Day 18: Ferrous Alloys, Aluminum and More

1. Day 18: Ferrous Alloys, Aluminum and More • Cast Iron • Aluminum • Titanium • Copper • Goals for tomorrow: Whatever we miss today + Zn, Mg, other metals. Review. Material selection discussion.

2. Types of Cast Iron • Grey • White • Malleable • Ductile • Compact Graphite • The form in which the carbon appears in the iron matrix is very important, and determines the use and properties.

3. Cast Iron • Notes: • Significance of Eutectic (ledeburite is the name of the eutectic mix of g and Fe3C.) • Silicon is often an important 3rd component steel Cast iron

4. T(°C) 1600 L 1400 Liquid + Graphite g +L 1200 g 1153°C Austenite 4.2 wt% C 1000  + Graphite a + g 800 740°C 0.65 600  + Graphite 400 90 0 100 1 2 3 4 Co, wt% C (Fe) Fe-C True Equilibrium Diagram • Graphite formation promoted by • Si > 1 wt% • slow cooling Adapted from Fig. 11.2,Callister 7e. (Fig. 11.2 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

5. Gray Cast Iron • By far the cheapest and most common. • Presence of Silicon + slow cooling rate causes graphite to form instead of cementite, Fe3C. • The graphite is in flake form. See micrograph. What we have: Graphite flakes. Look like cornflakes Pearlite matrix

6. http://www.nikonsmallworld.com/gallery/search/descriptions/cast+iron/1http://www.nikonsmallworld.com/gallery/search/descriptions/cast+iron/1

7. More on Gray Iron • The presence of the sharp-cornered flakes creates stress risers which make gray iron very brittle. • However, there are good results as well • Low shrinkage – very castable! • Excellent machinability • Excellent damping • This material is very useful in situations where no tensile strength is required.

8. Properties and Uses • About 3.5% C, 2% Si. • UTS about 40 Ksi • Ductility and yield strength not given, because it’s really brittle. How do the properties of cast iron make it such an ideal material for this application?

9. White Cast Iron • We cool fast enough and with less Silicon so that graphite flakes do not form. • The result is a material with lots of cementite, Fe3C. Extremely hard and brittle. Result: • Not easily machined • Often you have white iron on the surface and grey iron in the interior of the casting. This is a chilled casting and is desirable. • Uses: where very hard, wear resistant surface

10. White Cast Iron Massive cementite – white + Pearlite

11. Ductile Cast Iron • Nodular iron or Spherical. We take away as much S as possible and innoculate with a Mg alloy. This causes graphite to form in spherical clumps instead of flakes. This has a tremendous effect on mechanical properties. • Yield: 276 MPA, 40 Ksi • Ultimate: 414 MPA, 60 Ksi • Ductile: 18% EL • Please note that this stuff is castable. This is very important – has steel like properties in a casting!

12. Nodular / spherical Cast iron

13. Uses of Ductile Iron • Complex geometries + need for tensile strength. Valve body Shafts which take tension Pump housings Gears Engine blocks Ductile Iron can be really strengthened by austempering. That way the spherical graphite lives in a bainite matrix. http://www.ductile.org/didata/section4/4intro.htm

14. Malleable Iron - Blackheart • This has similar properties to the previous iron. But to get it, we start with white cast iron and reheat it for an extended period of time (hours). • The cementite decomposes into graphite in clumps. (Recall that grey iron has sharp flakes). • Consequently, we have ductility. • This approach is not used as frequently now as it was in the past. • White cast iron is hard to get in thick sections, therefore the same is true for malleable iron.

15. Compact Graphite iron • A relatively new approach – get somewhere in between gray and ductile as far as properties go. • I’ll let you do the research. This one will not be on the next quiz.

16. 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 Adapted from Fig. 11.3(a) & (b), Callister 7e.

17. 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 Adapted from Fig. 11.3(c) & (d), Callister 7e.

18. Production of Cast Iron Adapted from Fig.11.5, Callister 7e.

19. Aluminum Facts • Light metal: 2.7 g/cm3 vs 7.9 g/cm3 for steel. • Can be very strong: UTS up to 80 ksi. Competitive with steel at a fraction of the weight. • Very corrosion resistant. Forms a tenacious passive oxide layer on the surface. • Extremely plentiful in earth’s crust, yet very hard to extract from its ore. Hall-Heroult process requires extensive electricity for electrolysis. • Very recyclable. • Not good at high temperatures.

20. Precipitation Hardening in Aluminum Ageing- get coherent precips and get max strength Over-aging. Precipitates no longer coherent. Super-saturated

21. Ageing Curves UTS Ductility A precipitation hardened Al alloy will be sensitive to exposure to high temperatures over extended time, will be difficult to weld and will be more prone to corrode.

22. Types of Aluminum Alloys • Wrought, no heat treatment, but may be strengthed with cold work. • Wrought, heat-treatable and cold worked • Cast, Heat treatable. Aluminum is very castable. Lots of die casting is done. Due to low mp, about 660C. • Fact: Al has a modulus of elasticity about 1/3 that of steel. Al structures will be far less stiff than steel for the same geometry.

23. Wrought Alloy • 1100 series alloy. Basically commercially pure Al. According to Matweb • Yield: 103 Mpa, 15 Ksi. • UTS: 110 Mpa, 16 Ksi. • Ductility: 25%EL. (in 50mm) • Widely used in cheap aluminum products. Not for strength.

24. Wrought – Heat Treatable • The strongest Al is 7075. Alloys are Zn, Mg, and a little Cu. This alloy depends of precipitation hardening. • Yield: 462 Mpa, 67 Ksi. (Competitive with steel.) • UTS: 524 Mpa, 76 Ksi. • Ductility: 11%EL. (in 50mm) • Widely used in aerospace applications where a superior strength to weight ratio is needed. Aircraft structural parts.

25. How is Aluminum used in cars http://www.autoaluminum.org/main/news-releases/automotive-aluminum-use-reaches-all-time-high-in http://www.autoaluminum.org/main/aluminum-processes-and-materials/aluminum-processes-and-materials

26. Titanium • Light metal: 4.5 g/cm3 vs 7.9 g/cm3 for steel. • Can be very strong: UTS up to 200 ksi. Competitive with steel at a fraction of the weight, but you have to pay the price. • Very corrosion resistant. Forms a tenacious passive oxide layer on the surface. • Very reactive at high temperatures. Has to be produced in special process. Expensive.

27. More on Titanium • Wikipedia says up to 77 tons will be used in each A-380. Extremely corrosion resistant. Now also seeing commodity use. Be careful, it might be a fake. I.e. golf clubs! Komsomolets: Soviet era attack sub had Titanium hull. OR DID IT?

28. Copper • Heavier than Fe, about 9.0 gm/cc. • One of the earliest metals. MP 1100C. • In unalloyed form, not very strong, but extremely ductile. Yield: 5 Ksi, UTS: 30.5 Ksi, %EL = 60%. • Electical conductivity is key to its use in all kinds of electrical equipment and conductors. • Copper is also the base metal for two families of alloys: • Brasses (with Zn) • Bronzes (with Sn and other metals)

29. Copper

30. Copper

31. Brass • Very attractive metal. • Excellent corrosion resistance. • Excellent ductility Cartridge brass: 30%Zn Yield 63 Ksi UTS 76 Ksi Ductility %EL = 8% (After cold working.)

32. Bronze • Numerous alloys based on Sn, Ni, Aluminum, etc. • Tin bronze: UTS is 45 ksi, Yield is 22 ksi, %EL = 25%. (Cast alloy) • Used for bearings, bushings, fittings and gears.