WO1990004490A1

WO1990004490A1 - Brazing paste for joining materials with dissimilar thermal expansion rates

Info

Publication number: WO1990004490A1
Application number: PCT/US1989/004721
Authority: WO
Inventors: Dennis Beeferman
Original assignee: Handy & Harman
Priority date: 1988-10-24
Filing date: 1989-10-23
Publication date: 1990-05-03
Also published as: AU4517589A

Abstract

A brazing paste for brazing together materials having dissimilar thermal expansion rates which includes a powdered metal brazing alloy, powdered metal spacer spheres, an inorganic flux and a binder material. The spacer spheres are ductile metal which is not drawn into solution by the brazing alloy during the brazing operation and thereby provides a buffer which absorbs thermal stresses generated by the different materials upon cooling the joint. The brazing paste according to the present invention is equally well suited for brazing operations using directly applied heat, for example torch or induction.

Description

BRAZING PASTE FOR JOINING MATERIALS WITH DISSIMILAR THERMAL EXPANSION RATES

BACKGROUND OF THE INVENTION

The invention relates to brazing alloys and more particularly to a brazing paste for brazing together materials with dissimilar, thermal expansion properties.

In the brazing together of dissimilar materials such as tungsten carbide and steel, diamond and steel or in electrical contact applications, problems arise due to the dissimilar coefficients of thermal expansion of the materials. Upon cooling the joined parts after brazing, stresses develop due to the different expansion rates.

These thermal stresses can cause cracks m the joint limiting joint performance and possibly leading to ultimate failure.

In the area of carbide-steel brazing a number of solutions have been proposed. U.S. Patent No. 4,340,650 discloses a three-layer soli.d brazi.ng sheet wi.th a layer of ductile metal sandwiched between two layers of brazing alloy. The ductile center has a melting point higher than the melting point of the brazing alloy and helps to absorb thermal stresses as the joint is cooled. While good results have been achieved using this type of brazing sheet it has a significant disadvantage as compared to brazing

-pastes. The sheet must be stamped, cut or otherwise formed to the shape and size of the parts being joined. This entails additional fabricating steps and may necessitate a large inventory of preformed brazing sheets. An additional drawback is that such preforms are not well suited for high volume production brazing because they must be placed and temporarily secured on the parts to be joined. Brazing pastes have the distinct advantage of adapting to any shape, eliminating the need for an invent¬ ory of preforms, and they may be applied by existing automated paste dispensing equipment. In an attempt to 5 combine the advantages of the brazing alloy sheet with those of a brazing paste, U.S. Patent No. 4,431,465 provides a mixture of powered metal brazing alloy and a small quantity of ductile powdered metal w th a higher melting point in a water soluble resin. This mixture forms

1_Q a paste-like substance for brazing tungsten carbide to steel wherein the ductile powdered metal provides a buffer between the joined parts to alleviate thermal stress.^' However, this brazing paste compound has the disadvantage that the brazing must be done in an oven under a vacuum.

-[-This requirement severely limits the commercial practical¬ ity of the brazing paste because the majority of commercia] brazing is done by torch or induction at atmospheric pressure. Another drawback of this paste is that it contemplates the use of metals, such as nickel or copper,

„„ for the buffer which could be drawn into solution by the brazing alloy at temperatures below the actual melting point of the buffer metal. This would result in a loss of buffer material causing the gap between the parts joined to be decreased and thereby degrading joint performance.

Another application in which the difference 2b between the thermal expansion rates of the materials being joined causes problems is the brazing together of diamond and steel. U.S. Patents No. 4,560,853 and No. 3,056,400 provide brazing materials for joining diamond and steel - which include metal powders, but these patents are directed to other problems such as corrosion resistance and location of the diamond. Brazing pastes known in the art fail to address the problem of varying thermal expansion rates in the brazing of diamond to steel.

35 The brazing of electrical contact material to its supporting structure also presents similar problems, with dissimilar thermal expansion rates. Electrical contact material is generally a silver or silver-cadmium 5 oxide alloy, an example of such a material is disclosed in U.S. Patent No. 4,700,475. Another common electrical contact material is tungsten carbide impregnated with silver. The supporting structure for the electrical contact is often made of copper or steel.

₁₍- It is therefore desirable to provide a brazing paste which will provide a superior quality joint and can compensate for thermal stresses created in the brazing of materials with dissimilar thermal expansion rates, and which may be used in normal commercial brazing operations,

,- such as brazing by torch or induction.

SUMMARY

Accordingly, the brazing paste of the present invention includes a powdered metal brazing alloy, powdered metal spheres made of a material such as stainless steel that will not be drawn into solution by the brazing alloy, an inorganic flux and a binder material to provide a paste-like consistency. A brazing paste with this composition is equally well suited for brazing by directly

„ applied heating techniques such as torch or induction and 25 works especially well when brazing carbide to steel, diamond to steel, or electrical contact materials to steel or copper.

₃₀ DETAILED DESCRIPTION OF THE INVENTION

While the invention is described in connection with particular embodiments, it will be understood that it is not intended to limit the invention to these embodi¬ ments. On the contrary, it is intended to cover all alter- natives, modifications and equivalents as may be included in the spirit and scope of the invention as defined by the claims .

The brazing paste according to the invention comprises four basic components: a brazing alloy, a powdered metal spacer, an inorganic flux and a binder material to provide a paste-like consistency.

The brazing alloy is a powdered metal alloy, the exact composition can depend on the particular materials being joined. Three compositions of brazing alloy which work particularly well in the brazing of steel and carbide, diamond or electrical contact material are: 45Λg - 15Cu - 16Zn - 24Cd; 50Λg- 15.5CU - 15.5Zn - 16Cd - 3Ni, and 50Ag - 20Cu - 28Zn - 2Ni. The numbers preceding the element symbols refer to the weight percent of that particular element in the alloy mixture. Brazing alloys based on silver, copper and zinc are generally preferable because of their relatively lower melting points, in the range of 1100 to 1450°F depending on the particular percentages and other alloys included. Some other additional elements which might be included in the alloy are manganese, tin or phosphorous.

The powdered metal spacer is a critical compon¬ ent of the brazing paste because it provides a buffer between the parts being joined which compensates for thermal stresses developed during the brazing process. The powdered metal spacer is formed by known powdered metal technology, i.e., atomizing and screening to select the desired size particles. The particles in the form of spheres should be sized to provide the optimum gap between the parts to be joined. Generally, for the applications discussed this is approximately 0.003 inches. Depending on the particular application the ideal size may vary within a range of about 0.0029-0.005 inches in diameter for spheres.

At gaps beyond 0.005 inches joint strength becomes reduced and gaps below 0.0029 inches are generally not large enough to provide the necessary stress relief.- The size of the spheres is to be large enough so that when the materials being brazed together are pressed and held together during the brazing operation, the spheres function to hold the materials spaced from each other and prevent the paste and alloy from being squeezed out.

The powdered metal spacer should be provided in a quantity sufficient to maintain the desired gap in all areas of the joint. A quantity of spacer equal to 3% by weight of all metal components of the paste has been found to give excellent results. The quantity may be varied in the range of 0.5-12% by weight of the metal components of the paste. An excessive quantity of spacer metal will weaken the joint by actually reducing the brazed area, and inadequate amount of spacer may lead to the development of thermal stress and resulting cracks.

The material selected for the spacer should be a ductile metal with a melting point higher than that of the brazing alloys. But it is also necessary that the metal selected. be one which will maintain its size during the brazing operation and not be drawn into solution by the brazing alloy at the brazing temperature, even if that temperature is below the actual melting point of the spacer metal. For this reason metals such as nickel or copper are not suitable as spacer metals. Stainless steel, however, provides a good combination of ductility, high melting point and resistance to being drawn into solution. The only additional requirement is that the metal selected as spacer be wettable by the brazing alloy in order to provide maximum joint strength. AISI 316 stainless steel is an example of a spacer metal which exhibits the necessary properties and creates a good joint. The flux is an important component of the braz¬ ing paste because it permits the paste to be used in torch or induction brazing operations. The flux acts as a cleansing agent to remove oxides which would inhibit the 5 creation of a brazed joint. Suitable fluxes are commercially available which consist of inorganic cliemicals such as boric acid, fluorides and borates.

The binder material, holds the components together and gives the brazing paste its paste-like I_Q consistency, between 250,000 cps and 1,000,000 cps . This consistency allows the brazing paste to be applied by existing automated dispensing equipment. The binder material is made up of a polymer and a solvent such as mineral*, spirits and water. An emulsifier may be added to J5 the binder material if required for the particular applica¬ tion.

A brazing paste formulated as above has been ^' found to be effective in brazing carbide to steel, diamond to steel, and electrical contact silver and silver-cadmium „_n oxide or tungsten carbide to steel or copper by directly applied heating means such as torch, induction, electrical resistance or laser brazing. These are materials commonly joined in commercial applications, however, the present invention is not limited to joining only these materials. „ The present invention is generally suitable for brazing together materials having dissimilar thermal expansion rates. Materials having dissimilar thermal expansion rates, as used herein, contemplates materials which traditionally have been or could have been joined by _3Q brazing, but the differences in their thermal properties prevented the creation of a successful joint.

The method for creating a brazed joint using the - present invention is explained below in terms of carbide and steel alone for reasons of simplicity, but it applies ₃₅ equally to any of the materials disclosed above. The joint area of both parts must be cleaned to remove oil and dirt, etc., then a small amount of brazing paste is placed on each major surface plane of steel which is to be brazed. The spacer material must be spread evenly in all areas of the joint because it will not flow as does the brazing alloy. The carbide part to be brazed is then pressed firmly into place. Excess brazing paste which is forced from the joint should be removed prior to heating. Next, heat is applied directly to the joint area by torch or induction coil. Heating should be accomplished as quickly as possible, without overheating. By way of example the brazing temperature for the Ag-Cu-Zn-Cd brazing alloy disclosed above is approximately 1145"F. Temperatvires should be altered in accordance with the particular brazing alloy used. After the proper heating is complete, the joint should be allowed to cool in still air.

Claims

W1IAT IS CLAIMED IS:

1. A brazing paste for joining materials having dissimilar thermal expansion rates, comprising: (a) a powdered metal brazing alloy;

(b) ductile powdered metal spheres comprising 0.5-12% by weight of the metal components of the paste, the powdered metal spheres being a material which will not be brought into solution by the brazing alloy at the brazing ₍- temperature;

(c) an inorganic flux material; and

(d) a binder material.

2. The brazing paste as in claim 1, wherein: _τ5 (a) the powdered metal brazing alloy is a mixture which includes silver and copper; and

(b) the powdered metal spheres are stainless steel, and have a diameter between about .0029--005 inches.

3. The brazing paste as in claim 2, wherein:

(a) the stainless steel spheres have a diameter of approximately .003 inches and make up about 3% by weight of the metal components of the paste.

4. A method of brazing together carbide and 25 steel, which includes the step of placing between the carbide and steel a brazing paste, comprising:

(a) a powdered metal brazing alloy;

• (b) a plurality of ductile powdered metal spheres made of a material which will not be brought into solution by the brazing alloy at the brazing temperature, the spheres having a diameter in the range of approximately

.0029-.005 inches and comprising 1-12% by weight of the metal components of the paste;

__ (c) an inorganic flux material; and

Jfa (d) a binder material.

5. The method as in claim 4, further comprising the step of:

5 (a) heating the paste by torch to form a brazed joint.

6. The method as in claim 4, further comprising the step of:

₁₀ (a) heating the paste by induction coil to form a brazed joint.

7. A method of brazing together diamond and steel, which includes the step of placing between the _lf. diamond and steel a brazing paste, comprising:

(a) a powdered metal brazing alloy;

(b) a plurality of ductile powdered metal spheres made of a material which will not be brought into solution by the brazing alloy at the brazing temperature, the spheres having a diameter in the range of approximately .0029-.005 and comprising 1-12% by weight of the metal components of the paste;

(c) an inorganic flux material; and

(d) a binder material.

25

8. The method as in claim 7, further comprising the step of:

(a) heating the paste by torch to form a brazed joint.

30

9. The method as in claim 7, further comprising the step of :

(a) heating the paste by induction coil to form a brazed joint.

35

10. A method of brazing together electrical contact material and a metal selected from a group consist¬ ing of steel and copper which includes the step of placing between the electrical contact material and the metal a

5 brazing paste, comprising:

(a) a powdered metal brazing alloy;

(b) a plurality of ductile powdered metal spheres made of a material which will not be brought into solution by the brazing alloy at the brazing temperature, _ιn the spheres having a diameter in the range of approximately .0029-005 inches and comprising 1-12% by weight of the metal components of the paste;

(c) an inorganic flux material; and

(d) a binder material.

15

11. The method as in claim 10, further compris¬ ing the step of:

(a) heating the paste by torch to form a brazed joint.

20

12. The method as in claim 10, further compris-

^• ing the step of:

(a) heating the paste by induction coil to form a brazed joint.

25

30

35