Homogeneous low melting point copper based alloys

A copper based low melting point metal alloy composition consists essentially of about 2.5 to 11 atom percent tin and about 11 to 15 atom percent boron, the balance being essentially copper and incidental impurities. The composition is such that the total of copper and tin ranges from about 85 to 89 atom percent.

DESCRIPTION 
1. Field of the Invention 
This invention relates to copper based metal alloys and more particularly 
to a homogeneous, ductile brazing material useful for brazing metal 
articles such as those composed of copper and copper alloys. 
2. Description of the Prior Art 
Brazing is a process of joining metal parts, often of dissimilar 
composition, to each other. Typically, a filler metal that has a melting 
point lower than that of the metal parts to be joined together is 
interposed between the metal parts to form an assembly. The assembly is 
then heated to a temperature sufficient to melt the filler metal. Upon 
cooling, a strong, leak-tight joint is formed. Filler metals used are 
commonly in powder, wire or foil form depending on the type of 
application. Foil form provides the advantage of preplacing the filler 
metal in the joint area, thus permitting brazing of complex shapes with 
minimum rejection. 
The brazing alloys suitable for use with copper and copper alloys, 
designated AWS BAg are well known compositions. These alloys contain 
substantial amounts of the precious metal silver (19 to 86 weight percent) 
and hence are expensive. Most of the AWS BAg compositions are fabricated 
to a foil form through a lengthy sequence of rolling and annealing, 
thereby incurring substantial processing cost. 
Ductile glassy metal alloys have been disclosed in U.S. Pat. No. 3,856,513, 
issued Dec. 24, 1974 to H. S. Chen et al. These alloys include 
compositions having the formula T.sub.i X.sub.j, where T is at least one 
transition metal and X is an element selected from the group consisting of 
phosphorous, boron, carbon, aluminum, silicon, tin, germanium, indium, 
beryllium and antimony, "i" ranges from about 70 to about 87 atom percent 
and "j" ranges from about 13 to 30 atom percent. Such materials are 
conveniently prepared in powder, wire or foil form by rapid quenching from 
the melt using processing techniques that are now well-known in the art. 
However, no liquid-quenched glassy metal alloys of the family T.sub.i 
X.sub.j described above, containing copper as the principal transition 
metal have been reported. Chen et al. report only one copper containing 
composition (e.g. Pd.sub.77.5 Cu.sub.6 Si.sub.16.5) in U.S. Pat. No. 
3,856,513. H. Suto and H. Ishikawa, Trans. Japan Inst. of Metals, V. 17, 
1976, p. 596, report fabrication of glassy Cu-Si by vapor deposition. 
There remains a need in the art for a homogeneous brazing material for 
joining copper and copper alloys that is free of precious metals and can 
be produced in foil, powder or wire form. 
SUMMARY OF THE INVENTION 
The present invention provides a low melting point copper based metal alloy 
composition. Generally stated the composition consists essentially of 
about 2.5 to 11 atom percent Sn, about 11 to 15 atom percent B, balance 
being essentially Cu and incidental impurities. The composition being such 
that the total of Cu and Sn ranges from about 85 to 89 atom percent. 
Preferably, the metal alloy composition has at least partially glassy 
structure. 
In addition, the invention provides a homogeneous, ductile brazing foil 
having a composition consisting essentially of about 2.5 to 11 atom 
percent Sn, about 11 to 15 atom percent B, balance being essentially Cu 
and incidental impurities with total of Cu and Sn ranging from about 85 to 
89 atom percent. Preferably the brazing foil of this invention is at least 
partially glassy and consists essentially of about 75 to 78 atom percent 
copper, about 10 to 11 atom percent Sn and about 11 to 13 atom percent B. 
It has been found that the addition of Sn markedly increases the strength 
of joints brazed with alloys of this invention. The presence of the 
metalloid component, B, serves to depress the melting point of the Cu 
constituent and provides the alloy with self-fluxing capability. 
The homogeneous brazing foil of the invention is fabricated by a process 
which comprises forming a melt of the composition and quenching the melt 
on a rotating quench wheel at a rate of least about 10.sup.5 
.degree.C./sec. 
Further, there is provided in accordance with the invention, an improved 
process for joining two or more metal parts by brazing. The process 
comprises: 
(a) interposing a filler metal between the metal parts to form an assembly, 
the filler metal having a melting temperature less than that of any of the 
metal parts; 
(b) heating the assembly to at least the melting temperature of the filler 
metal; and 
(c) cooling the assembly. The improvement comprises employing, as the 
filler metal, a homogeneous, copper based foil that has the composition 
given above. 
The filler metal foil is easily fabricable as homogeneous, ductile ribbon, 
which is useful for brazing as cast. Advantageously, the copper based 
metal foil can be stamped into complex shapes to provide braze preforms. 
Advantageously, the homogeneous, ductile brazing foil of the invention can 
be placed inside the joint prior to the brazing operation. Use of the 
homogeneous, ductile copper based foil provided by this invention also 
permits brazing to be accomplished by processes such as dip brazing in 
molten salts, which are not readily accomplished with powder or rod-type 
fillers. 
DETAILED DESCRIPTION OF THE INVENTION 
Glassy metal alloys are formed by cooling a melt of the desired composition 
at a rate of at least about 10.sup.5 .degree.C./sec. A variety of rapid 
quenching techniques, well known to the glassy metal alloy art, are 
available for producing glassy metal powders, wires, ribbon and sheet. 
Typically, a particular composition is selected, powders or granules of 
the requisite elements in the desired portions are melted and homogenized, 
and the molten alloy is rapidly quenched on a chill surface, such as 
rapidly rotating cylinder, or in a suitable fluid medium, such as water. 
Copper based brazing alloys have been fabricated by processes such as those 
described above. 
In any brazing process, the brazing material must have a melting point that 
will be sufficiently high to provide strength to meet service requirements 
of the metal parts brazed together. However, the melting point must not be 
so high as to make difficult the brazing operation. Further, the filler 
material must be compatible, both chemically and metallurgically, with the 
materials being brazed. The brazing material must be more noble than the 
metals being brazed to avoid corrosion. Ideally, the brazing material must 
be in ductile foil form so that complex shapes may be stamped therefrom, 
Finally, the brazing foil should be homogeneous, that is, contain no 
binders or other materials that would otherwise form voids or 
contaminating residues during brazing. 
In accordance with the invention, a homogeneous, ductile brazing material 
in foil form is provided. The brazing foils include compositions ranging 
from about 2.5 to 11 atom percent Sn, about 11 to 15 atom percent B, 
balance being essentially Cu and incidental impurities. 
These compositions are compatible with copper and copper-based alloys and 
are particularly suited for joining these materials. 
By homogeneous is meant that the foil, as produced, is of substantially 
uniform composition in all dimensions. By ductile is meant that foil can 
be bent to a round radius as small as ten times the foil thickness without 
fracture. 
Examples of brazing alloy compositions within the scope of the invention 
are set forth in Table I. 
Within the broad range disclosed above, there is a preferred composition 
range that is compatible with and permits brazing of copper and a wide 
range of copper alloys under a wide range of atmospheric conditions. Such 
preferred composition range permits copper and copper alloys to be joined 
under substantially all brazing conditions. A specially preferred alloy 
composition of the present invention consists essentially of about 77 atom 
percent Cu, about 11 atom percent Sn and about 12 atom percent B. 
Further, in accordance with the invention, an improved process for joining 
two or more metal parts is disclosed. The process comprises: 
(a) interposing a filler metal between the metal parts to form an assembly, 
the filler metal having a melting temperature less than that of any of the 
metal parts; 
(b) heating the assembly to at least the melting temperature of the filler 
metal; and 
(c) cooling the assembly. The improvement comprises employing, as the 
filler metal, at least one homogeneous, copper based foil having a 
composition within the ranges given above. 
The brazing foils of the invention are prepared from the melt in the same 
manner as glassy metal foils. Under these quenching conditions, a 
metastable, homogeneous, ductile material is obtained. The metastable 
material may be glassy, in which case there is no long range order. X-ray 
diffraction patterns of glassy metal alloys show only a diffuse halo, 
similar to that observed for inorganic oxide glasses. Such glassy alloys 
should be at least 50% glassy to be sufficiently ductile to permit 
subsequent handling, such as stamping complex shapes from ribbons of the 
alloys. Preferably, the glassy metal alloys should be totally glassy, to 
attain superior ductility. 
The metastable phase may also be a solid solution of the constituent 
elements. In the case of the alloys of the invention, such metastable, 
solid solution phases are not ordinarily produced under conventional 
processing techniques employed in the art of fabricating crystalline 
alloys. X-ray diffraction patterns of the solid solution alloys show the 
sharp diffraction peaks characteristic of crystalline alloys, with some 
broadening of the peaks due to desired fine-grained size of crystallites. 
Such metastable materials may also be ductile when produced under the 
conditions described above. 
The brazing material of the invention is advantageously produced in foil 
(or ribbon) form, and may be used in brazing applications as cast, whether 
the material is glassy or a solid solution. Alternatively, foils of glassy 
metal alloys may be heat treated to obtain a crystalline phase, preferably 
fine-grained, in order to promote longer die life when stamping of complex 
shapes is contemplated. 
Foils as produced by the processing described above typically are about 
0.0010 to 0.0025 inch (25.4 to 63.5 .mu.m) thick, which is also the 
desired spacing between bodies being brazed. Such spacing maximizes the 
strength of the braze joint. Thinner foils stacked to form greater 
thicknesses may also be employed. Further, no fluxes are required during 
brazing, and no binders are present in the foil. Thus, formation of voids 
and contaminating residues is eliminated. Consequently, the ductile 
brazing ribbons of the invention provide both ease of brazing, by 
eliminating the need for spacers, and minimal post-brazing treatment. 
The brazing foils of the invention are also superior to various powder 
brazes of the same composition in providing good braze joints. This is 
probably due to the ability to apply the brazing foil where the braze is 
required, rather than depending on capillarity to transport braze filler 
metal from the edge of surfaces to be brazed.

EXAMPLE 1 
Ribbons about 2.5 to 6.5 mm (about 0.10 to 0.25 inch) wide and about 25 to 
60 m (about 0.0010 to 0.0025 inch) thick were formed by squirting a melt 
of the particular composition by overpressure of argon onto a rapidly 
rotating copper chill wheel (surface speed about 3000 to 6000 ft/min). 
Metastable, homogeneous alloy ribbons having at least partially glassy 
atomic structure were produced and the compositions of the ribbons are set 
forth in Table I. 
TABLE 1 
______________________________________ 
Sample No. Cu Sn B 
______________________________________ 
1 atom % 86.5 2.5 11.0 
wt. % 93.0 5.0 2.0 
2 atom % 77.0 11.0 12.0 
wt. % 77.0 21.0 2.0 
3 atom % 74.0 11.0 15.0 
wt. % 76.0 21.0 3.0 
______________________________________ 
EXAMPLE 2 
The liquidus and solidus temperatures, T.sub.L and T.sub.S of the selected 
composition (atom %) Cu.sub.77 Sn.sub.11 B.sub.12 were determined by 
Differential Thermal Analysis (DTA) techniques. The temperatures are set 
forth in Table II. 
TABLE 2 
______________________________________ 
Sample No. 
Composition 
T.sub.L .degree.C. (.degree.F.) 
T.sub.S .degree.C. (.degree.F.) 
______________________________________ 
2 atom % Cu.sub.77 Sn.sub.11 B.sub.12 
898 (1648) 784 (1443) 
______________________________________ 
EXAMPLE 3 
Lap shear test specimens were prepared according to the AWS C 3.2 "Standard 
Method for Evaluating the Strength of Brazed Joints." Copper sheet, 3.175 
mm (0.125") thick was used as the base metal. Ribbons of the selected 
composition (atom %) Cu.sub.77 Sn.sub.11 B.sub.12 having dimensions of 
about 25.4 .mu.m to 38.1 .mu.m (0.001"-0.0015") thick and about 6.35 mm 
(0.25") wide were used as the filler metal. Brazed joints were of the lap 
type with the lap dimension carefully controlled to 6.35 mm (0.25") and 
12.7 mm (0.5"). Specimens were then degreased in acetone and rinsed with 
alcohol. The mating surfaces of the blanks were fluxed using boric acid. 
Lap joints containing the selected brazing ribbon of the invention was 
then assembled by laying ribbons side by side to cover the entire length 
of the lap joint. Specimens were then clamped and torch brazed using 
oxyacetylene flame with 8 psi oxygen and 8 psi acetylene pressure. Brazed 
specimens were then air cooled to room temperature and the flux residue 
was removed by wire brushing. 
For comparative purposes identical joints were prepared using 25.4 .mu.m 
(0.001") thick BCuP-5 foil and 0.157 cm (0.064") dia BAg-1 and BAg-2 rod. 
The nominal compositions and brazing temperature ranges of these filler 
metals are given in Table IIIA and IIIB, respectively. 
TABLE IIIA 
______________________________________ 
Alloy Ag Cu P Zn Cd 
______________________________________ 
BCuP-5 atom % 8.92 80.73 
10.35 -- -- 
wt. % 15 80 5 -- -- 
BAg-1 atom % 37.53 21.24 
-- 22.02 
19.21 
wt. % 45 15 -- 16 24 
BAg-2 atom % 26.71 33.67 
-- 26.44 
13.18 
wt. % 35 26 -- 21 18 
______________________________________ 
TABLE IIIB 
______________________________________ 
Alloy Temp. .degree.C. (.degree.F.) 
______________________________________ 
BCuP-5 704-816 (1300-1500) 
BAg-1 618-760 (1145-1400) 
BAg-2 635-760 (1175-1400) 
______________________________________ 
When the applied filler metal was in rod form (BAg-1 and BAg-2 alloys), a 
clearance of 38.1 .mu.m (0.0015") was kept between the mating surfaces of 
the blank by placing stainless steel spacers at the two edges. The 
assembly was then heated to the brazing temperature range of these alloys 
and the filler metal was applied to one side only. The molten filler metal 
was then drawn by capillary action and covered the entire mating surfaces. 
Mechanical properties of brazed joints having an overlap of 12.7 mm (0.5 
inch) are listed in Table IVA, while mechanical properties of brazed 
joints having an overlap of 6.35 mm (0.25 inch) are set forth in Table 
IVB. 
TABLE IVA 
______________________________________ 
Shear Strength 
Tensile Strength 
Area of 
Alloy Mpa (psi) Mpa (psi) Failure 
______________________________________ 
BCuP-5 44 (6,320) 174 (25,280) Joint 
BAg-1 41 (6,660) 184 (26,640) Joint 
BAg-2 43 (6,240) 172 (24,960) Joint 
Sample 2 
46 (6,610) 182 (26,440) Base 
Metal 
______________________________________ 
TABLE IVB 
______________________________________ 
Shear Strength 
Tensile Strength 
Area of 
Alloy Mpa (psi) Mpa (psi) Failure 
______________________________________ 
BCuP-5 93 (13,440) 185 (26,880) Joint 
BAg-1 72 (10,440) 144 (20,880) Joint 
BAg-2 62 (9,040) 125 (18,080) Joint 
Sample 2 
94 (13,660) 188 (27,320) Base 
Metal 
______________________________________ 
At overlaps of both 12.7 mm (0.5 inch) and 6.35 mm (0.25 inch), the 
selected alloy of the present invention having the composition (atom 
percent) Cu.sub.77 Sn.sub.11 B.sub.12 failed in the base metal, indicating 
the strength of the brazed joint exceeded that of the base metal. On the 
contrary, identical brazements made with the silver containing alloys 
BCuP-5, BAg-1 and BAg-2 failed in the brazed joints at overlaps of 12.7 mm 
(0.5 inch) and 6.35 mm (0.25 inch). Therefore, the selected alloy of the 
present invention having the composition (atom percent) Cu.sub.77 
Sn.sub.11 B.sub.12 produced stronger joint compared to the silver 
containing alloys BCuP-5, BAg-1 and BAg-2. 
Having thus described the invention in rather full detail, it will be 
understood that such detail need not be strictly adhered to but that 
various changes and modifications may suggest themselves to one skilled in 
the art, all falling within the scope of the present invention as defined 
by the subjoined claims.