cr, Mn and S
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8. 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
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9. 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
Thin section: need fast cooling to form Fe3C. Slower cooling will form grey iron.
Dark area are Fe3C
Quiz: why unmachinable?
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10. Casting Alloys: Malleable Cast Irons
Made by heat treatment of white cast iron structures (10-30 hrs at °C followed by a slow cooling to 700°C)
Decomposes Fe3C to Fe + C
Consists of C aggregates (“temper nodules”) in ferrite matrix
Strength and ductility similar to steel
Only thin sections possible (why?)
Machinable due to the graphite
Good for electronic components
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11. Casting Alloys: Nodular/Ductile Iron
Graphite is made globular during solidification
small amount of Mg (0.1%) or Ce ( %) 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
Mg and Ce bonds to the edge of the graphite to deter flake formation
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12. 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
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13. 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
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14. 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)
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15. 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
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16. Casting Alloys: Other Alloys
Tin-based: bearings
Lead-based: battery grids, bearings
Zinc-based: die castings, automotive components, trim
Nickel and cobalt-based: superalloys, turbine blades
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17. Homework Reading Assignment: Chapters 22, 23, & 24
Quiz for online students: none
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