Flexible Brazing Alloy Tape and Method of Making Same

This invention relates to brazing alloys and more particularly to an improved flexible brazing alloy tape containing metal brazing powders including nickel-base alloys, manganese-base alloys, copper-base alloys, cobalt-base alloys and copper.

The advantage and utility of flexible brazing alloy tapes are in part measured by the organic content of the binder which holds the alloy powders together. Current technology, because of the brazing alloy and base metal chemistry, often requires that brazing be carried out in a vacuum, for example, in the order of 1.times.10.sup.-4 mm Hg. A major problem confronting the industry in the use of such tapes is the condensation of the organic elements on furnace chamber walls requiring expensive and time consuming cleaning operations at a freuquency generally proportional to quantity of organic material in the binder. Such organic condensation also tends to foul vacuum pump rotors and ultimately renders the pump inoperative and/or causes high overall maintenance or replacement costs of expensive pump and vacuum components.

In additin to the above, the amount of carbonaceous residue in a joint brazed with alloy powders held together with an organic binder is determined by the organic content of the binder. Since such residue restricts alloy flow and promotes carbon pickup in many alloy systems, the quality of the brazed joint is degraded as the organic content of the binder increases.

U.S. Pat. No. 2,833,030 describes binders having an organic content of 10% to 20% by weight of the composite system. In addition the patent specifies casting solvents of the hydrocarbon type which impose stringent ventilation requirements on the manufacturing process to meet safety standards for both fire and toxic inhalation hazards.

U.S. Pat. Nos. 3,166,402 and 3,171,734 describe preferred brazing alloy systems with binders having 8% by weight of organic material which limit the use of such systems to brazing applications which can tolerate this level of organic content.

This invention is directed to flexible composite brazing alloy tape systems which overcome the foregoing problems and disadvantages by substantially reducing the organic content from that of prior art systems.

A general object of the invention is the provision of a flexible brazing alloy tape having a minimum content of organic material.

Another object is the provision of a method of making a brazing alloy tape of this type.

These and other objects are achieved by use of low viscosity polyacrylic acid polymer together with glycerol as a binder system for brazing alloy powders. An alternative though less preferable agent for this purpose is polyvinyl alcohol and glycerol.

In accordance with the invention a binder that is water soluble and has a very low total organic content is used to prepare flexible brazing alloy tape. The binder comprises a resin selected from a group consisting of polyacrylic acid polymer and polyvinyl alcohol and a plasticizer consisting of glycerol. The preferred binder is a low viscosity polyacrylic acid polymer and glycerol, low viscosity as used herein meaning a molecular weight of approximately 450,000.

The metal brazing powders which constitute the major component of the brazing tape embodying this invention includes powders having nickel and copper bases and a copper powder.

Typical of such nickel-base alloy powders are those designated as AMS 4776-4779 inclusive and consist of powders in the following weight ranges:

In the examples given below the basic criteria for satisfactory brazing alloy tapes were tapes that

1. had less than 3% by weight total organic content,

2. were flexible and tough, and

3. produced very low ash (about 1% by weight of total organic content) when melted.

AMS 4777 was atomized and screened to approximately -150 mesh. AMS 4777 has the following nominal composition by weight:

A solution of 5% by weight glycerol, 10% by weight of polyacrylic acid polymer (molecular weight approximately 450,000) manufactured and sold by B. F. Goodrich Company as Carbopol 907, and 85% by weight water was prepared. The AMS 4777 powder was sprinkled slowly into this solution and gently stirred to form a slurry, the powdered metal being added to the solution in a 90:10 ratio. The slurry was then allowed to mix gently overnight. The slurry was then cast by the well-known doctor blade apparatus having a 0.75" blade opening onto a resin sheet such as Mylar and was thereafter dried. The resulting brazing alloy tape had a uniform thickness of 0.050".

The following systems were prepared and tested with the results as indicated, the amounts being given in percent by weight:

As noted above, the binders for satisfactory tapes contain approximately 1% to 3% by weight of total organic constituent with polyacrylic acid polymer present in the range of 0.87 to 1.15% by weight and glycerol present in the range of 0.5% to 0.6% by weight.

This low organic binder produced a strong flexible tape with a correspondingly low ash residue (approximately 1% by weight of organic content on melting of the alloy system. In addition the binder components are water soluble and produce a non-combustible, non-toxic casting slurry.

A binder system was made essentially as described in Example I using 0.5% to 1.5% by weight of polyvinyl alcohol and 1.0% to 1.5% by weight of glycerol.

The following binders were prepared using the following amountes, in percent by weight, of polyvinyl alcohol (PVA) and glycerol and the balance AMS 4777, with the results indicated:

The tapes of B, D, F and G were rated excellent. The tapes produced with polyvinyl alcohol had some cosmetic faults such as color variations and slight blistering and therefore are the less preferred form of the invention. However, polyvinyl alcohol is more economical than polyacrylic acid polymer and has utility and advantage in applications where such cosmetic faults can be tolerated.