Composite Bearings

Roll compacting also can be used for producing composite, or "sandwich," materials. An example of such a roll-compacted composite material is bimetallic strip used in producing main and connecting rod bearings (Ref 11). A change in automotive emission standards led to the use of these new materials to replace traditional copper-lead sleeve-bearing components. The rolled strip consists of a layer of Al-8.5Pb-4.0Si-1.5Sn-1.0Cu prealloyed powder "sandwiched" to a pure aluminum layer.

The production setup for roll compacting such a composite structure is shown in Fig. 13. Three powder hoppers are required, as well as powder flow blade that controls the flow of the powders into the roll gap. The coil of the composite strip is then sintered, and eventually the pure aluminum layer is roll bonded to a steel backing material—an operation that is common in the bearing industry.

Fig. 13 Rolling of strip for bearings. Using bonding powder, prealloyed bearing powder, and process powder

Powder Melting. An induction melting furnace is charged with elemental aluminum, copper, and silicon (Ref 11). At an intermediate elevated temperature, lead and tin are added to the melt. Furnace temperature is raised to the single-phase temperature (925 °C, or 1070 °F), and 38 °C (100 °F) of superheat is added to provide a safety margin. The induction current of the furnace creates a stirring action that ensures complete dissolution of the lead and tin to form a true singlephase solution.

From the induction furnace the molten alloy is poured through a launder into a gas-fired tundish furnace. An extension on the bottom of the tundish crucible holds a ceramic nozzle that meters the molten alloy at a controlled rate.

Atomization. The thin metal stream falls vertically into the atomizing chamber, where it is disintegrated into discrete particles and rapidly solidified. Powder particles fall into a 6 m (20 ft) high by 1 m (4 ft) diam collector. They then pass through a cyclone separator to remove the fines and a powder screen to separate oversized particles.

Small, finely dispersed lead-tin alloy particles are contained in a hypoeutectic aluminum-silicon-copper matrix. Because of solubility considerations, the lead-tin bearing constituent remains intact throughout subsequent processing. Controlled distribution of the lead-tin ensures the development of desired mechanical properties.

Consolidation. A consolidation process is selected to produce a strip of prealloyed aluminum-lead alloy that can be ultimately roll bonded to a low-carbon steel backing strip.

Previous experience with wrought aluminum-based bearing materials has proven them unsuitable for direct bonding to a steel liner. Ultimate fatigue resistance of the bearing material depends on both the intrinsic strength of the alloy and the integrity and strength of the bond between alloy layer and steel backing.

When aluminum alloys with soft phases such as lead or tin are bonded directly to steel, the bond interface necessarily contains microscopic discontinuities that occur where lead or tin precipitates are in direct contact with the steel. These imperfections create brittleness in the interface and act as sites for accelerated fatigue crack propagation and bond separation under service conditions. Consequently, wrought aluminum alloys containing appreciable amounts of soft bearing phase are almost always used with a pure aluminum or nickel bonding layer next to the steel. Powder rolling provides a convenient means of incorporating a pure aluminum bonding layer during the consolidation of the aluminum-lead prealloyed powder.

A third layer on the opposite side of the prealloyed layer from the bonding layer is necessary to ensure that the strip exits cleanly from the rolls of the powder rolling mill. This process layer, which is made from a blend of aluminum and lead-tin particles, is machined off in subsequent bearing finishing operations.

The three distinct compositions are fed into the compacting mill, as shown in Fig. 13. The work rolls compact these powders into a three-layer strip, which is close to 100% of theoretical density as it leaves the roll nip. A flowchart of the fabrication of powder rolled sleeve bearings is shown in Fig. 14. The production mill used to fabricate these bearings resembles a conventional four-high rolling mill (Fig. 1b). A cross-sectional view of the unsintered structure of the aluminum-lead alloy strip is shown in Fig. 15.

Fig. 14 Fabrication of powder-rolled sleeve bearing. (a) Flowchart. (b) Sleeve-bearing component

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Fig. 15 Cross section of umsintered powder rolled aluminum-lead strip. Original powder particle boundaries are visible. 25x

Sintering. The green strip is then sintered, which improves the morphology of one of the phases (silicon) and provides the necessary diffusion across particle boundaries to achieve excellent strength and ductility. The finely distributed lead phase is unaffected. Further processing consists of cladding the aluminum strip to a steel backing, followed by blanking, forming, and machining of the bearing halfshells.

Bearing materials must possess a unique combination of properties, including fatigue strength, seizure resistance, wear resistance, and corrosion resistance. The performance characteristics of several bearing alloys are given in Table 3.

Table 3 Typical properties of common bimetal bearing alloys

Alloy composition

Fatigue

Surface action at 150

C (300 °F)

(Underwood life)

Coefficient

Wear scar width

of friction

mm

in.

Al-8Pb-5Si-1.5Sn-2Cu (Clevite 66)

200 h at 49 MPa (7 ksi)

0.21

4.52

0.178

Cu-10Pb-10Sn (SAE 792)

200 h at 69 MPa (10 ksi)

0.22

5.15

0.203

Cu-23Pb-3Sn-2Zn (SAE 794)

200 h at 49 MPa (7 ksi)

0.17

5.08

0.200

Note: The Underwood test simulates the cyclic loading of a rod bearing resulting from the powder stroke in an internal combustion engine. Pressure is based on unit load. Wear scar and coefficient of fraction are obtained from the standard LFW-1 test machine,

Note: The Underwood test simulates the cyclic loading of a rod bearing resulting from the powder stroke in an internal combustion engine. Pressure is based on unit load. Wear scar and coefficient of fraction are obtained from the standard LFW-1 test machine,

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