Chapter 11: Metal Alloys Applications and Processing ISSUES TO ADDRESS... • How are metal alloys classified and how are they used? • What are some of the common fabrication techniques? • How do properties vary throughout a piece of material that has been quenched, for example? • How can properties be modified by post heat treatment?

Taxonomy of Metals Steels Cast Irons <1.4 wt% C 3-4.5 wt% C Alloys Steels Ferrous Nonferrous Cast Irons Cu Al Mg Ti <1.4wt%C 3-4.5 wt%C Steels <1.4 wt% C Cast Irons 3-4.5 wt% C microstructure: ferrite, graphite cementite Fe 3 C cementite 1600 1400 1200 1000 800 600 400 1 2 4 5 6 6.7 L g austenite +L +Fe3C a ferrite + L+Fe3C d (Fe) Co , wt% C Eutectic: Eutectoid: 0.76 4.30 727°C 1148°C T(°C)

Ferrous alloy: iron is the prime constituent Their widespread use is accounted for by three factors: iron-containing compounds exist in abundant quantities within the earth’s crust; metallic iron and steel alloys may be produced using relatively economical extraction, refining, alloying, and fabrication techniques; ferrous alloys are extremely versatile, they have a wide range of mechanical and physical properties Disadvantage: their susceptibility to corrosion

Steels Low Alloy High Alloy low carbon <0.25 wt% C Med carbon high carbon 0.6-1.4 wt% C plain HSLA heat treatable tool austenitic stainless Name Additions none Cr,V Ni, Mo Cr, Ni Mo Cr, V, Mo, W Cr, Ni, Mo Example 1010 4310 1040 43 40 1095 4190 310 Hardenability + ++ +++ TS - + ++ EL + + - - -- ++ Uses auto bridges crank pistons wear drills high T struc. towers shafts gears applic. saws applic. sheet press. bolts wear dies turbines vessels hammers applic. furnaces increasing strength, cost, decreasing ductility blades V. corros. resistant

Low Alloy Plain carbon steels: contain only residual concentrations of impurities other than carbon and a little manganese. Alloy steels: more alloying elements are intentionally added in specific concentrations ( Si, Cu, Al, V, Ni) Low Alloy Low carbon steel (<0.25 wt% C): unresponsive to heat treatments strengthening is accomplished by cold work microstructures consist of ferrite and pearlite constituents are relatively soft and weak have outstanding ductility and toughness they are machinable, weldable, and, of all steels, are the least expensive to produce

Medium carbon steel (0.25-0.6 wt% C): may be heat treated by austenitizing, quenching, and then tempering to improve their mechanical properties having microstructures of tempered martensite. have low hardenabilities additions of chromium, nickel, and molybdenum improve the capacity of these alloys to be heat treated stronger than the low-carbon steels, but at a sacrifice of ductility and toughness High carbon steel (0.6-1.4 wt% C): are the hardest, strongest, and yet least ductile of the carbon steels. usually containing chromium, vanadium, tungsten, and molybdenum.

Stainless Steels are highly resistant to corrosion (rusting) in a variety of environments their predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required. corrosion resistance may also be enhanced by nickel and molybdenum additions are divided into three classes—martensitic, ferritic, or austenitic a wide range of mechanical properties martensitic stainless steels are capable of being heat treated used at elevated temperatures and in severe environments because they resist oxidation and maintain their mechanical integrity under such conditions Equipment employing these steels includes gas turbines, high- temperature steam boilers, heat-treating furnaces, aircraft, missiles, and nuclear power generating units

Refinement of Steel from Ore Iron Ore Coke Limestone 3CO + Fe2O3 ® 2Fe +3CO2 C + O2 CO2 2CO CaCO3 CaO+CO2 CaO + SiO2 + Al2O3 slag purification reduction of iron ore to metal heat generation Molten iron BLAST FURNACE air layers of coke and iron ore gas refractory vessel

Ferrous Alloys Iron containing – Steels - cast irons Nomenclature AISI & SAE 10xx Plain Carbon Steels 11xx Plain Carbon Steels (resulfurized for machinability) 15xx Mn (10 ~ 20%) 40xx Mo (0.20 ~ 0.30%) 43xx Ni (1.65 - 2.00%), Cr (0.4 - 0.90%), Mo (0.2 - 0.3%) 44xx Mo (0.5%) where xx is wt% C x 100 example: 1060 steel – plain carbon steel with 0.60 wt% C Stainless Steel -- >11% Cr (SAE)The Society of Automotive Engineers (AISI)The American Iron and Steel Institute (ASTM)The American Society for Testing and Materials (UNS) unified numbering system

Cast Iron Ferrous alloys with > 2.1 wt% C more commonly 3 - 4.5 wt%C low melting (also brittle) so easiest to cast Cementite (Fe3C ) is a metastable compound, and under some circumstances it can be dissociate or decomposes to form ferrite + graphite Fe3C  3 Fe () + C (graphite) generally a slow process So phase diagram for this system is different (Fig 12.4)

Fe-C True Equilibrium Diagram 1600 1400 1200 1000 800 600 400 1 2 3 4 90 L g +L  + Graphite Liquid + Graphite (Fe) Co , wt% C 0.65 740°C T(°C)  + Graphite 100 1153°C Austenite 4.2 wt% C a + g Graphite formation promoted by Si > 1 wt% slow cooling during solidification Graphite is the carbon rich phase instead of cementite at 6.7 wt % C for most cast iron the carbon exists as graphite Cast irons have graphite

Types of Cast Iron Gray iron 2.5-4 wt% C, 1-3 wt% Si graphite flakes(on ferrite matrix) weak & brittle under tension stronger under compression excellent vibration dampening wear resistant The least expansive of all metallic materials Ductile (nodular) iron add Mg or Cerium (Ce) graphite in nodules(sphere)not flakes matrix often pearlite – better ductility and stronger than gray

Types of Cast Iron White iron <1wt% Si and fast cooling so harder but brittle more cementite instead of graphite is used as an intermediary in the production of malleable iron. Malleable iron heat white at 800-900ºC for prolonged time graphite in rosettes relatively high strength and appreciable ductility or malleability

Compacted graphite iron ( CGI) as gray, ductile, and malleable irons, carbon exists as graphite, which formation is promoted by the presence of silicon 1.7-3 wt % Si, 3.1-4 wt% C lower fracture and fatigue resistance of the materials, because of presence of sharp edges ( characteristic of flaks graphite) magnesium and/or cerium are added Desirable characteristics of CGI include: higher thermal conductivity better resistance to thermal shock lower oxidation at elevated temperatures applications: diesel engine blocks, exhaust manifolds, gearbox housing, flywheels

Gray cast iron Ductile iron Malleable iron Connecting rod, transmission gear, pipe fittings Diesel engine blocks, exhaust manifold, brake discs for high speed trains Compacted iron

Production of Cast Iron

Limitations of Ferrous Alloys Relatively high density Relatively low conductivity Poor corrosion resistance

Nonferrous Alloys cast alloys: alloys that are so brittle that forming or shaping by appreciable deformation is not possible ordinarily are cast wrought alloys: alloys that are amenable to mechanical deformation

Copper and its alloys: Unalloyed copper: it is difficult to machine because its soft and ductile it has an almost unlimited capacity to be cold worked it is highly resistant to corrosion in diverse environments including the ambient atmosphere, seawater, and some industrial chemicals. high electrical and thermal conductivity high ductility – EL 60% melting temperature 1080 C, density 8.9 g/cm3

Copper alloys: improved the mechanical and corrosion-resistance properties most copper alloys cannot be hardened or strengthened by heat- treating procedures copper – zinc alloys (brasses) - zinc up to 40% - quite ductile and formable - good corrosion resistance copper – tin alloys (bronzes) - more expensive because the high price of tin - up to 20% Sn - good strength, toughness, wear resistance, and corrosion resistance coppers – beryllium alloys the most common precipitation hardenable copper alloys. They possess a remarkable combination of properties: tensile strengths as high as 1400 Mpa, excellent electrical and corrosion properties, and wear resistance when properly lubricated; they may be cast, hot worked, or cold worked. These alloys are costly.

Aluminum and its alloys high electrical and thermal conductivities light weight , density 2.7 g/cm3 high resistance to corrosion in some common environments are easily formed by virtue of high ductility; limitation of aluminum is its low melting temperature 660 C the mechanical strength of aluminum may be enhanced by cold work and by alloying alloying elements: copper, magnesium, silicon, manganese, zinc A generation of new aluminum-lithium alloys -- for use by the aircraft and aerospace industries -- higher strength, greater stiffness, and lighter weight -- materials have relatively low densities 2.5g/cm3 -- high specific moduli (elastic modulus-specific gravity ratios) -- excellent fatigue and low-temperature toughness properties -- costly to manufacture than the conventional aluminum alloys -- are highly machinable, and can be welded

Magnesium and its alloys the most outstanding characteristic is its density, 1.7 g/cm3 is relatively soft, and has a low elastic modulus at room temperature magnesium and its alloys are difficult to deform most fabrication is by casting or hot working has a moderately low melting temperature 651C fine magnesium powder ignites easily when heated in air aluminum, zinc, manganese, and some of the rare earths are the major alloying elements Titanium and its alloys the pure metal has a relatively low density(4.5 g/cm3), a high melting point 1668C and an elastic modulus of 107GPa titanium alloys are extremely strong; room temperature tensile strengths as high as 1400 Mpa are attainable, yielding remarkable specific strengths. Furthermore, the alloys are highly ductile and easily forged and machined the major limitation of titanium is its chemical reactivity with other materials at elevated temperatures, high costs, fabrication difficult

The Refractory metals metals that have extremely high melting temperatures included in this group are niobium (Nb 2470C), molybdenum (Mo 2610), tungsten(W 3410), and tantalum (Ta 3000) melting temperatures range between 2470 C for niobium and 3410C the highest melting temperature of any metal, for tungsten applications: tantalum and molybdenum are alloyed with stainless steel to improve its corrosion resistance. Molybdenum alloys are utilized for extrusion dies and structural parts in space vehicles; incandescent light filaments x-ray tubes, and welding electrodes employ tungsten alloys.

The super alloys have superlative combinations of properties most are used in aircraft turbine components, which must withstand exposure to severely oxidizing environments and high temperatures for reasonable time periods density is an important consideration these materials are classified according to the predominant metal in the alloy, which may be cobalt, nickel, or iron.(7.8-8.9 g/cm3 ) applications: jet engine, gas turbine, rocket, nuclear reactors and petrochemical equipment. They posses high strength, creep resistance, and corrosion resistance at temperature up to and in excess of 1100 C are difficult to machine. Powder metallurgy techniques are also being used extensively in the manufacturing of super alloy components

The Noble metals Nickel and its alloys the noble or precious metals are a group of eight elements that have some physical characteristics in common they are expensive are superior or notable (noble) in properties characteristically soft, ductile, and oxidation resistant silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium Nickel and its alloys are highly resistant to corrosion in many environments(alkaline), salt water, high-velocity, high-temperature steam nickel is often coated or plated on some metals that are susceptible to corrosion as a protective measure nickel is one of the principal alloying elements in stainless steels, and one of the major constituents in the superalloy applications: pumps, valves, steam turbine blades

Nonferrous Alloys NonFerrous Alloys • Cu Alloys • Al Alloys Brass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant)35% Zn Bronze : Sn, Al, Si, Ni are subst. impurity (bushings, landing gear) Cu-Be : precip. hardened for strength • Al Alloys -lower r : 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct. aircraft parts & packaging) • Ti Alloys -lower r : 4.5g/cm3 7.9 for steel -reactive at high T - space application. NonFerrous Alloys • Mg Alloys -very low r : 1.7g/cm3 -ignites easily - aircraft, missiles • Refractory metals -high melting T -Nb, Mo, W, Ta • Noble metals -Ag, Au, Pt - oxid./corr. resistant

Metal Fabrication How do we fabricate metals? Blacksmith - hammer (forged) Molding - cast Forming Operations Rough stock formed to final shape Hot working vs. Cold working • T high enough for • well below Tm recrystallization • work hardening • Larger deformations • smaller deformations Material loss -- increase strength Poor final surface finish -- higher quality surface -- better mechanical properties -- closer dimensional control

Metal Fabrication Methods FORMING CASTING JOINING Forming: Forming operations are those in which the shape of a metal piece is changed by plastic deformation; the deformation must be induced by an external force or stress, the magnitude of which must exceed the yield strength of the material. Most metallic materials are especially amenable to these procedures, being at least moderately ductile and capable of some permanent deformation without cracking or fracturing. hot working: When deformation is achieved at a temperature above that at which recrystallization occurs. otherwise, it is cold working. With most of the forming techniques, both hot- and cold-working procedures are possible.

Joining: powder metallurgy and welding Casting: Casting is a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage. Casting techniques are employed when the finished shape is so large or complicated that any other method would be impractical, a particular alloy is so low in ductility that forming by either hot or cold working would be difficult, and (3) in comparison to other fabrication processes, casting is the most economical Joining: powder metallurgy and welding

Metal Fabrication Methods - I FORMING CASTING JOINING A o d force die blank • Forging (Hammering; Stamping) (wrenches, crankshafts) often at elev. T roll A o d • Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate) tensile force A o d die • Drawing (rods, wire, tubing) die must be well lubricated & clean ram billet container force die holder die A o d extrusion • Extrusion (rods, tubing) ductile metals, e.g. Cu, Al (hot)

Forging is mechanically working or deforming a single piece of a normally hot metal; this may be accomplished by the application of successive blows or by continuous squeezing. Forgings are classified as either closed or open die. Rolling is the most widely used deformation process, consists of passing a piece of metal between two rolls; a reduction in thickness results from compressive stresses exerted by the rolls. Cold rolling may be used. Extrusion a bar of metal is forced through a die orifice by a compressive force that is applied to a ram; the extruded piece that emerges has the desired shape and a reduced cross-sectional area. Drawing is the pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side. A reduction in cross section results, with a corresponding increase in length. The total drawing operation may consist of a number of dies in a series sequence.

Metal Fabrication Methods - II FORMING CASTING JOINING Casting- mold is filled with metal metal melted in furnace, perhaps alloying elements added. Then cast in a mold most common, cheapest method gives good production of shapes weaker products, internal defects good option for brittle materials

Metal Fabrication Methods - II FORMING CASTING JOINING • Sand Casting (large parts, e.g., auto engine blocks) trying to hold something that is hot what will withstand >1600ºC? cheap - easy to mold => sand!!! pack sand around form (pattern) of desired shape Sand molten metal

Metal Fabrication Methods - II FORMING CASTING JOINING • Sand Casting (large parts, e.g., auto engine blocks) Investment Casting pattern is made from paraffin. mold made by encasing in plaster of paris melt the wax & the hollow mold is left pour in metal Sand molten metal • Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades) plaster die formed around wax prototype wax

Metal Fabrication Methods - II FORMING CASTING JOINING • Sand Casting (large parts, e.g., auto engine blocks) • Die Casting (high volume, low T alloys) Sand molten metal • Continuous Casting (simple slab shapes) molten solidified • Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades) plaster die formed around wax prototype wax

Metal Fabrication Methods - III FORMING CASTING JOINING • Powder Metallurgy (materials w/low ductility) • Welding (when one large part is impractical) • Heat affected zone: (region in which the microstructure has been changed). piece 1 piece 2 fused base metal filler metal (melted) base metal (melted) unaffected heat affected zone pressure heat point contact at low T densification by diffusion at higher T area contact density

Powder metallurgy: the compaction of powdered metal, followed by a heat treatment to produce a more dense piece. Powder metallurgy makes it possible to produce a virtually nonporous piece having properties almost equivalent to the fully dense parent material. Diffusional processes during the heat treatment are central to the development of these properties. suitable for metals having low ductilities. Welding: two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient. Both similar and dissimilar metals may be welded.

Thermal Processing of Metals Annealing: a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Is used to negate the effects of cold work, that is, to soften and increase the ductility of a previously strain-hardened metal. It is commonly utilized during fabrication procedures that require extensive plastic deformation relieve stresses; increase softness, ductility, and toughness; produce a specific microstructure. Three steps: heating to the desired temperature, holding or ‘‘soaking’’ at that temperature, and cooling, usually to room temperature

Annealing of ferrous alloys: Normalizing Full annealing Spheroidizing Heat treatment of steels: quenching tempering The successful heat treating of steels to produce a predominantly martensitic microstructure throughout the cross section depends mainly on three factors: (1) the composition of the alloy, (2) the type and character of the quenching medium, and (3) the size and shape of the specimen.

Thermal Processing of Metals Annealing: Heat to Tanneal, then cool slowly. Types of Annealing • Stress Relief: Reduce stress caused by: -plastic deformation -nonuniform cooling -phase transform. • Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for 15-25 h. • Full Anneal (steels): Make soft steels for good forming by heating to get g , then cool in furnace to get coarse P. • Process Anneal: Negate effect of cold working by (recovery/ recrystallization) • Normalize (steels): Deform steel with large grains, then normalize to make grains small.

Heat Treatments Annealing Quenching Tempered Martensite a) b) c) TE 800 Austenite (stable) a) b) Annealing TE T(°C) A Quenching P 600 Tempered Martensite B 400 A 100% 50% 0% c) 0% 200 M + A 50% M + A 90% -1 3 5 10 10 10 10 time (s)

Hardenability--Steels • Ability to form martensite • Jominy end quench test to measure hardenability. 24°C water specimen (heated to g phase field) flat ground Rockwell C hardness tests • Hardness versus distance from the quenched end. Hardness, HRC Distance from quenched end

Why Hardness Changes W/Position • The cooling rate varies with position. 60 Martensite Martensite + Pearlite Fine Pearlite Pearlite Hardness, HRC 40 20 distance from quenched end (in) 1 2 3 600 400 200 A ® M P 0.1 1 10 100 1000 T(°C) M(start) Time (s) 0% 100% M(finish)

Hardenability vs Alloy Composition Cooling rate (°C/s) Hardness, HRC 20 40 60 10 30 50 Distance from quenched end (mm) 2 100 3 4140 8640 5140 1040 80 %M 4340 • Jominy end quench results, C = 0.4 wt% C • "Alloy Steels" (4140, 4340, 5140, 8640) --contain Ni, Cr, Mo (0.2 to 2wt%) --these elements shift the "nose". --martensite is easier to form. T(°C) 10 -1 3 5 200 400 600 800 Time (s) M(start) M(90%) shift from A to B due to alloying B A TE

Quenching Medium & Geometry • Effect of quenching medium: Medium air oil water Severity of Quench low moderate high Hardness low moderate high • Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increases Position center surface Cooling rate low high Hardness

Precipitation Hardening • Particles impede dislocations. • Ex: Al-Cu system • Procedure: 10 20 30 40 50 wt% Cu L +L a a+q q +L 300 400 500 600 700 (Al) T(°C) composition range needed for precipitation hardening CuAl2 A --Pt A: solution heat treat (get a solid solution) B Pt B --Pt B: quench to room temp. C --Pt C: reheat to nucleate small q crystals within a crystals. Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al Temp. Time Pt A (sol’n heat treat) Pt C (precipitate )

Precipitate Effect on TS, %EL • 2014 Al Alloy: • TS peaks with precipitation time. • Increasing T accelerates process. • %EL reaches minimum with precipitation time. %EL (2 in sample) 10 20 30 1min 1h 1day 1mo 1yr 204°C 149 °C precipitation heat treat time precipitation heat treat time tensile strength (MPa) 200 300 400 100 1min 1h 1day 1mo 1yr 204°C non-equil. solid solution many small precipitates “aged” fewer large “overaged” 149°C

Metal Alloy Crystal Stucture Alloys substitutional alloys can be ordered or disordered disordered solid solution ordered - periodic substitution example: CuAu FCC Cu Au

Metal Alloy Crystal Stucture Interstitial alloys (compounds) one metal much larger than the other smaller metal goes in ordered way into interstitial “holes” in the structure of larger metal Ex: Cementite – Fe3C

Metal Alloy Crystal Stucture Consider FCC structure --- what types of holes are there? Octahedron - octahedral site = OH Tetrahedron - tetrahedral site = TD

Metal Alloy Crystal Stucture Interstitials such as H, N, B, C FCC has 4 atoms per unit cell OH sites 4 OH sites TD sites 8 TD sites metal atoms

Summary • Steels: increase TS, Hardness (and cost) by adding --C (low alloy steels) --Cr, V, Ni, Mo, W (high alloy steels) --ductility usually decreases w/additions. • Non-ferrous: --Cu, Al, Ti, Mg, Refractory, and noble metals. • Fabrication techniques: --forming, casting, joining. • Hardenability --increases with alloy content. • Precipitation hardening --effective means to increase strength in Al, Cu, and Mg alloys.