GB2114152A - Aluminium silicon alloy - Google Patents

Abstract

An aluminium silicon alloy including zinc, for use as a bearing lining on a steel backing contains 8 to 14% by weight silicone and from 3 to 12% by weight zinc. Up to 3% lead and up to 0.2% copper may also be included.

Description

SPECIFICATION Aluminium silicon alloy The present invention relates to aluminium alloys for use as a bearing material, for example for use in internal combustion engines.

For these applications it is known to use a high silicon aluminium engine bearing alloy Al Si 1 1% Cu 1% in which the silicon is finely and uniformly distributed in the form of small platelets. The antiseizure properties of the alloy derive from the high silicon content, the function of the copper being to provide a high degree of fatigue strength. This Al Sill Cul alloy has become well established commercially as the lining of steel backed crankshaft bearings in internal combustion engines, particularly for high speed diesel engine.

Good anti-seizure properties and high fatigue strength are conflicting requirements in an engine bearing alloy, since the latter is associated with high hardness and the former is associated with low hardness. The 1% copper addition increases the bearing lining hardness to a value of approximately 65 HV. This degree of hardness may be greater than is required for optimum anti-seizure properties. As a result, the ability of the bearing lining to accept small misalignments between itself and the crankshaft may be relatively poor. This 'conformability" is generally recognised in the industry as an important aspect of crankshaft bearings.

Because of the lack of conformability in the Al Sill Cul alloy the bearings are generally provided with a thin soft overlay of a lead/tin alloy. The electro deposition process by which this overlay is laid down on the bearing surface is expensive and represents a major portion of the total cost of the -bearing.

Attempts to reduce the copper content to below 1% have not been successful in that both fatigue strength and "compatibility" tend to fall unacceptably. "Compatibility" is a second important aspect of seizure resistance -- a compatible bearing alloy is one which resists local welding between the alloy and the counterface in regions of asperity contact during rubbing. Test results have shown that the copper content contributes to compatibility as well as to fatigue resistance.

It is an object of the present invention to provide an alloy having a sufficiently high degree of fatigue strength and compatibility without the loss in conformability caused by copper.

According to the present invention there is provided an alloy comprising from 8 to 14% silicon by weight and from 3 to 12% zinc by weight, the balance being aluminium.

It has been found that this alloy may meet the above requirements, and that a high silicon-zinc alloy may possess the necessary attributes in respect of all three aspects of bearing performance, compatibility conformability and fatigue strength. It is believed that these good properties are only obtained whenYhe zinc is operating in conjunction with the relatively high silicon content of 814% by weight.

The zinc content is preferably 4 to 8% by weight and more preferably about 5%, though useful properties may be obtained within the range of from 3 to 12% by weight. The silicon content is preferably from 9 to 12% by weight and more preferably from 10 to 1 1%, for example, about 10%.

Minor additions of up to 3% lead and up to 0.5% copper have proved to be beneficial in certain instances, and these may optionally be included in the alloy.

Such an alloy may be used as a bearing lining on a backing, for example a backing of steel, aluminium or aluminium alloy. The bearing may be formed directly or first formed as a blank and subsequently formed into the desired shape.

The invention may be carried into practice in various ways and one embodiment will be described by way of illustration in the following Example.

EXAMPLE I In this example, an alloy in accordance with the invention is formed as the lining on a steel backed crankshaft bearing.

An alloy composition comprising 10.4% by weight silicon, 5.1% by weight zinc, 0.2% by weight copper, the balance being aluminium was established in a melting furnace. The melt was degassed and grain refined by conventional procedures. The melt was transferred to a continuous casting machine, and billet 200 mm x 25 mm was cast.

The billet-was cut up into lengths and the cast surface removed to a depth of 1 mm by machining.

The machined billet lengths were then heated to 480"C and rolled in a succession of passes to 6 mm, at which thickness the alloy was wound.into coil form. The coiled alloy was cold rolled in a succession of passes to 1.5 mm thickness. Inter-stage annealing was carried out to minimise edge cracking. The 1.5 mm thick alloy was degreased and brushed on one surface.

The brushed alloy surface was bonded to the prepared surface of a steel coil, by simultaneous extension of the steel and the alloy in the gap of a rolling mill. Reduction in the total thickness of steel plus alloy was 45%. The bimetal coil was then annealed at 3100C to consolidate the bond. Crankshaft bearings were formed from this coil by conventional techniques.

Comparative tests were carried out using a known aluminium alloy including 20% silicon and 1% Cu (Al Sn Zo Cu 1), the alloy of Example I (Al Si 10 Zu5) and the alloy of Example I with the addition of 1%lead (Al Sn 10 Zu5 Pb1).

LOAD CAPACITY TEST Four pairs of bearings were tested under dynamic loading conditions running against a shaft to which eccentrically positioned weights were attached. A fatigue rating was calculated from the extent of damage after 20 hours operation.

Material Fatigue Rating Ibf/in2 MNm-2 AS 1 0Zn5 5050-5150 34.8-35.6 AlSil 0Zn5Pb1 5200-5250 35.9-36.2 AlSn20Cu 1 5050-5100 34.8-35.2 The tests demonstrated the aluminium silicon zinc alloys to be at least as strong in fatigue as an aluminium bearing alloy containing 20% tin and 1% copper.

SEIZURE RESISTANCE TEST Bushes were tested under static loading conditions against a rotating shaft. The shaft ran for 1 minute and was stationary for 4 minutes during the test cycle. Lubrication was limited to Rpproximately 2 drops of light turbine oil per minute during the part of the test cycle when the shaft rotated. The load was increased each time 10 stop/start cycies had been completed until seizure occurred or until maximum load had been reached. The number of seizures were as follows:

Load (MNm'?) No. of "noseizure" Material 2 3 6 9 12 15 runs AlSn20Cl - - - 2 3 2 AlSilOZnS - - - - 1 2 9 AlSi lOZn5 - - - - - - 5 pbl

Claims (7)

1. An alloy comprising from 8 to 14% silicon by weight and from 3 to 12% zinc by weight, the balance being aluminium.

2. An alloy as claimed in Claim 1 in which the zinc content is about 5% by weight.

3. An alloy as claimed in Claim 1 or Claim 2 in which the silicon content is about 10% by weight.

4. An alloy as claimed in any preceding claim further including up to 3% by weight of lead.

5. An alloy as claimed in any preceding claim further including up to 0.5% by weight of copper.

6. A bearing comprising a metal backing having a bearing layer comprising an alloy as claimed in any preceding claim.

7. A bearing constructed and arranged substantially as herein specifically described in Example I.