GB2364946A

GB2364946A - Aluminium alloys and method for the production thereof

Info

Publication number: GB2364946A
Application number: GB0101375A
Authority: GB
Inventors: William Peter Brown; Kevin Nicholas Jupe; Ian Matthew Laing; Carl Perrin; Paul Aidan Shenton; Trevor Anthony Wright
Original assignee: Dana Inc
Current assignee: Dana Inc
Priority date: 2001-01-19
Filing date: 2001-01-19
Publication date: 2002-02-13
Also published as: GB0101375D0

Abstract

A method for the production of an aluminium alloy bearing material is described, the method comprises the steps of: providing an aluminium-based material in particulate form; providing a solid lubricant material in particulate form; mixing said aluminium-based material and said solid lubricant material together; feeding said mixture into an extrusion apparatus, the extrusion apparatus comprising fixed and moveable members defining an elongate passageway therebetween, an abutment 48 extending into the passageway and die means 56 associated with the abutment, the die means having at least one orifice leading from the passageway and means for feeding said mixture into said passageway; compacting said particulate mixture in said apparatus; and, extruding said compacted mixture through said die orifice of said apparatus to form a strip of the bearing alloy.

Description

<Desc/Clms Page number 1> ALUMINIUM ALLOYS AND METHOD FOR THE PRODUCTION THEREOF The present invention relates to aluminium alloys for the production of plain sliding bearings and particularly aluminium bearing alloys containing a so-called solid lubricant in the matrix thereof.

Bearing materials comprising a relatively soft matrix such as an aluminium alloy and having a dispersion of hard particles such as silicon for example therein are well known. Conversely, bearing materials comprising a relatively hard matrix, again such as an aluminium alloy and a soft phase such as tin, for example, dispersed therein are also well known. However, most of the known aluminium alloy materials are produced by the casting of a molten alloy wherein the constituents of the alloy are generally held in solution in a single phase liquid at the pouring temperature or by powder rolling. The hard particles or the soft phase are generally formed during cooling and solidification by precipitation from the liquid. Thus, the only influence that can be made upon the size, morphology and distribution of the second phase is that which is possible by the steps of mechanical working and heat treatment subsequent to casting.

In many current aluminium-based plain bearing materials, a low melting point soft-phase such as tin or lead is employed to enhance the tribological properties of the alloy, However, many high performance modern engines operate at such a high temperature that the strength of the bearing material is compromised by the presence of such a soft phase. Again, such soft phases are introduced

into the alloy mainly by being taken into solution during the melting of the aluminium matrix material and are precipitated out during the casting and solidification process.

Generally, it is not possible to introduce certain other materials such as so-called solid lubricant materials such as graphite or molybdenum disulphide, for example, which are capable of operating at higher temperatures, into the material by a casting route due to either segregation due to density differences, chemical reaction problems, decomposition problems and/or poor wetability. US-A-4361629 describes a process for the production of an aluminium alloy bearing material containing so-called solid lubricant material and also lead or tin or an alloy of these soft metals which, as noted above, are detrimental where bearing operating temperatures are relatively high. The process involves the production of a billet by compaction of particles of the required materials in a suitable form, heat treatment to consolidate the billet, extrusion of the billet into a shaped form such as a flat strip, for example, followed by rolling of the extruded material and subsequent roll- pressure bonding of the treated extruded material to a steel backing. Disadvantages of this material and method are that it contains a soft, low melting point phase; the need to heat treat the compacted billet to prevent cracking during extrusion and the fact that an extruded strip of only finite length is obtainable by the process due to the fact that billets of only a maximum size can be produced by mechanical compaction processes. Furthermore, the integrity of the final extruded material is dependent upon the quality of the initial heat treated billet which is of necessity very porous and which

porosity creates either residual porosity in the final processed strip or weak bonds between a substantial proportion of matrix grains.

US-A-4121928 describes a process for the manufacture of an aluminium alloy bearing material wherein two layers of aluminium material powder mixtures having solid lubricant materials mixed therein are deposited one on top of the other on a steel strip substrate. The powder layers are then compacted by passing the substrate and powders through a rolling mill to densify the powders onto the substrate, the rolled article then being heat treated to both improve the properties of the aluminium layers and to bond the aluminium material to the steel substrate. Processes such as this inevitably result in a relatively high degree of residual porosity and have an inherently low ability to impart high levels of mechanical work into the powder layer. This has consequences with regard to grain size and mechanical properties of the aluminium material lining layer.

According to the present invention there is provided a method for the production of an aluminium alloy bearing material, the method comprising the steps of: providing an aluminium-based material in particulate form; providing a solid lubricant material in particulate form; mixing said aluminium-based material and said solid lubricant material together; feeding said mixture into an extrusion apparatus, the extrusion apparatus comprising fixed and moveable members defining an elongate passageway therebetween, an abutment extending into the passageway and die means associated with the abutment, the die means having at least one orifice leading from the passageway; means for feeding said mixture into said passageway; compacting said particulate mixture in said

apparatus; and, extruding said compacted mixture through said die orifice of said apparatus to form a strip of the bearing alloy.

The extrusion apparatus may be of the continuous rotary type described in GB-A-1 370 894 or GB-A-1 434 201 for example and known to those people skilled in the extrusion art as a "CONFORM" (trade name) machine.

The die means may also be provided with heating means to raise the temperature of the material being extruded to a desired level. The apparatus works on the principle of a movable die member which forms for example three sides of the elongate passageway and is in the form of a continuous groove in the periphery of a circular wheel driven about an axis, and a fixed stationary shoe member which forms a fourth side of the arcuate passageway so formed. Since the moving driven wheel part possesses a greater proportion of the surface area of the passageway, the particulate material is drawn into the passageway by friction and dragged along the passageway whilst being heated (by friction and by optional additional heating) and simultaneously consolidated before reaching the abutment which is positioned along the passageway remote from the material feed entry point. On reaching the abutment, which extends into the passageway, the consolidated material is forced to change direction by plastically deforming and is extruded through the orifice of the die means.

The orifice of the die means can be any desired shape such as round or of rectangular cross section or the die may form a tube if desired so that round bush bearings may be formed directly from extruded tubular material. The die may alternatively be of a design which permits

the "canning" of an extrusion of the desired composition by another aluminium alloy or by substantially pure aluminium or cladding onto one side of a steel strip, for example.

Because no actual melting of the material occurs and the consolidation steps are all in the solid phase, no significant segregation occurs due to density differences when solid lubricant materials are employed in the mixture and because the temperatures are relatively low, no chemical reactions occur such as the decomposition of molybdenum disulphide or a reaction between molten aluminium and graphite, for example, as might happen if molten aluminium were present in a casting process. However, although the temperatures employed are below the melting points of the materials involved, the temperatures are sufficiently high due to the cumulative die heating and friction heating effects to cause simultaneous sintering, annealing and recrystallisation of the consolidated material during extrusion thereof. Use of the particular type of compaction and extrusion apparatus described provides a process where compaction, heating, sintering and extrusion are effected simultaneously in a continuous, seamless process. Furthermore, provided that that particulate material continues to be fed into the mouth of the passageway the resulting extruded strip may be any length desired with no arbitrary limit imposed by a finite sized billet of the prior art process. Thus, the extruded material issuing from the extrusion die may be directly cold or warm pressure welded, such as by roll-pressure bonding for example, to a strong backing material such as steel for example immediately following extrusion for the production of bimetal bearings (i.e. bearings comprising a lining of a bearing alloy bonded to a strong backing

material such as steel for example). Alternatively, the extrudate issuing from the die may be directly rolled to a desired, thinner thickness prior to bonding to a strong backing material or merely coiled for storing and future processing as appropriate. A yet further alternative may comprise the heat treatment of the strip, either as a continuous in line process or when in the form of a coil, prior to bonding to a strong backing so as to anneal the microstructure, for example.

Consequently, in addition to the alloy produced by the method of the present invention being able to be more closely suited in compositional and property terms to particular engine applications, much of the tedious, wasteful and expensive processing steps associated with prior art casting, working and heat treatment processes or billet compaction, heat treatment and extrusion processes may be avoided.

The aluminium-based material matrix may be pure aluminium (more usually so-called "commercially pure" aluminium) or a basic A1-Si-Cu alloy comprising in weighto: 1 to 20 Si; 0.1 to 4 Cu; balance A1 with optional additions selected from at least one of Zn, Mn, Ti, Cr, V and Mg up to a total of 10 weighto. In one embodiment of a matrix of the material made according to the method of the present invention, the composition may be Al-4Si-1Cu plus solid lubricant.

The form of the initial particulate material of the aluminium-based material may be extremely diverse and may range from a powder having a relatively small particle size from less than 10@un up to about l0mm diameter pellets for example. However, the preferred size range

may lie between 40 and 450N.m. The aluminium-based material may be in the form of a pre-alloyed powder or in the form of a mixture of elemental and/or pre-alloyed constituents, e.g. an A1-Si powder and an Al-Cu powder to form an overall A1-Si-Cu matrix having a solid lubricant material dispersed therein.

In one embodiment of the present invention, the mixture of aluminium material and solid lubricant may be formed by co-spraying and atomising a molten aluminium material into which the solid lubricant is incorporated. Such processing produces an intimate mixture of the aluminium alloy and the solid lubricant as processing trials with an A1-4Si-1Cu alloy co-sprayed with graphite have shown. However, plain mixtures of this alloy and graphite have also been tested and shown to have good properties.

The method according to the present invention will process swarf and shaving type particles as well as more regular shaped particles. Such wide ranges of particulate size and shape which may be accommodated by the method of the present invention gives great control over the material which is formed. For example, control over the size of the particulate input material gives control, to a large extent, over the manner in which the solid lubricant material is dispersed in the matrix.

The ability to utilise machining swarf, for example, from the boring of bearings of the alloy being produced, for example, leads to great economy of manufacture and the ability to recycle material which would otherwise be scrap. This is a further advantage of the present invention over prior art methods which could not utilise

such material and produce high quality material by conventional die pressing techniques.

A further advantage of the method of the present invention compared with, for example, the prior art technique of powder-rolling is that material with much higher density and lower porosity levels is produced giving higher strength and greater ductility.

Solid lubricant materials in the context of the present invention may constitute the common materials such as graphite, molybdenum disulphide, tungsten disulphide, for example, but may also encompass other materials such as fluorides, carbides, nitrides and oxides, examples of which may include calcium fluoride, silicon carbide, boron nitride and aluminium oxide.

Where the solid lubricant is graphite, the content may range from 0.1 to 30 weight % but a preferred range is 2 to 5 weight. Below 0.1% the effect of the lubricant is negligible whilst above 30% the strength of the material diminishes rapidly. The preferred range provides a good balance of improved bearing properties such as scuff and seizure resistance with good mechanical properties.

Where the solid lubricant is other than graphite, the content may lie in the range from 0.1 to 10 weight % but preferably from 1 to 3 weight. In principle the reasons for this are the same as for the limits on the graphite content.

Materials made according to the method of the present invention have exhibited improved seizure resistance compared with prior art materials made by die compaction and powder rolling techniques.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a schematic part-sectioned side view of a compaction and extrusion apparatus used in the method according to the present invention; Figure 2 shows a histogram of mechanical property test results at 25 C; Figure 3 shows a similar histogram to Fig. 2 but at 190 C; Figure 4 shows a partially sectioned view in elevation of a known Sapphire apparatus for the scuff and seizure testing of bearings; Figure 5 shows a trace of bearing temperature and load vs time for scuff and seizure tests; and Figure 6 which shows a histogram of scuff and seizure test results of various materials.

Figure 1 shows a schematic representation of the main parts of a so-called "CONFORM" (trade name) extrusion machine 30 and which is essentially similar to the apparatus described in GB-A-1 370 894 for example. The apparatus comprises a rotatable wheel 32 rotatable about a central axis 34. The wheel 32 has an endless groove 36 in the periphery thereof, the groove 36 forming the major surface area of a closed passageway 40 which is formed

between the groove 36 and the curved surface 42 of a shoe member 44 which fits closely against the edge 46 of the wheel 32. The shoe member 44 also includes an abutment block 48 which has a projection 50 which projects into and effectively blocks the passageway 40. The shoe member 44 and abutment block 48 also house and retain a die 52 with heater means (not shown) which has a chamber 54 and die orifice 56 of predetermined shape, e.g. a flat rectangular slot, for example, corresponding to the shape of the extruded product desired to be produced. Particulate material 60 to be compacted and extruded is fed into a hopper portion 62 in the shoe member 44. The wheel 32 is rotated by drive means (not shown) causing particulate material to be dragged into the passageway 40 due to the greater surface area of the groove 34 compared with the surface 42 which forms a stationary closure wall of the passageway 40. As the particulate material 60 is dragged into the passageway 40, it is compacted and heated by friction effects until it meets the projection 50 which blocks the passageway 40 against any further progress by the compacted material 60 which is forced to change direction by plastically deforming into the chamber 54 of the die 52 before being extruded through the die orifice 56 to form an extruded strip 66 for example.

Tests were performed utilising a common matrix material of Al-4Si-1Cu having an initial particle size in the range from 45 to 450Eun. Test materials were made by co- spraying and atomising a molten mixture of A1-4Si-1Cu with 10 weighto graphite with nitrogen gas. The powder so produced was diluted with further A1-4Si-1Cu powder to produce samples powder mixtures having graphite contents of 1, 2 and 3 weight. The sample materials were mixed,

compacted and extruded in the apparatus described with respect to Figure 1. Prior to mixing, the aluminium alloy powder was vacuum dried using a furnace schedule of 10 C/min to 400 C with a 1 hour dwell at temperature followed by .cooling at 10 C/min to room temperature. Further samples according to the present invention were made by mixing Al-4Si-1Cu with molybdenum disulphide. The molybdenum disulphide was also heated for 2 hours at 120 C for 2 hours to remove any moisture. The powders were then -blended in a double cone mixer, compacted and extruded in the apparatus of Fig.l to produce an extruded strip of 44mm x 3mm which was coiled on exit from the die. The die block was heated to 250 C but the inherent friction of the process increased the temperature during compaction and extrusion to 450 to 500 C. The extruded strip was then rolled to 0.9mm, a 70% reduction, and annealed at 300 C for 8 hours. Following annealing, the strip was cold pressure welded by roll-bonding to a steel backing strip and heat treated again to consolidate the bond therebetween.

From the steel backed material so produced, bearings for testing on a known Sapphire (trade name) scuff and seizure test apparatus were produced. The bearings had a nominal bore of about 52mm, a wall thickness of about 2mm including a lining thickness of about 0.3mm and an axial length of about 29mm.

A comparative material based on Al-4Si-1Cu having 3 weight% graphite therein but made by a prior art powder rolling technique was also produced. The production process for this material comprised the steps of incoporating 3 weight% graphite into an Al-4Si-1Cu powder followed by powder rolling into a strip material which was sintered under conditions of 14 C/hour to 530 C with a 12 hour dwell at temperature prior to roll-bonding.

Bearings were also made from AS124A (trade name and which is currently used in many engine applications) which is a cast and roll bonded alloy having a composition Al-12Sn- 4Si-1Cu for the purpose of comparison with materials made according to the present invention. The results of scuff and seizure tests are shown below in Table 1.

Table 1 Base Alloy A1-4Si-1Cu Means Ratings (6 bearings tested per composition) Average Number of Scuff Resistance Seizure Resistance co-sprayed Scuff Events (Mpa) (Mpa) feedstock lwt% Graphite 0 47 49 (1) 2wt% Graphite 4 47 69 (2) 3wt% Graphite 18 57 142 (3) 2wt% MOS2 - 108 108 Base Alloy A1-4Si-1Cu Means Ratings (6 bearings tested per composition) Average Number of Scuff Resistance Seizure Resistance Scuff Events (Mpa) (Mpa) Powder rolled 2 37 53 3wto Graphite (4) AS124A - 55 71

A Sapphire test machine, which is known in the bearing art and is shown in Figure 4, can be used to measure scuff and seizure performance of bearings. In the test, the bearing temperature is measured as the dynamic load on the bearing is steadily increased until seizure occurs when running against a nodular cast iron shaft. Figure 8 shows a graph of increasing load with time until scuffing or seizure occurs. The bearings were tested to determine the fatigue strength thereof, the load at which scuffing occurred and the ultimate load at which seizure occurred. The Sapphire apparatus 80 comprises a test shaft 82 having a central eccentric portion 84 supported by the test bearings 86, 88, the outer ends of the shaft are supported in slave bearings 90, 92. The shaft is rotated by a drive motor 96 and load is applied to the test bearings 86, 88 by a connecting rod 100 to which is applied a force by a piston 102 which is actuated by hydraulic means 106, 108. Strain gauges 110 measure the applied load. The fatigue load capacity is that load which causes fatigue at 200 hours running. In operation, the apparatus 80 applies a load to the test bearings 86, 88 by means of the eccentric portion 84 and the hydraulically loaded piston 102 thus imposing a sinusoidal dynamic load on the bearings. Via a computer control system (not shown), a programmed progressive load increase becomes the basis of the measurement of surface properties. In this mode of increasing load, the minimum oil film thickness steadily reduces and the test measures, via a temperature increase, the load at which the material is wiped or scuffed as it comes into contact with the geometrical inaccuracies or asperities on the shaft surface and/or the load at which the material welds itself to the shaft. Scuff resistance is a measure of material conformability whilst seizure resistance is a measure of compatibility.

A typical bearing temperature trace is shown in Figure 5 as a function of the applied load. This temperature trace shows the occurrence of temperature excursions that then self-correct as the load increases, ie as the oil film thickness decreases. These temperature excursions are indicative that the bearing system accommodates geometrically, ie possesses conformability. The load at which the first temperature excursion occurs has been used as a measure of conformability of the materials, i.e. scuff resistance. Under the most extreme operating conditions, when the material can no longer conform or condition the shaft journal sufficiently to avoid thermal runaway, seizure takes place. The components of the bearing alloy are chosen so as to avoid thermal welding of the bearing to the shaft, so building security into the alloy system whenever asperity contact between the bearing and the shaft journal occurs thus producing frictional heating. The load at which this takes place has been used as a measure of the compatibility of the material, i.e. seizure resistance.

From Figure 6 it may be seen that the aluminium alloy with 2 weight% graphite made by the method of the present invention possesses scuff and seizure resistance comparable to AS 124A material which is a production material made by conventional ingot casting, rolling/ heat treatment schedules and roll-pressure bonding currently in wide engine use. However, Figure 6 and Table 1 shows that by raising the graphite content to 3 weight, scuff resistance is improved by about 30$ but seizure resistance is improved by about 1000 over the conventional production AS124A material. Furthermore, the same material composition having 3 weight% graphite but made by prior art powder rolling techniques, shows both

scuff and seizure resistance significantly inferior to the conventional AS124A material and vastly inferior to the inventive material. The reason for the lower performance of the material produced by powder rolling is due to its greater residual porosity and less refined microstructure.

The material according to the present invention and containing 3 weighto graphite has the further advantage over the production AS124A material in that it does not contain a low melting point soft phase, in this case tin. Therefore, the temperature capability of this material according to the present invention is much greater than the conventional material.

Claims

CLAIMS 1. A method for the production of an aluminium alloy bearing material, the method comprising the steps of: providing an aluminium-based material in particulate form; providing a solid lubricant material in particulate form; mixing said aluminium -based material and said solid lubricant material together; feeding said mixture into an extrusion apparatus, the extrusion apparatus comprising fixed and moveable members defining an elongate passageway therebetween, an abutment extending into the passageway and die means associated with the abutment, the die means having at least one orifice leading from the passageway and means for feeding said mixture into said passageway; compacting said particulate mixture in said apparatus; and, extruding said compacted mixture through said die orifice of said apparatus to form a strip of the bearing alloy.
2. A method according to claim 1 wherein the extrusion apparatus is of the continuous rotary type.
3. A method according to either claim 1 or claim 2 wherein the die means are also provided with heating means to raise the temperature of the material being extruded to a desired level.
4. A method according to any one preceding claim wherein the orifice of the die means is selected from round or rectangular cross section or the die may form a tube.
5. A method according to any one preceding claim wherein the extrudate is cold pressure bonded to a strong backing material.
6. A method according to any one preceding claim wherein the aluminium-based material matrix material

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is selected from the group comprising: pure aluminium; A1-Si-Cu alloy comprising in weight: 1 to 20 Si; 0.1 to 4 Cu; balance A1 with optional additions selected from at least one of Zn, Mn, Ti, Cr, V and Mg up to a total of 10 weight.
7. A method according to claim 6 wherein the composition of the matrix is A1-4Si-1Cu.
8. A method according to any one preceding claim wherein the desired matrix composition is achieved by a mixture of powders.
9. A method according to any one preceding claim wherein the particle size of the aluminium matrix material powder ranges from less than 10m up to about 10mm diameter pellets.
10. A method according to claim 9 wherein the particle size range lies between 40 and 450Wm.
11. A method according to any one preceding claim wherein the solid lubricant material is selected from the group comprising: graphite, molybdenum disulphide, tungsten disulphide, fluorides and oxides.
12. A method according to claim 11 where the solid lubricant is graphite, the content ranges from 0.1 to 30 weight%
13. A method according to claim 12 wherein the graphite content is 2 to 5 weight.
14. A method according to claimll wherein the content of solid lubricants other than graphite lie in the range from 0.1 to 10 weight.
15. A method according to claim 14 wherein the content lies in the range from 1 to 3 weight.
16. A method according to any one preceding claim wherein at least a part of the aluminium-based

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material and the solid lubricant material are mixed by co-spraying and atomisation prior to extrusion.
17. A method substantially as hereinbefore described with reference to the accompanying description and drawings.