GB2429464A

GB2429464A - Manufacturing a ferrous article

Info

Publication number: GB2429464A
Application number: GB0517212A
Authority: GB
Inventors: Ian Robert Whitaker; Leslie John Farthing; Iain Robert Whitaker
Original assignee: Federal Mogul Sintered Products Ltd
Current assignee: Federal Mogul Coventry Ltd
Priority date: 2005-08-23
Filing date: 2005-08-23
Publication date: 2007-02-28
Anticipated expiration: 2025-08-23
Also published as: GB2429464B; GB0517212D0

Abstract

A method of manufacturing a ferrous article, with a high wear resistance, by a powder metallurgy route. A mixture of iron-based powders, is used which consists of a first powder, a second powder, and one or more functional additives which collectively form a maximum 5% by weight of the mixture. The first powder forms 35 to 60% by weight of the mixture and consists of iron particles to which copper particles have been diffusion bonded to give a copper content of the first powder of between 10 and 40% by weight. The second powder consists of substantially pure iron and forms the balance of the mixture. During manufacturing of the ferrous article, which preferably is a valve guide for the poppet valve of an internal combustion engine, the sintering takes place at a temperature below 1083 degrees C.

Description

MANUFACTURING A FERROUS ARTICLE

This invention is concerned with a method of manufacturing a ferrous article by a powder metallurgy route.

It is well known that ferrous articles can be manufactured by a powder metallurgy route. This method comprises preparing a powder which comprises iron-based particles, compressing the powder to form a compact which is generally in the shape of the article required, and heating the compact in a sintering furnace to cause sintering of the particles. Normally, the powder used in such a method is a mixture comprising at least one type of iron-based particles, pressing lubricants and specific functional additives which are incorporated for such benefits as enhanced wear resistance, machinability etc. It is well-known to incorporate copper in a powder mixture used in a powder metallurgy process since it is found that the presence of copper can improve mechanical properties of the article, for example wear resistance is frequently increased by the presence of copper. The copper may be added to the mixture as a substantially pure copper powder, ie the particles consist of copper and inevitable impurities. Alternatively the copper may be added as a powder formed from an alloy of copper. It is also possible to alloy the copper with iron so that the powder is "a pre-alloy" of copper and iron. However, the addition of elemental copper or copper alloy powders to an elemental iron powder, although satisfactory at lower copper levels, frequently leads to segregation of the copper within the powder mixture as the level of copper increases. The use of pre-alloy powders also has a disadvantage in that they frequently have low compressibility so that manufacture of a high density article is difficult.

It is known, for example from US 4238221, to form a powder for use in a powder metallurgy process which consists of iron particles with copper particles diffusion-bonded to the iron particles. In other words, copper particles are adhered to the surfaces of the iron particles by diffusion. Such a powder may be manufactured by heating a mixture of iron and copper particles so that diffusion takes place where the copper particles contact the iron particles. The aforementioned US 4238221 gives examples of how this can be done. The use of such a powder in a powder metallurgy process enables use of high levels of copper, reduces the risk of segregation, and improves compressibility.

Such a diffusion-bonded powder may contain 5 to 50% by weight of copper.

US 4238221 also suggests that the diffusion-bonded iron and copper particles should be mixed with pure iron powder to adjust the copper content of the powder mixture and also suggests the addition to the mixture of phosphorus in the form of ferrophosphorus. US 4238221 is concerned with the manufacture of high precision components such as bearing bushings and gives examples of sintering temperatures of 1120 degrees C which is above the melting point of copper so that, during sintering, the diffusion bonded copper wIl melt and is thus able to readily alloy with the iron based matrix.

GB 2023184 also describes diffusion bonded iron-copper powder (which it refers to as "diffusion-alloyed"). It, like US 4238221, refers to sintering temperatures above 1083 degrees C (the melting point of copper), specifically mentioning 1120 degrees C. Thus, the copper melts during sintering. GB 2023184 also refers to the presence of up to 1.5 wt% phosphorus.

The present invention has the objective of providing a method of manufacturing a ferrous article by a powder metallurgy route which enables articles to be manufactured which have a high wear resistance and, in particular, are suitable for use as valve guides for the poppet valves of an internal combustion engine. The invention is based on the discovery that significantly improved wear resistance can be achieved by the use of a diffusion bonded iron-copper powder which has a copper content within a specific defined range, when used in a mixture with a powder consisting of substantially pure iron and when the relative proportions of the two powders are within a specific defined range. The material produced by a method according to the invention has the copper substantially unalloyed with the iron particles in the sintered material, instead of the copper being alloyed with the matrix.

The invention provides a method of manufacturing a ferrous article by a powder metallurgy route, the method comprising preparing a mixture of iron- based powders, compressing the mixture to form a compact generally in the shape of the article required, and heating the compact to cause sintering of the particles forming the mixture, wherein the mixture consists of a first powder, a second powder, and one or more functional additives which collectively form a maximum 5% by weight of the mixture, the first powder forming 35 to 60% by weight of the mixture and consisting of iron particles to which copper particles have been diffusion bonded to give a copper content of the first powder of between 10 and 40% by weight, and the second powder consisting of substantially pure iron and forming the balance of the mixture and wherein said sintering takes place at a temperature below 1083 degrees C. It is found that a method according to the invention enables the manufacture of articles with surprisingly improved wear resistance when compared with articles having the same overall copper content but prepared from a powder mixture containing only one iron-based powder. It is considered that the wear improvements obtained if the overall copper content exceeds 15% are insignificant compared with that obtained at 15% and that only marginal advantages are obtained if the overall copper content is less than 8% by weight. Preferably the overall copper content is between 9 and 13%.

It is preferred that the proportions of the first powder and the second powder are approximately equal by weight. Accordingly, mixtures are prepared containing 45 to 55% by weight of the first powder and 45 to 55% of the second powder.

For the purposes of the present invention, it is preferred to use diffusion bonded iron-copper powders with 10 to 40% by weight of copper, especially powders having 15 to 30% by weight of copper. More preferred are powders with 18 to 27% by weight of copper and most preferred are powders with 23 to 26% by weight of copper.

The additives which may be included in the mixture may be pressing lubricant powders and/or other usual additive powders used in powder metallurgy methods. It is noted, however, that phosphorus is not a desirable constituent of an article made according to the invention and should be held below 0.05% by weight.

This invention has been found to be particularly suitable for the production of materials for use as valve guides for the poppet valves of internal combustion engines. For this type of application, it is desirable that the material has a microstructure having a network of inter-connected pores. This network can then be filled with oil to supply localised lubrication of the poppet valve during the operation of the engine. In the present invention, such a microstructure can be readily achieved by ensuring that the copper in the mixture does not melt during the sintering process. Thus, the material is sintered at temperatures below the melting point of copper, ie at below 1083 degrees C. It considered that articles made according to the invention are rendered suitable for use in such a valve guide of the type referred to by their micro- structure in which the copper particles remain substantially unalloyed with the iron particles as they were in the mixture, this being achieved by keeping the sintering temperature below 1083 degrees C so that the copper does not melt during sintering.

Examples of methods according to the invention and comparative examples are described hereinafter. The accompanying drawing illustrates the results obtained by using the examples and comparative examples.

The drawing is a graphical representation of the results of a reciprocating sliding wear test carried out on articles in the form of test pieces made by methods according to the invention (examples E and F) and by methods which are not according to the invention (examples A to D) but illustrate the advantages gained by the use of the invention.

In the drawing, the "x" axis represents the overall copper level in weight percentages in the powder mixture used in the methods described. The powder mixtures each consist of diffusion-bonded iron-copper particles, and/or pure iron particles. The "y" axis represents the results (expressed as mass loss in milligrams) of wear tests in which a fixed article to be tested is contacted by a reciprocating chromium steel counterface and the weight loss of the substrate is measured.

In the drawing, the line "L" joins points representing the test results on examples which are not within the scope of the present invention but are included as comparison examples. The point "A" represents a method in which a powder mixture was formed which consisted of: 1 % graphite powder, 1 % molybdenum disulphide powder, 0.7% pressing lubricant powder, and 97. 3% pure iron powder. This powder was compressed into the form of a test piece and sintered at 1035 degrees C in a mesh belt sintering furnace. The pieces formed by this method gave a wear mass loss of 10.0mg. The overall copper content was 0%.

Points "B, C and D" in the drawing represent the results of tests carried out on test pieces manufactured by further comparative methods which differ from one another in the percentage of copper contained in the powder mixture.

In examples B, C and D, the pure iron powder of example A was replaced by three different commercially available diffusion-bonded iron-copper powders with increasing copper content, ie the mixture contained 97.3% by weight of iron-copper powder. The percentage of copper contained in the mixture was hence varied in exampies B, C and D. In example B the powder used contained 10% of diffusion bonded copper by weight, giving an overall copper content of the mixture of 9.7% by weight. As can be seen from the drawing, example B gave test pieces which achieved a wear mass loss of 8.4mg which is an improvement over example A which is explained by the copper included in the mixture.

Example C was the same as example B except that the iron-copper powder used had a diffusion bonded copper content of 20% by weight, giving an overall copper content for the mixture of 19.5% by weight. The wear mass loss for example C was 6.3mg which is a further improvement over example B explained by the increased copper content. In example D, the diffusion bonded copper content was further increased by the use of iron-copper powder containing 25% by weight copper, the overall copper content of the mixture being 24.3% and the wear mass loss was 3.6mg.

As can be seen from the line L in the drawing with the use of a single iron based powder in the mixture, the wear mass loss improves as the copper content of the mixture is increased.

The line N in the drawing joins the point representing example A (pure iron without iron-copper powder) to points E and F which represent examples falling within the scope of the present invention.

The examples E and F differ from examples A to D in the composition of the mixture used. Specifically, in example E the method utilised a powder mixture having the following composition: 48.65% of a powder consisting of iron particles to which copper particles have been diffusion bonded to give a 20% by weight copper content in the powder, 48.56% of a powder consisting of substantially pure iron particles, 1% graphite powder, 1% molybdenum disuiphide powder, and 0.7% pressing lubricant. The overall copper content of the powder was therefore 9.7% by weight (the same as for example B). This would be expected to give a wear mass loss approximately equal to that of example B. However, as shown in the drawing, the test results for example E show a greater wear resistance, the actual figure being 6.9mg which compares favourably with the 8.4mg of example B. In example F, a mixture was used consisting of the same quantities (as in example E) of substantially pure iron powder, graphite powder, molybdenum disulphide powder and pressing lubricant and 48.65% by weight of powder consisting of iron particles to which copper particles have been diffusion bonded but the copper content of this powder was 25% by weight. The overall copper content of the mixture was therefore 12.16%. Although no precisely parallel comparative example is included herein, from the line L of the drawing it would be expected that the wear resistance obtained by this method would be approximately equal to a wear mass loss of 8.0mg. In fact, as illustrated in the drawing, the wear resistance was significantly greater and the wear mass loss was 2.8mg.

Claims

I A method of manufacturing a ferrous article by a powder metallurgy route, the method comprising preparing a mixture of iron-based powders, compressing the mixture to form a compact generally in the shape of the article required, and heating the compact thereby causing sintering of the particles forming the mixture, wherein the mixture consists of a first powder, a second powder, and one or more functional additives which collectively form a maximum 5% by weight of the mixture, the first powder forming 35 to 60% by weight of the mixture and consisting of iron particles to which copper particles have been diffusion bonded to give a copper content of the first powder of between 10 and 40% by weight, and the second powder consisting of substantially pure iron and forming the balance of the mixture and wherein said sintering takes place at a temperature below 1083 degrees C.
2 A method according to claim 1, wherein the overall copper content of the mixture is between 8 and 15%
3 A method according to claim 2, wherein the overall copper content of the mixture is between 9 and 13%.
4 A method according to any one of claims I to 3, wherein the first powder forms 45 to 55% by weight of the mixture.
A method according to any one of claims I to 4, wherein the second powder forms 45 to 55% of the mixture.
6 A method according to any one of claims I to 5, wherein the first powder has a copper content of 15 to 30% by weight.
7 A method according to claim 6, wherein the first powder has a copper content of 18 to 27% by weight.
8 A method according to claim 7, wherein the first powder has a copper content of 23 to 26% by weight.
9 A method of manufacturing a ferrous article by a powder metallurgy route substantially as hereinbefore described with reference to examples E and F. A method of manufacturing a valve guide for a poppet valve arrangement according to any one of claims I to 9.