Brazing of metal parts employing a low temperature, high strength metal alloy is disclosed. The alloy has a composition consisting essentially of about 6.10 to 6.66 atom percent chromium, about 2.43 to 2.46 atom percent iron, about 10.06 to 25.10 atom percent boron, about 3.22 to 12.85 atom percent silicon and the balance essentially nickel and incidental impurities, the composition being such that the total of nickel, chromium and iron ranges from about 71.68 to 74.58 atom percent and the total of boron and silicon ranges from about 25.42 to 28.32 atom percent. Such an alloy is suitable for brazing .gamma.' superalloys and stainless steels at temperatures ranging from about 927.degree.-1010.degree. C. to provide strong, low cost joints.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to brazing of metal parts, and in particular, to a 
homogeneous, ductile brazing material useful in brazing stainless steels 
and superalloys. 
2. Description of the Prior Art 
Brazing is a process for joining metal parts, often of dissimilar 
composition, to each other. Typically, a filler metal that has a melting 
point lower than that of the base 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, corrosion resistant, leak-tight joint is formed. 
Quite often the brazed assemblies are heat treated (or solution treated) 
after brazing. Alternatively, heat treatment or solution treatment of the 
base metal and brazing can be performed simultaneously. Heat treatment (or 
solution treatment) is a procedure that comprises heating the metal part 
to a preselected temperature followed by cooling at a preselected rate to 
achieve desired mechanical properties of the base metal. Solution 
treatment, frequently applied to strengthen superalloys, consists of 
several heating and cooling cycles. Some .gamma.' [Ni.sub.3 
(Al,Ti)]hardened superalloys (e.g., lnconel 718) require solution 
treatment below 1010.degree. C. (1850.degree. F.) to prevent excessive 
grain growth and dissolution of .gamma.', resulting in reduced mechanical 
properties. 
The most widely used brazing filler metal for joining superalloys such as 
lnconel 718 is a gold-nickel alloy (designated by the American Welding 
Society as BAu-4) consisting of 57.6 atom percent gold and 42.4 atom 
percent nickel (82 weight percent gold and 18 weight percent nickel). 
Brazing temperatures employed for this gold-nickel filler metal are in the 
vicinity of 996.degree. C. (1825.degree. F.), and joints formed using such 
filler metal provide good strength and corrosion resistance at elevated 
temperatures. The main drawback of this filler metal is its precious metal 
content, and hence its high price. For this reason, fabricators using 
.gamma.' superalloys have long been on the look out for less expensive 
substitutes. Certain of the brazing alloys designed in the AWS BNi family 
provide mechanical and metallurgical properties that are comparable to 
those of gold-nickel filler metal. However, the brazing temperatures of 
these BNi alloys are greater than 1010.degree. C. (1850.degree. F.). As a 
result, such BNi alloys are not suitable for joining .gamma.' superalloys. 
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 M.sub.a Y.sub.b Z.sub.c, where M is a metal selected 
from the group consisting of iron, nickel, cobalt, vanadium and chromium, 
Y is an element selected from the group consisting of phosphorus, boron 
and carbon, and Z is an element selected from the group consisting of 
aluminum, silicon, tin, germanium, indium, antimony and beryllium, "a" 
ranges from about 60 to 90 atom percent, "b" ranges from about 10 to 30 
atom percent and "c" ranges from about 0.1 to 15 atom percent. Also 
disclosed are glassy wires having the formula T.sub.i X.sub.j where T is a 
transition metal or mixture thereof and X is an element selected from the 
group consisting of aluminum, antimony, beryllium, boron, germanium, 
carbon, indium, phosphorous, silicon and tin and mixtures thereof and 
where the proportion in atomic percentages by i and j are respectively 
from about 70 to about 87 and from about 13 to about 30, with the proviso 
that i plus j equals 100. The transition metals T are those of Group IB, 
IIIB, IVB, VB, VIs, BIIV and VIII of the Periodic Chart of the Elements 
and include the following: scandium, yitrium, lanthanum, actinium, 
titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, 
molybdenum, thenium, osmium, cobalt, rhodium, iridum, nickel, palladium, 
platinum, copper, silver, and gold; preferably, Fe, Ni, Co, V, Cr, Pd, Pt 
and Ti. 
There remains a need in the art for an inexpensive brazing alloy in 
homogeneous foil or powder form, the mechanical and metallurgical 
properties of which would be comparable to those of the aforesaid BAu-4 
alloy. 
SUMMARY OF THE INVENTION 
The present invention provides a metal alloy which is suitable for joining 
.gamma.' superalloys at temperatures ranging from about 
1700.degree.-1850.degree. F. (926.degree.-1010.degree. C.) but which is 
much less expensive than alloys such as BAu-4 previously used. Generally 
stated, the alloy consists essentially of about 6.10 to 6.66 atom percent 
chromium, about 2.43 to about 2.66 atom percent iron, about 10.06 to 25.10 
atom percent boron, about 3.22 to 12.85 atom percent silicon, and the 
balance essentially nickel and incidental impurities, the composition 
being such that the total of nickel, chromium and iron ranges from about 
71.68 to 77.68 atom percent and the total of boron and silicon ranges from 
about 22.32 to 28.32 atom percent. 
In addition, the invention provides a process for joining together two or 
more metal parts comprising the steps of: 
(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, wherein the improvement comprises employing at 
least one homogeneous, ductile filler metal foil having a composition 
consisting essentially of about 6.10 to 6.66 atom percent chromium, about 
2.43 to about 2.66 atom percent iron, about 10.06 to 25.10 atom percent 
boron, about 3.22 to 12.85 atom percent silicon, the balance being 
essentially nickel and incidental impurities, the composition being such 
that the total of nickel, chromium and iron ranges from about 71.68 to 
77.68 atom percent and the total of boron and silicon ranges from about 
22.32 to 28.32 atom percent. 
The principal objective of the present invention is to provide brazing 
alloys having brazing temperatures below 1010.degree. C. (1850.degree. F.) 
and which do not contain noble metals. In addition, the mechanical 
properties of brazements made with compositions selected from these alloys 
are comparable to these made with BAu-4 filler metal. Since the alloys of 
the present invention contain substantial amounts of boron (10.06 to 25.10 
atom percent) and silicon (3.22 to 12.85 atom percent), which are present 
in the solid state as hard and brittle borides and silicides, the 
preferred method for fabrication of these alloys into a flexible thin foil 
form is that of rapid solidification on a moving chill surface. Foil 
produced in this manner is composed of metastable material having at least 
50% glassy structure with a thickness of less than 76 .mu.m (.003"). Other 
methods, such as (1) rolling (2) casting or (3) powder metallurgical 
techniques can be applied to fabricate these alloys to a foil form. It has 
been found that use of a thin flexible and homogeneous foil, is beneficial 
for joining wide areas with narrow clearances. The alloys of the invention 
can also be produced in powder form by atomization of the alloy or 
mechanical communication of a foil composed thereof.

DETAILED DESCRIPTION OF THE INVENTION 
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 to make the brazing operation difficult. 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 
metal 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 foil is less than 76 .mu.m (0.003") 
preferably about 38 .mu.m (0.0015") to 63.5 .mu.m (0.0025") thick and most 
preferably about 12.7 .mu.m (0.0005") to 38 .mu.m (0.0015") thick. 
Preferably the brazing foil has a composition consisting essentially of 
about 6.10 to 6.66 atom percent chromium, about 2.43 to 2.66 atom percent 
iron, about 10.06 to 25.10 atom percent boron and about 3.22 to 12.85 atom 
percent silicon, the balance being essentially nickel and incidental 
impurities. The composition is such that the total of nickel, chromium and 
iron ranges from about 71.68 to 77.68 atom percent and the total of boron 
and silicon comprises the balance, that is, about 22.32 to 28.32 atom 
percent. These compositions are compatible with all stainless steels, as 
well as nickel and cobalt based alloys. 
By homogeneous is meant that the foil, as produced, is of substantially 
uniform composition in all dimensions. By ductile is meant that the 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 below: 
TABLE I 
______________________________________ 
Example 
No. Ni Cr B Si Fe 
______________________________________ 
1 (wt %) 82 7 6 2 3 
(at %) 63.15 6.10 25.10 3.22 2.43 
2 (wt %) 82 7 5 3 3 
(at %) 64.83 6.25 21.47 4.96 2.49 
3 (wt %) 80.5 7 2.2 7.3 3 
(at %) 67.77 6.66 10.06 12.85 
2.66 
4 (wt %) 83 7 3 4 3 
(at %) 69.92 6.66 13.72 7.04 2.66 
5 (wt %) 82 7 3 5 3 
(at %) 68.45 6.60 13.60 8.72 2.63 
6 (wt %) 81 7 3 6 3 
(at %) 67.00 6.54 13.48 10.37 
2.61 
7 (wt %) 80.5 7 3 6.5 3 
(at %) 66.28 6.51 13.42 11.19 
2.60 
8 (wt %) 81.5 7 4.5 4 3 
(at %) 65.01 6.30 19.50 6.67 2.52 
9 (wt %) 82.7 7 5 2.3 3 
(at %) 65.78 6.29 21.60 3.82 2.51 
______________________________________ 
The brazing temperature of some of the brazing alloys of the invention are 
below 1010.degree. C. (1850.degree. F.). The temperature of brazing is 
thus in the solution treatment range of .gamma.' superalloys. This will 
also enable fabricators to braze and heat treat .gamma.' superalloys 
simultaneously. 
The brazing foils of the invention are prepared by cooling a melt of the 
desired composition at a rate of at least about 10.sup.5 .degree. C./sec, 
employing metal alloy quenching techniques well-known to the glassy metal 
alloy art; see, e.g., U.S. Pat. Nos. 3,856,513 and 4,148,973 discussed 
earlier. The purity of all compositions is that found in normal commercial 
practice. 
A variety of techniques are available for fabricating continuous ribbon, 
wire, sheet, etc. 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 metal cylinder. 
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 must 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 must be at least 80% glassy, and most preferably substantially (or 
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 
burdening of the peaks due to desired fine-grained size of crystallites. 
Such metastable materials are also 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 13 
.mu.m (0.0005) to 76 .mu.m (0.003) inch thick, which is also the desired 
spacing between bodies being brazed. Foil thickness, and hence spacing of 
about 13 .mu.m (0.005) to 36 .mu.m (0.0014) inch maximizes the strength of 
the braze joint. Thinner foils stacked to form a thickness of greater than 
0.0025 inch may also be employed. Further, no fluxes are required during 
brazing, and no binders are present in the foil. Thus, formation of voids 
and contamination 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. 
EXAMPLE 1 
Ribbons of about 2.54 to 25.4 mm (about 0.10 to 1.00 inch) wide and about 
13 to 76 .mu.m (about 0.0005 to 0.003 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 ribbons of substantially glassy alloys 
having the compositions set forth in Table 1 were produced. 
EXAMPLE 2 
The liquidus and solidus temperatures of the ribbons mentioned in Example 1 
were determined by Differential Thermal Analysis (DTA) Technique. The 
individual samples were heated side by side with an inert reference 
material at a uniform rate and the temperature difference between them was 
measured as a function of temperature. The resulting curve, known as a 
thermogram, was a plot of heat-energy change vs. temperature, from which 
the beginning of melting and end of melting, known respectively as solidus 
and liquidus temperatures, were determined. Values thus determined are set 
forth in Table 11 below. 
TABLE II 
______________________________________ 
Liquidus Solidus 
Sample No. .degree.C. 
.degree.C. 
______________________________________ 
1 1100 978 
2 1075 960 
3 1030 961 
4 1026 993 
5 1008 963 
6 993 960 
7 997.5 960.5 
8 1053 958 
9 1067 993 
______________________________________ 
EXAMPLE 3 
Tensile test specimens of dimensions 2.54 cm.times.15.24 cm .times.0.158 cm 
thick (1".times.6".times.0.0625") were cut from Inconel 718 superalloy. 
The brazing alloys of the invention, glassy ductile ribbons of nominal 
chemical compositions of samples 6 and 7 as in Table I having dimensions 
of about 38 .mu.m (0.0015") thick by 6.35-12.7 mm (0.25-0.5") wide, were 
used to braze test specimens. Braze joints were of the lap type, with lap 
dimension carefully controlled to 3.175 mm (0.125"). Brazing specimens 
were decreased in acetone and rinsed with alcohol. Lap joints containing 
brazing ribbons of the invention were assembled by laying ribbons side by 
side to cover the entire length of the lap joint. In the case of these 
brazing alloys, ribbons acted as joint spacers. Specimens were then tack 
welded by gas tungsten arc welding to hold the assembly together. 
For comparative purposes, samples were made in an identical manner to that 
described above using 38 .mu.m (0.0015") thick.times.2.54 cm (1") wide 
BAu-4 foil. 
Brazing was done in a vacuum furnace at a vacuum of about 10.sup.-4 torr 
for about 5 minutes. Brazing temperatures of the alloys of the invention 
were about 25.degree. C. (50.degree. F.) higher than the liquidus 
temperature of each alloy as given in Table Il. Each of the BAu-4 samples 
was brazed at 996.degree. C. (1825.degree. F.). After brazing, the 
specimens were machined to produce a specimen as shown in FIG. 1 having 
the following under dimensions: 
a.+-.1.050" (2.667 cm); b=0.812".+-..005" (2.072 cm.+-..0127); 
c=0.375R"(0.953R cm) where R is the radius of the arc; d=2.25".+-..010 
(5.715 cm.+-..0254; e-0.5" (1.27 cm); f=0.375R".+-..003/-0.000, where R is 
the radius o the hole; g=2b.+-.0.0625"(0.158 cm); and, h=0.0625" (0.158 
cm). 
Machined specimens were then solution treated in the following way to 
achieve optimum strength of the base metal: 980.degree. C. (1800.degree. 
F.)/1 hr/air cool +720.degree. C. (1325.degree. F.)/8 hr/furnace cool and 
620.degree. C. (1150.degree. F.)/8 hr/air cool to a base metal hardness of 
Rc 38-41. 
Tensile tests were conducted at 538.degree. C..+-.5.degree. C. 
(1000.degree. F. .+-.10.degree. F.) since superalloys such as lnconel 718 
are usually in service at elevated temperatures. The test results are 
reported in Table III. 
TABLE III 
______________________________________ 
Tensile 
Sample Shear Stress Stress Area of 
No. MPa (psi) MPa (psi) Failure 
______________________________________ 
6 362 725 Base 
(52,560) (105,170) Metal 
7 352 703 Base 
(51,040) (102,080) Metal 
BAu-4 337 687 Joint 
(48,813) (99,627) 
______________________________________ 
At an overlap of 0.3175 cm 0.125"), all the brazed samples made with the 
alloys of the invention failed in the base metal, indicating that the 
strength of the brazed joint was greater than that of the base metal. In 
other words, the base metal failed before the brazed joint. By way of 
contrast, each of the BAu-4 brazements failed in the brazed joint. This 
data indicates that joints constructed with an overlap of 0.317cm (0.125") 
and then brazed with the alloys of invention were stronger than joints 
having the aforesaid construction that were brazed with the BAu-4 alloy. 
EXAMPLE 4 
The joint stress rupture strength of some of the alloys of the present 
invention were determined in the following way. Test specimens of 
dimensions 2.5 cm.times.15.24 cm.times.0.158 cm thick 
(1".times.6".times.0.0625") were cut from Inconel-718 superalloy. The 
brazing alloys of the invention, glassy ductile ribbons of nominal 
chemical compositions of samples 6 and 7 as in Table I having dimensions 
of about 38 .mu.m (0.0015") thick by 6.35-12.7 mm (0.25-0.5") wide, were 
used to braze test specimens. Braze joints were of the butt type where the 
bonding area was equal to the cross-section of the test specimens. Brazing 
specimens were decreased in acetone and rinsed in alcohol. The selected 
brazing foils of the present invention were then placed between the mating 
cross-sections of the test specimens and the assemblies were lightly tack 
welded. As a result, after brazing, a clearance between the blanks was 
maintained which was the thickness of the particular brazing foil. Brazing 
was performed in a similar way to that described in Example 3. After 
brazing the specimens were machined to produce a specimen as shown in FIG. 
2 having the following dimensions: 
a'=1.05" (2.667 cm); c' =0.375R" (0.953R cm); d'=2.250" (5.715 cm); e'=0.5" 
(1.27 cm); f=0.375R" (0.953R cm); and h'=0.0625" (0.158 cm). Machined 
specimens were then solution treated in the manner described in Example 3. 
For comparative purposes, samples were made in an identical manner to that 
described above using 38 .mu.m (0.0015") thick.times.2.54 cm (1") wide 
BAu-4 foil. 
Stress rupture tests were conducted at 538.degree. C..+-.5.degree. C. 
(1000.degree. F..+-.10.degree. F.) at a static load of 455 kg (1000 lbs). 
The test results are reported in Table IV. 
TABLE IV 
______________________________________ 
Alloy Time of Failure (Hrs.) 
______________________________________ 
Sample 6 49 
Sample 7 140 
BAu-4 1 
______________________________________ 
It is evident from Table IV, that the joint rupture strength of the 
selected alloys of the present invention are much superior to that of the 
BAu-4 alloy.