Characteristics and Properties of Copper and Copper Alloy P/M Materials

• Pure Copper P/M Parts
• Bronze P/M Parts
• Brass And Nickel Silver P/M Parts
• Copper-Nickel P/M Materials
• Copper-Lead And Copper-Lead-Tin P/M Materials
• Dispersion-Strengthened P/M Materials
• P/M Friction Materials
• Copper-Tungsten P/M Materials

Pure Copper P/M Parts

The physical properties of pure copper in massive form are given in Table 3. Outstanding are the electrical and thermal conductivities which are markedly higher than those of any other base metal and are exceeded only by silver. A copper powder with a purity exceeding 99.95% is available, and, of course, the individual particles have the same properties as massive copper. However, it is impractical to achieve a density of 8.94 g/cm ¹ by pressing and sintering alone, and, therefore, the properties of P/M parts are influenced by the density attained. Densification can be increased by additional operations such as double pressing-double sintering or forging, for example, and the properties of the P/M part approach those of the massive metal as a limit.

The final sintered density has a significant effect on the conductivity of a P/M product. Conductivity is directly affected by porosity; the greater the void content, the lower the conductivity. Since the conductivity of a pore is zero, the relationship between porosity and conductivity is given by the equation: ²

K = K ^s(1-f)
where K = thermal or electrical conductivity of the P/M part
K ^s = intrinsic thermal or electrical conductivity of the massive metal
f = fractional porosity

As pressed and sintered, the electrical conductivity of pure copper parts can range from 80% to 90% IACS and higher conductivities can be achieved by additional working of the parts. The effect of sintered density on the electrical conductivity and mechanical properties of sintered copper is indicated in Figure 5.

Sintered Density, g/cm ³

Figure 5. Effect of Density on the Properties of Sintered Copper

Source: P.W. Taubenblat, W.E. Smith and C.E. Evans, " Production of P/M Parts from Copper Powder," Precision Metal 30(4):41 (1972).

The high electrical conductivity and excellent ductility that can be achieved in copper P/M compacts lead to the selection of pure copper powder for P/M parts for electronic and electrical applications where conductivity is essential. Such parts include commutator rings, contacts, shading coils, nose cones and twist type electrical plugs. A specific application is a diode used as the base of the silicon rectifier for the alternator charging systems in automobiles.

Copper powders are used in copper-graphite compositions, which have low contact resistance, high current-carrying capacity and high thermal conductivity for brushes in motors and generators and as moving parts of rheostats, switches and current-carrying washers. These powders are also used to produce electrode tools for electrical discharge machining of complex dies. Copper powder is selected for its high electrical and thermal conductivity.

Pure copper is also used in nonelectrical P/M applications. An interesting example is a copper blade shank which is impregnated with grease to increase the service life of a pocket knife.

Bronze P/M Parts

Most tin bronze parts are produced from premixes although some are made from pre-alloyed powder. Since prealloyed powders have higher yield strengths and work hardening rates than premixed powders, the pressing loads required to achieve a given green density are higher than those required when pressing elemental powders. The differences in pressing characteristics of premixed and prealloyed powders are indicated in Figure 6.

Density, g/cm ³

Compacting Pressure, ksi ³

Figure 6. Pressing Characteristics of Premixed and Prealloyed 90Cu-10Sn Powders

Source: A. Price and J. Oakley, "Factors in the Production of 90/10 Tin Bronze Compacts of Higher Density (7.49g/cm ³)," Powder Met. 8:201 (1965).

Processing variables influence the properties. In an investigation in which 90Cu-10Sn and 88.6Cu-9.9Sn-1.5C (graphite) premixed powders were used, the optimum strength was achieved when the tin-rich phase was completely alloyed with the copper but little grain growth had occurred. Figure 7 shows the effect of density and graphite content on the strength of the bronze.

Figure 7. Effect of Density on the Strength of Copper-Tin and Copper-Tin-Graphite Compacts

Source: A.K.S. Rowley, E.C.C. Wasser and M.J. Nash, "The Effect of Some Variables on the Structure and Mechanical Properties of Sintered Bronze," Powder Met. Int. 4(2):71 (1971).

Properties of tin bronze P/M parts are influenced also by such factors as heating rate and sintering time and temperature. Faster heating rates tend to produce greater growth than slow heating rates. Sintering temperature influences both growth and strength. Sintering time influences dimensional control and strength; rapid growth occurs at the beginning of sintering and is followed by a period of predictable slow shrinkage. By completing sintering in the shrinkage range, it is possible to maintain dimensional control over the bronze P/M product.

Bearings

A unique attribute of powder metallurgy is the ability to produce porous products with interconnected porosity. This attribute made possible the development of the self-lubricating bronze bearing, an early P/M product, the first having been used in a Buick automobile in the 1920s. Depending on the sintered density, these bearings can absorb from 10% to 30% by volume of oil and can supply a continuous lubricating film even at low speeds. Porous bronze bearings also have the advantage that they are sufficiently ductile to permit assembly by ring staking.

Development of these bearings revolutionized the home appliance industry. By eliminating the requirement of periodic lubrication, the self-lubricating bearing assured many years of trouble-free operation of home appliances and led to a great expansion of the industry. New applications continue to be found and the self-lubricating bronze bearing industry consumes a major portion of the copper powder produced each year.

Self-lubricating porous bronze bearings depend on conduction and convection for heat dissipation during service. The frictional heat developed is proportional to PVµ where P is the pressure on the bearing, V is the surface velocity and µ is the coefficient of friction. Practical limits for safe operation of these bearings are often set at a PV factor of 50-60 ksi (345-414 MPa). These bearings are installed by pressing into rigid reamed or bored housings.

Porous bronze bearings are used widely in automotive service, household appliances, automatic machines and industrial equipment in two types of applications:

1. For low-duty shaft bearings where the static load-carrying capacity is adequate; where lubrication is impossible; and where the only requirement is low cost and avoidance of heating, seizure or squeaking throughout the life of the appliance or machine.
2. As an alternative to an oil bottle or ball bearing in medium to heavy duty applications. In these applications, facilities for relubrication must be supplied. ³

There are many other uses for these bearings. For example, in space vehicles, bronze P/M bearings have been used as sleeve bearings for attitude control mechanisms, solar panel hinges and stepping device bushings in tape recorders and commutators.

Filters

The ability to achieve close control of porosity and pore size is the basis for the use of metal powders as filters. Most producers prefer spherical powder of closely controlled particle size to permit the production of filters within the desired pore size range. Tin bronze is probably the most widely used filter material but nickel silver and copper-nickel-tin alloys are also used. The effective pore size can be varied widely but for P/M filters generally ranges from 5-125 microns. P/M bronze filters can be obtained with tensile strengths ranging from 3-20 ksi (21-138 MPa) and appreciable ductility, up to 20% elongation. In addition, P/M bronze has the same corrosion resistance as cast bronze of the same composition and, therefore, can be used in a wide range of environments.

P/M bronze filters are used to filter gases, oils, refrigerants and chemical solutions. They have been used in fluid systems of space vehicles to remove particles as small as one micron. Bronze diaphragms can be used to separate air from liquids or mixtures of liquids that are not emulsified. Only liquids capable of wetting the pore surface can pass through the porous metal part.

Bronze filter materials can be used as flame arrestors on electrical equipment operating in flammable atmospheres where the high thermal conductivity of the bronze prevents ignition. They can also be used on vent pipes on tanks containing flammable liquids. Here again, heat is conducted away so rapidly that the ignition temperature is not reached.

Aluminum bronze P/M parts containing from 5% to 11% aluminum are prepared from blends of the elemental powders. Alloys containing from 5% to 9% aluminum are single-phase materials and have excellent ductility. They can be strengthened by cold working. Alloys containing from 9% to 11% are two-phase materials which are less ductile than the alloys of lower aluminum content. However, they can be heat treated to increase their strengths.

The sintered yield strength increases from 11 ksi (26 MPa) at 7% aluminum to 40 ksi (276 MPa) at 11% aluminum; heat treatment of the latter alloy increases the yield strength to 60 ksi (414 MPa). Tensile strengths increase uniformly from 32 ksi (221 MPa) for the 7% alloy to 65 ksi (448 MPa) for the heat treated 11% alloy. Elongations of the 5% to 9% alloys are in the 25-35% range; the two phase alloys are considerably less ductile. ⁴ These properties make the P/M aluminum bronzes suitable for the production of parts where the strength requirements are too high to be met by the tin bronzes.

Limited corrosion data indicate that these P/M aluminum bronzes have properties similar to those of the cast and wrought counterparts. With this combination of strength and corrosion resistance, the alloys can be used for the production of P/M parts such as impellers, gears, connecting rods and similar components.

Brass and Nickel Silver P/M Parts

Commercial brass powders are available in the simple brasses ranging from 95Cu-5Zn to 60Cu-40Zn and leaded versions of these brasses, and in modified brasses containing such elements as phosphorus, manganese and silicon. Nickel silver powders containing 64Cu-18Ni-18Zn and 64Cu-18Ni-16.5Zn-1.5Pb are also available on the commercial market. These powders are produced by atomizing alloy melts.

Optimum properties are attained by preheating to drive off lubricants and sintering in a cracked ammonia atmosphere. The P/M parts produced by such procedures have mechanical properties comparable with those of the corresponding cast alloys. Typical properties of representative brasses and nickel silvers are given in Table 4. These P/M alloys have moderate strength with good ductility.

Next to bronze bearings, the brasses and nickel silvers are the most widely used materials for structural P/M parts. Examples of the many applications are hardware for latch bolts and cylinders for locks; shutter components for cameras; gears, cams and actuator bars in timing assemblies; small generator drive assemblies; and decorative trim and medallions.

Copper-Nickel P/M Materials

Copper-nickel P/M alloys containing 75Cu-25Ni and 90Cu-10Ni have been developed for coinage and corrosion resisting applications. The 75Cu-25Ni alloy pressed at 112 ksi (690 MPa) has a green density of 89% of theoretical. After sintering at 2000 F (1090 C) in dissociated ammonia, the elongation was 14%, and the apparent Rockwell hardness B20. A repressing at 112 ksi (690 MPa) increased the density to 95%. This alloy has the color of stainless steel and can be burnished to a high luster. The 90Cu-10Ni, under similar pressing and sintering conditions, has a final density of 99.4%. It has a bright bronze color and also can be burnished to a high luster. ⁵

In one method of producing coins, medals and medallions, a mixture of 75Cu-25Ni powders with zinc stearate lubricant is compressed, sintered coined and resintered to produce blanks suitable for striking. These blanks have the advantage over rolled blanks of being softer because they are produced from high purity material. Therefore, they can be coined at relatively low pressures and achieve greater relief depth with decreased die wear.

In another procedure, organic binder is mixed with copper or copper-nickel powders and rolled into "green" sheets. Individual copper and copper-nickel sheets are pressed together to form a laminate and blanks are punched from it. The blanks are heated in hydrogen to remove the organic binder and sinter the material. The density of the "green" blanks is low, being only about 45% of theoretical, but coining increases the density to 97%. After pressing, the blanks are annealed to improve ductility and coinability. ⁶

Copper-Lead and Copper-Lead-Tin P/M Materials

Metals such as copper and lead which have very limited solubilities in each other are difficult to alloy by conventional means but copper-lead powder mixtures have excellent cold pressing properties. They can be compacted at pressures as low as 11 ksi (76 MPa) to densities as high as 80% and, after sintering, can be repressed at pressures as low as 22 ksi (152 MPa) to produce essentially nonporous bearings.

Copper-lead sintered bearing materials with a lead content of 40-45% have tensile strengths of about 11 ksi (76 MPa), Vickers hardness values of about 32 and a fatigue strength of 3 ksi (21 MPa), which is almost double that of a white metal bearing. The surface properties are good enough to permit use in an automobile engine without an overlay.

Copper-lead alloys containing about 30% lead are stronger but have less satisfactory surface properties and are usually used with a thin lead-tin overlay.

If the copper-lead alloys do not have sufficient load-carrying capacity, the lead content is reduced, and tin is added to improve the strength. Typical is a 74Cu-22Pb-4Sn composite. This material has a tensile strength of 17 ksi (117 MPa) and a Vickers hardness of 50. Its fatigue strength of 5 ksi (34 MPa) is almost three times that of white metal liners. However, an overlay is required, if this alloy is to be used in an automobile engine.

Where still greater strength and hardness are required, an 80Cu-10Pb-10Sn alloy is used. This composition usually has a Vickers hardness of 60-80 but can be cold worked to a hardness as high as Vickers 130. It has a tendency to seize, however, and is normally used with grease rather than oil lubrication.

Steel-backed copper or copper-lead-tin P/M materials are being used in an increasing number of applications to replace solid bronze bearings. They are produced by spreading the powder in a predetermined thickness on a steel strip, sintering, rolling to theoretical density, resintering and annealing. The final product has a residual porosity of about 0.25%. Blanks of suitable size are cut from the bimetallic strip, formed and drilled with oil holes or machined to form suitable grooves. These materials are represented by four groups:

1. A Cu-25Pb-0.5Sn alloy is used with an overlay plate for high load applications.
2. A Cu-25Pb-3.5Sn alloy is used widely for such applications as cam bearings, turbine bearings, pump bushings and high speed thrust washers.
3. A Cu-10Pb-10Sn alloy is used for shock and oscillating loading applications such as piston pin bushings, rocker arm bushings, wear plates and thrust washers.
4. A Cu-50Pb-1.5Sn alloy is used for intermediate duty applications. ⁷

Dispersion-Strengthened P/M Materials

Copper P/M products can be strengthened by incorporating finely dispersed particles of oxides such as alumina, titania, beryllia, thoria or yttria in the matrix. Dispersions can be made by mechanical mixing, internal oxidation or coprecipitation. For example, the Bureau of Mines prepared copper alumina dispersions by coprecipitation of the nitrates of copper and aluminum with ammonia, conversion of the product to oxides, reduction by hydrogen, compaction and extrusion. ⁸ Others have consolidated dispersion-strengthened copper by hot forging or rolling.

Dispersion strengthening has a number of advantages. Since the oxides are inert, they reduce the electrical conductivity only to the extent that they reduce the cross-section of the material. Thus, electrical conductivities on the order of 80% to 95% IACS can be achieved. However, the major value of dispersion strengthening is to produce a material that resists softening and grain growth at temperatures approaching the melting point of copper. Dispersion-strengthened materials are superior in structural stability to the precipitation hardenable alloys, such as copper chromium or copper-beryllium, because the oxides have no tendency to dissolve at high temperatures, a characteristic of the precipitation hardenable alloys.

For example, a commercial copper-alumina alloy now available has an electrical conductivity of 85% IACS and a room-temperature tensile strength of 85 ksi (586 MPa). Approximately 90% of the strength is retained with no loss in conductivity after one hour exposure at 1700 F (925 C). The precipitation hardenable alloys would be completely soft after a similar treatment.

P/M Friction Materials

A basic attribute of powder metallurgy is the ability to combine materials in powder form that are otherwise immiscible. This unique advantage allows the production of friction materials in which copper and other metal powders are combined with solid lubricants, oxides and other compounds. Metallic friction materials can be operated at higher loads and temperatures than organic friction materials.

P/M friction materials are used as clutches and brakes. Dry applications may include both, but wet applications are normally confined to clutches. For brake and clutch facings, powders having high green strength are essential. Such powders characteristically also have high internal porosity, low apparent density and irregular shapes.

There is no definite relation between the physical properties of the brake material and its performance as a friction material. Further, there are so many intangibles that influence friction and wear that the selection of a P/M friction material is still empirical.

Generally, the major portion of the matrix is copper with about 5-15% low melting metal such as tin; 5-25% lubricant which may be lead, litharge, graphite, or galena; up to 20% friction material such as silica, alumina, magnetite, silicon carbide or aluminum silicide; and up to 10% wear-resistant materials such as cast iron grit or shot.

Typical compositions are:

• For dry clutches and brakes: 75Cu-6Pb-7Sn-5graphite-4molybdenum disulfide-3feldspar.
• For wet clutches and brakes: 74Cu-3.5Sn-2Sb-16graphite-4.5galena.

Copper-base friction materials perform best under wet conditions. They are also suitable for dry friction applications under relatively mild operating conditions with moderate loads, speeds and temperatures.

Dry clutches are used in highway trucks, machine tools, farm tractors and industrial presses. Dry brakes are used in automobiles and industrial presses. Wet clutches are used for automatic transmissions, machine tools and tractors. Wet brakes are used for off-highway vehicles and military service.

Copper-Tungsten P/M Material

Copper, nickel and tungsten powders are used in the production of so called heavy metal, which contains from 80% to 95% tungsten. The alloys are prepared by the liquid-phase sintering of mixed elemental powders during which part of the tungsten dissolves in the copper-nickel liquid. The product is a two-phase material consisting of rounded tungsten grains and a matrix of copper-nickel-tungsten containing up to 17% tungsten.

The density of the alloys ranges from 17-18 g/cm ³ and the electrical conductivity is quite low, on the order of 17% IACS. The mechanical properties are strongly influenced by the nickel-copper ratio and by the post-sintering heat treatment. Tensile strengths range from 45-125 ksi (310-862 MPa) and elongations from 2% to 8%.

These alloys are used in such applications as gyro rotors, instrument counterweights, airframe counterweights, jet aircraft wing edges and balancing weights for rotating elements in machinery, golf clubs and self winding wrist watches.

Footnotes

1. P.W. Taubenblat, W.E. Smith and C.E. Evans, "Production of P/M Parts from Copper Powder," Precision Metal 30(4):41 (1972).
2. Hirschhorn, Introduction to Powder Metallurgy. New York, American Powder Metallurgy Institute, 1969.
3. V. Morgan, "Applications of Porous Metal Bearings," Industrial Lubrication & Tribology 24(3):129-138 (1972).
4. P.E. Matthews, "Cubraloy, A New Development in Aluminum Bronze Powder Metallurgy," Proc. Fall 1971 Powder Metallurgy Conference, Metal Powder Industries Federation.
5. P.E. Matthews, "The Mechanical Properties of Brass and Developmental Nonferrous P/M Materials," Int. J. Powder Met. & Powder Technology 5(4):59 (1969).
6. T.R. Bergdtrom and B.G. Harrison, "Laminated Cupronickel/Copper Coin Blanks from Metal Powders," Int. J. Powder Met. & Powder Technology 3(4):47 (1967).
7. D.N. lisson, "A Metallurgical Review of Plain Bearings," paper presented at Coppermetal Bearings Symposium, Melbourne, Australia, Oct. 29, 1969.
8. D.H. Desy, "Dispersion Strengthened Copper," Bureau of Mines R.I. 7228 (1969).