WO2001088219A1

WO2001088219A1 - Surface coated motor component and process for producing it

Info

Publication number: WO2001088219A1
Application number: PCT/SE2000/000982
Authority: WO
Inventors: Kent Strand; Sven-Åke KARLSSON; Stefan Ekblad
Original assignee: Saab Ab
Priority date: 2000-05-17
Filing date: 2000-05-17
Publication date: 2001-11-22
Also published as: EP1297195A1

Abstract

An engine component made of a metal matrix material (MMC), wherein at least one of the tolerance-critical surfaces of the engine component is coated with a metallic layer that is easier to work by machining than the metal matrix material itself. For example, the interior hollow surfaces of a connecting rod are thus coated with bronze or a corresponding tribologically functional metal. The invention also provides a method for coating the component with the metallic layer, and in particular a method wherein the component is pre-machined from a raw billet of MMC by means of high-speed machining, HSM.

Description

SURPACE , COATED MOTOR COMPONENT AND PROCESS FO > PRODUCING IT.

TECHNICALFIELD

This invention concerns engine components made of MMC materials (Metal-matrix composite), where the component surfaces that have rotational contact or sliding contact with other engine components are coated with a friction-reducing tribologically functional material by means of a surface treatment process.

STATE OF THE ART

Legislation and competition are pushing vehicle makers to constantly improve performance in terms of fuel consumption, emissions, vibrations, noise, comfort, etc. Decreased weight is essential in all of these contexts, particularly with respect to unsprung mass and in fast-moving components. Increased power combined with lower weight and acceptable useful life are naturally sought after in the world of motor racing. Along with pistons, connecting rods comprise the components wherein reduced mass has the most obvious beneficial effect in engines that run at relatively high speeds (above 5000 rpm). This applies in particular to the weight of the small end of the connecting rod. Light materials such as aluminum, titanium or carbon fiber composite are used instead of steel in components such as connecting rods in advanced racing contexts. One decisive property of the material used in connecting rods is its specific stiffness, E/p. This value is the same for aluminum, titanium and steel. The advantages of making connecting rods from aluminum or titanium are thus limited. A composite material must be used to achieve high specific stiffness values, which is why carbon-fiber-reinforced polymers are used, as noted above. This type of material is of limited usefulness in the fabrication of other types of components, due to its pronounced anisotropy, limited heat tolerance and high production costs.

The use of MMC material is proposed in the invention described below. Structural materials of the type known collectively as MMC materials, ("metall-matris-kompositer" in Swedish) became known during the 1990s. MMC materials are composites that consist of a binding material in the form of a metal such as aluminum, magnesium, titanium or alloys thereof that is reinforced through the admixture of fibers or particles in the form of ceramic substances such as silicon carbide, boron carbide or aluminum oxide.

MMC materials exhibit extremely interesting properties, which can be further adapted depending on the area of application, thus offering advantages in that the component can be made lighter, stronger, stiffer and possessed of better fatigue properties than can be achieved using conventional materials within the specific area of application in question.

One significant disadvantage associated with use of MMC materials is that they are very difficult to machine. To create a component made of MMC material, methods are usually used in which the component is cast in a shape that is closely akin to the final shape of the component. Another method involves using a forged billet or a piece of extruded bar, so that spark machining of the surface and conventional cutting techniques can be used to produce the final component shape. Attempts have been made to produce e.g. connecting rods for motorcycles by means of such conventional fabricating and machining methods. In this way, the goal of achieving the desired component and its desired properties, such as lower weight, has been achieved, and the use of such connecting rods in an engine results in an engine that turns more readily and vibrates less. However, the problem is that the cost of producing engine components by conventional means has been extremely high, thereby limiting use to areas where cost is of lesser importance.

A number of patents document various methods for the final forming of components based on MMC materials. US 5 765 667 may be cited as an example of such a patent, wherein a method is described for fabricating a component, in this case a brake disk, by casting to a format that is as close as possible to the shape of the finished component in order, as is clearly described, to avoid the need for machine cutting to the greatest possible extent. It is obvious to one skilled in the art to avoid the need for cutting operations, since MMC material contains, when consisting of e.g. an aluminum base and reinforcing particles of silicon carbide, the very substances that are commonly used to grind cutting tools. The silicon carbide particles imbedded in the MMC material have a destructive effect on cutting tools when conventional cutting methods are used, since the edges of the cutting tools are rapidly worn down by the abrasive particles in the composite material.

DESCRIPTION OF THE INVENTION

Patent application PCT/SE/02007, which was not yet published at the time of the submission of the present application, presents a method that shows that MMC materials can be machined by HSM (High-Speed Machining), and this method was used in the fabrication of products related to the present invention. Everything described in patent application PCT/SE/02007 is hereby incorporated into the present patent application.

One aspect of the present invention concerns engine components made of an MMC material (Metal-matrix composite) provided with a surface treatment in the form of an applied tribologically functional layer on surfaces that are subject to abrasive friction through contact with rotating or sliding elements. The applied layer preferably consists of a thin, easy-to-work metallic layer of, e.g. a bronze.

According to another aspect of the invention, a method is provided for surface treating an engine component made of MMC material, wherein surfaces of the component that are subject to abrasive friction are coated with a metallic layer. The applied layer is preferably applied by spraying.

Yet another aspect of the invention comprises a method for fabricating an engine component with surface treatment of designated surfaces of the component wherein, in a first step, the component is machined by cutting via high-speed machining of a raw billet of MMC material. When tolerance-sensitive MMC component surfaces are coated with a material layer that is easy to work in accordance with the invention, it becomes further advantageous from a production engineering standpoint to form the components from the raw billets by means of high-speed machining (HSM), at which point the component dimensions will not be so critical from a tolerance standpoint.

Because the reinforcement phase in the MMC material consists of ceramic particles, which are commonly used as abrasives, these particles can have a wearing effect on materials in affected surfaces on adjacent engine elements. The layer applied on the MMC component in accordance with the invention prevents such wear against other engine elements. The foregoing assertions concerning the use of MMC components apply to all types of means of conveyance, i.e. not only within the automobile industry, but to an equally great extent in e.g. airplanes and helicopters.

DESCRIPTION OF EMBODIMENTS

A number of exemplary embodiments of the invention are described below.

A first example of an application of the invention is based on the fact the MMC materials make possible an optimized connecting rod design achieved by choosing the materials and geometrical configuration used based on the set requirements for the engine application in question, i.e. whether the engine is intended for, e.g. a motorcycle, car, race car, boat, truck, work machine, etc. Connecting rods made of aluminum-based, particle-reinforced metal-matrix composite offer the following general advantages in comparison with connecting rods made of other materials:

- Relative to steel:

Lower mass, better heat conduction - Relative to titanium:

Lower mass, better head conduction Higher specific stiffness - Relative to aluminum:

Higher stiffness Higher yield point Higher fatigue strength Better adaptation to thermal linear expansion of the steel in a connecting rod

Better high-temperature strength

- Relative to fiber composite:

Lower price Isotropic properties Better adaptation to thermal linear expansion of the steel in a connecting rod

Higher resistance to hot engine oil Better damage tolerance

Because of its particular structure, MMC material causes problems both during fabrication and in operation:

• The aluminum alloy in the matrix is, as in all Al-based materials, very difficult to grind, which creates problems in achieving the narrow dimensional tolerances that apply to the openings at the large and small ends of the rod. Because of their hardness, the ceramic particles that comprise the reinforcement phase make the MMC material highly resistant to wear, but the opposing surface in the wear couple, e.g. the steel in a piston, may be subject to rapid wear. This can be prevented by providing the small end with a bronze bushing, although that does entail an increase in weight.

Both problems are solved according to an aspect of the invention via modern surface-treatment technology. This is illustrated here through its application to a connecting rod made of aluminum-based MMC, wherein the following are achieved:

• Compliance with dimensional tolerances is facilitated in that a grindable 0.1 mm - 2 mm-thick layer of bronze, preferably on the order of 0.4 mm, is applied to the coarse-machined opening surfaces, which are then finished by grinding.

• The coating can, if necessary, also cover the end plane of the large end to prevent the occurrence of galling damage from unintentional contact with adjacent components such as balance shafts.

• The bronze coating at the small end replaces the bronze bushing, thus providing a weight reduction of roughly 7%.

Other suitable coating metals include bearing metal, molybdenum, nickel, nickel-graphite and hard chrome.

The method used to apply the layer is chosen from among chemical and thermal coating methods, based on the functional requirements in each individual case, and on production engineering and financial considerations. The thickness of the applied metal coating according to the invention is preferably between 0.1 and 2 mm.

Comprehensive testing has shown that MMC materials of the type in question can be surface- coated both chemically, e.g. with nickel, and via thermal spraying. In the latter case, the following known processes have been tested with good results:

Arc

Plasma HNOF

Detonation Gun Super D-gun

The coating materials have consisted of, e.g. stainless steel, hard carbide steel, aluminum oxide and chromium oxide.

Aluminum-based, particle-reinforced metal-matrix composite (MMC) is used as the structural material in the invention currently in question. The properties of this type of material can be tailored specifically through the choices made in terms of matrix material, type and volume proportion of reinforcement phase, and fabrication method. In the event that the component in question is a connecting rod, high stiffness combined with low weight and high fatigue strength are required, as is thermal linear expansion on a par with that of the steel in a connected crankshaft. A number of MMC materials that can be used in the connecting rod in question are currently commercially available. The metal matrix in this case consists of an aluminum alloy plus a reinforcement phase consisting of particles of aluminum oxide, silicon carbide or boron carbide. The properties are determined by the type of MMC chosen, and by the fabrication method used.

An example of a high-quality MMC material is produced by powder metallurgy, using a matrix from the AA2000 series reinforced with silicon carbide particles. The following properties are achieved:

• Modulus of elasticity 110 - 140 GPa

• Yield strength •430 MPa

• Fatigue limit, notched •370 MPa

• Fatigue limit, unnotched •210 MPa

• Density 3 g/cm

• Thermal expansion 11 - 14 ppm/°K

The design, which is optimized based on the material properties, pertains to a connecting rod for a racing motorcycle.

The advantages from a weight standpoint are given in the table below:

ight, total, g Weight, small end, g

Steel 312 128 Titanium 237 74 MMC (AA6013 + 40% SiC particles) 196 46

Two MMC connecting rods are test run in a racing motorcycle. The result is an engine that turns more readily and with clearly less vibration, on a par with engines equipped with balance shafts, and with less gyro effect, which is important to maneuverability in, e.g. moto-cross. The bearing surfaces of these MMC connecting rods are provided with bushings to prevent any abrasion problems from affecting motor elements connected to the connecting rods as a result of the silicon carbide particles that the material contains. Even greater weight reduction can be achieved if the bushings are eliminated in accordance with the invention.

One aspect of the invention is based on a method of working a billet of MMC material by means of what is referred to here as HSM, or High-Speed Machining, and a component can be given its final form directly from the billet by means of this method. The billet can be forge or cast, or may consist of a piece of extruded bar or a raw material produced in some other way.

High-speed machining is characterized in that the cutting tool achieves extremely high speeds in relation to the machined billet in comparison with conventional techniques. The cutting tools that are relevant in this context are preferably milling tools and drills.

In this document, the term high-speed machining (HSM) is used to denote a method that differs from conventional machining methods. It so happens that this term is also sometimes used to denote conventional machining in which new methods are being developed to push the limits of conventional machining data upwards. This is not the sense of the term as it is used herein.

HSM is characterized by:

- extremely high cutting speeds

- a high rate of shear elongation (ability to separate a cutting from the billet)

- a very high power density is achieved in front of the cut (typical values: MW/mm₃)

- extremely high local temperatures prevail at the cutting point during the cutting-producing process

- the cuttings flow without coming into contact with the cut,

- the cutting forces tend asymptotically toward zero.

The following examples of the high cutting speeds associated with the machining of various substances may be noted:

- aluminum: ca. 3000 m/minute (conventionally ca. 100 - 400 m/minute),

- titanium: ca. 15,000 m/minute (conventionally ca. 15 - 100 m/minute).

Finding the right cutting speed to produce the foregoing conditions that characterize HSM thus depends entirely on the material to be machined. The cutting forces can be studied in connection with testing to determine the optimum cutting speed for HSM of a new material. These forces tend asymptotically toward zero as the criteria for HSM conditions are met. HSM conditions may thus be said to prevail when the cutting forces are diminishing. Under such conditions, it then remains to determine an optimum cutting speed for the material being machined. In conventional machining, the cutting forces increase with increasing cutting speeds.

Yet another advantage of using HSM is that the cuttings absorb the bulk, typically about 80%, of the heat generated at the cutting point, so that a work piece will be left largely unaffected by the heat generated during machining.

HSM has been shown to yield unexpectedly good results when used on MMC materials. Despite the high proportion of abrasive particles in such material, the cutting tools are found to retain their sharpness for a long time, as though unaffected by the abrasive in the MMC material. The reason for this has not been fully explained, since the internal process, i.e. what actually happens to the MMC material at the cutting point during machining, is not entirely understood. One theory is that a cutting being cut from the material is brought to some extent into a liquid state in a small area immediately in front of the beak of the cutting tool, and that the abrasive particles imbedded in the material in the form of e.g. silicon carbide, boron carbide or aluminum oxide are carried away in the molten material and thus do not come into direct contact with the cut. This could explain why the cutting tools retaining their sharpness, in direct contrast to what occurs during conventional cutting operations.

A number of tests have been conducted to evaluate the method according to the invention. Among other tests, a flange was produced from a raw billet of an MMC material, with a mill being used to remove all the material from the raw billet around the remaining flange. The flange in this case was L-shaped, with a final thickness of 1 mm of material, and the sides of the flange measured 45 and 15 mm, respectively. The values used during the machining in this example were: spindle speed 15,000 rpm, cutting speed 565 m/minute and feed speed 300 mm/minute. It took 2.5 minutes to create the flange. The tool life of the cutting tool was measured in hours. The proportion of SiC in the material was 40% in this example.

Tests were also conducted in which holes were drilled in MMC material containing 40% SiC. A number of holes were drilled using 6.9 mm HM bits, with a spindle speed of 15,000 rpm and a feed speed of 3000 mm/minute. The wear-out times for the drill bits were such that one bit could be used to drill 1000 holes.

The good results obtained in connection with machining according to the proposed method were produced using cutting tools of coated hard metal with internal duct cooling, and with diamond tools. When diamond tools are used, the tool lives of the tools are long at carbide contents of up to 40% in the MMC material. Good results are still obtained at carbide contents of as high as 70% in the MMC material.

The method according to the invention is applicable to all types of components that are to be made from MMC materials in cases where machine cutting is feasible in view of the final shape of the component. The method is thus not limited to the depicted embodiments, but can be used in connection with any components where the choice of MMC as the material is advantageous. The description above concerns a connecting rod whose surfaces that are to be corrected to < tolerance and/or surfaces that will be subject to wear are coated with an easy-to-work metallic layer. A corresponding procedure may be applied to surfaces of corresponding type on components used in combustion engines, such as pistons, connecting rods, crankshafts, valve mechanism components and other fast-moving machine elements.

Claims

1. An engine component made of a metal matrix material (MMC), characterized in that at least one of the tolerance-critical surfaces of the engine component is coated with a metallic layer that is easier to work by machining than the metal matrix material itself.

2. An engine component according to claim 1, characterized in that the thickness of the metallic layer is between 0.1 and 2 mm.

3. An engine component according to claim 2, characterized in that the material in the metallic layer contains a substance from the group consisting of bronze, bearing metal, molybdenum, nickel, nickel-graphite and hard chrome.

4. An engine component according to claim 3, characterized in that the material in the metallic layer is a bronze.

5. An engine component according to any of claims 1- 4, characterized in that the tolerance- critical surface of the engine component is a surface that is in contact with and exposed to friction against an opposing surface of another engine element.

6. An engine component according to any of claims 1 - 4, characterized in that it consists of a component from the group consisting of a connecting rod, piston, crankshaft part of a valve mechanism or other fast-moving machine element.

7. A method for fabricating an engine component made of metal-matrix composite (MMC) with at least one tolerance-critical component surface coated with a metallic layer that is easier to machine than the metal-matrix composite, characterized in that the metallic layer is applied in a chemical way or by means of thermal spraying.

8. A method according to claim 7, characterized in that the thermal spraying is performed by means of any of the methods: arc spraying, plasma spraying, HNOF, Detonation Gun or Super D-gun.

9. A method according to claim 8, characterized in that the application of the metallic layer by chemical means occurs through the use of autocatalytic or electrolytic methods.

1.0. A method according to claim 8 or 9, characterized in that the engine component, which is made of a metal-matrix composite that may consist of, e.g. a base material of aluminum, titanium, steel or an alloy of said material and at least one ceramic reinforcement material such as silicon carbide, boron carbide or aluminum oxide, wherein the reinforcement material accounts for 10 to 70% of the MMC material, is machined by means of High Speed Machining, so-called HSM machining, wherein HSM machining is present when the cutting forces during machining are decreasing as a function of the cutting speed, so that the component assumes predetermined dimensions via said machining, whereupon the metallic layer is applied.

11. A method according to claims 7 - 10, characterized in that the applied metallic layer is finished to the final dimensions by means of fine machining, e.g. grinding.