Silver-Copper-Titanium Brazing Alloy

This invention relates to brazing alloys. More particularly it relates to a silver-copper-titanium brazing alloy.

Prealloyed brazing foils of silver-copper-titanium are not commercially available because titanium and/or titanium containing intermetallic phase(s) are distributed unevenly throughout a silver-copper phase during ingot casting thereby rendering subsequent ingot rolling and/or punching to produce braze preforms very difficult; especially when the titanium composition is greater than two weight percent.

Because of the processing difficulties, silver-copper-titanium alloy foils are produced in a laminated form where the titanium layer is bonded between two silver-copper alloy layers to form the silver-copper-titanium laminated brazing foil.

There is a need to produce a silver-copper-titanium brazing alloy which can be rolled to a thin foil and/or punched from a foil into braze preforms.

Accordingly, it is an object of the present invention to provide a new and improved silver-copper-titanium brazing alloy which can be rolled into a thin foil.

Further objects of the present invention will become apparent from the description contained herein.

In accordance with the present invention, there is provided a new and improved silver-copper-titanium brazing alloy foil. The foil comprises from about 10 to about 50 weight percent copper, from about 0.1 to about 9.5 weight percent titanium and the balance silver.

The foil is at least 0.0008 inches thick and the titanium is uniformly distributed throughout the foil.

In the drawing:

FIG. 1 is a scanning electron photomicrograph (SEPM) of a longitudinal cross section of a silver-copper-titanium brazing foil of the present invention at a magnification of 750 times (750.times.).

FIG. 2 is a (SEPM) of a cross section of a silver-copper-titanium ingot at a magnification of 330 times (330.times.).

FIG. 3 is a x-ray dot map of the titanium distribution in the silver-copper-titanium brazing foil depicted in FIG. 1.

FIG. 4 is a x-ray dot map of the titanium distribution in the silver-copper-titanium ingot depicted in FIG. 2.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawing.

Using a rapid solidification processing method, a homogeneous Ag-Cu-Ti alloy ductile foil was produced. The alloy composition ranges from about 10 to about 50 weight percent copper, 0.1 to 9.5 weight percent titanium and the balance silver. Preferably, this alloy ranges from 20 to 40 weight percent copper, one to five weight percent titanium and the balance silver The foil thickness was at least 0.0008 inch, preferably 0.0008 to 0.006 inch thick, more preferably 0.001 to 0.004 inch thick. As even distribution of fine titanium and/or titanium containing phase(s) was obtained in the rapidly solidified foil. This foil can be used directly for brazing, or this foil can be further rolled down to a thinner dimension having a more uniform foil surface finish. Rolling and/or punching the foil into braze preforms can be carried out surprisingly easily because of the uniform distribution of titanium and/or titanium containing hard phase(s) within the silver-copper alloy matrix rendering the foil ductile rather than brittle.

A mixture of 68.8 grams of silver, 26.7 grams of copper and 4.5 grams of titanium were heated to 980.degree. C. in a melt spinning quartz crucible, preferably a graphite coated quartz crucible having a yttria coating on the graphite. The molten mixture was melt spun to form a ductile foil. The crucible has a lmm diameter orifice, and an ejecting overpressure of 2 psi was used. The crucible was positioned 0.25 inches above and perpendicular to a chill wheel. The wheel is made from copper and has an 8" diameter and is 2" wide. The wheel was run at 1519 rpm, with internal cooling water. The overpressure gas was argon. The resulting 0.001 inch thick ductile foil was punched to make braze preform without cracking.

An example of the homogeneous morphology of a silver-copper-titanium brazing foil of the present invention is shown in FIG. 1. FIG. 1 is a scanning electron photomicrograph (SEPM) of a longitudinal cross section of a silver-copper-titanium brazing foil at a magnification of 750 times (750.times.). This foil was made by melting 1 7 w/o titanium, 70.8 w/o silver, and 27.5 w/o copper at 980.degree. C. and rapid quenched by a melt spinning technique as described above. As shown in FIG. 1 the grains are very fine and uniformly distributed throughout the cross section. An x-ray dot map of the titanium distribution of this brazing foil is shown in FIG. 3 illustrating the uniform distribution of fine grains of titanium and/or titanium containing phase(S) throughout the longitudinal cross section of the foil.

This is compared to the SEPM cross section of an ingot of the same starting material, 1.7 w/o titanium, 70.8 w/o silver, and 27.5 w/o copper, which was melted at 980.degree. C. and cast as an ingot shown in FIG. 2. The grains of the ingot are much larger and have a much broader grain size distribution than the material shown in FIG. 1.

The magnification of the SEPM in FIG. 2 is only 330 times (330.times.) compared to the magnification of the SEPM in FIG. 1 which is 750 times (750.times.) and yet the grains in FIG. 1 are very small in comparison. The titanium distribution of the ingot is illustrated in the x-ray dot map shown in FIG. 4 at a magnification of 500 times (500.times.). FIG. 4 illustrates the non-uniform distribution of titanium throughout the cross section of the ingot.

The cast ingot is very brittle and can not be rolled into a foil or made into preforms by a punch technique without cracking. This is attributed to the non-uniform distribution of the titanium throughout the ingot.

However, the disadvantage of a brittle, unworkable alloy has been overcome by the present invention which has the titanium uniformly and finely distributed throughout the alloy as shown in FIG. 3 and can be worked by rolling and/or punching to make preforms without cracking.

While there have been shown what are at present considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.