CHAPTER Non-ferrous Metals and Alloys

6

CHAPTER Non-ferrous Metals and Alloys 6.1

INTRODUCTION

Modern technology has been highly dependent upon non-ferrous metals and alloys for in certain cases they present the advantages of high strength and low weight and for certain other cases they surpass the mechanical strength of ferrous metals. In certain cases the nonferrous metals like copper and aluminium alloys have no alternative in wide range of steel. Electrical conduction and aircraft bodies are examples. A jet turbine engine is a good example of application of these materials. A typical engine of this type contains 38% titanium, 12% chromium, 37% nickel, 6% cobalt, 5% aluminium, 1% niobium and 0.02% tantalum. Though steel is the largest consumed metal, good amounts of non-ferrous metals are coming into demand for mechanical, electrical, elevated temperature and corrosion resistance. Typically aluminium alloys are used for cooking utensils, aircraft bodies and as building materials, copper is used as electrical conductor in electrical machines and power transmissions, copper alloys are also used as tubing wherever good thermal conductivity is desired. And there are several other examples. Table 6.1 compares the prices on the basis of both weight and volume. Table 6.1 Metal Mo alloys Ti alloys Cu alloys Zn alloys Stainless steel Mg alloys Al alloys Low alloy steel Gray cast iron Carbon steels

Comparison of Prices of Various Non-ferrous Alloys per vol.

Price

3.3–4.170 0.33–0.660 0.083–0.166 0.025–0.115 0.055–0.150 0.032–0.640 0.032–0.048 0.023 0.020 0.0167 Gold—1000 (per wt and per vol).

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per wt. 6.24–7.8 1.37–3.71 0.30–0.36 0.67–0.158 0.082–0.37 0.31–0.74 2.65–3.97 0.097 0.050 0.041

80 6.2

MANUFACTURING SCIENCE

ALUMINIUM

Aluminium was first produced in 1825. Presently it is produced in quantity second only to steel. It is the most abundent metallic element on the crust of the earth, easily comprising about 8% of the crust. Bauxite, an hydrous oxide of aluminium and several other oxides, is the principal ore of aluminium. Aluminium is extracted from its ore mainly through electrolytic process. The ore is first washed off to remove clay and dirt, the ore is crushed into powder and treated with hot sodium hydroxide (caustic soda) to remove impurities. Alumina (the oxide of aluminium) extracted from this solution is dissolved in molten sodium fluoride and aluminium fluoride both at 940-980°C. This mixture is then subjected to direct current electrolysis by passing direct current between carbon anode and cathode. The metallic aluminium forms in liquid state and sinks to bottom of the cell. This liquid aluminium is tapped off from time to time. The aluminium so obtained is 99.5 to 99.9% pure with iron and silicon as the major impurities. Aluminium, then is taken to large refractory lined furnaces for refining before casting. The chlorine gas is used as purging agent to remove the dissolved hydrogen gas, and the liquid metal surface is skimmed off to remove oxidized metal. The molten metal is then cast into ingots for remelting or rolling.

6.2.1 Wrought Aluminium Alloys Sheets and extrusion ingots are cast through semicontinuous direct chill method. The sheet ingots are scalpped wherein about 12 mm of ingot surface is removed. The scalpped ingots are preheated to homogenize the structure by heating to a high temperature and soaking there for 10-24 hours. The preheating is done at a temperature below the lowest melting point of the constituents. The ingots are then hot rolled to about 75 mm thickness in 4 high reversed rolling stand. Thereafter the rolled sheet is further reheated to the same temperature and further hot rolled to 18 mm to 25 mm thickness. Further thickness reduction may be achieved through cold rolling. The products obtained this way are termed wrought alloys and normally are in form of sheet, plate, rod, wire and extruded sections. The wrought alloys are identified by a four digit code out of which the first digit signifies the aluminium purity (if pure aluminium) or the major alloying element. The second digit indicates the modification of alloy. The third and fourth digits indicate the minimum amount of aluminium in the alloy. The first digit indicates following: 1. 2. 3. 4. 5. 6. 7.

Aluminium is pure on alloying element. Alloying element copper but magnesium is also added. Alloying element manganese. Alloying element silicon. Alloying element magnesium. Main alloying elements are magnesium and silicon. Main alloying elements are zinc, magnesium and copper.

NON-FERROUS METALS AND ALLOYS

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6.2.2 Aluminium Cast Alloys Aluminium alloys are cast by any one of the following processes Sand casting is the simplest and most versatile process for small castings, complex castings with intricate cores. Large castings and structural castings are produced by sand casting with equal ease. In permanent mould casting a metallic mould is used which may be gravity filled or rotated for centrifugal action. The castings from permanent mould are fine grained as compared to sand cast products. In die casting maximum rate of production is achieved. The molten metal is forced into die which is spilt but sufficiently strong to withstand pressure. One important characteristic of die casting is close tolerance in parts. Fine grained structure and automation of process are other advantages. Aluminium casting alloys need such element for alloying which will not only impart mechanical strength but will also increase fluidity and feeding ability. Silicon in aluminium cast alloys improves fluidity and feeding ability as well its mechanical strength. Normal silicon content varies between 5 and 12%. Magnesium in the range of 0.3 to 1% provides strength mainly through precipitation. Zn, Sn, Ti are also added sometimes.

6.3 PROPERTIES OF ALUMINIUM ALLOYS Among the various properties of aluminium alloys following are notable: (i) (ii) (iii) (iv) (v) (vi)

Low density (2.7 gm/cc). High electrical and thermal conductivity, only next to Cu. Good resistance to atmospheric, water and seawater corrosion. Good machinability, formability and castability. Maintains good light reflectivity. Non-toxic, non-magnetic and non-sparking.

Aluminium is a soft but weak material whose strength is increased by strain hardening and several heat treatments. Aluminium is used as a matrix in several fibre reinforced composites. A12O3, an oxide of Al is very hard and strong and can be dispersed in the matrix of Al by powder metallurgy to produce SAP (sintered aluminium product). Other reinforcing elements used in softer aluminium matrix are boron whiskers, stainless steel fibres and whiskers of Al3Ni. Aluminium alloys are divisible in three groups: 1. Cast Al alloys. 2. Wrought Al alloys. 3. Aluminium composite reinforced with fibers or particles.

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6.3.1 Cast Al Alloys Low melting temperature, insolubility to gases except H2 and good surface finish are characteristics of these alloys. Important drawback of cast aluminium alloys is their shrinkage after solidification and hence careful mould design is called for. Mechanical properties are inferior to wrought alloys except in creep. Alloys can be sand cast gravity die cast, and cold chamber pressure die cast. Si, Cu, Mg and Sn increase fluidity when casting thin sections. Mechanical properties of cast Al alloys is improved by adding Cu which induces age hardening to impart hardness and stability up to 250°C. Alloys used for die casting are: 380 (Al, 8.5 Si, 3.5 Cu) and 413 (Al, 11.5 Si). Alloys preferred for permanent mould casting are: 332 (Al, 9 Si, 3 Cu, 1 Mg) and 319 (Al, 6 Si, 4 Cu). Y-alloy containing 4% Cu and 2% Ni retain strength at high temperatures. It is used for piston and cylinders of I.C. engines.

6.3.2 Wrought Al Alloys Wrought aluminium alloys are obtained by addition of Mn and Mg. The Al-Mn and Al-Mg alloys cannot be heat treated. Al-Mn alloy combines high ductility with excellent corrosion resistance. Beverage cans, cooking utensils and roofing sheets are made in Al-Mn alloy. Some alloys of Al with Cu, Cu and Mg and Mg and Si can be hardened by process of age hardening. Duralumin is one such alloy which contains 4% Cu and small amounts of Mg, Mn and Si. After heat treatment this alloy develops a UTS of 450-550, MPa and finds use in air craft structures. Some Al alloys are described in Table 6.2. Table 6.2 Alloy Designation

Some Aluminium Alloys — Properties and Applications

Composition %

UTS/Elong. % N/mm2

Characteristics and Applications

EC—O

99.5 Al (min)

75/50

Ductile, high elect conductivity.

3003—O 3003—H16

98.8 Al, 1.2 Mn 98.8 Al, 1.2 Mn

130/40 190/40

Good formability and corrosion resistance weldable, storage tanks and utensils.

2024—T4

93 Al, 4.5 Cu, 1.5 Mg, 0.5 Si, 0.5 Mn

500/19

High strength, aircraft parts, bridges, rivets.

5056—H18 5056—O

94.6 Al, 5.2 Mg, 0.3 Mn 94.6 Al, 5.2 Mg, 0.3 Mn

450/10 300/35

Good corrosion resistance to sea water, good finish when buffed or anodized. Marine parts, cooking utensils, bus bodies.

6061—T6 6061—O

98Al, lMg, 0.6Si, 0.4Cu 98Al, 1Mg, 0.651, 0.4Cu

320/17

Good corrosion resistance and formability, general structure, anodized articles, marine and transport parts.

7075—T6

90 Al, 5.5 Zn, 2.5 Mg, 1.7 Cu, 0.3 Cr 90 Al, 5.5 Zn, 2.5 Mg, 1.7 Cu, 0.3 Cr

600/11

High strength and corrosion resistance, aircraft parts, bridges.

7075—O

240/16

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Apart from cast and wrought alloys the greater tonnage (about 85%) of Al is used in commercially pure form in which impurities are less than 1%. Al extrusions, tube, rods, wire, electrical conductors, chemical process equipment, foils and many architectural fittings are made in commercially pure Al. The properties of aluminium are described in Table 6.3. Table 6.3 Typical Properties of Aluminium S. No. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Property Purity % Melting point °C Sp. gravity Tensile strength, N/mm2 O—Temper H—18 Temper Elongation % O—temper H—18 Temper Hardness BHN O—Temper H—18 Temper Electrical conductivity* % IACS O—Temper H—18 Temper Thermal conductivity J/m2/m/°C/s at 25°C Corrosion resistance

Value 99.5 Al, 0.25 Si, 0.25 Fe 660 2.70 72 135 60 17 19 35 62 61 234 Very good in rural, marine and industrial atmosphere.

*Compares with copper. 62% of copper electrical conductivity.

In the Table 6.3 aluminium alloys have been assigned certain temper like O-Temper and H-18 Temper. The temper designation indicates the condition and heat treatment of any given alloy. Generally the temper designation must follow the alloy designation and separated by a dash. For example the alloys in Table 6.3 must be described as 3003—O, 2004— T4. The temper designations are described below. There are four basic tempers: (i) F—As fabricated (ii) O—Annealed (iii) H—Strain hardened (iv) T—Heat treated H is always followed by two or more digits. The first digit indicates basic operations while the following digit stands for the final degree of strain hardening. H1—only strain hardened, H2—strain hardened and partial annealed, H3—strain hardened followed by stabilization. The second digit stands for amount of cold work. The digit 8 represents fully cold worked or full hard. The digit of 4 means half hard and 2 means quarter hard. Thus, H18 means full hard by strain hardening only.

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T designation is followed by numbers 2 to 9. Their meanings are: T2—Annealed (only for castings) T3—Solution heat treated and then cold worked T4—Solution heat treated and naturally aged to stable condition T5—Artificial ageing after any one of the following: Elevated temperature, rapid cool fabrication such as casting or extrusion T6—Solution heat treated and fabricated T7—Solution heat treated and stabilized T8—Solution heat treated, cold worked and then artificially aged T9—Solution heat treated, artificially aged and then cold worked

6.4

AGE-HARDENING OF ALUMINIUM ALLOYS

In certain alloys precipitation from a single phase may occur. The precipitate phase may be in form of fine submicroscopic particles distributed both around the grain boundaries and throughout the grains. In certain alloys of Al-Cu, Mg-Si and Be-Cu such phases precipitate after suitable heat treatment. These precipitated phases have strengthening effects on the alloys. This hardening of alloys is termed age hardening or precipitation hardening. Here the process of age-hardening will be described with particular reference to aluminium alloys containing 4% Cu. Fig. 6.1 shows the equilibrium diagram of Al-Cu system. It is seen that the solubility of Cu in α-phase solid solution decreases steadily and quite considerably with decrease in temperature. At temperature corresponding to point 3, copper forms copper aluminide (CuAl2) which is deposited as coarse particles in and around the grains of α-solid solution. CuAl2 is extremely hard and brittle. If the alloy is now reheated to about 500°C, between the points 2 and 3, CuAl2 is reabsorbed in α-solid solution resulting into single-phase alloy. If alloy from this state is quenched to room temperature, there is insufficient time for CuAl2 to form and Cu atoms are now held in a super-saturated solid solution within the aluminium. When this alloy is allowed to stay at room temperature for five to seven days, the strength improves significantly because of slow precipitation of fine submicroscopic particles. These particles are almost uniformly distributed around the grains. The time of this precipitation may be reduced to a few hours by heating the quenched alloy to 120°C. This is known as artificial age-hardening. Close control of both time and temperature is essential in precipitation hardening for this purpose. Salt baths at constant temperatures are used. 4% Cu aluminium alloy is most suitable for this type of treatment. However, this alloy loses its corrosion resistance in hardened state and must be protected by cladding. Age-hardening alloys containing Si and Mg behave in a similar manner. However, the submicroscopic particles that provide strengthening are made of magnesium silicide (Mg2Si). Thus, the age-hardening effect of CuAl2 is reinforced by Mg2Si.


Fig. 6.1

6.5

85

The aluminium-rich portion of the copper aluminium equilibrium diagram showing the mechanism of precipitation hardening for a 4% copper alloy. Over ageing causes a coalescence of the CuAl2 particles and a consequent loss of strength in the alloy.

COPPER AND ITS PRODUCTION

Copper is marked by a host of good engineering properties. The foremost is its good electrical conductivity and bulk of copper is used as electrical conductor. It also has a high thermal conductivity and coupled with its resistance to corrosion it is largely used as heat exchanger tubes particularly under circumstances when corrosive atmosphere exists. Its medium tensile strength and ease of fabrication are added advantages in its industrial application. Copper is extracted from its sulphide ore. Such ores also contain sulphides of iron. Low grade ore is converted into sulphide concentrate which is smelted in reverberatory furnace to produce a mixture of sulphides of iron and copper, called matte. The slag is separated from matte. The copper sulphide is then chemically converted into impure or blister copper of 98% purity, by blowing air through the matte. The iron sulphide is oxidized and converted into slag. The blister copper is then transferred to refining furnace where most of impurities are converted into slag and removed. This fire refined copper is called tough pitch copper and is further refined electrolytically to produce 99.95% pure copper called electrolytic tough pitch (ETP) copper. ETP copper is used for production of wire, rod, plate and strip. These products serve several industrial purposes. But ETP copper contains 0.04% oxygen which forms interdendritic Cu2O when copper is cast. If copper is heated to a temperature of 400°C in the atmosphere of hydrogen, then hydrogen reacts with dendritic Cu2O and produces steam. These H2O molecules being large in size do not diffuse readily and cluster around grain

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boundaries thus causing internal holes. This phenomenon is called hydrogen embrittlement. The methods of avoiding hydrogen embrittlement are adding phosphorous in the alloy copper and thus allowing P2O5 to form. The other method is to cast ETP copper under a controlled reducing atmosphere to produce copper which is oxygen free high conductivity (OFHC) copper.

6.6 COPPER ALLOYS Several alloys of copper are used in industry for varying purposes. Copper forms alloys with zinc (the brasses), tin (the bronzes), with tin and phosphorous (the phosphor bronzes), aluminium (the aluminium bronzes) and with nickel (the cupronickels).

6.6.1 The Brasses 70/30 brass also known as cartridge brass contains 70% Cu and 30% Zn. It is used for cartridge cases, condenser tubes, sheet fabrication and for general purposes. Its ultimate tensile strength varies between 350 and 600 N/mm2. It is soft and ductile and in annealed form can withstand severe cold working. 60/40 brass or Muntz metal contains 60% Cu and 40% Zn. Its UTS varies between 400 and 850 N/mm2. It is suitable for hot working operations as well as for casting. Many cast valves and marine fittings are made out of this brass. Addition of 2% Pb improves its machinability. Small additions of Fe, Al, Sn, Mn and Ni to 60/40 brass improves its strength considerably. Marine propellers and shafts, pump rods, autoclaves, switch gears and high strength fittings are made out of these brasses. Brazing alloys are essentially the brasses of 50/50 composition with small additions of Sn, Mn and Al. These brasses are hard and brittle.

6.6.2 The Bronzes The coinage bronze used for making coins in earlier days contains 95% Cu, 4% Sn and 1% Zn. The Zn acts as a deoxidiser. This alloy is soft and ductile. Admiralty gun metal contains 88% Cu, 10% Sn and 2% Zn. This bronze is normally cast to produce steam and water fittings and bearings. The addition of Pb improves the pressure tightness of the alloy. Phosphor bronzes are commonly used in manufacture of bearings, hard drawn wires and springs. In addition to tin they contain small percentage of phosphorous as alloying element. 0.2% P forms Cu3P which is a hard compound. It acts as deoxidizer and improves fluidity. Copper aluminium alloys possess high strength with good resistance to fatigue, corrosion and abrasion are golden in colour. Aluminium can dissolve in copper to the extent of 9% and greater content than this induces brittleness. Wrought alloys which are good for hot and cold working applications contain 5 to 7% Al. Casting alloys contain 10% Al. Small percentage of Fe, Ni and Mn are added to casting alloys to make them more easily heat

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treatable. Aluminium bronze is well known for its colour and often called Imitation gold. Al bronze compares well with the strength of steel. Bronzes in general are known for the following characteristics : (i) Costlier than brass. (ii) Better corrosion resistance. (iii) Stronger than brass. (iv) Bearing material. Tables 6.4 and 6.5 respectively describe Brasses and Bronzes with their applications. Table 6.4

Composition, Properties and Applications of Brasses

1. Gliding metal (95 Cu 5 Zn)

High ductility and corrosion resistance, coins, medals, gold platings

2. Red brass (85 Cu 15 Zn)

Good corrosion resistance, workability, heat exchanger tube, plumbing pipes

3. Cartidge brass (70 Cu 30 Zn)

Good strength and ductility, rivets springs, automotive radiator cores

4. Yellow brass (65 Cu 35 Zn)

Screws, rivets, reflectors, plumbing accessories, automotive radiator cores

5. Muntz metal (60 Cu 40 Zn)

Soundness and good machinability; condenser tubes, architectural work

6. Leaded red brass (85 Cu 5 Zn 5 Sn 5 Pb)

`fair strength, soundness and good machining in cast state; pressure valves, pipe fittings, pump testings

7. Leaded commercial bronze (89 Cu, 9.25 Zn, 1.75 Pb)

Screws, screw machine parts, electrical connectors, builder’s hardware

8. Admiralty brass (71 Cu 28 Zn 1 Sn)

Condensor, evaporator and heat exchanger tubes, marine applications

9. High leaded brass (65 Cu 33 Zn 2 Pb)

Flat products, gears, wheels

Table 6.5

Composition and Applications of a Few Bronzes

1. Phosphor bronze (94.8 Cu 5 Sn 0.2 P)

Bolts, electric contacts, spring, bearing

2. Phosphor bronze (89.8 Cu 10 Sn 0.2 P)

Such applications where high strength and resistance to salt water is desired, bushings and gears

3. Gun metal (88 Cu 10 Sn 2 Zn)

Sand cast, used under heavy pressure such as gears and bearings

4. Aluminium bronze (86 Cu 10.5 Al 3.5 Fe)

High UTS

5. Beryllium bronze (98 Cu 1.7 Be 0.3 Co)

Very high mechanical strength, springs, used against fatigue, wear and corrosion (UTS—1200 MPa)

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6.6.3 Copper-Nickel Alloys Complete solubility occurs between copper and nickel. All alloys have similar microstructure and can be cold or hot worked. Cupro-nickel also known as German silver is extremely malleable and ductile. It is good against corrosion due to salt water. Condenser tubes are main parts made out of this alloy. It is also used for coinage. 70/30, 80/20 and 75/25 alloys are very common. Monel metal is essentially 70% Ni and 30% Cu with small amounts of iron and other elements. Alloy is well known for its high strength and corrosion resistance. This alloy is largely used for chemical and food processing plants. It also finds great use as turbine blades, valves, corrosion resistance bolts, screws and nails. It is known for its characteristic silver lustre.

6.6.4 Copper-Beryllium Alloys Copper-beryllium alloys contain between 0.6 to 2% Be and 0.2 to 2.5% Co. These alloys can be precipitation hardened and cold worked to develop a tensile strength as high as 1460 MPa. This is the highest strength among the copper alloys. Cu-Be alloys are used as tools requiring high hardness and non-sparking characteristics for the chemical industry. These alloys are very useful for making springs, gears, valves and diaphragms for their excellent corrosion resistance, fatigue properties and strength. These alloys, however, are costlier.

6.7

MAGNESIUM AND ITS ALLOYS

Magnesium is a light metal with density of 1.74 g/cm3. Magnesium is much costlier than aluminium (density 2.74 g/cm3) with which it compares for lightness. Magnesium in its molten state burns readily, hence it is difficult to cast the alloys of magnesium. Magnesium alloys have low corrosion resistance and show poor fatigue and creep behaviour. Their h.c.p. structure does not permit to deform readily at room temperature since only three slip systems exist in h.c.p. at room temperature. The best advantage that magnesium alloys offer is that of low density and many aircraft parts are made in these alloys. Al when added to Mg in the range of 3 to 10% with small amounts of Zn and Mn increases strength, hardness and castability. Addition of Mn (1.2%) with small amount of C does not increase strength but improves corrosion resistance. Mg-Al-Zn alloys have good mechanical strength and corrosion resistance. These alloys are good casting material and generally used at high temperature like 250°C. Extrusions and forgings for general purpose are made in these alloys and used in aircraft, automotive, radio and instrument industries.

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Some magnesium alloys are described in Table 6.6 alongwith their application. Table 6.6

Y.S. N/mm2

Elong.%

Application Air borne cargo equipment Missile and aircraft sheets upto 427°C Highly stressed aerospace uses, extrusions, forgings

Wrought Alloys

Condition UTS N/mm2

Mg, 3 Al, 1 Zn, 0.2 Mn Annealed Mg, 2 Th, 0.8 Mn T8

228 228

— 198

11 6

Mg, 6Zn, 0.5 Zr

T5

310

235

5

Cast alloys

Composition

Magnesium Alloys

Mg, 6 Zn, 3 Al, 0.15 Mn

As cast

179

76

4

Mg, 3 Re, 3 Zn, 0.7 Zr

T6 T5

235 138

110 97

3 2

6.8

Sand casting requiring good room temperature strength Pressure tight sand and permanent mould castings used at 150-260°C

TITANIUM ALLOYS

Pure titanium is a strong ductile and light weight metal. It is very strong, highly resistant to corrosion of all types but has the drawback that it readily reacts with common gases at around 300°C. It reacts readily with C, O2, N2 and these elements cause embrittlement of Ti. It melts at 1725°C, has a UTS of 600-800 MPa and per cent elongation of 25%. Ti 6 Al 4 V alloy develops a UTS of 1300 MPa and has good creep, fatigue and oxidation resistance. Aero engine gas turbine blades and other parts of engine and components of air frame are made of this alloy. Ti 5 Al 2.5 Sn is also a strong alloy (900 MPa UTS) which is used in aircraft engine components at 470 to 500°C.

6.9 BEARING MATERIALS In general, it can be said that a good bearing material should possess following characteristics: (i) (ii) (iii) (iv) (v)

It should be strong enough to sustain bearing load. It should not heat rapidly. It should show a small coefficient of friction. It should wear less, having long service life. It should work in foundry.

Generally it is expected that the journal and bearing would be made of dissimilar materials although there are examples where same materials for journals and bearings have been used. When the two parts are made in the same material the friction and hence the wear is high. Cast iron has been used as bearing material with steel shafts in several situations. However, the various non-ferrous bearing alloys are now being used largely as bearing material because they satisfy the conditions outlined above more satisfactorily.

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Bronzes, babbitts and copper-lead alloys are the important bearing materials that are widely used in service. Certain copper zinc alloys, that is brasses, have been used as bearing materials, but only to limited extent. The babbitts are alloys of Sn, Cu, Pb and Sb. They are extensively used as bearing liners. Since brass in general is cheaper, it has replaced bronze in several light duty bearings. Bronzes and babbitts are described in Table 6.7 and 6.8 respectively. Table 6.7 Bronze and SAE

Bearing Bronzes

Composition, %

number

Mechanical Properties U.T.S. Y.S. MPa MPa

Applications

% Elong.

Leaded gun metal, 63

Cu, 86-89; Sn, 9-11; Pb, 1-2.5; P, 0.25 max. impurities, 0.5 max.

200

80

10

Bushing

Phosphor bronze, 64

Cu, 78.5-81.5; Sn, 9-11; Pb, 9-11; P, 0.05-0.25; Zn, 0.75 max, imp, 0.25 max.

167

80

8

Heavy loads

Bronze backing for lined bearings 66

Cu,83-86;Sn,4.5-6.0, Pb, 8-10; Zn. 2.0; imp. 0.25 max.

167

80

8

Bronze backed bearings

Semi plastic bronze, 67

Cu, 76.5-79.5; Sn, 5-7; Pb, 14.5-17.5; Zn, 4.0 max; Sb, 0.4; max Fe, 0.4 max; imp, 1.0 max

133

—

10

Soft and good antifriction . properties

Some babbitt material are described in Table 6.8. Table 6.8 Babbitts (White Bearing Metals) SAE No.

Composition, %

Applications

10

Sn, 90; Cu, 4-5; Sb, 4-5; Pb, Fe, 0.08; max.; As, 0.1, max; Bi, 0.08 max.

Thin linear on bronze backing

11

Sn, 86; Cu, 5-6.5; Sb, 6-7.5; Pb, 0.35 max.; Fe, 0.08max.; As,. 0.1 max.; Bi, 0.08 max.

Hard babbitt good for heavy pressures.

12

Sn, 59.5; Cu, 2.25-3.75; Sb, 9.5-11..5; Pb, 26.0 max; Fe, 0.08; Bi, 0.08 max.

Cheap babbitt, good for large bearings under moderate loads.

13

Sn, 4.5-5.5; Cu, 0.5 max; Sb, 9.25-10.75; Pb, 86.0 max.; As, 0.2, max

Cheap babbitt for large bearing under light load.


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6.9.1 Copper-Lead Alloys Copper-lead alloys, containing a larger percentage of lead have found a considerable use as bearing material lately. Straight copper-lead alloys of this type have only half the strength of regular bearing bronzes. They are particularly advantageous over babbitt at high temperature as they can retain their tensile strength at such temperature. Most babbitts have low melting point and lose practically all tensile strength at about 200°C. Typical copper-lead alloys contain about 75% copper and 25% lead and melt at 980°C. The room temperature tensile strength of copper-lead alloy is about 73 MPa and reduces to about 33 MPa at about 200°C.

6.10

ALLOYS FOR CUTTING TOOLS

Apart from tool steels described in chapter 5 many alloys which contain wholly non-ferrous elements have been developed. Such alloys behave better than tool steel in many respects and are widely used in industry. These alloys are mainly divided in two groups: stellites and cemented carbides. Stellite: Stellite is an alloy of Co (40-60%), Cr (25-35%), W (4—25%) and C (1—3%). It is a cast alloy containing C, Cr and W in Cobalt matrix. Its main characteristic is low coefficient of friction and it possesses high hardness, red hardness, high wear and corrosion resistance. Desired size and shape is achieved by casting and no heat treatment is required. They are mainly used for cutting tools and can cut steel at twice the cutting speed of H.S.S. Stellite can be used to cut all types of materials like steels, cast iron, stainless steels, non ferrous metals and plastics. They are not as tough as high speed steels because they are cast but perform better than H.S.S. with higher life. Stellites are used for cutting hard die faces, cam surfaces, wear plates and crushers. The hardness varies between 40 to 60 RC and they retain their hardness upto high temperature because they do not undergo phase changes. Cemented Carbide: These are small pieces with cutting edges and mechanically jointed or brazed to tool shank. Cemented Carbide tool tips are produced by process of powder metallurgy by sintering the powder carbides of W, Ta, Ti in Co powder. The contents are 40-95% WC, 3-30% Co, 0-30% TaC and TiC and hardness of tips is in excess of 65 RC compared to 60 RC of stellite. High hardness, high compressive strength at high temperatures are the main characteristics. Cermets are the variation of cemented carbides when the carbides of W and Ti are solidified in the softer matrix of Co and Ni to obtain high hardness, resistance to oxidation and thermal shock and resistance to high temperature abraison. Ceramic Tools: Aluminium oxide (Al2O3) is pressed and sintered in a powder metallurgy technique in various shapes of cutting edges which are fastened to mechanical shanks. The hardness of this ceramic tool is above 65 RC and has chemical inertness and high resistance to wear. Ceramic tools are made in small pieces of various geometrical shapes and can be disposed of when not usable.

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6.

QUESTIONS

1. How is aluminium produced commercially? Discuss various applications of aluminium. 2. What is the role of silicon as alloying element in cast aluminium alloys? 3. Describe aluminium alloys commonly used for engineering applications. Give their properties and applications. 4. Discuss age hardening of aluminium alloys taking example of Al 4% Cu alloy? 5. Describe production and applications of copper. What is hydrogen embrittiement of copper and how can it be avoided? 6. Distinguish between brasses and bronzes and discuss their applications. 7. Which copper alloy is used in food processing plant? What are the main characteristics of this alloy and its applications? 8. Discuss the applications of magnesium and titanium alloys? What are the problems of using titanium and magnesium in unalloyed conditions? 9. Which alloys are used as major bearing materials? Which properties make them suitable for this application? 10. Compare the properties of bearing bronzes, babbitt materials and copper-lead alloys for bearings. 11. Discuss tool materials which are non-ferrous alloys. What are the advantages offered by these alloys? 12. List cutting tool alloys with increasing hardness from 60 RC.