MSE 440/540: Processing of Metallic Materials

1. 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 1

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 2

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 3

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. 4

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? 5

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?) 6

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 7

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 8

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 Quiz: why unmachinable? 9

10. Casting Alloys: Malleable Cast Irons • Made by heat treatment of white cast iron structures (10-30 hrs at 850-1000°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 10

11. 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 11

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 12

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 13

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) 14

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 15

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 16

17. Homework • Reading Assignment: Chapters 22, 23, & 24 • Quiz for online students: none 17