WO1999015294A1

WO1999015294A1 - Method of liquid phase bonding

Info

Publication number: WO1999015294A1
Application number: PCT/GB1998/002847
Authority: WO
Inventors: Mark Andrew Miodownik; Brian Derby
Original assignee: Isis Innovation Limited
Priority date: 1997-09-19
Filing date: 1998-09-18
Publication date: 1999-04-01
Also published as: AU9174098A; GB9720059D0

Abstract

A method of bonding together two cermet bodies comprises heating the two bodies preferably under vacuum to a temperature where a cermet material melts and encapsulates a bond interface between the two bodies; then subjecting the two bodies to hot isostatic pressing to bond them together. The method is useful for bonding together approach and bearing sections of a floating plug for use in drawing copper tubing.

Description

METHOD OF LIQUID PHASE BONDING

This invention is concerned with bodies composed of interpenetrating or interconnected networks of generally high melting materials. Typically two such materials are present in the body, each as a 3-D network, with the two networks being interconnected or interpenetrating; bodies formed of dispersed particles of one material in a continuous matrix of another are not included. The bodies are industrially important and include:

• Cermets, in which one material is a ceramic and the other is a metal. When the ceramic is a carbide these are sometimes known as cemented carbides. An example is the product hardmetal, which is based on a ceramic (usually tungsten carbide or titanium carbide) and a metal (usually cobalt in the case of tungsten carbide but other alloys are also used). The material is extremely hard, moderately tough and can be fabricated in bulk. Virtually every screw, nut, bolt and nail is produced using hardmetal heading dies. Engine components such as gear boxes, transmissions and cylinder heads are all machined using hardmetal tooling. References 1 to 4 below describe the manufacture and properties of hardmetal and other cermets.

• Reaction bonded silicon carbide.

Although this invention relates generally to the bonding of bodies of this kind, it resulted from an initial study of how to bond two components used for fabricating copper tube.

Copper tube is made by drawing copper through a die. The inner bore of the tube is shaped by a floating plug or mandrel made from hardmetal. It is desirable to make longitudinal ribs on the inside of the copper tube to increase its heat transfer coefficient and this is achieved by means of a correspondingly profiled plug. This technology suffers one outstanding problem, the design of a grooved floating plug. The grooved components of the plug, the approach and the bearing which are currently bolted together, become misaligned during drawing and cause failure. This problem would be overcome by manufacturing the plug from a single piece of hardmetal, however this exceeds the capabilities of current grinding technology. There follows a summary of existing bonding technology as applied to this problem.

Mechanical Fastening. The approach and the bearing can be bolted together - this allows good alignment of the grooves between the approach and the bearing. However they become progressively misaligned during drawing, which causes draw failure. This technique is current practice.

Brazing. Brazing is the preferred method for joining hardmetal tips to steels in the manufacture of machine tools. However for the copper tube application it is impossible to form the join without contaminating the grooves with brazing alloy. In addition the approach and the bearing become separated by a thick brazing fillet.

Diffusion Bonding. Diffusion bonding⁵⁶ is a joining process where high temperatures and pressures are used to bring together two surfaces. so that there is atom to atom contact throughout the bond area. Under these conditions bond strengths comparable with the bulk strength of the material can be achieved. It is important to realise that most surfaces, even those polished to a mirror finish, are rough at the atomic level. So when two surfaces are brought together they will contact first where the asperities meet. Therefore the initial interface will consist of a number of point contacts surrounded by voids where the surface 'hills' and 'valleys' do not match up. Diffusion bonding in the solid state involves three mechanisms which act to reduce this mis-match and so reduce the volume of voids: plastic deformation, creep deformation and atomic diffusion. The effectiveness of such processes is dependent on the process variables, time, temperature, pressure, vacuum quality and surface roughness. The final bond strength of the joint will critically depend on the volume and size distribution of voids at the interface. Thus successful diffusion bonding is achieved when the process variables are optimised to minimise the volume fraction of interfacial voids. In practice there are two problems with this technique:

(a) It is difficult to ensure a uniform pressure during bonding, resulting in a high void density and inferior bond strengths.

(b) Accurate alignment of the grooves between the approach and bearing is difficult to achieve without the use of a mechanical jig. Hot isostatic Pressing (HIP)-Bonding. HIP-Bonding⁷ can be used as a method of diffusion bonding by encapsulating the parts to be bonded into a mould so that the two interfaces are brought together. During the HI Ping the mould is compressed by a gas transmitting a pressure of preferably at least 30 MPa to the interface allowing diffusion bonding to occur. The method of encapsulation will depend on the geometry of parts to be joined, the encapsulation material is usually mild steel or glass. The advantage over direct bonding is that the pressure applied to the bonding face is uniform. However there are several problems with this technique, the first being that exact alignment of the two parts within the encapsulation mould is very difficult to achieve. During HIPing the encapsulation material flows around the parts causing further misalignment. Finally in the case of steel encapsulation, the steel bonds to the Hardmetal, contaminating the grooves. This problem is avoided by using glass because it can be removed from the Hardmetal easily using Hydrofluoric Acid.

The invention provides a method of bonding together two bodies, each composed of interconnected networks of a higher melting material and of a lower melting material, which method comprises a) heating the two bodies, while keeping them adjacent one another at a bond interface, to a temperature intermediate the melting points of the higher melting material and the lower melting material, whereby the bond interface between the two bodies is encapsulated, b) then subjecting the two bodies to hot isostatic pressing to bond them together.

Preferably, but not necessarily, the two bodies to be bonded are formed of the same materials. The facing surfaces of the two bodies, through which the bodies are to be bonded together, should preferably be as flat as possible. Preferably the average Ra is no more than 2.0 μm and the mean peak roughness is no more than 20 μm. Suitably flat surfaces may be prepared by grinding. Polishing or lapping of the facing surfaces may produce even better results.

In step a), the two bodies are heated to a temperature intermediate the melting points of the higher melting material and the lower melting material. The heating temperature may be slightly above the melting point of the lower melting material, or of the lowest melting material if the two bodies are formed of different materials. The heating is preferably performed under vacuum e.g. below 0.1 Pa in order to extract gas from between the facing surfaces of the two bodies. Alternatively, heating is performed in an inert gas atmosphere e.g. argon. The heating temperature is maintained for a time to permit a bond to be formed by a combination of liquid flow (of the lower melting material) and rapid diffusion. This flow results in the local encapsulation of the bond interface. That is to say, any remaining spaces between the facing surfaces of the two bodies become encapsulated by the bonding of the bodies, so that when a vacuum is removed gas cannot enter the spaces. These liquid flow and diffusion effects are so rapid that it is generally sufficient simply to heat the two bodies to the required temperature and then immediately cool them without any period of hold at temperature.

During the performance of step a) and prior to the formation of a bond, it is necessary that the two bodies be kept adjacent one another at the bond interface. A convenient way of achieving this is by the use of an adhesive; alternatively mechanical external fixtures may be used. Under the high temperatures required for cermets or glass ceramics, an organic adhesive will decompose and the decomposition products are expected to volatilise. Because any remaining decomposition products of adhesive might weaken the bond between the two bodies, it is preferred to use the minimum amount of adhesive needed to hold the two bodies together. As shown in the experimental section below, very strong bonds can be obtained notwithstanding the use of an adhesive.

After step a), the two bodies are bonded together, but by bonds which are relatively weak. In step b) the bodies are subjected to hot isostatic pressing in order to increase the bond strength. The temperature and time conditions for HIP-bonding may be conventional; in particular, the temperature is generally below the melting point of the lower or lowest- melting material. What is unusual about the method of this invention is that an encapsulating material is unnecessary and is preferably not used. In conventional HIP-bonding an encapsulating material is needed to prevent the ingress of gas into spaces between facing surfaces of two bodies to be bonded. • In the method of the present invention, since the bond interface has already been encapsulated in step a) an encapsulating material is not required for step b). Step b) may conveniently be performed directly after step a) without intermediate cooling of the two bodies.

Reference is directed to the accompanying drawings in which:-

Figure 1 shows test specimen geometry. Figure 2 is a graph showing HIP bonding conditions.

Figure 3 is a schematic diagram of the 4-point bending jig used in the example.

Figure 4 is a bar chart showing bend strength data. EXAMPLE 1 1.1 Specimen Fabrication

Square cross section bars of dimensions 65 mm x 6 mm x 6 mm were cut from bulk material, see Figure 1. The beams were cut in half, ground and (in some cases) polished on their bonding surfaces to a 1 μm finish. Cutting and sectioning was carried out using a Shipman & Jones grinding machine, with a diamond impregnated cutting wheel. Some specimens were polished with a Kent polishing system, using Kemet liquid diamond type 'K' fluids, of 25 μm, 8 μm, 3 μm, 1 μm grit size. The ground specimen had an average roughness Ra of

0.3 μm, and a mean peak roughness (average of 5 traverses of 0.8 mm) of 2.5 μm. The polished specimen had an average roughness Ra of 0.2 μm and a mean peak roughness of 0.3 μm.

1.2 Steel Encapsulation

A 6 mm square hole was cut into set of steel washers. These were placed in a steel tube to make a square mould into which two specimens were placed with their bonding faces (see Figure 1) in contact. The mould was evacuated to 10^"3 Pa and welded shut.

1.3 Self Encapsulation

Two specimens with their bonding faces in contact were joined together using cyano-acrylate adhesive. The bonded bars were placed in a furnace, standing with their longitudinal direction vertical (graphite furnace furniture was used to prevent them falling over). In some experiments the furnace was evacuated to between 10^"3 - 10^"4 Pa. In other experiments, heating was performed under an atmosphere of argon. The temperature was increased to 300°C and held (in the vacuum experiments) for 1 hour to re-establish a vacuum of at least 10^"3 Pa. The temperature was then increased at a rate of 200°C/hour to 1400°C (a temperature at which the cobalt phase is liquid) and held for 1 hour, after which it was allowed to air cool.

1.4 HIP Bonding

HIP bonding was carried under the conditions shown in Figure 2. These are not unique bonding conditions, there is a wide temperature and pressure window which will allow sufficient diffusion to cause bonding.

1.5 Bend Testing Bonded bars were ground flat on each side to 5 mm square, chamfered, and then each face was polished to a 1 μm finish.

The bond strength was evaluated using a 4-point bend test. These were carried out on an Instron 8600 series machine. Figure 3 shows a schematic of the bending jig. The bend strength (σ_bs) of the specimen is given by the expression:

_ 3Pa σbs bh² where P is the ultimate load before failure, a is the span, h and b being height and breadth of the bending specimen respectively. The values of the key parameters are a = 16 mm, b = 5 mm, d = 5mm. Each test was controlled in the 'Displacement' mode with the rate of 0.25 mm/min.

1.6 Results Results are shown in Figure 4:-

The bar entitled "Bulk" shows that the bulk material had a bend strength of just over 3 GPa.

The bar entitled "HIP (steel)" refers to ground and polished specimens which were subjected to steel encapsulation and HIP bonding. The bend strength was much weaker than the bulk material. The bar entitled "ASE" refers to ground and polished specimens that were subjected to self encapsulation in an argon atmosphere. The bend strength was again much less than of the bulk material. The bar entitled "ASE+HIP" refers to ground and polished specimens which were subjected to self encapsulation under an argon atmosphere followed by HIP bonding. The bend strength was only slightly less than of the bulk material, showing that self encapsulation under an argon atmosphere is an effective way of performing step a) of the method of this invention.

The bar entitled "VSE+HIP" refers to ground and polished specimens that were subjected to vacuum self encapsulation followed by HIP bonding. The bend strength of the bonded product was marginally greater than of the bulk material. The bar entitled "VSE^*+HIP" refers to ground but unpolished specimens which were subjected to vacuum self encapsulation followed by HIP bonding. The bend strength of the bonded product was equal to that produced by grinding and polishing the facing surfaces, and marginally greater than of the bulk material. The use of vacuum self encapsulation (not followed by HIP bonding) gave a result similar to that shown for argon self encapsulation.

EXAMPLE 2

The method of the invention has been used with success for bonding the approach and bearing sections of a floating plug for use in drawing copper tubing, wherein the approach and bearing sections of the floating plug have aligned ribs or grooves for imparting longitudinal grooves or ribs to the inner surface of the copper tubing. REFERENCES

1. "Physical and chemical nature of cemented carbides". H E Exner (1979) International Metals Reviews 4 pp.149.

2. "Densification mechanisms in the tungsten carbide-cobalt system". B Meredith and D R Milner (1976) Journal of Powder Metallurgy 1 pp.39.

3. "Tungsten Carbide Technologies". S Ragunathan, R Caron, J Friederichs, P Sandell )1996) Advanced Materials and Processes 149-4 pp. 21-23. 4. "WC-(Fe,Ni,C) hardmetals with improved toughness through isothermal heat treatments". R Gonzalez, J Echeberria, J M Sanchez, F Castro (1995) Journal of Materials Science 30 pp. 3435-3439.

5. "Diffusion Bonding of Materials". Ed N F Kazakov, Translated from Russian by Boris V Kuznetsov. Pergamon Press 1995. ISBN 0-08-032550-5.

6. "Review diffusion bonding of ceramics". O M Akelsen (1992) Journal of Materials Science 27 pp. 569-579.

7. "Strength improvement of cemented carbides by hot isostatic pressing (HIP). Ulf Engel and Heinz Hubner. (1978) Journal of Materials Science 13 pp. 2003-2012.

Claims

1. A method of bonding together two bodies, each composed of interconnected networks of a higher melting material and of a lower melting material, which method comprises a) heating the two bodies, while keeping them adjacent one another at a bond interface, to a temperature intermediate the melting points of the higher melting material and the lower melting material, whereby the bond interface between the two bodies is encapsulated, b) then subjecting the two bodies to hot isostatic pressing to bond them together.

2. A method as claimed in claim 1 , wherein each body is composed of a cermet.

3. A method as claimed in claim 1 or claim 2, wherein each body is composed of hardmetal.

4. A method as claimed in any one of claims 1 to 3, wherein the two bodies are heated in step a) under vacuum.

5. A method as claimed in any one of claims 1 to 4, wherein step b) is performed directly after step a) without intermediate cooling.

6. A method as claimed in any one of claims 1 to 5, wherein the two bodies are held together at the bond interface at the start of step a) by means of an adhesive.

7. A method as claimed in any one of claims 1 to 6, wherein the two bodies are approach and bearing sections of a floating plug for use in drawing copper tubing.

8. A method as claimed in claim 7, wherein the approach and bearing sections of the floating plug have aligned ribs or grooves for imparting longitudinal grooves or ribs to the inner surface of the copper tubing.