WO2020187918A1

WO2020187918A1 - Method of attaching a strike tip holder to a pick tool body of a pick tool

Info

Publication number: WO2020187918A1
Application number: PCT/EP2020/057307
Authority: WO
Inventors: Amanda Mckie; Maweja Kasonde; Teresa Rodriguez Suarez; Serena Bonetti
Original assignee: Element Six (Uk) Limited
Priority date: 2019-03-20
Filing date: 2020-03-17
Publication date: 2020-09-24
Also published as: GB2583411A; GB202003963D0; GB201903805D0

Abstract

This disclosure relates to a method of attaching a strike tip holder to a pick tool body of a pick tool using linear friction welding.

Description

METHOD OF ATTACHING A STRIKE TIP HOLDER TO A PICK TOOL BODY OF A

PICK TOOL

FIELD OF THE INVENTION

The field of the invention is pick tools. In particular, the invention relates to a method of joining carbide to steel for polycrystalline diamond (PCD) pick tools, particularly but not exclusively for road and mining applications. It also relates to pick tools having a solid-state diffusion bonded joint.

BACKGROUND ART

Conventional continuous mining machines include a rotatable drum with a plurality of pick tools attached to it. Each pick tool is attached to a pick tool holder (sometimes referred to as a bit block or holder block), and each pick tool holder is attached to the rotating drum. When the drum is rotated, a striking surface on each pick tool is brought into contact with a formation (such as a rock formation) and this action mechanically breaks down and degrades the formation.

A known pick tool 100 is illustrated in Figure 1. The pick tool 100 comprises a strike tip 101 that has a working surface formed at least in part from a superhard material. The strike tip 101 is non-rotationally attached to a strike tip holder 102. In this example, the strike tip holder 102 is formed from cemented tungsten carbide. The strike tip holder 102 has a projection 103 that extends from an end opposite to the strike tip 101. The projection 103 is not a necessary feature of the strike tip holder 101, as it could be brazed to a surface of the pick tool body 100 without needing a projection 103.

A steel pick tool body 104 is provided that has a bore into which the strike tip holder projection 103 is shrink fitted or press fitted, or press fitted with a spacer or brazed, to ensure that the strike tip holder 102 is firmly and non-rotationally affixed to the pick tool body 104. The strike tip holder 102 is located at a first end of the pick tool body 104, and the pick tool body 104 flares out to a shoulder at an opposite end of the pick tool body 104. A lower surface of the shoulder is termed an abutment surface 105, as the abutment surface 105 is in contact with an upper surface of a pick tool holder 201, as explained below. Areas of the steel pick tool body 104 that are expected to undergo significant wear in use may be provided with a hard face coating.

A hard face coating is formed from a harder material, for example one based on tungsten carbide.

A shaft 106 extends from the abutment surface 105. The shaft 106 is arranged to pass into or through a bore in a pick tool holder. The shaft 106 also comprises a mechanism 107 for attaching the pick tool 100 to the pick tool holder 102. In Figure 1, the mechanism comprises a threaded bolt configured to engage with a corresponding nut, but any suitable type of attachment mechanism may be used. Examples include a twist lock mechanism, an attachment pin, a threaded portion configured to engage with a corresponding threaded attachment bolt, an interference fit with an angled ring and so on.

Figures 2 and 3 show the pick tool 100 mounted to a pick tool holder 201. The pick tool holder 201 has a bore through which the shaft 106 passes. The abutment surface 105 of the pick tool 100 abuts a surface of the pick tool holder 201. A spacer 202 is optionally included in the bore to improve the fitting of the shaft 106 within the pick tool holder 201. In Figure 2, a nut 203 connects with the corresponding threaded attachment mechanism 107. A washer 204 is located between the nut 203 and the shaft 106. An angled ring 205 is provided above the washer. The angled ring 205 has an angled internal surface that corresponds with an angled surface of the shaft 106. When the nut 203 is tightened on the threaded attachment mechanism 106, it pushes the washer 204 upwards which, in turn, pushes the angled ring 205 toward the strike end of the pick body 100. As the angled ring 205 is pushed upwards, it forms a strong interference fit between the shaft 106, the spacer 202 and the tool holder 201, and consequently forms a strong interference fit between the shaft 106 and the tool holder 201.

To further illustrate the concept, Figure 3 is a perspective view of the pick tool 100 attached to the pick tool holder 201, with the pick tool holder 301 shown in cutaway view to illustrate how the pick tool 100 attaches to the pick tool holder 201.

During the design development of the pick tool 100, field trials were conducted. Several failure modes were identified, of which a significant portion (>70%) of failures seemed to originate at the shaft region of the carbide strike tip holder 102 where it is shrink- fitted into the steel pick tool body 104. The two main failure modes showed fracture of the carbide strike tip holder 102 at the shaft region due to stresses under predominately normal or side forces and the shoulder region due to stresses under predominately cutting force.

Carbide is inherently a brittle material that cannot support high impact (tensile) loads. The carbide strike tip holder 102 is required to protect the tool body 104 from excessive wear for longer tool life. The carbide strike tip holder 102 is joined to the steel tool body 104 by shrink fitting, which is currently the only method that offers high enough strength to withstand the extremely high impact loads that the pick tool 100 will experience during application. As the primary source of most of the failures observed during the field trials, improvements to the structural integrity of this joint is necessary.

It is an object of the invention to address the above mentioned problem and to provide an alternative to shrink- fitting as a method of joining carbide to steel for super-hard pick tools.

The challenge is to develop a joining method that provides much greater strength and toughness than currently achieved with brazing or shrink- fitting. A technique for producing a high quality joint between dissimilar materials, for example carbide to steel, is required.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a method of attaching a strike tip holder to a pick tool body of a pick tool, the method comprising: rubbing the strike tip holder against the pick tool body under an applied compressive load L, and oscillating the strike tip holder and/or pick tool body back and forth along a linear path relative to the other of the pick tool body and/or strike tip holder respectively for at least one oscillation and for a frequency f, each oscillation having an amplitude A, the applied compressive load L, frequency f and amplitude A being sufficient to produce a solid state diffusion bond joining the strike tip holder to the pick tool body.

Preferable and/or optional features of the first aspect of the invention are provided in dependent claims 2 to 25. In a second aspect of the invention, there is provided a pick tool comprising a carbide strike tip holder joined to a steel pick tool body with a diffusion bonded joint obtained by linear friction welding.

Preferable and/or optional features of the second aspect of the invention are provided in dependent claims 26 to 31.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting example arrangements to illustrate the present disclosure are described with reference to the accompanying drawings, in which:

Figures 1(a) and 1(b) are front and side elevation views of a prior art pick tool;

Figure 2 is a front elevation view of the pick tool of Figure 1 mounted in a prior art pick tool holder;

Figure 3 is a front perspective view of the pick tool and pick tool holder of Figure 2;

Figures 4(a) and 4(b) are schematic illustrations of the linear friction welding (LFW) process;

Figure 5 is a front perspective view of two workpieces used in the LFW trial, the upper component representing the carbide strike tip holder of the pick tool, and the lower component representing the steel body of the pick tool to which the strike tip holder will be joined;

Figure 6 is a cross-sectional view from above of the lower component of Figure 5, indicating the surface area that bonds to the upper component during LFW;

Figure 7 is a close up view of Figure 7, and in particular indicates the direction of oscillation;

Figure 8(a) shows an example of a good LFW weld joint and Figure 8(b) shows an example of a poor LFW weld joint which has delaminated; Figure 9(a) shows an image of test sample WC10 after LFW and Figure 9(b) is a close-up image of test sample WC10;

Figure 10 indicates cross-sectional directions of each LFW sample for microstructural analysis;

Figures 11(a) and 11(b) are scanning electron micrographs of test sample WC2 across the LFW weld, in the‘F’ or 0° orientation (Fig. 11a) and the‘A’ or 90° orientation (Fig. 1 lb);

Figure 12(a) and 12(b) are close up scanning electron micrographs of test sample WC2 across the LFW weld, in the‘F’ or 0° orientation (Fig. 12a) and the‘A’ or 90° orientation (Fig. 12b), and show in particular the interface bonding of carbide to steel;

Figure 13(a) and 13(b) are scanning electron micrographs of test sample WC10 across the LFW weld, in the‘F’ or 0° orientation (Fig. 13a) and the‘A’ or 90° orientation (Fig. 13b);

Figure 14 is a graph showing the Vickers hardness profile of sample WC2 in orientation directions A and F;

Figure 15 is a graph providing a comparison of shear strength achieved for LFW compared to traditional brazing and shrink fitting of components with similar geometries; and

Figure 16 is an annotated scanning electron micrograph illustrating the diffusion path of certain elements during solid-state diffusion bonding.

DETAILED DESCRIPTION

Friction welding processes have become key manufacturing solutions in joining of metals together, which is difficult to achieve compared to conventional welding processes, especially in the aerospace and automotive industries. Joining occurs through mechanically induced friction between two parts. The main friction welding methods are Linear Friction Welding (LFW), Rotary Friction Welding (RFW) and Friction Stir Welding (FSW). Each method has its own advantages and limitations. This disclosure concerns only LFW as a joining method for mining and road planning pick tools, selected due to the geometrical shape of the tools being commonly axially symmetric in one direction. LFW is a solid-state joining process, which works by oscillating one workpiece relative to another while under a large, compressive force as indicated in Figures 4a and 4b. The friction between the oscillating surfaces produces heat, causing the interface material to plasticise. The plasticised material is then expelled from the interface in a step known as‘burn-off , thereby causing the work pieces to shorten in the direction of the compressive force. During the burn- off stage, the interface contaminants, such as oxides and foreign particles, which can affect the properties and possibly the service life of a weld, are expelled into the weld flash. Once free from contaminants, pure metal-to-metal contact occurs, resulting in a bond.

The LFW joining process is typically used for joining metals, particularly those that have good high-temperature properties such as compressive yield, shear strength and low thermal conductivities. Materials such as titanium-based alloys are particularly suitable for LFW, although aluminium alloys, nickel-based super alloys and steels are also often used. LFW has become an established technology for several niche aerospace applications, pioneered by TWI Ltd (The Welding Institute) in the UK, for example, the fabrication of titanium alloy bladed disk (blisk) assemblies in aero-engines. LFW can produce joints that are similar or superior in strength to the parent material.

Other sectors using LFW includes automotive, shipbuilding, rail, oil and gas industry as well as the energy and construction areas. In some applications dissimilar material combinations can be bonded together using LFW such as aluminium alloys to copper, aluminium to steel or nickel alloys to steel. In most cases, the bonding of dissimilar materials has been through metals or between two plastically deformable materials.

Bonding of very dissimilar materials, such as metals to hard phases, is much less common, as in the case of joining steel to carbide. It is still theoretically feasible for bonding to occur because at least one of the materials is a metal. An interlayer may be required to form an intermediate bond between the materials for improved bonding strength.

LFW offers several advantages over competing manufacturing processes:

(1) The weld remains in the solid-state, avoiding many of the defects associated with melting and solidification during fusion welding, such as pores and solidification cracks. Minimal joint preparation is required, which reduces preparation time.

(2) Imperfections and impurities in the surfaces is removed during the process. (3) The distortion of the welded component is reduced.

(4) The process has lower peak temperatures than fusion welding, reducing intermetallic formation and allowing for a range of dissimilar materials to be joined.

(5) The process does not need a filler metal, flux and shielding gas.

(6) The process is easily automated, making the process highly repeatable and not dependant on human influence, resulting in very low defect rates.

(7) LFW reduces the material required to make a component by joining smaller work pieces to produce a preform, which is subsequently machined to the desired dimensions. This brings substantial improvements to the buy-to-fly ratios, which significantly reduces manufacturing costs.

Parameter Condition Study

Development trials were carried out on a servo-hydraulic E20 model linear friction welder, manufactured by Thompson, with a 25 tons forging capability. This LFW machine is digitally controlled, with multi-stage capability and with weld cycle recording for key welding parameters.

Welding parameters that were monitored during the joining process:

^■ Axially applied forge force (kN)

^■ Axially applied friction force (kN)

^■ Oscillation amplitude (±mm)

^■ Oscillation frequency (Hz)

^■ Axial displacement (mm)

^■ Welding duration (seconds)

The grade of carbide used for tool holder was a standard proprietary grade, Έ40’, developed by Element Six™. Basic material properties are provided in Table 1.

Table 1

This grade of carbide has good hardness and toughness and is especially suitable for mining applications. The grade of steel used for the pick tool body was a medium carbon low alloy tool steel, 42CrMo4, with 0.4 wt.% C. This grade of steel is a good choice for machining. Due to its excellent properties such as strength, hardenability, fatigue resistance, and impact toughness, it is also suitable for a wide range of applications, in particular, mining parts. However, the steel grade has limited weldability, which makes it unsuitable for conventional welding process, because it has a high carbon equivalent (CE) value of 0.87. This suggests that the material will suffer post weld defects such as cracking and a high amount of martensitic formation, which would then require pre- and post- welding heat treatments to minimize the effects. Preferably, steels utilized in this LFW application have a carbon equivalent greater than 0.40% as determined by either the International Institute of Welding (IIW) formula or the critical metal parameter (PCM) from the Japanese Welding Engineering Society.

The formula of the IIW is:

%Mn + %Si %Cr + %Mo + %V (%C + %Ni )

CE = %C + - - - + - - - + -

6 5 15

The critical metal parameter is calculated using the following equation:

%Si %Mn + %Cu + %Cr %Ni %Mo (%V)

PCM = %C + + + + + + 5 B

30 20 60 15 10 Two samples (also known as‘workpieces’), 304, 302 representing the pick tool body and the strike tip holder respectively were designed and produced for the trial to replicate the full-scale cross-sectional contact area dimensions required for the commercial pick tool. Evaluation of the interfacial bond strength of the joint was achieved using a shear test method. In Figure 5, the smaller upper component 302 represents the carbide strike tip holder 102, maintaining the overall cross-sectional area and a small flat 305 at the top at 45° for the shear test. The larger lower component 304 represents the steel pick tool body 104, a small cross- sectional area 307 (best seen in Figure 6) of the steel being present for bonding to the carbide strike tip holder 102.

A set of machining tool holders was also produced to clamp the work pieces in the LFW machine during welding. A keyhole was machined in the base of the bottom steel component to allow for accurate and repeatable locating in the tool holder during welding.

The workpieces were orientated in the tool holder on the LFW machine as indicated in Figure 7, with the direction of oscillation 306 along the shorter of the two interface dimensions (shorter relative motion). Hydraulic clamps were used to prevent movement of the tooling or the workpieces. The workpieces were also cleaned with acetone prior to welding to remove any potential weld contaminants.

Testing consisted of evaluating a range of applied forging pressures and oscillation amplitudes. The oscillation frequency and pre-set bum-off was kept constant at 50 Hz and 3.5 mm respectively. The oscillation cycle duration and the‘upset’ was measured. The term‘upset’ refers to the total axial shortening that occur during the oscillation and forging phases.

The initial starting point was an applied forging pressure of 75 MPa and oscillation amplitude at 1.5 mm; sample WC1. During testing, the pre-set bum-off target was not reached and the weld did not bond. It was determined that the applied forging pressure was insufficient for bonding to be achieved during the cycle; essentially the process was cold. This was also evident in the weld flash generated. Subsequent tests used applied forging pressure greater than 150 MPa and increased to maximum forging pressure for the LFW equipment of 200 MPa. A summary of the experiments is given in the Table 2 below. The oscillation amplitude was also varied between 1.0 mm and 2.5 mm to determine the best operating window.

Table 2: Weld parameters and results

Post-weld visual inspection revealed that there were noticeable differences bonding of the workpieces; ‘Good’ refers to the workpieces remaining intact after cooling with good morphology of the flash across entire perimeter of the weld region and is relatively consistent throughout the profile.‘Poor’ indicates poor bonding, that the workpieces delaminated after cooling at the interface or that there was fracture of the carbide during the test cycle. An example of a‘Good’ or‘Poor’ weld bond is shown in Figure 8. Figure 8(a) shows an example of a good weld joint and Figure 8(b) shows a delaminated poor weld joint.

Further optimization experiments were run. A summary of the parameter operating window is highlighted in Table 3;‘x’ indicates the parameters tested while the‘solid’ or‘patterned’ box indicates which parameters showed‘Good’ or‘Poor’ weld conditions respectively.

Good bonding

Table 3: Summary of effective parameter window

From this, it is noted that the optimal range of bonding is with an applied forging pressure of 150 MPa or higher and an oscillation amplitude of 1.5 mm to 2.0 mm. The oscillation duration ranges between 5.7 seconds and 11.8 seconds, but more typically is between 5.9 seconds and 7.2 seconds with an average of 6.3 seconds. Oscillation duration increases with oscillation amplitude from 5.7 seconds to 11.8 seconds.

High applied forging pressures (>175 MPa) and high oscillation amplitudes (2.5 mm) resulted in‘Poor’ welding bonds and long weld times. It is hypothesized that thermal cycling occurred during the oscillation process. High amplitudes result in too much hot metal being replaced with cold metal per oscillation cycle and limited the amount of bonding occurring at the interface. This results in a higher residual stress state at the interface after the process has been completed, causing the workpieces to delaminate after cooling.

The combination of high applied forging pressures (200 MPa) and small oscillation amplitude (1.0 mm) tool also resulted in‘Poor’ weld bonds; in this case cold welding of the workpieces effectively occurred. Not enough heat was generated during the oscillation cycle. This can be seen in the limited amount of flash 308 created and fracture of the carbide strike tip holder at the interface shown in Figures 9a and 9b (sample WC10). The pre-set burn-off valve was not achieved and resulted in longer than usual oscillation durations. In general, WC10 remained intact after cooling despite fracture of the carbide and insufficient flash formation, suggesting bonding still occurred. Microstructure Analysis

For microstructural analysis of the carbide to steel welds, two weld samples with significantly different performance results from Table 2, were sectioned and polished for analysis, these being samples WC2 and WC10. The samples were sectioned into three orientations with respect to the direction of oscillation, detailed in Figure 10, although only the F (0°) and A (90°) orientations were analysed. Examination of the macro-structure and microstructure was completed optically, using scanning electron microscopy (SEM).

An optical analysis of sample WC2 can be seen in Figure 11(a) and Figure 11(b); a clear distinction is observed between carbide and steel interface. The steel component 304, which plasticises and extrudes, forms a weld interface 310 and flash 308 while the carbide component 302 shows no deformation. In orientation F (0°), a large flash 308 is produced while it is minimal in orientation A (90°). The thickness of the weld zone towards the centre of the sample is about 1.5mm for both directions, but tapers towards the edge for the orientation A (90°). Analogous with conventional steel-to-steel welds, a weld central zone (WCZ) 312, thermo- mechanically affected zone (TMAZ) and a heat-affected zone (HAZ) are evident but only on the steel side 304.

Closer examination of the interface 310 of the weld using SEM was also carried out, showing bonding of carbide to steel in Figure 12a (WC2F) and Figure 12b (WC2A). Steel material appears to have bonded to both the cobalt phase and to the carbide particles within the carbide.

Analysis of sample WC10, the cold welded sample which fractured during the welding process, revealed the weld was intact despite the carbide having been fractured, as seen in Figures 13a and 13b. The images show that very little flash was generated at either orientation directions but especially in the direction of oscillation (F). An interesting feature was noted in the examination, metal flowed into a crack at the interface, indicated generally at 314. This suggests that fracture occurred during plasticisation of the steel material. The flow of metal into a feature like a crack can lead to future mechanical interlocking at the interface that would be beneficial in improving the interfacial bonding strength of the weld. Cracks 316 in the steel were also observed in the steel, which can also show that significant deformation stress was applied to the sample to cause the carbide to fracture and the steel to crack. Hardness measurements

Vickers hardness measurements were performed across the weld interface of each weld sample. Measurements were taken at 0.5 mm intervals with a load of 1kg and a dwell time of 15 s. The Hardness profile of sample WC2 for orientations‘A’ and‘F is shown in Figure 14 where there is very little difference between the two orientations. The average bulk Hardness of B40 grade carbide is 1050 ± 50 HV, while for 42CrMo4 steel was measured to be 334 ± 34 HV. A similar hardness of the weld central zone of 600 HV was attained; this is much higher than the bulk steel and is indicative of a martensitic material. This value is intermediate of that of steel and carbide, which could act as a gradient between the materials. Unfortunately, the toughness of the weld zone can make the material very brittle and prone to cracking in this area.

Shear Tests

Shear tests were carried out on the carbide to steel LFW samples to evaluate the bond strength between the two material surfaces. The tool is placed into a holder, which is inclined at 45 degree so that the flat portion of the carbide section is horizontal. A load is applied with a steel anvil to the flat potion of the carbide section until failure occurs. A shear strength of around 180 MPa was achieved.

A comparison between the shear strength of a tool produced with LFW and a traditional induction brazed tool with a similar flat interface geometry is shown in Figure 15. The shear strength of the LFW tool shows an 87% improvement compared to the brazed tool.

Delamination at the interface was found to be a typical failure pattern.

Solid State Diffusion

Referring to Figure 16, for good bonding of carbide to steel, diffusion of sufficient amount of elements needs to occur at the interface, elements such as Fe, Co, C. Initial electron dispersive spectroscopy (EDS) analysis of the interface showed limited diffusion of Fe into Co phase and Co into Fe. The rate of diffusion can be estimated by calculating the diffusion coefficient of inter-diffusion of each of the respective elements (i.e. Co into Fe, Fe into Co and C into both Fe and Co). The diffusion coefficient (D) was calculated using the Arrhenius type equation below which uses activation energy (Q), absolute temperature (T), gas constant (R) and the temperature-dependant pre-exponential (DO). From this, the approximate diffusion length (x) was determined for each element during time (t) for welding. A summary of the diffusion coefficient and estimated diffusion length is given in Table 4.

x » VUt The calculated diffusion coefficient and diffusion length of Fe into Co is greater than for Co into Fe, which suggests that Fe diffuses more quickly into Co than Co into Fe. Similarly, the diffusion length for Carbon into g-Fe and Co was calculated, this shows that the diffusion of carbon into both solutes is similar 28.15 pm and 25.6 pm This is in the order of lOOx greater than the inter-diffusion of Co and Fe, which shows that carbon diffuses much faster especially at high temperatures.

Diffusion Coefficient Estimated diffusion

Diffusion Path Temperature (K) Time for diffusion (s)

(cm¾) distance (pm)

Fe into Co 1.03 x 10 ¹⁰ 1473 6 0.25

Co into Fe 4.36 x 10^~11 1473 ~ 6 0.16

C into Y-Fe 1.32 x 10-e 1473 6 28.15

C into Co 1.1 x 1Q ^e 1473 6 25.6

Table 4 Bond Strength

A comparison of the shear strength achieved for LFW compared to traditional induction heating brazing method using an Ag-based braze alloy on the same flat interfacial geometry and two shrink- fit tool geometries is shown in Figure 16. An improvement of 87% was achieved using the LFW method compared to brazing method. The shear strength reached for the LFW was not as high as that achieved in direct shrink-fitting of carbide shaft into the steel holder. The brazing and LFW results can be directly compared as the same flat interface geometry was used. The failure pattern of the brazed tool occurred through the brazed layer. The results attained on the shrink-fit tools, around 300 MPa, is more difficult to compare directly as the strength is related to the strength of the carbide material. Fracture occurs through the carbide bolster shaft.

SUMMARY

This investigation demonstrates that it is possible to provide an effective alternative method of joining dissimilar materials together to conventional brazing. The inventors have established that LFW is viable as a method of joining a carbide strike tip holder to a steel body of a pick tool and that a shear strength of 180 MPa is achievable.

TERMINOLOGY

Certain terms and concepts as used herein are briefly explained below.

Synthetic and natural diamond, PCD, cubic boron nitride (cBN) and PCBN material are current examples of super-hard materials. As used herein, a super-hard material has a Vickers hardness of at least about 25 GPa. Synthetic diamond, which is also called man-made diamond, is diamond material that has been manufactured. PCD material comprises an aggregation of a plurality of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume per cent of the material. Interstices between the diamond grains may be at least partly filled with a filler material that may comprise catalyst material for synthetic diamond, may be substantially empty, or may include a material introduced to the PCD after removal of a catalyst. A catalyst material (which may also be referred to as a solvent / catalyst material) for synthetic diamond is capable of promoting the growth of synthetic diamond grains and or the direct inter-growth of synthetic or natural diamond grains at a temperature and pressure at which synthetic or natural diamond is thermodynamically stable. Examples of catalyst materials for diamond are Fe, Ni, Co and Mn, and certain alloys including these. Bodies comprising PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains. Note that the voids may be subsequently infiltrated with another material such as tungsten carbide, silicon carbide, silicon nitride, titanium carbide, titanium nitride, CBN or diamond. A PCD grade is a variant of PCD material characterized in terms of the volume content and/or size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. Different PCD grades may have different microstructures and different mechanical properties, such as elastic (or Young’s) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K1C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

Other examples of super-hard materials include certain composite materials comprising diamond or cBN grains held together by a matrix comprising ceramic material, such as silicon carbide (SiC), or cemented carbide material, such as Co-bonded WC material. For example, certain SiC-bonded diamond materials may comprise at least about 30 volume per cent diamond grains dispersed in a SiC matrix (which may contain a minor amount of Si in a form other than SiC). A further example is thermally stable polycrystalline diamond composite (TSP), which uses silicon carbide (SiC) binders. Such composites are stable up to 1200°C, but have reduced fracture toughness owing to the brittleness of the SiC and diamond.

As used herein, a shrink fit is a kind of interference fit between components achieved by a relative size change in at least one of the components (the shape may also change somewhat). This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after assembly. Shrink-fitting is understood to be contrasted with press-fitting, in which a component is forced into a bore or recess within another component, which may involve generating substantial frictional stress between the components.

While this invention has been particularly shown and described with reference to certain embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of attaching a strike tip holder to a pick tool body of a pick tool, comprising: rubbing the strike tip holder against the pick tool body under an applied compressive load L,

oscillating the strike tip holder and/or pick tool body back and forth along a linear path relative to the other of the pick tool body and/or strike tip holder respectively for at least one oscillation and for a frequency f, each oscillation having an amplitude A, the applied compressive load L, frequency f and amplitude A being sufficient to produce a solid state diffusion bond joining the strike tip holder to the pick tool body.

2. A method as claimed in claim 1, the applied compressive load being at least 150 MPa.

3. A method as claimed in claim 1 or claim 2, the applied compressive load being between 150 MPa and 200 MPa.

4. A method as claimed in any one of claims 1 to 3, the amplitude A being no greater than 2.0 mm.

5. A method as claimed in any one of the preceding claims, the amplitude A being between 1.5 mm and 2.0 mm.

6. A method as claimed in any preceding claim, wherein the strike tip holder and the pick tool body comprises dissimilar materials.

7. A method as claimed in any preceding claim, wherein the strike tip holder and/or the pick tool body comprise metal.

8. A method as claimed in any preceding claim, wherein the pick tool body comprises steel.

9. A method as claimed in claim any one of claims 6 to 8, wherein the strike tip holder and/or the pick tool body has a carbon equivalent greater than 0.4%.

10. A method as claimed in any preceding claim, wherein the strike tip holder comprises carbide material.

11. A method as claimed in claim 10, wherein the strike tip holder comprises cemented tungsten carbide material.

12. A method as claimed in any preceding claim, wherein the strike tip holder is a substrate for a super-hard material.

13. A method as claimed in claim 12, wherein the super-hard material is poly crystalline diamond, PCD.

14. A method as claimed in any preceding claim, further comprising the step of pre-heating either the strike tip holder or the pick tool body, or both.

15. A method as claimed in claim 14, wherein the step of pre-heating occurs in-situ.

16. A method as claimed in claim 14 or 15, wherein the step of pre-heating comprises heating to a temperature of 100° C ± 50° C.

17. A method as claimed in any one of claims 14 to 16, wherein the step of pre-heating comprises anodic pre-heating.

18. A method as claimed in any preceding claim, further comprising the step of post heating either the strike tip holder or the pick tool body, or both.

19. A method as claimed in claim 18, wherein the step of post-heating occurs in-situ.

20. A method as claimed in claim 18 or 19, wherein the step of post-heating comprises heating to a temperature of up to 100° C above the Martensitic transition (Ms) temperature of steel.

21. A method as claimed in claim 20, wherein the step of post-heating comprises heating to a target temperature of 25° C to 75° C above the Martensitic transition (Ms) temperature of steel and holding said target temperature for a period of time.

22. A method as claimed in claim 20, wherein the step of post-heating comprises heating to a target temperature of 25° C to 75° C below the Martensitic transition (Ms) temperature of steel and holding said target temperature for a period of time.

23. A method as claimed in any preceding claim, further comprising the step of subsequently air-cooling the strike tip holder and/or the pick tool body.

24. A method as claimed in any preceding claim, further comprising the step of subsequently removing weld flash created during generation of the solid state diffusion bond.

25. A method as claimed in claim 24, wherein the weld flash is removed by a machining e.g. grinding.

26. A pick tool comprising a carbide strike tip holder joined to a steel pick tool body with a solid state diffusion bonded joint obtained by linear friction welding.

27. A pick tool comprising a carbide strike tip holder joined to a steel pick tool body with a solid state diffusion bonded joint produced according to the method of any one of claims 1 to 25.

28. A pick tool as claimed in claim 26 or 27, in which the solid state diffusion bonded joint has a shear strength of around 180 MPa.

29. A pick tool as claimed in claim 26, 27 or 28, further comprising one or more interlayers between the pick tool body and the strike tip holder.

30. A pick tool as claimed in any one of claims 26 to 29, further comprising a super-hard strike tip joined to the carbide strike tip holder.

31. A pick tool as claimed in claim 30, wherein the super-hard material is polycrystalline diamond, PCD.

32. A pick tool as claimed in any one of claims 26 to 31, for road milling or mining.