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Brazing of carbide-tipped cutting tools Marian Bronny; Max Schimpfermann, Umicore AG & Co. KG; Hanau

Brazing of cemented carbides is not always a success. Either the brazing alloy does not „bind“ properly to the cemented carbide or the strength of the brazing alloy is inadequate. Not uncommonly, cracks or detachments occur in the cemented carbide in the cooling phase. Such defects can be avoided by selecting the right brazing alloy and brazing flux. One of the decisive factors determining the operating

efficiency of a cutting tool is the way the cutting material is

processed. One of the cutting materials frequently used for

machining and dissecting manufacturing processes is

cemented carbide (Figure 1). Due to the hardness and

temperature stability of cemented carbides, they are

eminently suitable for milling, turning, drilling and sawing of

metal, wood, plastics and other materials.

In many cutting tools, the specific characteristic properties

of cemented carbides come into their own by combining

them with tool steels as support material. Crucial for the

success of the tool is the bond between the cemented

carbide and the support material. Brazing is one of the

most important bonding techniques [1]. Depending on the

silver brazing alloy selected and the quality of the brazing,

tensile strengths in the joint of approximately 150 to 300

MPa can be achieved.

Soldering of cemented carbide-faced tools takes place at

temperatures above 450°C. In terms of definition, a brazing

process is consequently involved. In order to ensure a

reliable, high strength braze joint between the cemented

carbide and steel a few important points need to be taken

into consideration when choosing the brazing alloy and

flux.

The need for special brazing alloys and fluxes

Cemented carbides are sintered materials consisting of

hard materials, usually tungsten carbide, which are

embedded in a metallic binder matrix, usually cobalt. Due

to the high proportion of metal carbide, cemented carbides

are considered to be materials difficult to wet. For this

reason, silver brazing alloys together with alloying

elements that promote wetting, such as manganese, are

preferably used for brazing of these materials. The brazing

alloy Ag 449 according to DIN EN ISO 17672 (e.g.

BrazeTec 4900) is a typical representative of these brazing

alloys (Table 1).

Since universal fluxes (type FH 10 according to DIN EN

1045) are usually insufficiently active, brazing of cemented

carbides requires special fluxes (e.g. BrazeTec spezial h or

BrazeTec h 900) (Table 2). These correspond to type FH

12 according to DIN EN 1045. By using these fluxes, it is

possible as a result of their oxide-dissolving properties to

effect brazing in the air by flame or induction heating.

The wettability of cemented carbides is considerably

improved by a cobalt or nickel layer applied by

electroplating. This is true in particular in the case of

cemented carbides with a very low proportion of binder.

The above-mentioned metal coatings have a further

important effect: they prevent the oxidation of cemented

carbides. The same applies to brazing of cemented

carbides as to any other brazing operation: the surfaces to

be joined should be free from oxide and grease as far as

possible.

Thermal effects and states of stress

During brazing, it is essential to take the fact into account

that the thermal coefficients of expansion of the materials

to be joined, namely cemented carbide and steel, are quite

different. As a rule, they are 5 to 7 × 10-6 K-1 in the case of

cemented carbides and 11 to 14 × 10-6 K-1 in the case of

steels. The thermal expansion of steel is thus two to three

times higher than that of cemented carbide [1].

Cemented carbides are basically sensitive to tensile

stresses. The impact of this characteristic varies,

depending on the type of cemented carbide and the

geometry of the structural part, and affects the bond during

the cooling phase following brazing (Figure 3). Figure 1: Section of a cemented carbide-faced saw blade Source: Umicore

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When the combination of cemented carbide and steel is

heated the steel expands more strongly due to its

considerably higher coefficient of thermal expansion. At the

brazing temperature, the parts to be bonded are still

loosely connected through the liquid brazing alloy; they are

unstressed. Once the brazing alloy solidifies, the cemented

carbide is firmly bonded to the steel. Thus a direct force

transmission can occur between the two materials. During

cooling of the structural part, the cemented carbide

contracts considerably less than the steel with the result

that tensile stress is transferred through the steel to the

cemented carbide by a type of “bi-metal effect”. This can

cause irreversible damage to the cemented carbide

evidenced by cracks directly after brazing, grinding or use.

Sandwich alloys reduce stresses

The use of special sandwich alloy foil allows crack-free

joining of cemented carbides. Brazing alloys sheets have a

sandwich construction. The core is formed by a ductile

intermediate layer which is plated with silver base brazing

alloy on both sides. The diagrammatic representation of a

cross-section through a sandwich alloy sheet is shown in

Figure 2. The intermediate

layers generally consist of

copper or copper based

alloys which absorb the

stresses occurring during

cooling.

Figure 3 shows the simulated state of stress in cemented

carbide following brazing with brazing alloy in the cooled

state. As can be clearly seen, when a sandwich alloy is

used, a distinctly smaller area with lower tensile stress is

formed in the central area than without the use of sandwich

alloy. Moreover, the forces acting laterally on the cemented

carbide are equally reduced.

The shear strength of the bond is determined by the

strength of the intermediate brazing alloy layer. By using

special intermediate alloy layers (e.g. BrazeTec 49/Cuplus)

the shear strength of the bond can be increased by more

than 20 % compared with a standard intermediate copper

layer (e.g. BrazeTec 49/Cu).

The optimum width of the sandwich alloy strip with respect

to the widths of the parts to be joined is often a matter for

discussion. Numerous tests and simulation calculations

have been carried out on cemented carbide-faced blades

of circular saws. Basically, a continuous intermediate layer

should be present across the entire joining surface for an

optimal reduction of stress. (Figure 4, illustration A) [2].

Figure 4 shows how the absence of intermediate layers

affects the states of stress in cemented carbide on the

example of cemented carbide teeth on circular saws.

In practice, it has been found that in certain cases

cemented carbides with small joining surfaces can be

brazed only with low-melting silver brazing alloys without

an intermediate layer. The stresses occurring in the

cemented carbide increase substantially with increasing

size of the joining surface. Higher stresses in the join are

counteracted by using greater intermediate layer

thicknesses [3]. Another possibility is to conduct the

stresses to the generally ductile brazing alloy by widening

the soldering gap. The gap width can be adjusted by a

nickel network (e.g. BrazeTec 49/NiN) incorporated into the

brazing alloy. The way the stresses increase in the

narrowing brazing gap is illustrated by the error patterns B

to E in Figure 4. The gap decreases in the tooth ridge from

0.2 mm (B) until the press fit (E) is reached.

Particularities achieved by special applications

The multitude of tasks is achieved by way of different

cutting materials, cutting material modifications or

additional coatings. This can lead to special aspects for

brazing.

If the anti-wear protection of cemented carbides is

improved by vapour depositing a layer of hard material

consisting of titanium carbide (TiC), titanium nitride (TiN) or

titanium aluminium nitride (TiAlN), for example, allowance

must be made for the process conditions of CVD (chemical

vapour deposition) or PVD (physical vapour deposition)

coating processes. If the process temperatures are above

Figure 2: Diagrammatic cross-section through a sandwich alloy; Source: Umicore

Steel

Cemented carbide

Alloy layer

Sandwich alloy

Time Figure 3: State of stress in the join: left: brazing alloy; right: sandwich alloy; Source: Umicore

Temperature Temperature

Brazing temperature

Room temperature

Room temperature

Time

Brazing temperature

Time

Steel

Cemented carbide

Alloy layer

Sandwich alloy

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400°C or if the operation takes place under vacuum, this

must be taken into account when selecting the brazing

alloy. Zinc, a typical alloying element of low-melting, silver-

containing brazing alloys begins to evaporate at 400°C as

a result of its relatively high vapour pressure, with a

consequent negative influence on the quality of the coating

and the strength of the braze joint. Against this

background it is recommended to braze cemented

carbides which are to be used in such a coating process

with a zinc-free silver based brazing alloy such as

BrazeTec 6488 or BrazeTec 64/Cu.

By using polycrystalline diamonds (PCD, abbreviation: DP),

monocrystalline diamonds (MCD, abbreviation: MD) or

polycrystalline cubic boron nitride (CBN, abbreviation: BN)

sintered onto one side of a cemented carbide base body,

the anti-wear protection of cemented carbides can be

further improved. Due to the temperature sensitivity of the

hard material layers applied by sintering it is advisable to

avoid brazing temperatures above 700°C. Strict

temperature control during soldering is essential.

If fairly small cemented carbide areas such as those

typically used on turning tools and milling cutters for wood

working are involved the use of sandwich alloys, as

described above, can be dispensed with. If this is the case,

use can be made of the low melting silver brazing alloys

BrazeTec 5600 or BrazeTec 5507. Their brazing

temperature is 30 to 40°C below that of BrazeTec 4900. It

deserves to be mentioned in passing that diamonds (e.g.

CVD thick layer diamonds) can be soldered directly with

so-called active brazing alloys (e.g. BrazeTec CB4).

For special applications such as in medical or plastics

engineering the stress-equalising intermediate layer must

satisfy anticorrosion requirements. These are often not

satisfied by pure copper. In order to be able to braze

cemented carbides nevertheless successfully, intermediate

layers consisting of copper-nickel-iron-alloys are available

(e.g. BrazeTec 49/CuNiFe).

As illustrated, brazing technology provides solutions for brazing cemented carbide cutting materials. The materials of first choice are the brazing alloys which are capable of reducing the thermally induced tensile stresses acting onto the joint as a result of their excellent ductility. When used in combination with the “right“ flux, cemented carbide brazing operations which are considered to be difficult, will be successful. Marian Bronny, Regional Sales Manager, Umicore AG & Co. KG, Hanau, marian.bronny@eu.umicore.com Max Schimpfermann, Manager Applied Technology Brazing Center, Umicore AG & Co. KG, Hanau, max.schimpfermann@eu.umicore.com

Literature [1] Weise, W.; Koschlig, M.; Herzog, H.; Beuers, J.:

Broschüre „Einsatz innovativer Lote in der Schneidetechnik“. Degussa-Hüls, 1995

[2] Magin, M.; Rassbach, S.: Stress Analysis on Brazed Hartmetal Saw Teetf, 17. Plansee Seminar, Reutte 2009

[3] N.N., Brochure „Technik die Verbindet“, Volume 30, Degussa AG

A B C D E F G perfect bottom bottom bottom bottom top AgCu Sandwich no Cu no Cu no Cu no Cu no Cu braze braze t=0.2 t=0.1 t=0.05 t=0.0 t=0.3

Figure 4: Influence of the intermediate copper layer on the stress distribution in cemented carbide, source [2]

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Table 1. Selection of brazing alloy for brazing cutting materials of cemented carbide based on tungsten carbide

Composition [% by wt.] Brazing alloy Remarks

Brazing temperature

[°C] Ag Cu Zn Mn Ni In

BrazeTec 49/Cu Sandwich alloy; Intermediate copper layer

690 49 27,5 20,5 2,5 0,5

BrazeTec 49/Cuplus Sandwich alloy; Intermediate alloy layer of increased strength

690 49 27,5 20,5 2,5 0,5

BrazeTec 49/NiN Sandwich alloy; Intermediate nickel network layer

690 49 27,5 20,5 2,5 0,5

BrazeTec 49/CuNiFe Sandwich alloy; Intermediate CuNiFe layer

690 49 27,5 20,5 2,5 0,5

BrazeTec 64/Cu Sandwich alloy; Intermediate copper layer, TiN coatable

770 64 26 2 2 6

BrazeTec 2700 Silver-based brazing alloy Ag 427*

840 27 38 20 9,5 5,5

BrazeTec 4900 Silver-based brazing alloy Ag 449*

690 49 16 23 7,5 4,5

BrazeTec 4900 A Silver-based brazing alloy 690 49 27.5 20.5 2.5 0,5

BrazeTec 6488 Silver-based brazing alloy 770 64 26 2 2 6

*according to DIN EN ISO 17672

Table 2. Selection of flux for brazing cutting materials of cemented carbide based on tungsten carbide

Flux Remarks Flux colour Designation according to DIN EN 1045

Effective temperature[°C]

BrazeTec spezial h For brazing cemented carbides and higher alloyed steels

brown FH 12 520 to 1030

BrazeTec h 285 Binder-stabilised type for machine application optimised

brown FH 12 520 to 910

BrazeTec h 900 Binder-stabilised type for machine application optimised; specially chemically activated for brazing special cemented carbides

brown FH 12 520 to 850

BrazeTec h 80 For large surface soldering and short brazing times white FH 10 550 to 850