Lecture alloys 2014
- 1. 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? CHAPTER 11: METAL ALLOYS APPLICATIONS AND PROCESSING
- 2. Classifications of Metal Alloys Fe3C cementite Metal Alloys Steels Ferrous Nonferrous Cast Irons Cu Al Mg Ti <1.4wt%C 3-4.5wt%C 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 L g austenite g+L g+Fe3C a ferrite a+Fe3C L+Fe3C d (Fe) Carbon concentration, wt.% C Eutectic Eutectoid 0.77 4.30 727°C 1148°C T(°C) Steels <1.4wt%C Cast Irons 3-4.5tic • Ferrous alloys: iron is the prime constituent • Ferrous alloys are relatively inexpensive and extremely versatile • Thus these alloys are wide spread engineering materials • Alloys that are so brittle that forming by deformation is not possible ordinary are cast • Alloys that are amenable to mechanical deformation are termed wrought • Heat-treatable - are alloys whose mechanical strength can be improved by heat-treatment (e.g. precipitation hardening or martensitic transformations). Some definitions:
- 3. Name: High Alloy Low carbon <0.25wt%C Medium-carbon 0.25-0.6wt%C High carbon 0.6-1.4wt%C Applications: auto struc. sheet bridges towers press. vessels crank shafts bolts hammers blades pistons gears wear applications wear applications drills saws dies high T applications turbines furnaces corrosion resistant Example (ASTM#):1010 A633 1040 4340 1095 4190 304 Additions: noneCu,V,Ni,Monone Cr, Ni,Mo none Cr, V,Mo,W Cr, Ni, Mo Hardenability 0 + + ++ ++ +++ 0 TS - 0 + ++ + ++ 0 EL + + 0 - - -- ++ increasing strength, cost, decreasing ductility Classification of Steels Low Alloy Plain Plain Plain High strength Heat treatable Tool Stainless 10-plane 0.1 or 0.4 C wt.%
- 4. CAST IRON • The cast irons are the ferrous alloys with greater that 2.14 wt. % carbon, but typically contain 3-4.5 wt. % of C as well as other alloying elements, such as silicon (~3 wt..%) which controls kinetics of carbide formation Fe3C cementite 3-4.5wt%C 1600 1400 1200 1000 600 400 0 1 2 3 4 5 6 6.7 L g austenite g+L g+Fe3C a+Fe3C L+Fe3C d (Fe) Carbon concentration, wt.% C Eutectic Eutectoid 0.77 4.30 727°C 1148°C T(°C) 3-4.5wt%C • These alloys have relatively low melting points (1150-1300°C), do not formed undesirable surface films when poured, and undergo moderate shrinkage during solidification. Thus can be easily melted and amenable to casting • There are four general types of cast irons: 1. White iron has a characteristics white, crystalline fracture surface. Large amount of Fe3C are formed during casting, giving hard brittle material 2. Gray iron has a gray fracture surface with finely faced structure. A large Si content (2-3 wt. %) promotes C flakes precipitation rather than carbide 3. Ductile iron: small addition (0.05 wt..%) of Mg to gray iron changes the flake C microstructure to spheroidal that increases (by factor ~20) steel ductility 4. Malleable iron: traditional form of cast iron with reasonable ductility. First cast to white iron and then heat-treated to produce nodular graphite precipitates.
- 5. Equilibrium and Metastable Phases • Cementite (Fe3C) is a metastable phase and after long term treatment decompose to form a-ferrite and carbon: Fe3C 3Fe(S) + C(graphite) • Slow cooling and addition of some elements (e.g. Si) promote graphite formation • Properties of cast irons are defined by the amount and microstructure of existing carbon phase. •Equilibrium iron-carbon phase diagram
- 6. White and Malleable Cast Irons • The low-silicon cast irons (<1.0wt.%), produced under rapid cooling conditions • Microstructure: most of cementite • Properties: extremely hard very but brittle • White iron is an intermediate for the production of malleable iron White iron: light Fe3C regions surrounded by pearlite Malleable iron: dark graphite rosettes in a-Fe matrix
- 7. Gray and Ductile Cast Irons • The gray irons contain 1-31.0 wt..% of Si • Microstructure: flake –shape graphite in ferrite matrix • Properties: relatively weak and brittle in tension BUT very effective in damping vibrational energy an high resistive to wear!! Gray iron: Dark graphite flakes In a-Fe matrix Ductile iron: dark graphite nodules in a-Fe matrix •Ductile (or Nodular) iron : small addition of Mg or/and Ce to the gray iron composition before casting • Microstructure: Nodular or spherical-like graphite structure in pearlite or ferric matrix • Properties: Significant increase in material ductility !! • Applications: valves, pump bodies, gears and other auto and machine components.
- 8. RAPIDLY SOLIDIFIED FERROUS ALLOYS • Eutectic compositions that permit cooling to a glass transition temperature at practically reachable quench rate (105 and 106 C°/s) – - rapidly solidified alloys • Boron, B, rather than carbon is a primary alloying element for amorphous ferrous alloys • Properties: (a) absence of grain boundaries – easy magnetized materials (b) extremely fine structure – exceptional strength and toughness Compositions (wt. %) B Si Cr Ni Mo P 20 10 28 6 10 6 40 6 14 •Some Amorphous Ferrous Alloys
- 9. Nonferrous Alloys • Cu Alloys Brass : Zn is prime impurity (costume jewelry, coins, corrosion resistant) Bronze : Sn, Al, Si, Ni are prime impurities (bushings, landing gear) Cu-Be precipitation-hardened for strength • Al Alloys -lower r: 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid solutions or precipitation strengthened (structural aircraft parts & packaging) • 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 - oxidation/corrosion resistant • Ti Alloys -lower r: 4.5g/cm3 vs 7.9 for steel -reactive at high T -space applications NONFERROUS ALLOYS
- 10. Cooper and its Alloys • Cooper: soft and ductile; unlimited cold-work capacity, but difficult to machine. • Cold-working and solid solution alloying • Main types of Copper Alloys: – Brasses: zinc (Zn) is main substitutional impurity; applications: cartridges, auto-radiator. Musical instruments, coins – Bronzes: tin (Sn), aluminum (Al), Silicon (Si) and nickel (Ni); stronger than brasses with high degree of corrosion resistance – Heat-treated (precipitation hardening) Cu-alloys: beryllium coopers; relatively high strength, excellent electrical and corrosion properties BUT expensive; applications: jet aircraft landing gear bearing, surgical and dental instruments. • Copper’s advantages as primary metal and recycled metal, for brazed, long-life radiators and radiator parts for cars and trucks:
- 11. Aluminum and its Alloys • Low density (~2.7 g/cm3), high ductility (even at room temperature), high electrical and thermal conductivity and resistance to corrosion BUT law melting point (~660°C) • Main types of Aluminum Alloys: - Wrought Alloys - Cast Alloys - Others: e.g. Aluminum-Lithium Alloys • Applications: form food/chemical handling to aircraft structural parts Numerals Major Alloying Element(s) 1XXX 2XXX 3XXX 4XXX 5XXX 6XXX 7XXX 8XXX None (>99.00 %Al) Cu Mn Si Mg Mg an Si Zn Other elements (e.g. Li) Typical alloying elements and alloy designation systems for Aluminum Alloys Temper Definition F As fabricated O Annealed H1 Strain-hardened only H2 Strain-hardened and partially annealed H3 Strain-hardened and stabilized T1 Cooled from elevated-T shaping and aged T2 Cooled from elevated-T shaping, cold-work, aged T3 Solution heat-treat., cold-work, naturally aged T4 Solution heat-treat and naturally aged T5 Cooled from elevated-T shaping, artificially aged T6 Solution heat-treat. and artificially aged T7 Solution heat-treat and stabilized T8 Solution heat-treat, cold-work, artificially aged T9 Solution heat-treat, artificially aged, cold-work Temper designation systems for Aluminum Alloys
- 12. Magnesium and its Alloys • Key Properties: • Light weight · Low density (1.74 g/cm3 two thirds that of aluminium) · Good high temperature mechanical properties · Good to excellent corrosion resistance • Very high strength-to density ratios (specific strength) • In contrast with Al alloys that have fcc structure with (12 ) slip systems and thus high ductility, hcp structure of Mg with only three slip systems leads to its brittleness. • Applications: from tennis rockets to aircraft and missiles Example: Aerospace RZ5 (Zn 3.5 - 5,0 SE 0.8 - 1,7 Zr 0.4 - 1,0 Mg remainder), MSR (AG 2.0 - 3,0 SE 1.8 - 2,5Zr 0.4 - 1,0 Mg remainder) alloys are widely used for aircraft engine and gearbox casings. Very large magnesium castings can be made, such as intermediate compressor casings for turbine engines. These include the Rolls Royce Tay casing in MSR, which weighs 130kg and the BMW Rolls Royce BR710 casing in RZ5. Other aerospace applications include auxiliary gearboxes (F16, Euro-fighter 2000, Tornado) in MSR or RZ5, generator housings (A320 Airbus, Tornado and Concorde in MSR) and canopies, generally in RZ5. Formula 1 Gearbox Casting
- 13. Titanium and its Alloys (1) • Titanium and its alloys have proven to be technically superior and cost-effective materials of construction for a wide variety of aerospace, industrial, marine and commercial applications. • The properties and characteristics of titanium which are important to design engineers in a broad spectrum of industries are: - Excellent Corrosion Resistance: Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals. - Superior Erosion Resistance: Titanium offers superior resistance to erosion, cavitation or impingement attack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys. - High Heat Transfer Efficiency: Under "in service" conditions, the heat transfer properties of titanium approximate those of admiralty brass and copper-nickel.
- 14. Other Alloys • Miscellaneous Nonferrous Alloys: - Nickel and its alloy: high corrosion resistant (Example: monel – 65Ni/28Cu/7wt%Fe – pumps valves in aggressive environment) - Lead, tin and their alloys: soft, low recrystallization temperature, corrosion resistant (Applications: solders, x-ray shields, protecting coatings) • The Refractory Metals: Nb (m.p.=2468°C); Mo (°C); W (°C); Ta(3410°C) - Also: large elastic modulus, strength, hardness in wide range of temperatures - Applications: • The Super alloys – possess the superlative combination of properties - Examples: - Applications: aircraft turbines; nuclear reactors, petrochemical equipment • The Noble Metal Alloys: Ru(44), Rh (45), Pd (46), Ag (47), Os (75), Ir (77), Pt (78), Au (79) - expensive are notable in properties: soft, ductile, oxidation resistant - Applications: jewelry (Ag, Au, Pt), catalyst (Pt, Pd, Ru), thermocouples (Pt, Ru), dental materials etc.
- 15. Fabrication of Metals • Fabrication methods chosen depend on: - properties of metal - size and shape of final piece - cost
- 16. Ao Ad force die blank force • Forging (wrenches, crankshafts) FORMING • Drawing (rods, wire, tubing) often at elev. T • Rolling (I-beams, rails) • Extrusion (rods, tubing) tensile force Ao Ad die die METAL FABRICATION METHODS-I
- 17. • Hot working: deformation at T > T(recrystallization) + less energy to deform + large repeatable deform. - surface oxidation: poor finish • Cold working: deformation at T < T (recrystallization) + higher quality surface + better mechanical properties + closer dimension control - expensive and inconvenient • Cold worked microstructures --generally are very anisotropic! --Forged --Fracture resistant! --Swaged FORMING TEMPERATURE
- 18. Extrusion and Rolling • The advantages of extrusion over rolling are as follows: - Pieces having more complicated cross-sectional geometries may be formed. - Seamless tubing may be produced. • The disadvantages of extrusion over rolling are as follows: - Nonuniform deformation over the cross-section. - A variation in properties may result over the cross-section of an extruded piece.
- 19. plaster die formed around wax prototype CASTING • Sand Casting (large parts, e.g., auto engine blocks) • Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades) • Die Casting (high volume, low T alloys) • Continuous Casting (simple slab shapes) METAL FABRICATION METHODS-II
- 20. Casting • The situations in which casting is the preferred fabrication technique are: - For large pieces and/or complicated shapes. - When mechanical strength is not an important consideration. - For alloys having low ductility. - When it is the most economical fabrication technique. Different casting techniques: • Sand casting: a two-piece mold made of send is used, the surface finish is not an important consideration, casting rates are low, and large pieces are usually cast. • Die casting: a permanent two-piece mold is used, casting rates are high, the molten metal is forced into the mold under pressure, and small pieces are normally cast. • Investment casting: a single-piece mold is used, which is not reusable; it results in high dimensional accuracy, good reproduction of detail, a fine surface finish; casting rates are low. • Continuous casting: at the conclusion of the extraction process, the molten metal is cast into a continuous strand having either a rectangular or circular cross-section; these shapes are desirable for secondary metal-forming operations. The chemical composition and mechanical properties are uniform throughout the cross-section.
- 21. CASTING FORMING Miscellaneous • Powder Processing • Joining: Welding, brazing, soldering • 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 unaffected heat affected zone METAL FABRICATION METHODS-III
- 22. Powder Processing • Some of the advantages of powder metallurgy over casting are as follows: - It is used for materials having high melting temperatures. - Better dimensional tolerances result. - Porosity may be introduced, the degree of which may be controlled (which is desirable in some applications such as self-lubricating bearings). • Some of the disadvantages of powder metallurgy over casting are as follows: - Production of the powder is expensive. - Heat treatment after compaction is necessary.
- 23. Process: heat alloy to TAnneal, for extended period of time then cool slowly. Goals: (1) relieve stresses; (2) increase ductility and toughness; (3) produce specific microstructure Annealing Types of Annealing • Process Anneal: To eliminate negate effect of cold working by recovery/recrystallization • Stress Relief : To reduce stress caused by: -plastic deformation -non-uniform cooling -phase transform. • Normalize (steels): Deformed steel with large grains heat-treated to make grains small. • Full Anneal (steels): Make soft steels for good forming by heating to get g, then cool in furnace to get coarse P. • Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for 15-25h.
- 24. Thermal Processing of Metals: Steels • Normalizing: Heat to between 55 and 85°C above the upper critical temperature until the specimen has fully transformed to austenite, then cool in air. The final microstructure is fine pearlite. • Quenching: Heat to a temperature within the austenite phase region and allow the specimen to fully austenite, then quench to room temperature in oil or water. The final microstructure is martensite. • Full annealing: Heat to between 15 and 40°C above the A3 line (if the concentration of carbon is less than the eutectoid) or above the A1 line (if the concentration of carbon is greater than the eutectoid) until the alloy comes to equilibrium; then furnace cool to room temperature. The final microstructure is coarse pearlite. • Tempering: Heat a quenched (martensitic) specimen, to a temperature between 450 and 650°C, for the time necessary to achieve the desired hardness. The final microstructure is tempered martensite.
- 25. Hardness versus distance from the quenched end HARDENABILITY: STEELS • Full annealing and Spheroidizing: to produce softer steel for good machining and forming • Normalization: to produce more uniform fine structure that tougher than coarse-grained one • Quenching: to produce harder alloy by forming martensitic structure Jominy end-quenching test Rockwell Hardness test Hardness, HRC Distance from quenched end 24°C water specimen (heated to g-phase field) flat ground 4” 1”
- 26. Because the cooling rate varies with position !! WHY HARDNESS CHANGES W/POSITION? • Note: cooling rates before reaching Austenite – Martensite transformation are in the range 1 -50 C/s • Measuring cooling rates at every point (e.g. by thermocouples) and finding rates correlations with the hardness one may develop quenching rate – hardness diagram •But how one can change quenching rate?
- 27. • Effect of geometry: Position center surface Cooling rate small large Hardness small large QUENCHING GEOMETRY When surface-to-volume ratio increases: --cooling rate increases --hardness increases
- 28. • Effect of quenching medium: Medium air oil water Severity of Quench small moderate large Hardness small moderate large QUENCHING MEDIUM Water Oil
- 29. • Jominy end quench results, C = 0.4wt%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. HARDENABILITY VS ALLOY CONTENT
- 30. • Ex: Round bar, 1040 steel, water quenched, 2" diam. PREDICTING HARDNESS PROFILES
- 31. • 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. SUMMARY