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 Text book, Office hour, by appointment Casting alloys Department of Materials Science and Engineering 1

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 Department of Materials Science and Engineering 2

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

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

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? Department of Materials Science and Engineering 5

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

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 Department of Materials Science and Engineering 7

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

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? Department of Materials Science and Engineering 9

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 Department of Materials Science and Engineering 10

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 Mg and Ce bonds to the edge of the graphite to deter flake formation Department of Materials Science and Engineering 11

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

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

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

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

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 Department of Materials Science and Engineering 16

Homework Reading Assignment: Chapters 22, 23, & 24 Quiz for online students: none Department of Materials Science and Engineering 17