WO2008099347A1

WO2008099347A1 - Electro discharge sintering manufacturing

Info

Publication number: WO2008099347A1
Application number: PCT/IB2008/050513
Authority: WO
Inventors: David Patrick Egan; Seamus Melody
Original assignee: Element Six Ltd; Donald, Heather, June
Priority date: 2007-02-13
Filing date: 2008-02-13
Publication date: 2008-08-21

Abstract

The invention relates to a method of making an abrasive/cutting tool comprising a tool blank and a working portion which comprises a plurality of abrasive particles in a metallic matrix, the method including the step of subjecting a mass of abrasive particles and powdered matrix to electro discharge sintering (EDS) to either bond the working portion to a holder adapted to key with the tool blank or to bond the working portion or a holder for the working portion directly to the tool blank. The invention extends to a saw blade, drilling core bit, grinder tip, cutting point or wire saw manufactured by a method according to the invention.

Description

ELECTRO DISCHARGE SINTERING MANUFACTURING

BACKGROUND TO THE INVENTION

This invention relates to a method of manufacture of abrasive cutting tools. In particular, this invention relates to the use of electro discharge sintering (EDS) to manufacture abrasive cutting tools.

In the prior art, wear resistant parts more often than not need to be attached to a mechanical system (tool) in a distinct step by brazing or other means. This step incurs several disadvantages, such as i) additional cost and manufacture time, and ii) the need to subject the tool (mechanical system) to high temperatures during brazing. The latter tends to impair certain of the desired mechanical properties of the tool and may compromise the structural integrity of the wear resistant part. This is especially important where the wear resistant part is enhanced with diamond, which is degraded by high temperatures.

Laser welding of wear resistant parts to tools is growing in popularity but sometimes necessitates including a backing on the wear resistant part as the wear resistant (bond) material cannot be welded. This consideration is also applicable to the brazing of some wear resistant parts.

Different materials have different properties. For example, steels typically are strong and tough but their wear resistance is significantly lower than that of tungsten carbide or diamond materials. A complete abrasive or cutting tool requires a strong and wear resistant material to handle the abrasive/cutting forces while supported in a strong material, all of which is attached to a mechanical support mechanism.

Typically, two different materials (one supporting the wear resistant material, the other comprising the tool) do not join together easily using conventional sintering technologies.

Using powder metallurgical technology it is not easy to join the two very different materials together to form a useable product. For example, when using hot pressing (HP) or hipping it is not possible to bond sintered carbide to an iron powder metal section. The sintering procedure for each of these materials is completely different.

JP 08-25141 teaches using plasma discharge sintering to bond a pre-formed holder to a pre-formed tool insert.

US 3,670,137 discloses a method of spark discharge sintering of metal powders to form a sintered layer on a substrate. Much of the disclosure is directed to sintering metal powders on a substrate. One example, Example VII, describes a method of coating a substrate with a layer of tungsten carbide by spark discharge sintering.

Attempts to use HP to attach a diamond enhanced cutting/abrasive part to a tool is not expected to work as bonding of the steel of the part to the steel of the tool is unlikely to succeed as the holding would be physical only. Also, the steel of both components would have to be heated to a temperature which would affect the strength/stiffness of the steel, thereby compromising both the tool and the part,

Abrasive and cutting tools, regardless of operation, will eventually wear out requiring replacement. The options are to replace either the complete tool or just the abrasive/cutting elements (wear resistant parts) on the tool. For very large tools replacing only the wear resistant parts is preferred as removing and replacing the tool is time consuming and/or costly. Sometimes, the actual tool body itself is more costly than a wear resistant part.

Replaceable wear resistant parts typically consist of a shaped holder which fits onto the tool and holds or receives the abrasive portion. Typically, the wear resistant parts and holder materials are very different requiring interference joints which are costly to manufacture because of the tolerances needed. Alternatively the wear resistant parts may be brazed to the holder, a procedure which is costly. The two materials may be so different that joining is very difficult and may require special processes.

A need exists to attach the wear resistant (enhanced) part to the tool when the enhanced part is being made, thereby combining the steps of manufacturing the enhanced part and attaching it to the tool.

A further need exists for a method to sinter very dissimilar powder metallurgical materials that can be sintered at the same time to produce a complete product previously unreal isable. One material could be iron powder, for example, which would form the main body of a tool and the other material a WC/Co powder including diamond particles which form the abrasive resistant element. A need exists to realise a complete tool using this technique.

Also, a need exists for powder materials which are not sinterable/processable using conventional techniques to be sintered into useable parts by using EDS.

For example, powder particles which are too hard/strong to allow pressing and thus sintering can be used as a bond to allow a diamond bearing element to be manufactured. Some high carbon steel powders are not processable using conventional techniques but would make effective bond materials for abrasive applications. SUMMARY OF THE INVENTION

According to the invention, a method of making an abrasive/cutting tool comprising a tool blank and a working portion which comprises a plurality of abrasive particles in a metallic matrix, the method including the step of subjecting a mass of abrasive particles and powdered matrix to electro discharge sintering (EDS) to either bond the working portion to a holder adapted to key with the tool blank or to bond the working portion or a holder for the working portion directly to the tool blank. Typically, the working portion is bonded in srt^' u to a support or holder.

The term 'key with the tool' is intended to encompass engagement of the holder and working portion with a tool blank other than by screw threading, clipping or studding the holder to the tool. EDS is defined as sintering achieved by passing one or more pulses of high current through a porous metallic based medium while also under compressive force.

Details of EDS are as follows:

Force applied (range):

Preferably greater than 500kg/cm², more preferably greater than 600 kg/cm², more preferably greater than 800 kg/cm², more preferably greater than 1000 kg/cm², more preferably greater than 1500 kg/cm², most preferably greater than 2000 kg/cm². Preferably less than 6000kg/cm², more preferably less than 5500kg/cm² , more preferably less than 5000kg/cm² , more preferably less than 4500kg/cm² , most preferably less than 4000kg/cm² ,

Amperage (range):

Preferably greater than 0.5kA, more preferably greater than 1.OkA, more preferably greater than 5.OkA, more preferably greater than 10.OkA, more preferably greater than 20.OkA, more preferably greater than 50.OkA, most preferably greater than 80.OkA. Preferably less than 100OkA, more preferably less than 90OkA, more preferably less than 80OkA, more preferably less than 70OkA, more preferably less than 60OkA, more preferably less than 55OkA, most preferably less than 50OkA₁

Voltage (range): Preferably greater than 1V, more preferably greater than 2V, more preferably greater than 3V, more preferably greater than 5V, more preferably greater than 8V, more preferably greater than 10V, most preferably greater than 20V. preferably less than 10OV₁ more preferably less than 90V, more preferably less than 80V, more preferably less than 70V, more preferably less than 60V, more preferably less than 55V, most preferably less than 50V.

No. of pulses (range):

Preferably greater than 1 , for example 2. Preferably less than 10, more preferably less than 9, more preferably less than 8, more preferably less than 7, more preferably less than 6, more preferably less than 5, most preferably less than 4.

Duration of each pulse (range):

Preferably greater than 0.1 ms, for example 0.2ms. Preferably less than 10ms, more preferably less than 9ms, more preferably less than 8ms, more preferably less than 7ms, more preferably less than 6ms, more preferably less than 5ms, most preferably less than 4ms.

The development of electro discharge sintered bodies comprising metal and particles of a hard phase, such as diamond, makes it possible to rapidly and economically produce working portions with superior wear resistance. The superior wear resistance is due to the presence of the harder phases incorporated. Such working portions are usually used as components of mechanical systems such as saw blades.

According to the invention, the working portion preferably comprises two or more regions which differ from each other in their abrasive properties. For example, one of the regions may comprise abrasive particles in a metal matrix and the other region the metal matrix alone. Alternatively, the abrasive particle size or type can differ in each region. The use of EDS allows such regions differing in properties and/or sinterability to be sintered and bonded in a single operation. This aspect of the invention has particular application to saw blades, drilling core bits and wire saws.

According to the invention, the working portion may replace a working portion of a tool which has worn away through use. The working portion can be created on the tool blank and/or holder to which the original working portion was bonded. Using EDS the new working portion is sintered and bonded to the tool blank and/or holder in the same operation. This aspect of the invention has particular application to the replacement of wear parts in chain saws, twist drills, saw segments and floor grinding apparatus.

According to another aspect of the present the invention, the working portion may be bonded to a holder which is produced by EDS at the same time as the working portion. Thus, a tool insert as a whole designed to key with a tool blank is produced in a single operation.

The abrasive particles can be carbide, nitride, oxide or suicide particles or ultra-hard particles such as diamond or cubic boron nitride. The diamond may be natural diamond or it may be synthetic diamond manufactured by high temperature/high pressure or chemical vapour deposition techniques.

The use of EDS allows for a range of difficult to sinter materials and materials of dissimilar metallurgical properties to be sintered.

For wear resistant parts (working portions) made using EDS, the wear resistant part is bonded to a tool blank during the sintering step. This is achieved by positioning the blank in the EDS reaction volume together with the precursor material for the wear resistant (enhanced) part. The same mechanism that consolidates the precursor material into a sintered compact wear resistant part also bonds the wear resistant parts to the blank, thus eliminating a separate attachment step. In such an instance, the wear resistant part may be directly bonded to the blank, or it may be bonded to a holder which is in turn engaged with the blank. In any event, the sintering and bonding to the blank occurs as a single step.

When EDS is used to sinter and bond a wear resistant part to a tool blank (body) or other support there is no significant temperature increase in the tool blank to speak of, at least, none that would affect the properties of the tool blank, for example, no warping or bending.

The method according to the present invention can also be used to bond conventional segments to blanks as an alternative to brazing/laser welding.

By using EDS according to the present invention, the abrasive part of a replaceable cutter can be sintered directly onto a pre-manufactured holder shaped/keyed to fit the tool blank. As the abrasive material begins life as a metal powder with abrasive particles and is sintered to high density directly onto the holder, this negates the need for any subsequent joining operation as discussed above.

Alternatively, the complete keyable/exchangeable holder and working portion can be manufactured from metal powder and diamond or any other abrasive material at the same time in a single step.

Typically, high abrasion resistant materials cannot be worked to fashion internal keys for attachment of the abrasion resistant component to a tool blank. With EDS, keys which protrude from the part can be incorporated into the actual wear resistant material and/or a holder which can then be engaged individually to the tool blank.

In a chain saw application, for example, a rectangular disk of enhanced metal (the working portion) can be directly bonded to one end of a partially or fully keyed metal rod/shaft. This joined (composite) part can then be engaged with the chain by attaching it to a part that is attached/engaged to the chain. The teachings of the present invention apply to any abrasion resistant part (working portion) of a tool connected to a holder which is exchangeable. The invention therefore includes but is not limited to large diameter saw blades, coal picks, mining applications, road planning applications, grinder tips and small cutting points which can be keyed on to ailow fast exchange.

Applicant's co-pending application entitled 'Abrasive Segments' and claiming priority from ZA 2007/01268 is incorporated herein by reference.

The invention will now be described with reference to the following non- limiting examples and figures in which:

Figures 1A and 1 B are photographs of the remains of a working portion of a saw blade;

Figures 2A and 2B are photographs of a new working portion directly bonded to a saw blade blank;

Figure 3 shows three dimensional placement of diamond abrasive particles in a working portion according to the present invention; and

Figure 4 shows the expected wear patterns on the working portions as shown in Figure 3.

In Figure 1A and 1 B (1 B is an angled view of the working portion of 1A) approximately 0.5mm of a originally 7mm sized working portion can be seen bonded to the saw blade blank.

In Figures 2A and 2B (2B is a top view of the working portion shown in 2A) the worn working portion has been replaced in situ with a 7mm iron powder/diamond material working portion directly bonded to the saw biade blank in accordance with the present invention, i.e. using EDS. The braze material used to braze the first working portion (now discarded and not shown) can be seen to be smeared on the tool blank as a result it having been melted locally and dispersed under the EDS conditions.

EXAMPLE 1

Bonding a replacement or enhanced part onto an existing worn insert.

1A) Re-tipping a saw blade with new segments

The worn saw blade is held vertically in the machine, supported by a shaft through the centre. From either side, mould halves are moved in to surround the blade. Each half consists of a conductor to carry the current and an insulator into which a working portion segment precursor is placed. The conductor further supports any vertical forces on the blade. The segment precursor (a mixture of bond powder and diamond) is preferably pre-pressed.

in the present example, 0.13g of SDBTCH 4050# diamond was mixed with 6.13g of Hoeganaes Ancorsteel 737H iron powder which had been heat treated at 700 deg C for 30 minutes with 0.8 weight % carbon graphite. The segment dimensions desired in this case is 40mm long, 3.4mm wide and 6mm tall. The iron and diamond were mixed thoroughly to ensure a good distribution. This was charged into the mould opening where it was placed on top of the worn saw blade segment area. A conductive plunger was then inserted into the mouid opening and pressed firmly down. Using a hydraulic ram, 27kN of force was exerted onto the conductive plunger. With the pressure constant, a pulse of current 30OkA @ 24V was passed through the powder diamond mixture. A second pulse can also be used. 0.5-10ms (Energy of 35% of 38kJ) was used for each pulse.

The moulds are opened and the disc rotated to the next segment area where the process is repeated for the next segment.

The above can be achieved on a new blade or where the old segments have been completely removed or worn to an extent where they cannot be removed. 1 B) Producing a BQ (core) bit.

The steel tube onto which the BQ (core) segment areas are to be produced is held vertically in the machine. Ceramic linings to surround the tube top are placed around as are the ceramic parts beneath where the diamond bearing areas are to be produced. Copper conductors are pushed in from the side to the steel tube surface. The diamond bearing area dimensions desired in this case is a curved 15mm long, 10mm wide and 10mm tall A mixture of 0.18g SDBTCH 40/60# diamond with 4.36g of a Co/WC bond material (such as Dr. FritschVI 7-785) is added to each chamber. A copper/Cu-W electrode punch is inserted into the chamber and a force of 20 to 30 kN is applied. With the pressure constant, a pulse of current 30OkA @ 24V was passed through the powder diamond mixture. A second pulse can also be used. 0.5-10ms

The conductive electrode punch is lifted and the tube rotated to align with the next chamber where the process is repeated.

EXAMPLE 2

Creation of the whole insert in one operation, i.e. the tool support and the working portion.

This description is of a shaped holder with a diamond bearing working portion which is designed to be fitted to the outside of a blade shaped to accept the holder. This design is a simple angled holder where the blade has a corresponding slot cut on the circumference. The mould to manufacture these inserts is of split design to allow easy removal of the finished insert. The mould is closed, the bottom conductive electrode punch is inserted and the unit placed on the machine. Iron powder (13g) to produce the shaped holder portion of the insert is poured into the chamber. Alternatively, a pre-pressed iron shape could be inserted from the bottom and the conductive electrode punch replaced. When iron powder is used, the top conductive electrode punch is then inserted and the bottom punch raised to compress the iron powder using a force of 3OkN. The segment dimensions desired in this case is 40mm long, 3.4mm wide and 6mm tall. The top punch is lifted and 0.13g of SDBTCH 4050# diamond mixed with 6.13g of Hoeganaes Ancorsteel 737H iron powder which had been heat treated at 700 deg C for 30 minutes with 0.8 weight % carbon graphite is added. The top punch is replaced and a force of 3OkN is used to force both the top and bottom punches in toward the centre. With the pressure constant, a pulse of current 30OkA @ 24V was passed through the powder diamond mixture. A second pulse can also be used (0.5- 10ms).

The die is opened and the insert removed. The process is repeated for the next insert.

EXAMPLE 3

Working portions which have regions or zones differing in properties, 1 D, 2D and 3D examples.

The 1 , 2 and 3D structures hereinafter described are considered to cover not only segments as described but also single step bonding/sintering to a tool blank and single step bonding/sintering to a keyable holder/working portion.

A 1 D example.

The segment dimensions desired in this case is 40mm long, 3.5mm wide and 6mm tall. Strips of iron powder which contain diamond at a concentration of 16 have been produced. These strips are 40 mm long, 0.5mm thick and 7.5mm tall, with a curvature the same as a typical 400mm segment. They typically have a porosity of 20%. Two types of strips are used for this example, one containing 40/50# diamond and another containing 50/60# diamond.

A segment mould of correct size with a conductive bottom punch is used. Two strips containing the 40/60# diamond are placed at the outer edges of the mould. 4 strips of containing the 40/50# diamond are then placed between these. The top punch is inserted and a force of 25 to 3OkN applied. With the pressure constant, a puise of current 30OkA @ 24V was passed through the powder diamond mixture. A second pulse can also be used (0.5-10ms).

The die is opened and the segment removed. The process is repeated for the next segment. This can also be done to a saw blank or interchangeable piece.

A 2D example.

The segment dimensions desired in this case is 40mm long, 3.5mm wide and

6mm tall. Strips of iron powder which contain diamond at a concentration of 30 have been produced. These strips are 40 mm long and 0.5mm square, with a curvature the same as a typical 400mm segment. Another strip not containing diamond has also been produced. They typically have a porosity of 20%. Seven of these strips can be placed side by side in the mould, alternating the two strips (one containing diamond, the other not) across. A second layer can be built on top, ensuring that the strips are alternating in the vertical direction also. This is continued until a height of 7.5mm. The top conductive plunger is inserted and a force of 27kN is applied. With the pressure constant, a pulse of current 30OkA @ 24V was passed through the powder diamond mixture. A second pulse of 0.5-10ms can also be used.

The die is opened and the segment removed. The process is repeated for the next segment.

A 3D example. The segment dimensions desired in this case is 40mm long, 3.5mm wide and 6mm tall. Cubes of iron powder (0.5mm in dimension for example) which contain SDBTCH 4050# diamond have been produced. Other cubes of equivalent dimension which do not contain diamond have also been produced. The wear resistance of the material which surrounds the diamond particles is noticeably greater than that which forms the cubes that do not contain diamond. These cubes are placed in the mould, 7 units wide and 80 long. They can be placed in an ordered fashion or randomly. The ratio of cubes which contain diamond to those that do not will determine the concentration of diamond in the segment. For example, to achieve a concentration of 16 using 40/50 # diamond, 1 in 5 cubes contain diamond particles. This arrangement of cubes is sandwiched between the conductive electrode punches, and a force of 25 to 3OkN applied. With the pressure constant, a pulse of current 30OkA @ 24V was passed through the powder diamond mixture. A second pulse can also be used (0.1 -10ms).

Claims

1. A method of making an abrasive/cutting tool comprising a tool blank and a working portion which comprises a plurality of abrasive particles in a metallic matrix, the method including the step of subjecting a mass of abrasive particles and powdered matrix to electro discharge sintering (EDS) to either bond the working portion to a holder adapted to key with the tool blank or to bond the working portion or a holder for the working portion directly to the tool blank.

2. The method according to claim 1 wherein the working portion is bonded in situ to a support or holder.

3. The method according to claim 1 or 2 wherein the working portion is a component of mechanical systems such as saw blades, drilling core bits and wire saws.

4. The method according to any previous claim wherein the working portion comprises two or more regions which differ from each other in their abrasive properties.

5. The method according to claim 4 wherein one of the regions comprises abrasive particles in a metal matrix and the other region the metal matrix alone.

6. The method according to claim 4 wherein the abrasive particle size or type is different in each region.

7. The method according to any previous claim wherein the working portion replaces a working portion of a tool which is inutile.

8. The method according to any previous claim wherein the working portion is created on a tool blank and/or holder to which the original working portion was bonded.

9. The method according to claim 8 wherein the new working portion is sintered and bonded to the tool blank and/or holder in the same operation.

10. The method according to any previous claim used in the replacement of wear parts in chain saws, twist drills, saw segments and floor grinding apparatus.

11. The method according to any previous claim wherein the working portion is bonded to a holder which is produced by EDS at the same time as the working portion.

12. The method according to any previous claim wherein the abrasive particles are selected from carbide, nitride, oxide or suicide particles or ultra-hard particles such as diamond or cubic boron nitride.

13. The method according to claim 12 wherein the diamond is natural diamond or synthetic diamond manufactured by high temperature/high pressure or chemical vapour deposition techniques.

14. The method according to any previous claim wherein the wear resistant part is bonded to a tool blank during the sintering step by positioning the blank in the EDS reaction volume together with the precursor material for the enhanced wear resistant part.

15. The method according to any previous claim wherein there is no significant temperature increase in the tool blank that would affect the properties of the tool blank.

16. The method according to any previous claim wherein the abrasive part of a replaceable cutter is sintered directly onto a pre-manufactured holder shaped/keyed to fit the tool blank.

17. The method according to any previous claim wherein a complete keyable/exchangeabie holder and working portion is manufactured from metal powder and diamond or any other abrasive material in a single step.

18. The method according to any previous claim wherein, in a chain saw application, a rectangular disk of enhanced metal {the working portion) can be directly bonded to one end of a partially or fully keyed metal rod/shaft which is then engaged with the chain by attaching it to a part that is attached/engaged to the chain.

19. The method according to any previous claim applicable to large diameter saw blades, coal picks, mining applications, road planning applications, grinder tips and small cutting points which can be keyed on to allow fast exchange.

20. A saw blade, drilling core bit, grinder tip, cutting point or wire saw manufactured by a method according to any one of claims 1 to 19.