NC State University Department of Materials Science and Engineering1 MSE 440/540: Processing of Metallic Materials Instructors: Yuntian Zhu/Suveen Mathaudhu Office: 308 RBII Ph: Lecture 6: Casting IV Casting alloys

NC State University Department of Materials Science and Engineering2 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 2.Surface Effects – Problematic when melt flows through small channels or into corners; modification of oxides changes this. 3.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

NC State University Department of Materials Science and Engineering3 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

NC State University Department of Materials Science and Engineering4 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.

NC State University Department of Materials Science and Engineering5 Casting Alloys: Fe-Fe 3 C 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?

NC State University Department of Materials Science and Engineering6 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?)

NC State University Department of Materials Science and Engineering7 Casting Alloys: Chemical Composition Effects - The approximate range in carbon and silicon contents of ferrous alloys - Carbon Equivalent - Carbide (Fe 3 C) stablizers are Cr, Mn and S

NC State University Department of Materials Science and Engineering8 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

NC State University Department of Materials Science and Engineering9 Casting Alloys: White Cast Iron - More rapid solidification and lower C content (C.E. <3%) and Si content - All C is in the form of Fe 3 C - Very brittle but hard; excellent wear resistance - Used for thin section products - Virtually unmachinable Quiz: why unmachinable?

NC State University Department of Materials Science and Engineering10 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 700C) - Decomposes Fe 3 C to Fe + C - Consists of C aggregates (“temper nodules”) in ferrite matrix - Strength and ductility similar to steel, but with lower melting point and better castability - Only thin sections possible - Machinable due to the graphite - Good for electronic components

NC State University Department of Materials Science and Engineering11 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

NC State University Department of Materials Science and Engineering12 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

NC State University Department of Materials Science and Engineering13 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 Mg 2 Si 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

NC State University Department of Materials Science and Engineering14 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)

NC State University Department of Materials Science and Engineering15 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

NC State University Department of Materials Science and Engineering16 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

NC State University Homework Reading Assignment: Chapters 22, 23, & 24 Quiz for online students: none Department of Materials Science and Engineering17