2747 tungsten carbide

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1 TUNGSTEN CARBIDE Filler Metals and Fluxes for Brazing Tungsten Carbide By Jack Willingham, Manager Quality & Technical Service Johnson Matthey Metal Joining This article reviews the development of the filler metals and fluxes used when brazing tungsten carbide. Metal Joining

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Filler Metals and Fluxes for brazing

Tungsten CarbideBy Jack Willingham, Manager Quality & Technical Service Johnson Matthey Metal Joining

This article reviews the development of the filler metals and fluxes used when brazing tungsten carbide.

Metal Joining

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Contents

Brazing filler metals - 3 The background

Brazing filler metals - 5 The historical perspective

Brazing filler metals - 9 The technical considerations

Brazing fluxes - 17 The technical considerations

Brazing filler metals - 22 The common applications

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Brazing filler metals - The backgroundMany different types of brazing filler metal are used to join sintered tungsten carbide to a supporting backing material. Although many different filler metal compositions are used they can be divided into two principal groups.

Group 1Copper and copper based filler metals including the copper-zinc (brazing brasses) and materials in the copper-zinc-nickel alloy system, (widely known as nickel-silver, and often used for bronze welding mild steel).

Group 2Low melting point brazing filler metals containing silver.

In addition to these two main groups there are a number of other precious metal filler metals that contain either gold or palladium that are used in some specialised applications where filler metals from the two main groups cannot meet the demands of the application.

Of the two main groups, the silver containing brazing filler metals are the ones most widely used, particularly those that have a melting point below 750˚C. Such filler metals are used not only because of their technical suitability, but also because they are easy to use due to their conveniently low brazing temperature.

A further advantage of the low temperature silver brazing filler metals is their compatibility with the induction heating process, one of the most commonly used heating methods in applications involving the joining of tungsten carbide to steel. This compatibility is as a result of the low temperature silver brazing filler metals having working temperatures below the Curie point of iron. At the Curie point (approximately 770˚C), iron changes from being a magnetic- to becoming a non-

magnetic material. This change results in a loss of heating efficiency, a reduction in the speed of heating and a need to increase the energy input to maintain the rise in temperature.

Functions of the brazing filler metalThe primary function of any brazing filler metal is to wet and bond with the parent materials to be joined, and to form a joint that is robust enough to withstand the loads that will be imposed upon it in service. When brazing tungsten carbide the brazing filler metal has a further function to perform. It needs to accommodate the stresses that develop in the joint during cooling, as a result of the significantly different coefficients of expansion that almost always exist between the tungsten carbide and the backing material to which it is brazed.

Wetting and bondingIn most applications involving the brazing of tungsten carbide, obtaining good wetting and bonding of the filler metal to the backing material is not normally a consideration. This is because the most commonly used backing material steel, is readily wetted by most brazing filler metals that are used to braze tungsten carbide. The ability of a brazing filler metal to wet and bond to tungsten carbide is somewhat more problematic and is a function of the composition of the tungsten carbide, namely:

1. Its cobalt content.2. The nature and level of any other

metallic carbide added to its composition other than tungsten carbide.

3. Whether it contains any free graphite / carbon on its surface or within the matrix of the tungsten carbide itself.

The primary function of any brazing filler metal is to wet and bond with the parent materials to be joined.

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Accommodating the inevitable post-braze stressIf the brazing filler metal is unable to accommodate the stresses that inevitably develop during the cooling of the joint, one or more of four outcomes are possible:

1. The part distorts– See Figure 1.2. The tungsten carbide cracks

– See Figure 2.3. The joint fails completely and the

tungsten carbide tip comes off – See Figure 3.

4. Nothing happens on initial cooling, but the tungsten carbide cracks or comes off in service or when the part is ground – See Figure 4.

Outcomes 1, 2 and 3 present themselves as obvious problems that immediately point to the fact that there is something wrong with the brazing process. Outcome 4, the ‘nothing happens’ situation is the most concerning, since unless the stresses that developed on cooling of the joint have been accommodated satisfactorily, the joints could contain a high level of residual stress. Any load applied to the joint during use or grinding increases the level of stress in the joint resulting in the tungsten carbide cracking or coming off. Tungsten carbide can generally be considered to be a brittle material, although this does depend upon its composition. It does not like

to be subject to tensile-, shear- or compound-bending stresses. In most applications involving the brazing of tungsten carbide, preventing it from cracking by accommodating the cooling stress is the primary consideration.

Figure 1: Example of a joint that distorted on cooling

Figure 3: Example of a joint where tip has come off during cooling

Figure 2: Example of a joint that cracked on cooling

Figure 4: Example of a joint that cracked during grinding

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Nickel and manganese enhance the filler metal’s wetting and bonding onto tungsten carbide.

It would appear that tungsten carbide as a material was first developed in the mid 1920’s. It seems that pure copper and 60/40 type brass brazing filler metals were the first filler metal types to be used to join tungsten carbide to a steel backing. In the mid to late 1930’s low temperature, cadmium- bearing, quaternary silver brazing filler metals had become available and also started to be used. Between 1940 and 1944 the first specialised silver brazing filler metal for the brazing of tungsten carbide appeared. This was the addition of 3% nickel being made to a low temperature, cadmium-bearing, quaternary silver brazing filler material. The nickel addition enhances the filler metal’s wetting and bonding capability onto tungsten carbide. It also produces a thicker layer of brazing filler metal within the joint, which helps to accommodate the cooling stresses. Nickel is now a common addition made to the specialised tungsten carbide brazing filler metals.

Manganese is another element commonly present in many brazing filler metals used for brazing tungsten carbide since it also improves wetting and bonding. Again it would appear, that while a filler metal with a composition of 65% silver- 28% copper- 5% manganese and 2% nickel existed in 1948, its use would have been in applications involving elevated temperature service, not for the brazing of tungsten carbide. One reference from 1953 indicates the use of ‘high silver-content brazing filler materials containing manganese’ as they have ‘added readiness to wet certain types of carbide’. Whether this is a reference to the 68% silver-content filler metal, or to the 15% manganese – silver alloy is not clear. In the 1st edition of the AWS Brazing Manual of 1955, the 15% manganese-silver alloy is mentioned for use in applications where subsequent heat treatment of the backing material is required. In the early- to mid- 1960’s a cadmium-bearing quaternary based filler metal containing both

nickel and manganese came onto the market. This filler metal was used initially for the brazing of rock-drills before being adopted more widely for the brazing of tungsten carbide. Shortly after the appearance of the cadmium-bearing filler metal with additions of nickel and manganese, a ternary silver-copper-zinc alloy, with additions of nickel and manganese, was also being used to braze rock-drills. Both these filler metals were subsequently adopted more widely for the brazing of tungsten carbide, with the 49% silver-copper-zinc-nickel-manganese now being one of the most commonly used filler metals for the brazing of tungsten carbide.

Running parallel with the development of the silver-base filler metals were the copper-base filler metals. The historical information available concerning the development of the copper-base filler metals is much less abundant than for those with a silver-base. Most of the filler metals seem to have been developed to meet the requirements of rock-drill brazing. The sketchy nature of the information freely available is, perhaps, not surprising, since the various companies involved in the manufacture of rock-drills between the 1960’s and 1980’s were very secretive about their product development. The main reason for the development of the copper-base filler metals arose from the need to replace the high silver content filler metals in order to reduce material costs. (At that time the 49 to 50% silver-content were the favoured brazing filler metals). There are also indications that in some cases the silver containing filler metals were being overheated in order to allow for simultaneous heat treatment of the shank material. The overheating above 750˚C would result in some deterioration of the brazing filler metals due to the vaporisation of cadmium and zinc. In other cases the inference is that the silver-base filler metals were used because of their low brazing temperatures. This feature, together with rapid induction heating, allowed the brazing of fully

Brazing filler metals - The historical perspective

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heat-treated shanks, as the short duration of the brazing operation only had a minimal effect on the already heat treated properties of the shank material.

As already mentioned, pure copper was one of the first brazing filler metals used to braze tungsten carbide, although it has not been possible to uncover any evidence that pure copper was used to braze rock-drills. Some evidence suggests that copper nickel filler metals were used and certainly prior to 1967 a 96.9% copper-2.5% nickel-0.6% silicon alloy, widely known as Corson Bronze, was in extensive use. Since this material already existed as a precipitation hardenable copper-base filler metal it seems likely that brazing tests carried out with it showed that it worked acceptably. The high brazing temperature and properties of this filler metal, while not ideal, did allow for simultaneous brazing and air hardening of the shank material, followed by a lower temperature tempering heat treatment. As indicated, the copper-nickel-silicon filler metal is of a composition that can be subject to precipitation hardening and there are some suggestions that the high strength and hardness of the filler metal prolonged the life of drills used in certain drilling environments. While the copper-nickel-silicon filler metal was reasonably successful, it was not ideal. It was found to be ‘hot short’ on cooling, and go through a ductile, brittle transition at around 600˚C. On larger diameter drills this was known to cause internal cracking of the filler metal due to the stresses that developed during post-braze cooling of the joint, and this resulted in premature service failure of the drills due to fatigue.

The failings of the copper-nickel-silicon filler metal prompted the development of copper-manganese- cobalt filler metals. Their lower brazing temperatures reduced the amount of deterioration suffered by the shank material as a result of the high brazing temperatures of

the copper- nickel-silicon filler material, while also helping to minimise the cooling stress. The manganese in the filler metal acted as a melting point depressant while also providing good filler metal wetting. Cobalt was added to the filler metal composition, as opposed to nickel, as it appeared that cobalt interalloyed with the cobalt-bonded matrix of the tungsten carbide more freely than nickel did. In the finished braze joint, the cobalt is present as a fine cobalt rich phase dispersed throughout the filler metal, and this is said to produce joints with a high resistance to fatigue failure together with a good level of ductility.

While the copper-manganese-cobalt filler metal overcame many of the shortcomings of the copper- nickel-silicon filler metal, ideally the rock-drill manufacturers were seeking inexpensive brazing filler materials with brazing temperatures as close to 850˚C as possible. Brazing temperatures around 850˚C more closely match the heat treatment temperature of the steel shank and minimise the cooling stresses. This requirement eventually resulted in the early 1970’s in the development of a brass type brazing filler metal with additions of manganese and cobalt. The filler metal produced joints, which greatly extended the life of drills compared to those that had been brazed with the previously used types of copper-base filler metal.

A further important development in the field of brazing tungsten carbide was the ‘sandwich’ or tri-metal product. These filler materials have a central core of material metallurgically bonded on either side with a layer of brazing filler material. In the early 1950’s it was common practice when brazing large sections of carbide to form these types of product in situ. Either a plain sheet of material, woven soft iron wire gauze or a corrugated metal foil was used to thicken the brazed joint. The joint was prepared by first laying a section of fluxed brazing filler metal in foil form onto the joint area, then the

Rock drills

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interlayer material, and finally by another piece of fluxed brazing filler metal foil on top. The reason for thickening the joint by introducing a spacing layer was to allow the joint to deform more readily such that it could dissipate the greater levels of cooling stresses that occur when brazing large sections of carbide.

It has not been possible to establish precisely when purpose made tri-metal products, where the brazing filler metal had been pre-bonded to the interlayer, first became commercially available. However, the 1st Edition of the AWS Brazing Manual from 1955 states that “A special brazing sheet is available for making sandwich brazes. This consists of a copper core clad on both sides with a thin layer of nickel-bearing BAg filler metal.” By the mid 1960’s such products were in common use. Typically, the product consisted of a copper or copper, nickel interlayer, with a silver-base brazing filler metal bonded on either side. Some of the high temperature copper-base filler metals were also available as tri-metal products, with gauze woven from nickel wire being used as the interlayer material.

Today copper in the form of a solid sheet of material is most commonly used as the core or interlayer material in most tri-metal products. Copper is used not only because it is soft and ductile and can deform easily to dissipate the cooling stresses, but also because it makes for easy bonding of the brazing filler metal layers during production of the products. The common ratio of the layers in the products is 1:2:1. For example, in a 0.4 mm thick product the first layer of brazing filler metal will be 0.1 mm thick, followed by the interlayer with a thickness of 0.2 mm and then another layer of filler metal 0.1 mm thick. Other ratios with a thicker interlayer or thicker brazing filler metal layers are also available. Recently there has been a re-emergence of tri-metal products with copper alloy cores. Products with a copper-nickel core had traditionally

been available up until the later 1970’s, early 1980’s, but they have now started to re-appear with cores described as ‘copper alloy’. The products would seem to offer joints with higher shear strengths, as the copper alloy core will be stronger than products using the more common ‘pure copper’ core. This additional strength is seen as being advantageous in applications where carbide tips are subject to high impact service loads.

It is also possible to obtain what are commonly referred to as ‘tri-metal’ brazing pastes. This description is used to describe brazing pastes that produce joints with a thick layer of filler metal, similar to the tri-metal products by introducing into the paste formulation a metallic powder that is not easily dissolved and taken into solution when the brazing filler is molten. Typically nickel powder is used for this.

Circular saw blade

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The most recently developed filler metals are silver containing, and are free from both zinc and cadmium. These have been developed for use when the tungsten carbide and its backing are to be coated using a Physical Vapour Deposition (PVD) process to apply TiN (titanium nitride) and similar types of coatings (See Figure 5). Under the coating conditions, if present, the cadmium and zinc will tend to vaporise out of the brazing filler metal affecting the coating process. They act as melting point depressants within the filler metals and while it is possible to have silver-copper-nickel-manganese filler metals without these volatile elements, their brazing temperatures are quite high. Filler metals with lower brazing temperatures, but still containing the beneficial elements nickel and/or manganese, have been produced where indium or tin have been added as melting

point depressants. (See Argo-braze® 64 and Argo-braze® 57 in Table 4). While both tin and indium are low melting point elements they have high vaporisation temperatures.

The above comments cannot be considered all inclusive, but they do at least chart the major filler metal product developments made in connection with the need to braze tungsten carbide while attempting to attribute some approximate dates and chronological order to those developments.

It is interesting to note that most of the brazing filler metals and products specially developed for the brazing of tungsten carbide are still being used today. However, the use of the cadmium-bearing filler metals is in major decline due to the health and safety issues that arise from their use and environmental pressures.

Figure 5: Titanium nitride (TiN) coated gun drill

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Filler metal wetting and bondingIt has already been highlighted, that obtaining good wetting and bonding of a brazing filler metal onto tungsten carbide is not as straightforward a proposition as it can be for many other materials. Where the cobalt content of the tungsten carbide is above approximately 6%, both the common quaternary silver-copper-zinc-cadmium and the ternary silver-copper-zinc filler metals will wet and bond with tungsten carbide relatively easily. However, wetting becomes more difficult when other metallic carbides are added to the carbide, particularly the refractory metal carbides of titanium and tantalum. In such cases levels of only 1 to 2% have very serious effects on filler metal wetting. Free graphite or carbon within the matrix of the tungsten carbide can also impair filler metal wetting.

It is apparent from the historical perspective, that improved wetting and bonding of the filler metals for use when brazing tungsten carbide has been one of the main development themes. We have seen that nickel was the first element to be added to the silver brazing

alloys in an attempt to improve wetting and bonding. Nickel is known to change the nature of the bond that is created from one of an intermetallic nature to that of an interalloying type. Cobalt is also known to have a similar effect. Manganese is another element that has been found to improve the wetting and bonding of filler metals. Filler metals containing manganese are particularly useful when brazing carbides that contain additions of titanium and tantalum carbides, as they have a much greater ability to wet such carbides than filler metals that contain only nickel. Manganese also seems to have the ability to wet surfaces containing graphite, so again filler metals that contain manganese show improved wetting on those tungsten carbides that contain free graphite / carbon on their surfaces or within their matrix. Nickel and manganese also improve filler metal wetting and bonding on tungsten carbides having low cobalt contents.

It has not been possible to find information that suggests that any real scientific or metallurgical thought processes (at least in the early development of the specialised

Brazing filler metals - The technical considerations

Figure 6: Colour comparison AWS A5.8 B-Ag24 manganese free and EN 1044 AG502 manganese containing filler metals

Nickel and manganese also improve filler metal wetting and bonding on tungsten carbides having low cobalt contents.

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brazing filler metals for use on tungsten carbide) existed in relation to the additions of nickel and subsequently manganese. It would seem that their beneficial effects were established on an empirical basis early on and this information was then used to formulate subsequent filler metal compositions.

When selecting a brazing filler metal for a tungsten carbide brazing operation it is established practice to use filler metals that at least have an addition of nickel or better still additions of nickel and manganese. However, some resistance does exist to the use of the manganese bearing filler metals. This is sometimes as a result of the surface finish of the filler metal that can have a rough and dark reddish brown appearance (See Figure 6). Some operators dislike these filler metals, as they are more ‘sticky’ than those that are manganese-free. In many tungsten carbide brazing operations an operator is required to move the tungsten carbide tip around when

the brazing filler metal is in its molten state to help remove any flux and gas trapped in the joint before finally positioning the tip in its correct location. The term ‘sticky’ is a reference to the fact that the tungsten carbide tips do not float and move so easily on the molten brazing filler metal, so making it difficult for the operator to move and position them. Some do not like the nickel bearing filler metals either, since they do not flow as well as the straight quaternary silver-copper-zinc-cadmium or the cadmium-free silver-copper-zinc-tin filler metals.

It is easy to check how a particular filler metal and flux combination will wet onto a piece of tungsten carbide. Simply clean the face of the tungsten carbide piece that would be brazed in the normal way. Flux the surface, and then cut a section of brazing filler metal foil, rod or wire and place it on top of the fluxed surface of the tungsten carbide. Finally apply more flux over the top of the cut section of filler

Figure 7: Example of a wetting test

Position 1. EN1044 AG305 & standard flux EN1045 Type FH10

Position 2. EN1044 AG103 & boron modified flux EN1045 Type FH12

Position 3. A5.8 B-Ag24 & standard flux EN1045 Type FH10

Position 4. EN 1044 AG103 & standard flux EN1045 Type FH10

Most specialist tungsten carbide brazing filler metals contain at least an addition of nickel, and many contain additions of both nickel and manganese or cobalt and manganese.

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metal. It is then simply a question of heating the tungsten carbide until the filler metal melts. Once the piece of tungsten carbide has cooled down remove the flux residues. A visual assessment shows how well the filler metal has wetted and spread. By using different filler metal and flux combinations it would be possible to determine which combination gives the best results for the carbide in question. (See Figure 7)

This very basic test can throw up issues when the pieces of tungsten carbide start coming off rather than finding there is a problem when a whole batch of parts has been brazed and is in use. By using a controlled size and shape of filler metal this standard wetting test can be used as a pre-production check on batches of tungsten carbide. It can also be used to check the effectiveness of the pre-braze cleaning of the tungsten carbide.

Accommodating the cooling stressesAs previously mentioned, due to the significant mismatch in expansion coefficients that typically exist between the tungsten carbide and the backing material to which it is brazed, whichever brazing filler metal is used it needs to be able to accommodate the stresses that develop within the joints on cooling,

When cooling a joint, the backing material wants to contract faster and further than the piece of tungsten carbide to which it is now securely brazed. (See Figure 8) This mismatch results in the development of shear stresses within the joint. If the brazing filler metal has a high strength it will transmit these stresses directly into the relatively brittle tungsten carbide and cause it to crack. What is required is brazing filler metal that has a low yield point that will deform plastically allowing the stress to be dissipated. Unfortunately, the strength

Figure 8: Diagram depicting mismatch in expansion between a tungsten carbide tip and its support backing at brazing temperature

Expansion of Tungsten Carbide

Expansion of Backing

Tungsten Carbide

Backing of Material

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of brazing filler metal in a brazed joint is not primarily a function of the strength of that filler metal, but the strength of the parent materials and the thickness of the brazing filler metal layer within the joint.

Practical testing has shown that if, for example, an ordinary carbon steel and high strength carbon steel are brazed with the same brazing filler metal, the ordinary carbon steel joint will be weaker than that made in the high strength steel (See Figure 9). It is also understood that the thickness of brazing filler metal in a joint has a strong influence on the strength of a brazed joint - the thinner the joint the higher its strength. It is possible to develop joint strengths that are 2 to 3 times greater than that of the ‘as cast’ strength of

the brazing filler material.

The situation that is found in a typical brazed joint is that there is a thin layer of a relatively weak, ductile, filler metal bonded securely to two much stronger, less ductile parent materials. When such a joint is subjected to a tensile load, if the load applied to the joint is above the yield point of the brazing filler metal, then the filler metal would be expected to begin to deform in the same manner as a tensile test specimen by ‘necking in’. (See Figure 10) However, the thin layer of filler metal is securely bonded to the two parent materials, which prevents it from doing so. Brazed joints therefore typically fail under tri-axial stress conditions and show very little ductile deformation, failing in an almost brittle mode.

Figure 9: Results of tensile tests showing how joint clearance and the strength of the parent materials affects joint strength.

U.T.S of Drill Rod

Tens

ile S

tren

gth

Kgf/

mm

2

U.T.S of 1020 Steel

U.T.S of Pure Silver

84

70

56

42

28

14

0

U.T.S of Drill Rod

U.T.S of 1020 Steel

U.T.S of Pure Copper

Joint Thickness mm

0 0.127

a

AA

B

B

E

E

FF

GG

H

H

IC

CD D

b

0.254 0.267 0.508 0.635

Joint Thickness mm

0 0.127 0.254 0.267 0.508 0.635

The effect of joint gap (joint thickness) on the tensile strength of brazed butt joints (after Bredzs). The strength of silver (a) and copper (b) brazed joints in steel 1020 is represented by curves ABCD, curves EFGH relating to joints brazed in drill rod steel. The U.T.S. of silver, copper and parent metals is indicated in the graphs

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Taking the above into account, the normal perception that a strong joint is required when brazing tungsten carbide could be questioned. It can be understood that if too strong a joint is produced it will not be able to deform to dissipate the cooling stresses. If the filler metal is too strong it will simply transmit the stress into the component causing it to distort or the piece of tungsten carbide to crack to relieve the stresses that have developed. If the levels of stress developed during cooling are extremely high, and the component sufficiently robust such that it cannot distort or crack, the brazing filler metal itself can rupture resulting in the piece of tungsten carbide becoming detached. It is also possible in circumstances where the component is sufficiently robust for the stresses developed on cooling to be contained as residual stresses within the component. The residual stresses locked up in the joint only show themselves when some additional stress is added to the component by the application of some external force,

either a physical mechanical load or thermally generated stresses.

It follows from the preceding text, that when brazing tungsten carbide, joints with a good level of ductility are required, such that they can deform readily and allow the stresses that arise on cooling to be dissipated. It also follows, that strong joints, where only a thin layer of brazing filler metal is present between the tungsten carbide and its backing material, will not allow the brazing filler metal to deform in a ductile fashion, and therefore will not allow dissipation of the cooling stresses. Joints that have a thick layer of brazing filler metal between the tungsten carbide and the backing material will be more able to deform and therefore allow dissipation of the cooling stresses. In essence ductile joints are required, which usually means thick joints, which in turn means joints are going to be lower in strength than might be the case for other joints.

Figure 10: Results from Tensile Loading

Necking under tensile load

Necking constrained by bond to parent material

‘…when brazing tungsten carbide, joints with a good level of ductility are required, such that they can deform readily and allow the stresses that arise on cooling to be dissipated.’

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The general rules

1.

2.

For pieces of tungsten carbide where the maximum dimension does not exceed 10 mm the use of conventional, ductile silver brazing filler metals like EN 1044 AG102, 103 (56 & 55% silver cadmium-free with tin) and 301, 302 and 303 (50, 45 & 42% silver with cadmium) can be used (See Tables 1 & 2).

These free flowing filler metals that are relatively ductile tend to produce joints containing a relatively thin layer of brazing filler metal, which limits the size of carbides

that can be brazed successfully. A further advantage of these filler metals is their low brazing temperatures, which help to minimise the amount of differential expansion developed between the tungsten carbide and the backing material. The high silver content cadmium-bearing filler metals tend to have higher levels of ductility than the cadmium-free filler metals, and therefore may be capable of brazing a wider range of sizes and shapes of tungsten carbide than cadmium-free filler metals.

For pieces of carbide where the maximum dimension is between 10 and 20 mm, the use of the nickel or nickel and manganese bearing filler metals is recommended. EN1044 filler metals AG502, 351 and AWS A5.8 BAg-24 (See Table 3)

The addition of nickel to these filler metals reduces their flow properties and helps them to produce joints that have a naturally thicker layer of brazing filler metal than the standard filler metals. Because of the reduced flow properties of these filler metals they are best pre-placed in the joint as a section of foil as opposed to being hand fed into the joint from wire or

rod. Pre-placing the filler metal in the joint allows it to produce the thickest possible joints, so allowing the filler metals to be used to their full potential.

The additions of nickel and manganese to the filler metals increase their ‘as cast’ strengths, which to some extent are offset by the thicker joints they produce. In a few cases problems have been encountered when changing from the cadmium-bearing filler metals with additions of nickel and manganese to the cadmium-free filler metals containing nickel and manganese. While there had been no problems using the cadmium-bearing filler metals, with the change to cadmium-free materials

From the need to produce ductile joints three approaches in relation to the selection of brazing filler metal for applications involving the brazing of tungsten carbide have evolved:

These three approaches are particularly applied in the case of selecting silver-base filler metals, where some general rules have been developed, based on the size of carbide that can be brazed successfully.

1. The use of filler metals that possess a high level of ductility.

2. The use of filler metals that produce joints with thicker than normal layers of filler metal.

3. The use of products or methods that produce artificially thick brazed joints.

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cracking problems started to occur. These problems tended to be in applications that were at the extreme of the capabilities of the cadmium-bearing filler metals. It was clear that the filler metal, EN 1044 AG502 (49% silver with nickel and manganese) was stronger and less ductile than the cadmium-bearing filler metals previously used.

While the AG502 filler metal with its high levels of nickel and manganese is the filler metal most widely used for brazing tungsten carbide in Europe and other

parts of the world, the AWS A5.8 BAg-24 filler metal (50% silver-copper-zinc-nickel) is equally common in the United States. As previously mentioned the nickel and manganese bearing AG502 filler metal is disliked by some due to the colour of the finished braze - a dark red brown, and it is ‘sticky’ making it difficult to move and position pieces of tungsten carbide. The BAg-24 filler metal does not contain manganese and therefore does not exhibit these characteristics, and as a result some users in Europe are now favouring it.

Where the largest dimension of the tungsten carbide is greater than 20 mm then the use of the tri-metal products is recommended.

The interlayer in the tri-metal products produces a thick, ductile joint capable of dissipating significant amounts of stress. The tri-metal products come in various thicknesses; the thickness used in any application being a function of the size of the piece of tungsten carbide. i.e the larger the piece of tungsten carbide, the greater the thickness of tri-metal. In the past, the tri-metal products were coated with the cadmium containing filler metals, EN 1044 AG301 or the nickel containing AG351. By far the most commonly used product today is the cadmium-free 49% silver-copper-zinc alloy containing additions of nickel and manganese. The filler metal used is a modified version of the EN 1044 AG502, where the nickel and manganese

contents have been significantly reduced. The reason why this modified filler metal is used, as opposed to high nickel and manganese AG502 filler metal is to facilitate manufacture of the product. When the need arose for cadmium-free tri-metal products, attempts were first made to use the standard AG502 filler metal. However, problems with de-lamination of the layers and edge cracking during rolling (due to the different work hardening characteristics of the copper interlayer and the filler metal) resulted in the development of the modified filler metal.

As previously mentioned, the manganese containing filler metals are disliked by some due to the colour of the finished braze and their stickiness. Tri-metals using the manganese free AWS A5.8 BAg-24 are available and are now preferred in some cases.

2.

3.

The above can only be seen as general guidelines to the selection of a filler metal, as they are based only on the size of the piece of tungsten carbide. Another factor that also needs to be considered is the ductility of the tungsten carbide. This is a function of the cobalt content of the tungsten carbide,

the higher it is the more ductile it will be. The thickness of the tungsten carbide is also important as the thicker it is the more robust it will be. The shape of the tip is also important, since if it has a complex shape with changes in width, thickness and includes sharp corners, as for example in certain router bits,

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then these changes in form can be points of stress concentration, increasing the likelihood of the tungsten carbide cracking. These additional factors may mean that although it is possible to braze the piece of tungsten carbide based simply on its size with say a standard ductile filler metal, due to its shape it would be prudent to use a nickel bearing filler metal that produces a thicker joint with more stress dissipating capacity.

A tri-metal product could be used, although the size of the piece of tungsten carbide does not really warrant its use, in applications where the tungsten carbide in use experiences impact or percussive loads. In these applications the tri-metal can act to cushion the loads applied to the tungsten carbide and stop it from cracking. In some applications, while it prevents cracking of the tungsten carbide, deformation of the interlayer under the service loads can result in cracking problems or joint failures. In such circumstances the use of a tri-metal product with a copper alloy interlayer,

as opposed to a pure copper one, is likely to prove beneficial.

In addition to using specific brazing filler metals or products as an aid to dissipating the cooling stresses, slow controlled cooling of joints following brazing is important. Also of importance are those methods that seek to produce thick layers of brazing filler metal between the tungsten carbide and its backing material. As already mentioned, joints were historically thickened by using a layer of woven wire mesh or foil and while tri-metal products are commercially available, the ‘do it yourself approach’ could be considered if a suitable commercial product is not available. Simple spacer wires can also be used to provide a controlled and thick joint clearance. Raised bars or pips formed on either the tungsten carbide or the backing can be another way in which to produce a suitably large, yet controlled, joint clearance.

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The vast majority of joints made between tungsten carbide and a backing material are made in air using flame- or induction heating. This means that the filler metal needs to be used in conjunction with a suitable flux.

1. The flux must be able to remove the oxides found and formed on the parent materials and the filler metal during the brazing operation.

2. Its active range should be compatible with the brazing temperature of the filler metal.

3. The flux should become active at least 50˚C below the filler metal’s solidus temperature.

4. The flux should have a molten viscosity such that it can be easily displaced from the joint by the molten filler metal at the brazing temperature.

5. It should have sufficient time / stability to survive the brazing operation.

Removing the oxides from the parent materials and filler metalIt is perhaps obvious that a flux should remove the oxides found and formed on the parent materials. However, not all fluxes are capable of removing all oxides. For example, traditional borax, boric acid type fluxes are not good at removing nickel, zinc or refractory metal oxides.

Most modern fluoroborate containing fluxes are very good at removing most oxide types and are used in most tungsten carbide brazing applications quite successfully. However, the standard fluxes still find it difficult to deal with refractory metal oxides, and where titanium and tantalum carbides are present within the formulation of the tungsten carbide then the use of the special boron-modified fluxes is recommended. These fluxes are recognisable by their brown or dark brown to almost black colour. The boron-modified fluxes contain as part of their formulation elemental boron powder. The addition of the elemental boron not only changes their colour, but also makes them highly active against refractory

Figure 11: Lathe tool coated with a boron modified EN1045 Type FH12 flux ready for brazing

Brazing fluxes - The technical considerations

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oxides. There is also some practical evidence that boron containing brazing fluxes aid the wetting of filler metals where the parent materials contain some free graphite, which as previously explained can be present in some tungsten carbides. Tungsten carbides with low cobalt contents also exhibit improved wetting when brazed using boron-modified fluxes. One criticism of the boron-modified fluxes is that their dark colour prevents operators from seeing what is happening during the brazing operation, masking the flow of the filler metal. Many of those engaged in the brazing of tungsten carbide will use nothing other than the boron-modified fluxes. The combination of a nickel and manganese bearing filler metal together with a boron-modified flux will give the best wetting and bonding performance whether the carbide is straightforward tungsten carbide, a carbide with a low cobalt content, one that contains additions of titanium and tantalum carbide or one where free graphite is present.

Compatible active rangeIt is important when selecting a flux that the active range of the flux is compatible with the brazing temperature of the filler metal. The flux needs to be active and removing the oxides on the parent materials, and any filler metal pre-placed in the joint, before the filler metal begins to melt. Likewise, using a flux where the filler metal melts beyond the stated active range of the flux is likely to prove problematic, because it will tend to end up in a burnt and blackened condition and incapable of removing any oxides. Typical examples of such a mismatch are the use of the borax, boric acid based fluxes designed for use with the brass brazing filler metals, with the low temperature silver brazing filler metals. The borax boric acid based fluxes are not active much below 750˚C, which is of course well above the melting point of silver brazing alloys. The reverse of this situation also applies, that is the use of the fluoroborate based fluxes formulated for

use with the silver brazing filler metals being used with the brass brazing filler metals. The upper active range of the silver brazing filler metals’ fluxes tends to be in the order of 800˚C, but the brass brazing filler metals have brazing temperatures around 900˚C.

The active temperature range of a flux for best practice resultsAs already mentioned, in a brazing operation a flux needs to be active and removing the oxides from the parent materials and any filler metal pre-placed in the joint before the filler metal melts, and maintain its activity until the filler metal has flowed and made the joint. In practice, it has been found that the flux needs to become active and efficiently removing oxides at a temperature at least 50˚C below the filler metal’s solidus temperature, (the temperature at which it starts to melt), and remain active at a temperature which is at least 50˚C above the liquidus temperature, (the temperature where the filler material becomes completely liquid). The physical melting of a flux does not necessarily indicate that it has become active, or at least active enough to start removing oxides efficiently. The requirement for the flux to become active at a temperature 50˚C below the solidus of the filler metal is to allow the flux enough time to remove those oxides that have formed during the heating of the joint from room temperature to the brazing temperature.

The joint will be subject to oxidation from the onset of heating until the flux becomes active. This will mean that for typical fluxes used with a silver brazing filler material the joint will have already attained a temperature of about 550˚C before the flux becomes active. Clearly, the oxidation that has occurred has to be removed before brazing can occur. Consequently, when the flux becomes active it will begin to remove the oxidation that has built up during the pre-heating stage. Naturally, time is needed to do this, and if insufficient is provided the oxide

It is commonly understood that fluxes remove the oxides from the parent material and then prevent further oxidation from taking place. However, this is not strictly the case.

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layer that remains on the work when the filler material melts will impede the wetting and flow of the molten filler material.

Viscosity of a molten flux It is important that when the brazing filler metal attains brazing temperature the viscosity of the molten flux is such that that it can be easily displaced from the joint by the advancing front of the molten filler material. If the viscosity of the flux is too high at brazing temperature, and the filler metal is unable to displace it the flux will almost certainly become trapped in the joint. This type of problem can result in joints that contain a high level of flux inclusions. Flux manufacturers formulate their fluxes with this problem in mind. However, there may be technical circumstances where due to the size and mass of the parts a flux with good time-temperature stability is being used with a low melting point filler metal. While the flux may have good time-temperature stability, its molten viscosity at the lower end of its active range may be such that it cannot be easily displaced by the molten filler metal. Molten viscosity curves for fluxes are not linear; thus it might be that at 650˚C the flux is quite viscous, whereas at 700 it is very fluid. Every flux is different, so in cases where joints exhibit a high level of flux inclusions this is one factor to be considered when trying to reduce the occurrence of the problem.

The viscosity of a flux will also change during a brazing operation due to the absorption of oxides. Changes in viscosity tend to become an issue where a flux is being used at the upper end of its active range or where it has been subjected to prolonged heating. This situation may suggest that a flux with a higher active range or with greater time-temperature stability should be used.

Time-temperature stabilityIt is commonly understood that fluxes remove the oxides from the parent material and then prevent further oxidation from taking place. However, this is not strictly the case. While fluxes do remove oxides from the parent materials they do not prevent further oxidation of the parent materials taking place. In fact there is a continual process during a brazing operation of oxygen diffusing through the layer of molten flux, oxidising the parent materials beneath and the flux removing the oxide that has been formed. Molten fluxes do tend to act as a partial barrier to the diffusion of oxygen, but this is a variable characteristic dependent upon the flux formulation and its temperature.

This continual process of oxygen diffusing through the molten flux and the flux removing the oxide that has been formed eventually results in the flux becoming spent / exhausted, as it becomes saturated with oxides and so unable to remove any more. Clearly, if the flux has become spent or is losing its ability to remove oxides efficiently before the filler metal is molten, the filler metal will not wet the parent materials successfully.

The time-temperature stability of a flux is not something that can be defined numerically, but is rather a concept to explain a characteristic of a flux. For example, if a standard flux with an active range of 550 to 800oC is used with a silver brazing filler metal with a brazing temperature of 750oC, then immediately the joint starts to be heated the parent materials will begin to oxidise. When the flux becomes active at 550˚C it starts to remove the oxides that have formed up to that point, and more are still continuing to form. The phase is then reached where oxygen diffuses through the molten flux, oxidation of the parent material continues, and the flux continues to remove the oxides as they form.

This continual process of oxygen diffusing through the molten flux and the flux removing the oxide that has been formed eventually results in the flux becoming spent / exhausted, as it becomes saturated with oxides and so unable to remove any more.

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It can be understood that more oxide will be formed if it takes 5 minutes to get to the brazing temperature of 750˚C as opposed to 2 minutes. If after 4 minutes, the flux has become loaded with oxide and has become exhausted, then when the brazing filler metal melts the surface will not be oxide-free and will not wet successfully. In such an example, the use of a filler metal with a brazing temperature of 650˚C would be possible, assuming that the joint could be heated to 650˚C in less than 4 minutes.

The speed at which a flux will become exhausted is a function of how much oxide is formed during a brazing operation. If the thermal masses of the parts to be heated are significant and it takes an extended period to achieve the brazing temperature then the flux will be required to remove oxides from the parent materials over an extended period. It can be appreciated that heating a joint may not involve a linear temperature rise. For example, while it might be possible to heat a component to a temperature of 500˚C in two minutes, it may take a further 4 minutes to achieve a temperature of 750˚C. Furthermore, since oxide formation is a chemical reaction, the higher the temperature the faster it goes. In simple terms this means that a flux when used at the lower end of its active range will have a longer life than at the higher end of its temperature range. In practical terms this means that while a flux with an upper active range of 800˚C could be used to make a joint with a filler metal melting at 800˚C, there would only be seconds available to make the joint. This might be possible for small parts using HF induction heating. In general, a flux with an upper active range of 800˚C should not be considered for use with filler metals melting above 750˚C. Applying this restriction means that the flux will still have some useful life available when the filler material melts and flows.

Flux selectionUnlike brazing filler metals there are no standard flux compositions. Fluxes are all made to proprietary formulations and will therefore all have different properties and characteristics. EN 1045 Brazing “Fluxes for brazing – Classification and technical delivery conditions” is, as it indicates, a means by which manufacturers can apply some standard classification to their flux products, but the standard does not go beyond specifying some basic flux characteristics and uses. In manufacturers’ literature, where reference is made to EN1045, many fluxes are given the same classification, yet they clearly have different properties and characteristics.

Unlike a brazing filler metal, where it is an easy matter to conduct a chemical analysis to determine its make up, it is not possible to do this with fluxes. A chemical analysis will not be able to determine the specific chemical compounds that have been used to formulate the flux, as many of the compounds react during manufacture, and it is how they are proportioned and reacted together that determines the components, properties and characteristics of the finished flux. In selecting a flux for any particular application the above points need to be considered. Most manufacturers provide information in their literature about the active range of the flux and give some indication about its life or overheat resistance. There may also be additional information related to some special characteristics of the flux, for example its suitability of use when using HF induction heating. This basic information should allow anyone to select a flux suitable for an application. However, a discussion with the flux manufacturer is likely to be beneficial as they will be aware of the subtle differences in their products and may, based on the details of the application, be able to suggest a flux that offers specific advantages in that application. Rather than just selecting one flux

Unlike brazing filler metals there are no standard flux compositions. Fluxes are all made to proprietary formulations and will therefore all have different properties and characteristics.

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from a manufacturer’s range, it could well be worth testing two or three that appear to be suitable, to see which provides the best ‘on-the-job’ performance.

The fact that fluxes are proprietary formulations often causes problems if one wants to change from one manufacturer’s product to another’s. Operators will say that the flux does not work as well. This could be the case, but in many cases what the operator is really saying is that it works differently or perhaps more likely that it reacts differently when heated. This is to be expected, as each formulation will result in a flux with different characteristics. What must be assessed is whether the differences are good, bad or indifferent, and whether the joints produced are of an acceptable quality.

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The cadmium-bearing filler metals were for many years the principle filler metals employed for the brazing of tungsten carbide in a wide range of applications from lathe tools to rock-drills.

The use of cadmium-bearing filler metals is in decline due to the health and safety implications associated with their use and environmental and product stewardship issues. Indeed in many countries of the EU, cadmium-bearing filler metals are no longer being used. EU Directives and legislation have already effectively banned the use of cadmium-bearing filler metals in any automotive application and in the manufacture of any electrical or electronic equipment, since a maximum cadmium level of 0.01% has been imposed on any materials used in the manufacture of such items. It has to be considered how long it will be before other EU Directives or legislation prevent their use in other applications or completely bans the sale of cadmium-bearing filler metals. In addition to the Directives and legislation, the growing interest by major companies in the subject of ‘Product Stewardship’ is leading to individual companies applying their own restrictions based upon the Directives and legislation already in place. As a supplier or manufacturer it may, for example, not be possible to supply a company with a cutting tool that has been brazed with a cadmium-bearing filler metal.

In view of the above it would seem prudent to select a cadmium-free brazing filler metal for use in any tungsten carbide application, and to phase out the use of cadmium-bearing filler metals as soon as possible before EU Directives, legislation or individual companies prevent their use.

The two main filler metals used from this group are Easy-flo® and Easy-flo® No. 3 in Table 1. Easy-flo® is a filler metal that possesses a level of ductility that is around 35% in the ‘as-cast’ condition. Mattibraze™ 45, Easy-flo® No. 2 and DIN Argo-flo™, possess a lower ductility, the range extending from 20-25% for Mattibraze™ 45 and Easy-flo® No. 2 with DIN Argo-flo™ being only 15%. As a result, Easy-flo®, Mattibraze™ 45 and Easy-flo® No. 2 represent the first choice filler metals for brazing most of the easily wetted types of tungsten carbide, and where the largest dimension of the carbide is less than 10mm.

These five filler metals are extremely free flowing and therefore demonstrate good capillary flow, which together with their low brazing temperature make them ‘user friendly’ and easy to use. Typically these filler metals are hand fed into the joint as rod or filler wire, but can be pre-placed as sections of foil. When applied as a foil the production of a thicker joint, which will provide better dissipation of

Brazing filler metals - The common applications

Table 1: Nominal compositions of the common silver-base, cadmium-bearing filler metals used for the brazing of tungsten carbide

Filler Metal Silver %

Copper %

Zinc %

Cadmium %

Nickel %

Manganese %

Melting Range ˚C EN 1044

Easy-flo® 50 15 16 19 - - 620-630 AG301

Mattibraze™ 45 45 15 16 24 - - 605-620 AG302

Easy-flo® No.2 42 17 16 25 - - 610-620 AG303

DIN Argo-flo™ 40 19 21 20 - - 595-630 AG304

Easy-flo® No.3 50 15.5 15.5 16 3 - 635-655 AG351

Argo-braze ® 50 50 13.5 15.5 16 3 2 639-668 Prop

Most of the brazing filler metals and products specially developed for the brazing of tungsten carbide are still being used today. However, the use of the cadmium-bearing filler metals is in major decline due to the health and safety issues that arise from their use and environmental pressures.

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the cooling stresses, is much more likely to result than when hand feeding rod or wire.

Mattibraze™ 45, Easy-flo® No. 2 and DIN Argo-flo™ represent less expensive alternatives to Easy-flo®, but due to their lower ductility (particularly DIN Argo-flo™) there is some restriction on the size of tungsten carbide that can be brazed in comparison to those that might be brazed with Easy-flo®.

For applications involving pieces of tungsten carbide, with dimensions greater than 10mm, but less than 20mm, Easy-flo® No. 3 and Argo-braze® 50 are the natural choice. They would also be chosen where one of the ‘more difficult to wet’ grades of tungsten carbide is to be brazed, this being particularly the case with Argo-braze® 50 since it contains manganese. The nickel and manganese additions to these filler metals significantly

reduce their flow properties, which can be best described as sluggish. They therefore exhibit limited capacity for capillary flow. Thus to fully utilise their ability to braze larger sections of tungsten carbide they need to be pre-placed in the joint as pieces of foil, as opposed to being hand fed to the joint. Easy-flo® No. 3 and Argo-braze® 50 find particular use in the brazing of large diameter rock-drills.

Easy-flo® No. 3 would also be chosen where tungsten carbide is to be brazed to stainless steel and the finished joint is likely to be exposed to a wet or damp service environment where interfacial corrosion would be a service hazard. Clearly, in brazing tungsten carbide to stainless steel a whole set of new rules (due to the much greater difference in the expansion coefficients between tungsten carbide and stainless steel) needs to be applied.

Table 2: Nominal compositions of the common cadmium-free filler metals

Filler Metal Silver % Copper % Zinc % Tin % Gallium % Melting Range ˚C EN 1044

A* 56 19 17 5 3 608-630 Prop

Silver-flo® 56 56 22 17 5 - 620-655 AG102

Silver-flo® 55 55 21 22 2 - 630-660 AG103

* This filler metal currently not available from Johnson Matthey

The standard cadmium-free filler metals, Silver-flo® 56 and Silver-flo® 55, are the ones that have replaced the first four cadmium-bearing filler metals in Table 1. However, their ductility is generally lower than the cadmium containing filler metals, typically in the order of 20-25% in the as cast condition. This means that the sizes of tungsten carbide that they will be able to braze successfully are likely be somewhat less than those than for the cadmium-bearing filler metals. The general rule of the tip being less than 10mm in size can however still be applied.

All three filler metals listed in Table 2 have relatively low brazing temperatures and free flowing properties. The general comments made against the free flowing cadmium-bearing filler metals are also applicable here.

Filler metal A has a patented and proprietary composition, and was developed to have a melting range / brazing temperature close to that of the cadmium containing filler metals. Silver-flo® 56 and Silver-flo® 55, while having marginally higher brazing temperatures than the cadmium containing filler metals, have been

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for many years the accepted replacements for the first four filler metals in Table 1.

There are of course a number of lower silver content silver-copper-zinc and silver-copper-zinc-tin filler metals available that could potentially be used for the purpose of brazing tungsten carbide but none appear to be in common use and hence they are not included in Table 2. If seeking a less expensive alternative to the relatively high silver content filler metals of this group, it is important to appreciate that while filler metals may be of a similar composition, their properties, particularly their ductility, could well be significantly less.

For example, Silver-flo® 56 and Silver-flo® 55 are both members of the silver-copper-

zinc-tin family and might seem very similar to a filler metal with only 45% silver and 3% tin, however, the lower silver content, silver-copper-zinc-tin filler metals go through a ductile, brittle transition at about 300C. This transition could result in cracks forming in the filler metal on cooling as it tries to dissipate the cooling stresses. Likewise, while some silver-copper-zinc filler metals are very ductile, others are not. A filler metal’s ductility cannot be assessed just by its silver content, as it is the combination / percentages of silver, copper and zinc that determine this property. If a less expensive product is required then this should be discussed with a brazing filler metal supplier, as they will have experience and knowledge of which products within their range exhibit the required properties.

Table 3: Nominal compositions of the common cadmium-free nickel and manganese containing filler metals

Filler Metal Silver %

Copper %

Zinc %

Nickel %

Manganese %

Melting Range ˚C EN 1044

Argo-braze® 502 50 20 28 2 - 670-750

Argo-braze® 49H 49 16 23 4.5 7.5 680-705 AG502

Argo-braze® 49LM 49 27.5 20.5 0.5 2.5 670-710 Prop

Argo-braze® 40 40 30 28 2 - 670-780

Argo-braze® 27 27 39 22 5.5 9.5 680-830 AG503

Argo-braze® 25 25 38 33 2 2 710-810 Prop

= AWS A5.8 BAg-24 = AWS A5.8 BAg-4

The filler metals in Table 3, (or at least the first 4) are the most commonly used for the brazing of tungsten carbide. In Europe, Argo-braze® 49H is by far the most common, and therefore can currently be regarded as the ‘standard filler metal’ used for brazing tungsten carbide. All the filler metals listed contain nickel, and the majority also contain manganese. They are therefore suitable for brazing pieces of tungsten carbide with dimensions up to 20 mm as well

as those carbides that are more difficult to wet, due to them having a low cobalt-content, containing titanium or tantalum carbides, or free graphite. None of the filler metals are free flowing and as a general rule need to be pre-placed in the joint as a foil pre-form to achieve the best results.

Argo-braze® 49H can be seen as a replacement for cadmium-containing filler

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metals, Easy-flo® No. 3 and Argo-braze® 50 in Table 1. However, Argo-braze® 49H is stronger and less ductile than both of them, and this can lead to some problems when moving from one of the cadmium-containing filler metals to Argo-braze® 49H. The high strength and lower ductility of this filler metal can sometimes result in cracking problems in applications where no such problems existed when using a cadmium-bearing filler metal. A solution to such problems can often be found by using Argo-braze® 502 or Argo-braze® 49LM. Argo-braze® 49H is disliked by some users due to the finished colour of the filler metal and its sticky feel when attempting to move tips around. The solution to both of these issues is to use Argo-braze® 502. One advantage that Argo-braze® 49H has over similar filler metals is that it offers improved elevated temperature properties. Where most silver based filler metals tend to start losing strength at around 200C, Argo-braze® 49H extends this drop off temperature to around 300C. This can be useful when tools are being made that could be subject to elevated temperatures in service.

Argo-braze® 49H is used as the brazing filler metal in applications ranging from the manufacture of rock-drills to dental burrs, lathe tools to mining tools and router bits to tungsten carbide tipped circular saws. It is currently the first choice filler metal for most tungsten carbide brazing applications where the size of the tip is less than 20 mm. It would also be the first choice in applications where the grade of tungsten carbide was one identified as being difficult to wet, due to its low cobalt content, having additions of titanium and or tantalum carbide or where free graphite was present.

Argo-braze® 502 is free from manganese and is seen by some to offer certain advantages. This filler metal is commonly used in the United States and is listed within the AWS A5.8

specification (as indicated in Table 3). One of its main advantages is that it is more free flowing than Argo-braze® 49H and as a result is often preferred by operators. It can be used in most applications where using Argo-braze® 49H would be considered, but as it does not contain manganese, this would exclude it from being used on the more difficult to wet grades of tungsten carbide. Some disadvantage is also seen due to its slightly higher brazing temperature.

Argo-braze® 49LM was developed as a modified version of Argo-braze® 49H for use in the manufacture of a cadmium-free tri-metal product. Although it was developed specifically for this application, it can be used as a filler metal in its own right. Its lower nickel and manganese levels compared to Argo-braze® 49H result in a filler metal that is less strong and more ductile. In certain applications, where cracking problems have been experienced with Argo-braze® 49H it can provide a solution. Where a tri-metal product coated with Argo-braze® 49H is being used and a need arises for some additional filler metal to be added to the joint then it would be logical to use this filler metal if available. However, it is also acceptable to use Argo-braze® 49H in such circumstances. Although it is lower in nickel and manganese, the filler metal still shows improved wetting characteristics over the manganese free filler metals in the table.

Argo-braze® 40 (another AWS A5.8 listed filler metal) represents a more economic filler metal for use when brazing tungsten carbide than the first three filler metals in this table because it contains only 40% silver. Its nickel content provides enhanced wetting characteristics and joint thickening properties making it suitable for many of the applications where Argo-braze® 502 might be used. Its main drawback is its higher brazing temperature, which makes it less ‘user-friendly’ than the first three filler

Argo-braze® 49H is the first choice filler metal for most tungsten carbide brazing applications where the size of the tip is less than 20 mm… and also for applications where the grade of tungsten carbide is difficult to wet, due to its low cobalt content, having additions of titanium and or tantalum carbide or where free graphite was present.

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metals in Table 3. Nevertheless where a need exists for a less expensive filler metal then Argo-braze® 40 would be a good choice.

Argo-braze® 27 was developed as a less expensive alternative to Argo-braze® 49H. However, its high brazing temperature and long melting range make it much less user friendly than Argo-braze® 49H, and even though it is a less expensive alternative it is not extensively employed.

Argo-braze® 25 was originally developed as a lower cost rock-drill brazing filler

metal. However, the success of the silver-free copper-zinc-cobalt-manganese filler metal for the same application means that currently this filler metal is rarely used. It provides a less expensive alternative to Argo-braze® 49H, while still offering very good wetting characteristics due to its nickel and manganese contents. Its slightly lower brazing temperature, narrow melting range and lower nickel and manganese content mean that it will flow more freely than Argo-braze® 27 and therefore may prove somewhat easier to use.

Argo-braze® 85 was a filler metal originally suggested for use in applications requiring the simultaneous heat treatment of a backing material, and it was also identified as having enhanced wetting capabilities on difficult to wet carbides.

Table 4: Nominal compositions of some specialised silver containing filler metals

Filler Metal Silver %

Copper %

Others %

Nickel %

Manganese %

Melting Range ˚C EN 1044

Argo-braze® 85 85 - - - 15 960-970 AG501

B* 65 28 - 2 5 750-850 Prop

Argo-braze® 64 64 26 Indium 6 2 2 730-780 Prop

C* 57.5 32.5 Tin 7 - 3 605-730 Prop

* These filler metals currently not available from Johnson Matthey

Argo-braze® 85 was a filler metal originally suggested for use in applications requiring the simultaneous heat treatment of a backing material, and it was also identified as having enhanced wetting capabilities on difficult to wet carbides. Additionally, the filler metal has good elevated temperature properties up to 400oC and can be used in applications where joints are exposed to elevated temperature conditions in service. However, the filler metal is rarely used as the development of copper based filler metals with similar brazing temperatures and enhanced elevated temperature strength has replaced it.

Filler metal B and Argo-braze® 64 can or have been specially formulated for use when brazing tungsten carbide items that will be

subject to a Physical Vapour Deposition (PVD) coating with TiN (titanium nitride) and other similar types of coating. They contain no elements such as cadmium and zinc that are likely to volatilise during the coating process. The nickel and manganese additions obviously impart the required characteristics for the brazing of tungsten carbide, and indium or tin are used as melting point depressants. Filler metal B might also be considered for use in applications where the joints would be subject to elevated service temperatures.

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Pure copper will wet and bond with tungsten carbide very successfully and together with brass was one of the first filler metals to be used for the brazing of tungsten carbide. Its use today is as an inexpensive filler metal for reducing atmosphere or vacuum furnace brazing of tungsten carbide. However, its natural ability to produce joints with a thin layer of brazing filler metal limits its use to applications where small sections of carbide need to be brazed. A typical application could be the brazing of tungsten carbide tips into drills to be sold in the DIY market.

B Bronze™ is another filler metal for use in reducing atmosphere or vacuum furnace brazing of tungsten carbide. It was originally developed to be a filler metal with improved gap filling properties that could be used in reducing atmosphere furnace brazing applications where the joint fits were too large

to use copper. Its nickel and boron additions give it better wetting characteristics, and will also allow the formation of joints containing a thicker layer of brazing filler metal. B Bronze™ has the ability to bridge gaps up to 0.5 mm and is able to fill joints where spacer wires or other means have been used to create a larger controlled joint clearance. This means that it is possible to braze quite large sections of tungsten carbide, since it is possible to produce relatively thick ductile joints with this filler metal.

A Bronze™ was extensively used for the brazing of rock-drills and is still used in some cases for that type of application. It is used in air with a flux and induction heating. While it could be used for other applications involving the brazing of tungsten carbide it has never been widely used.

Table 5: Nominal compositions of some common copper based brazing filler metals

Filler Metal Copper %

Zinc %

Cobalt %

Nickel %

Manganese %

Others %

Melting Range ˚C EN 1044

Copper 100 - - - - 1083 CU103

B Bronze™ Bal. - - 3 - Boron 0.035 1081-1101 CU105

A Bronze™ 96.9 - - 2.5 - Silicon 0.6 1090-1101 Prop

C Bronze™ 86.85 - - 2.15 11 965-995 Prop

D Bronze™ 86 - 4 - 10 - 980-1030 Prop

J Bronze™ 67.5 - - 9 23.5 - 925-955 Prop

Argentel™ No. 1 60 39.75 - - - Silicon 0.3 875-895 CU301

F Bronze™ 58 38 2 - 2 890-930 Prop

D* 55 35 - 6 4 - 880-920 Prop

E* 54.85 25 - 8 12 Silicon 0.15 855-915 Prop

H Bronze™ 52.5 - - 9.5 38 - 880-920 ▲

Argentel™ 48 42.25 - 9.5 - Silicon 0.25 870-890 CU306

▲ = AMS 4764 * These filler metals currently not available from Johnson Matthey

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C Bronze™ was originally developed to allow for step or sequential furnace brazing of steel components, where pure copper was used as the first step brazing filler metal. The nickel and manganese contents of this filler metal make it highly suitable for the brazing of tungsten carbide, and its melting range makes it suitable for the brazing and simultaneous heat treatment of certain grades of steel. It is used in reducing atmosphere furnace brazing applications, but due to its manganese content, low dew point atmospheres must be used to reduce the manganese oxide and prevent the formation of further quantities during the brazing process. It is also used in vacuum brazing applications, but it needs to be used with a partial pressure brazing technique to suppress the volatilisation of the manganese. This filler metal has found extensive use in the brazing of tungsten carbide tipped drills for use by professional trades people, where brazing of the tip and heat treatment of the shank are carried out in one operation.

D Bronze™ is another filler metal specifically developed for the brazing of rock-drills. It has found little or no use outside of this area.

J Bronze™ with its high manganese content, provides filler metal with a lower brazing temperature, and which is higher in strength than C Bronze™. Some manufacturers of professional type tungsten carbide tipped drills use it in preference to C Bronze™ since it is said to offer good wear resistance. At the same time it has a brazing temperature which matches the heat treatment of the high strength steel frequently used as the shank material.

Argentel™ No.1 represents the classic brass brazing filler metal, a 60-40 copper-zinc filler metal with a small addition of silicon. This filler metal tends to be used in the manufacture of

low cost, DIY market, tungsten carbide tipped drills and circular saw blades. Brazing with this filler metal is often carried out using HF induction - the filler metal being applied to the joint in the form of a flux-bearing brazing paste. Flame brazing using a separate flux and automatic feeding of the joint with wire is also widely practiced. Due to the fact that it produces joints containing a relatively thin layer of brazing filler metal its use is restricted to brazing relatively small pieces of tungsten carbide.

F Bronze™ is another filler metal specifically developed for the brazing of rock-drills, but unlike some of the other filler metals developed for this purpose it has found a wider application for brazing tungsten carbide. It is typically applied in air using HF induction heating and a separate flux. The filler metal is most commonly pre-placed in the joint as a main mass and allowed to flow through the joint. Hand torch brazing is also possible with this filler metal.

Filler metals D and E are based on the classic nickel silver / bronze welding filler metal composition of Argentel™ where an addition of manganese has been made to the basic copper-zinc-nickel composition to enhance their wetting performance. These two filler metals represent low cost, high strength materials for torch brazing applications.

H Bronze™ is similar to J Bronze™ but its high manganese content makes for a lower brazing temperature. This filler metal is recommended for use in elevated temperature applications up to 400C and in the field of brazing tungsten carbide has been used to produce joints that will be exposed to such temperature. It has found use in the brazing of road-planing tools.

Argentel™ is the copper-zinc-nickel-silicon filler metal widely used for ‘bronze welding’.

F Bronze™ is another filler metal specifically developed for the brazing of rock-drills, but unlike some of the other filler metals developed for this purpose it has found a wider application for brazing tungsten carbide.

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Table 6: Nominal compositions of the brazing filler metals used on the common tri-metal products

Filler Metal Silver %

Copper %

Zinc % Other % Nickel

%Manganese

%Melting Range ˚C

Core Material

F* 64 26 - Indium 6 2 2 730-780 Copper

Easy-flo®** 50 15 16 Cadmium 19 - - 620-630 Copper

Easy-flo® No.3 50 15.5 15.5 Cadmium 16 3 - 635-655 Copper

Argo-braze® 502 50 20 28 - 2 - 660-750 Copper

Argo-braze® 49LM 49 27.5 20.5 - 0.5 2.5 670-710 Copper †

G* - 100 - - - - 1083 Nickel ‡

† Also available with copper alloy core and nickel mesh core ‡ With nickel mesh core ** Obsolete alloy for Johnson Matthey * These filler metals currently not available from Johnson Matthey

The filler metals detailed in Table 6 are those that can be found in the form of tri-metal products, primarily with a copper core and supplied in a ratio of 1:2:1. The characteristics of the individual filler metals have been covered in the relevant tables above.

By far the most commonly used products these days are the ones coated with Argo-braze® 49LM. The product coated with Filler metal F is obviously intended for use in applications where Physical Vapour Deposition (PVD), TiN (titanium nitride) coatings or similar are to be applied to the part. The products coated with the cadmium containing filler metals, Easy-flo® and Easy-flo® No.3, are rarely used for the reasons previously mentioned. The cadmium-free product coated with Argo-braze® 502 provides a manganese-free alternative to Argo-braze® 49LM, and is preferred by some users as it allows for easier movement and positioning of tips. It is recommended when using the tri-metal products to move and slide the tips around when the filler metal is in the molten condition to reduce flux and gas entrapment in the joints. Filler metal G with a nickel mesh core is a furnace brazing filler

metal with the ability to braze larger sections of tungsten carbide.

As indicated in the table, Argo-braze® 49LM while commonly supplied with a copper core can also be supplied with a copper alloy core. The alloy core, often a copper-nickel alloy, provides joints with a higher level of toughness able to cope more adequately with impact loads in service.

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The filler metals listed in Table 7 find use in applications where joints of extremely high strength are required or where the joints need to exhibit high strength at elevated temperatures. Many are patented or have their applications patented. Some are used for making carbide-to-carbide joints, whereas others are used because they allow for simultaneous heat treatment and vacuum brazing of parts. Many of the applications in which such filler metals are used are considered confidential so it is not possible to provide any specific details or information about their uses.

Pallabraze® 830 was a filler metal developed for the brazing of PCD’s where the finished joints were going to be subject to service at elevated temperature. The requirement was for a filler metal to have the lowest possible brazing temperature to minimise the damage to the diamond layer, but have adequate high temperature properties. Filler metal screening suggested the standardised silver-copper- 5% palladium filler metal EN 1044 PD106 would be a possible candidate. However, when this was tested it showed very poor wetting on

the tungsten carbide backing of the PCD. This was surprising, since palladium is an element known to improve the wetting characteristics of filler metals. Modified filler metals, with additions of nickel and nickel and manganese were produced and tested. While the nickel containing filler metal showed an improvement in wetting, the nickel and manganese bearing filler metal was by far superior.

Table 7: Specialised noble metal containing filler metals

Filler Metal Silver %

Copper %

Gold %

Palladium %

Nickel %

Manganese %

Silicon %

Melting Range ˚C

Pallabraze® 1120 75 - - 20 - 5 - 1000-1120

Pallabraze® 830 66.5 24.5 - 5 2 2 - 810-830

Orobraze® 1004 - 31.5 35 10 14 9.5 - 971-1004

H* - 33.5 31 9.75 9.75 16 - 927-949

Orobraze® 1052 - 31 25 15 18 11 - 1017-1052

Orobraze® 1013 - 37 25 15 10 13 - 970-1013

Orobraze® 950 - - 82 - 18 - - 950

Pallabraze® 851 - - - 47 47 - 6 810-851

* These filler metals currently not available from Johnson Matthey

The filler metals listed in Table 7 find use in applications where joints of extremely high strength are required or where the joints need to exhibit high strength at elevated temperatures. Many of the applications are considered confidential so it is not possible to provide any specific details or information about their uses.

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Table 8: EN 1045 Flux classes for use when brazing tungsten carbide

EN 1045 Classification

Temperature Range ˚C

For Brazing

Above ˚C

Composition Type

Comments

Type FH10 500 to 800 600 Boron compounds

Simple & complex fluorides

Compatible for use with most low temperature silver brazing filler metals. Suitable for use in most applications involving the brazing of tungsten carbide, except in those applications where the tungsten carbide contains titanium of tantalum carbides or has a low cobalt content.

Type FH12 550 to 850 600 Boron compounds

Simple & complex fluorides with the addition of elemental boron

Compatible for use with most low temperature silver brazing filler metals. Suitable for use in most applications involving the brazing of tungsten carbide, especially in those applications where the tungsten carbide contains titanium of tantalum carbides, has a low cobalt content or contains free graphite.

Type FH20 700 to 1000 750 Boron compounds

Fluorides

Compatible for use with the medium temperature brass and nickel silver type filler metals. Flux will not be suitable for brazing tungsten carbides where titanium or tantalum carbides are present.

Type FH21 750 to 1000 800 Boron compounds

Compatible with the high temperature copper based filler metals. Flux will not be suitable for brazing tungsten carbides where titanium or tantalum carbides are present.

Type FH30 +1000 +1000 Boron compounds phosphates and silicates

Compatible with the high temperature copper based filler metals. Flux will not be suitable for brazing tungsten carbides where titanium or tantalum carbides are present.

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Johnson Matthey Metal Joining is a global supplier of brazing filler metals and fluxes.

If you would like to find out more about the products and services that we can offer email Metal Joining on [email protected]

or visit our website at www.jm-metaljoining.com

for more information, or contact us on + 44(0)1763 253200.

Johnson Matthey plc cannot anticipate all conditions under which this information and our products or the products of other manufacturers in combination with our products will be used.

This information relates only to the specific material designated and may not be valid for such material used in combination with any other materials or in any process. Such information is given in good faith, being based on the latest information available to Johnson Matthey Plc and is, to the best of Johnson Matthey plc’s knowledge and belief, accurate and reliable at the time of preparation. However, no representation, warranty or guarantee is made as to the accuracy or completeness of the information and Johnson Matthey plc assumes no responsibility therefore and disclaims any liability for any loss, damage or injury howsoever arising (including in respect of any claim brought by any third party) incurred using this information. The product is supplied on the condition that the user accepts responsibility to satisfy himself as to the suitability and completeness of such information for his own particular use. Freedom from patent or any other proprietary rights of any third party must not be assumed. The text and images on this document are Copyright and property of Johnson Matthey.

This datasheet may only be reproduced as information, for use with or for resale of Johnson Matthey products. The JM logo©, Johnson Matthey name© and product names referred to in this document are trademarks of Johnson Matthey. Easy-flo® and Silver-flo® are registered to JM in the EU. Sil-fos™ is registered to JM in the UK and certain other countries but is marketed as Mattiphos™ in Germany and the USA.

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