Nickel base brazing alloy and method

A nickel base brazing alloy, particularly suitable for diffusion brazing of superalloys, including gamma prime nickel base superalloys. The improved brazing alloy consists essentially of the following in weight percent: 12 to 14% chromium, 1 to 3.5% boron, 1.5 to 5% iron, less than 0.06% carbon, and the balance nickel. The brazing alloy and method of this invention is able to fill gaps in the brazed joint up to 0.02 inches, without adversely affecting the joint microstructure. The resultant microstructure of the brazed joint exhibits minorsecondary phase of ultrafine spherical secondary precipitates, in a predominant solid solution matrix resulting in improved ductility, high temperature oxidation and sulfidation resistance and elimination of the requirement for pressure brazing fixtures.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to an improved nickel base brazing alloy, 
particularly suitable for diffusion brazing superalloys, including gamma 
prime superalloys and a method of nonpressure diffusion brazing using the 
improved brazing alloy of this invention. 
Diffusion brazing relies upon solid-state diffusion or movement of metal 
atoms across the interface of the brazed joint between the brazing alloy 
and the base metals. It necessarily follows that diffusion brazing alloys 
are formulated to complement the chemistry of the parts to be joined. 
Diffusion brazing allows are thus generally nickel, iron or cobalt base 
alloys, depending upon the composition of the parts to be joined. High 
strength superalloys have presented a particular problem for diffusion 
brazing because of the limited wettability. The problem is to formulate a 
diffusion brazing alloy which complements the base metals having the 
requisite properties including microstructure and which may be brazed at 
temperatures low enough for commercial application. In general, high 
brazing temperatures adversely affect the properties of the brazed joint. 
Reference is made herein to my copending application for U.S. patent filed 
Dec. 1, 1982, Ser. No. 445,818, now U.S. Pat. No. 4,507,264, which 
discloses a nickel base brazing alloy including chromium, tantalum, boron, 
aluminum and a rare earth, preferably yttrium or lanthanum. The improved 
brazing alloy of this invention has similar concentration of chromium but 
differs in other material respects and permits lower temperature brazing 
and improved wetting. In addition to the prior art patents cited in my 
above referenced copending application, the prior art includes commercial 
diffusion brazing alloys having greater concentrations of chromium, 
without iron. These commercial alloys do not, however, exhibit the 
improved microstructure of the brazing alloy of the present invention, as 
described more fully hereinbelow. Further, the elimination of silicon 
eliminates the precipitation of secondary phases of silicon which may 
result in hard spots and lack of uniformity in the microstructure of the 
brazed joint. 
The applicant also previously developed a nickel base commercial brazing 
alloy including similar concentrations of chromium and iron, with extra 
low carbon, but with a higher level of boron and also including silicon 
for improved braze wetting, as described below. This alloy is not, 
however, suitable for many diffusion brazing applications, especially thin 
sections, where joint ductility and strength are critical, nor on wide gap 
conditions where the silicon does not go into solid solution. Further, the 
brazing temperature of this alloy is relatively high, about 2100.degree. 
F. This alloy also has a very wide spread between the solidus and liquidus 
temperatures, making the alloy more sensitive and difficult to control 
when "superheating" above 2100.degree. F. is utilized, e.g., 
diffusion-brazing of MM-002.degree. blade components at 2175.degree. to 
2225.degree. F., the alloy's primary secondary solution treatment 
temperature. It was, therefore, determined that a more sluggish flowing, 
silicon-free braze alloys would be required for use on superalloys at 
these higher braze temperatures. 
Conventional diffusion brazing alloys require a gap in the parts to be 
joined of less than 0.002 inches to get a good or acceptable 
microstructure. This limitation handicaps and limits joint design freedom 
and braze flow, requiring preplaced braze foils or tapes. In fact, 
comemrcial applications in the aerospace industry have gaps of 0.002 to 
0.006 inches which results in a poor microstructure with conventional 
brazing alloys. Further, microhardness surveys of the brazed joint with 
conventional diffusion brazing alloys exhibit coarse, brittle "Chinese 
script" borides as secondary phases and often the gaps are not filled. To 
obtain an acceptable braze, it is necessary to use a long brazing cycle at 
excessive temperatures or a post braze soak of as much as 24 hours, or 
more. Further, a pressure fixture is recommended to obtain an optimum 
microstructure. The brazing alloy of the present invention eliminates the 
requirements for a pressure fixture and is capable of brazing gaps of 
0.020 inches or more, without adversely affecting the joint 
microstructure. 
SUMMARY OF THE INVENTION 
The brazing alloy of the present invention is suitable for high temperature 
brazing and diffusion brazing of superalloys, at 2225.degree. F., 
including gamma prime strengthened superalloys as well as stainless 
steels, carbon and low alloy steels at 1975.degree. F. As brazed, the 
microstructure of the joint includes ultrafine precipitates surrounded and 
separated by a fine, thick, ductile solid solution interface. The as 
brazed joint has improved ductility, improved high temperature oxidation 
and sulfidation resistance and eliminates the requirement for pressure 
brazing fixtures. 
When brazing is followed by a short diffusion cycle, e.g. four hours at 
1975.degree. F., the joint becomes substantially 100% homogeneous in both 
feed and exit fillets, as well as the joint interface, resulting in 
improved ductility, fatigue and stress rupture properties, increased 
remelt temperature, and shear strength, as will be understood by those 
skilled in the art. 
The nickel base brazing alloy of this invention consists essentially of the 
following composition in weight percent: 
Chromium: 12 to 14% 
Boron: 1 to 3.5% 
Iron: 1.5 to 5% 
Nickel: Balance 
The brazing alloy preferably includes less than 0.06% carbon, more 
preferably less than 0.03% carbon, and may include incidental impurities 
and additions of less than about 0.05%. 
The more preferred embodiment of the brazing alloy of this invention 
includes 1.5 to 2.5% boron and 3 to 4.5% iron. The concentration of boron 
appears to have an affect upon the applicability of the brazing alloy. For 
example, the most preferred embodiment of the brazing alloy of the present 
invention has a nominal concentration of boron of about 2% and is 
particularly suitable for joining honeycomb sections of Inco 617, wherein 
the core and face sheets have a thickness of 0.002 to 0.005 inches, 
respectively, such as used in alternate thermal panels for the space 
shuttle. This alloy brazes at 1975.degree. to 2050.degree. C. and is 
particularly suitable for brazing stainless steels, e.g. AISI-316, 304, 
etc; superalloys, e.g. Inco 625, 617 and 713LC and Mar-M; and low alloy 
steels, e.g. 4130. Short post brazing diffusion cycles, e.g. three to four 
hours, may be used for superalloys and stainless steels as the "as brazed" 
microstructure is predominantly a solid solution in the interface areas. 
Another embodiment of the brazing alloy of this invention has a nominal 
concentration of boron of about 1% and is particularly suitable for 
non-erosive brazing and diffusion brazing of thin section superalloys, 
including Hastelloy X, Inco 625, Hastelloy, etc. and diffusion brazing 
many superalloys at a temperature of 1975.degree. F. to 2100.degree. F. or 
Mar-M 246 at its solution temperature of 2225.degree. F., followed by 
short post braze diffusion cycles at 2175.degree. F. for three to four 
hours. 
The more commercial embodiment of the brazing alloy of this invention, 
having a nominal concentration of boron of about 2%, has the following 
composition, in weight percent: 
Chromium: 12 to 13.5% 
Boron: 1.7 to 2.2% 
Iron: 3 to 4% 
Nickel: Balance 
wherein the concentration of carbon is preferably less than about 0.03%. 
The brazing alloy may include impurities less than about 0.03%, e.g. Mn, 
s, P, Al, Ti and Zr and normally less than 1% Co from the nickel base. The 
nominal composition of one preferred embodiment includes 13% chromium, 2% 
boron, 3.5% iron and the balance nickel. Another preferred embodiment 
includes 12% chromium, 1.8% boron, 3.5% iron and the balance nickel. The 
embodiment of the brazing alloy having a nominal concentration of about 1% 
boron is 13% chromium, 1% boron, 3.5% iron and the balance nickel, also 
preferably including less than about 0.03% carbon. It will be understood 
by those skilled in the art that in commercial practice the concentration 
of chromium will be .+-.0.5% chromium, .+-.0.2% boron and .+-.0.5% iron. 
The brazing alloy of the present invention is silicon free and essentially 
eutectic, exhibiting a very narrow spread between the solidus and liquidus 
temperatures and making the alloy particularly suitable for diffusion 
brazing. Differential thermal analyses of the brazing alloy of this 
invention establishes a spread between the liquidus and solidus 
temperatures of only about 50.degree. F. or less, compared to 150.degree. 
to 350.degree. F. for similar alloys including silicon. This substantially 
reduces the required diffusion time for the brazed joint and makes the 
brazing alloy less sensitive to gap and the process cycle in diffusion 
brazing applications and resulting in a more uniform and improved 
microstructure. The improved microstructure of the as brazed joint with 
the brazing alloy of this invention has a minor secondary phase, which is 
fine grained and very uniform. The primary phase is comprised primarily of 
alpha nickel, exhibiting large ductile eyebrow loops having uniform 
microstructure. The secondary precipitates in narrow gap joints are very 
narrow, uniform and include primarily nickel chromium and iron borides. 
This secondary phase is ultrafine, surrounded and separated by a thick and 
ductile solid solution interface. Further, as described, the brazing alloy 
of the present invention is capable of brazing gaps of 0.020 inches, or 
greater with improved uniform and fine microstructure. This permits the 
use of brazing powders and external reservoir feeding, which is not 
possible in most applications with the prior art brazing alloys which need 
interface preplacement. Other advantages and meritorious features will be 
more fully understood from the following detailed description of the 
preferred embodiments and the appended claims. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS AND METHOD OF THE INVENTION 
Having generally described the compositions of the brazing alloy of this 
invention, the preferred embodiments will now be described with reference 
to specific examples comparing brazed joints formed with the brazing alloy 
of this invention and the prior art. In each of the following examples, 
the composition of the brazing alloy was formulated, melted, and the 
molten alloy was atomized in an inert atmosphere or a vacuum. As described 
above, the brazing alloy may then be applied to the joint in a fine powder 
or paste form, or the brazing alloy may be applied in a continuous tape, 
as will be understood by those skilled in the art. The joint and brazing 
alloy is then heated in a braze furnace, forming a brazed joint, which may 
be followed by a post-braze diffusion heat treatment cycle. The brazed 
joint was then cross-sectioned and the microstructure examined and 
described hereinbelow.

EXAMPLE 1 
A brazing alloy having the following composition in weight percent was 
formulated: 
Chromium: 13.11% 
Boron: 1.19% 
Iron: 3.58% 
Carbon: 0.009% 
Nickel: Balance 
This composition was found to have a solidus temperature of 1065.degree. C. 
or 1949.degree. F. and a liquidus temperature of 1092.degree. C. or 
1998.degree. F. 
The joint was a U-Tee joint of Inconel 625. The brazing alloy was applied 
in a powdered form to the U-Tee joint and brazed in an argon furnace at 
2100.degree. F. for thirty minutes. The furnace was then cooled to 
1975.degree. F. and the post braze diffusion cycle was held for three 
hours. The brazed joint was then sectioned through the fillet area and 
photomicrographs established that the braze was a uniform 100% solid 
solution free of secondary precipitates. 
EXAMPLE 2 
A brazing alloy having the following composition was formulated: 
Chromium: 13.3% 
Boron: 1.8% 
Iron: 3.54% 
Carbon: 0.02% 
Nickel: Balance 
This composition of the brazing alloy had a solidus temperature of 
1067.degree. C. or 1953.degree. F. and a liquidus temperature of 
1080.degree. C. or 1976.degree. F. 
The brazed joint was a U-tee joint of Hastelloy X. Hastelloy X is a nickel, 
chromium, molybdenum alloy strengthened with iron and cobalt. The brazing 
alloy was applied in a powder form to the U-tee joint and brazed in an 
argon furnace at a temperature of 1975.degree. F. for thirty minutes. 
Sections were made to study the as-brazed microstructure and the balance 
of the specimen replaced in the vacuum furnace for PBD (post-braze 
diffusion) studies. The furnace was then reset to 1975.degree. F. and the 
post-braze diffusion cycle was held for three hours. The as-brazed joint 
was then sectioned through the fillet area and the photomicrographs 
established that the braze prior to the post braze diffusion cycle was 
nearly a 100% solid solution of alpha nickel, however, the braze included 
some very fine secondary phases of nickel, iron, chromium and boron. 
Following the post brazed diffusion cycle, the secondary phases were 
eliminated and the braze was a uniform 100% solid solution, free of 
secondary precipitates. 
EXAMPLE 3 
A brazing alloy having the following composition in weight percent was 
formulated: 
Chromium: 13.13% 
Boron: 1.97% 
Iron: 3.45% 
Carbon: 0.020% 
Nickel: Balance 
This composition had a solidus temperature of 1067.degree. C. or 
1953.degree. F. and a liquidus temperature of 1080.degree. C. or 
1976.degree. F. 
The joint was a U-tee joint of Inconel 625. The brazing alloy was applied 
in a powder form to the U-tee joint and brazed in an argon furnace at 
2050.degree. F. for thirty minutes. The furnace was then cooled to 
1975.degree. F. and the postbrazed diffusion cycle was held for three 
hours. The brazed joint was then sectioned through the fillet area and 
photomicrographs established that the braze included a solid solution zone 
at the interface between the parts joined with a thick solid solution at 
the fillet interface and a very uniform matrix with fine rounded borides 
in the matrix secondary phase, providing an excellent brazed joint with 
good toughness. 
EXAMPLE 4 
A brazing alloy having the following composition in weight percent was 
formulated: 
Chromium: 12.19% 
Boron: 2.08% 
Iron: 3.39% 
Carbon: 0.010 
Nickel: Balance 
This composition had a solidus temperature of 1925.degree. F. and a 
liquidus temperature of 1994.degree. F. 
The joint was formed of a Hastelloy X base and a 304 stainless steel 
washer. The brazing alloy was applied in a powder form to the joint and 
vacuum braze at 2025.degree. F. for twenty minutes. The brazed joint was 
then sectioned and examined as described above. A further sample as 
described above was subjected to a postbraze diffusion cycle for four 
hours at 1975.degree. F. The braze, prior to the postbraze diffusion 
cycle, included a secondary matrix in the feed fillet consisting of alpha 
nickel spheres in a very fine matrix and the balance being a solid 
solution. These spheres were quite soft and measured RB 76-80 
micro-hardness. This structure is unique in any nickel base braze alloy 
studied to date and is one of the principal claims of this patent 
application. After the diffusion cycle, the brase was a substantially 100% 
solid solution of alpha nickel. A hardness survey indicated that the 
hardness was substantially uniform across the braze and an exceptionally 
soft, RB 80. 
EXAMPLE 5 
A brazing alloy having the following composition in weight percent was 
formulated: 
Chromium: 13.71% 
Boron: 2.57% 
Iron: 3.29% 
Carbon: 0.027% 
Nickel: Balance 
This composition had a solidus temperature of 1013.degree. C. or 
1855.degree. F. and a liquidus temperature of 1060.degree. C. or 
1940.degree. F. 
The first braze was a U-tee joint of Hastelloy X. The brazing alloy was 
applied in a powder form to the U-tee joint and vacumn brazed at a 
temperature of 2050.degree. F. for twenty minutes. The solid solution 
interface had a very fine microstructure, even at high magnifications. The 
secondary matrix structure was very fine and spheridized. A microhardness 
survey was also run across the braze which established that the hardness 
was substantially uniform across the braze and the joint had good 
ductility. 
The same brazing alloy was used to braze a yoke assembly of 416 stainless 
steel, wherein the joint was brazed at 1975.degree. F. for twenty minutes, 
without a postbraze diffusion cycle. The fillet area included a fine 
spheridized structure including nickel chrome and nickel iron chrome solid 
solutions. It will be understood by those skilled in the art that 
conventional nickel base brazing alloys cannot be used to braze 416 
stainless steel as the braze generally becomes porous resulting in a poor 
microstructure. 
Other examples of the brazing alloy of this invention were formulated and 
tested by brazing joints of Hastelloy X and similar superalloys. For 
example, a brazing alloy having the following composition in weight 
percent was formulated: 
Chromium: 12.87% 
Boron: 2.19% 
Iron: 3.29% 
Carbon: 0.011% 
Nickel: Balance. 
The brazing alloy exhibited similar properties to the brazing alloys of 
this invention described above, including an improved microstructure and 
improved ductility. Other examples of the brazing alloy of the present 
invention actually formulated which exhibited the improved microstructure 
and physical properties of the brazing alloy of this invention are as 
follows. A brazing alloy having the following composition in weight 
percent was formulated and brazed: 
Chromium: 13.32% 
Boron: 3.32% 
Iron: 3.8% 
Carbon: 0.021% 
Nickel: Balance. 
Another example of the brazing alloy of this invention included 13.45% 
chromium, 2.89% boron, 2.01% iron, 0.05% carbon and the balance nickel. It 
is believed that these brazing alloys compositions illustrate the range of 
compositions which exhibit the desired improved microstructures and 
properties of this invention. 
It has been found that the concentrations of chromium of less than 12% by 
weight results in inferior corrosion resistence. Alloy composition having 
concentrations of chromium greater than about 15% are not as tough and the 
resultant brazed joints are generally too hard and not as ductile. The 
preferred concentration of chromium results in high temperature oxidation 
and sulfidation resistance and resistance to chemical corrosion. It has 
also been found that boron is a critical temperature suppressant in the 
brazing alloy of this invention and improves the shear strength and stress 
rupture of the brazed joints in the concentrations covered by this patent 
application. Concentrations of boron of less than about 0.75% results in a 
melting temperature which is generally too high for most commercial 
applications. Concentrations of boron greater than about 3.5%, when 
brazed, form brittle borides of nickel, chromium, and iron. Greater 
concentrations of boron also diffuse excessively into the base metal, 
which may be detrimental to the brazed joint, especially in the joining of 
thin section components. 
It is believed that the iron controls the cross diffusion between the braze 
alloy and the base metal in the brazing alloy of this invention as well as 
forming borides and adding to the alloy's solid solution strengthening. 
Concentrations of iron of about 1.5% to 4.0% blocks detrimental 
cross-diffusion. Concentrations of iron greater than about 5%, however, 
results in excessive precipitates with boron and carbon, resulting in 
embrittlement of the joint. 
Having described the preferred compositions of the brazing alloy of this 
invention and the preferred method of brazing, it is now possible to 
compare the brazing alloy of this invention with some present or 
commercial nickel based brazing alloys, which are described more fully in 
the following examples. 
EXAMPLE 6 
AMDRY 770 is a commercial brazing alloy offered by the assignee of the 
present invention although this alloy is generally not considered a 
diffusion brazing alloy. AMDRY 770A was formulated having the following 
composition in weight percent: 
Chromium: 7.12% 
Boron: 3.13% 
Iron: 3.13% 
Silicon: 4.25% 
Carbon: 0.022% 
Nickel: Balance 
This composition had a solidus temperature of 1760.degree. F. and a 
liquidus temperature of 1881.degree. F. It is noted that the difference 
between the solidus and liquidus temperatures is over 120.degree. F., 
whereas the difference between the solidus and the liquidus temperatures 
of the brazing alloys of the present invention is less than about 
50.degree. F. 
The brazing alloy was applied to the joint to be brazed in a powdered form, 
as described above, and the joint was brazed in an argon furnace. 
In one test, this brazing alloy was utilized to braze a U-tee joint of 
Hastelloy X, wherein the parts were brazed in an argon furnace at 
1950.degree. F. for twenty minutes. The resultant brazed joint includes an 
alpha nickel solid solution at the interface between the braze and the 
face members, however, the braze included an extremely rough exit fillet, 
which included voids and coarse secondary precipitates of nickel silicides 
and nickel, iron chrome borides. The secondary phase included multiple 
cooling cracks in these brittle continuous phases. This composition was 
also utilized to braze a U-tee joint of Hastelloy X, wherein the joint was 
brazed in hydrogen at 1930.degree. F. for thirty minutes. The brazed joint 
included coarse secondary precipitates in the braze in the exit fillet, 
which are subject to cracking during cooling. Finally, this composition 
was utilized to braze a T-section of Inconel 625, wherein the braze 
included a solid solution of alpha nickel and a secondary phase of coarse 
borides and silicide precipitates, as described above. 
EXAMPLE 7 
AMDRY 775 is a commercial nickel base brazing alloy of the assignee of the 
present invention. AMDRY 775 has the following nominal composition in 
weight percent: 
Chromium: 15.50% 
Boron: 3.50% 
Carbon: 0.06% or greater 
Nickel: Balance 
A commercial sample of AMDRY 775 was analyzed and found to have the 
following composition, in weight percent: 
Chromium: 15.37% 
Boron: 3.95% 
Iron: 0.25% (impurity) 
Carbon: 0.062% 
Nickel: Balance 
A stainless steel lap flow washer was brazed to a Hastelloy X base, as 
described above, and cross-sections were made. The braze had three phases, 
including a solid solution of alpha nickel phase adjacent to each at 
interface of the base metals, a very coarse and a gross "Chinese script" 
secondary phase in both the feed and exit fillets. The braze also included 
a void at the exit fillet and the stainless steel washer showed strong 
nickel, chromium, boron, bulky grainboundry diffusion into the stainless 
steel. The secondary phases was relatively coarse. Post brazed diffusion 
did not eliminate the secondary phases. 
EXAMPLE 8 
AMDRY 915 is a commercial nickel based brazing alloy available from the 
Assignee of the present invention. AMDRY 915 has the following nominal 
compositions, in weight percent: 
Chromium: 13% 
Boron: 2.8% 
Iron: 4% 
Silicon: 4% 
Carbon: 0.030% 
Nickel: Balance 
A commercial analysis of AMDRY 915 was as follows, in weight percent: 
Chromium: 12.65% 
Boron: 2.74% 
Iron: 4.31% 
Silicon: 4.18% 
Cobalt: 0.19% (minor trace element from the nickel base) 
Carbon: 0.044% 
Nickel: Balance 
A stainless steel lap-flow washer was brazed to a Hastelloy X base, using 
this alloy, as described above, and the brazed joint was cross-sectioned. 
The brazed joint included an alpha nickel primary phase, adjacent to the 
base members, and intermittent fine secondary phases of 
nickel-iron-chromium silicides, plus nickel-chrome-iron borides. A 
post-braze diffusion cycle reduced the secondary phases; however, the 
braze retained considerable secondary precipitates in the 0.006 inch joint 
interface and fillets. 
Based upon the above, the broad composition of the brazing alloy of this 
invention preferably includes the following, in weight percent: 12-14% 
chromium, 1-3.5% boron, 1.5-5% iron, less than about 0.06% carbon and the 
balance nickel. It will be understood, however, that the brazing alloy 
will necessarily include some impurities and may include additions, 
including rare earth, up to about 0.05% total, by weight. The most 
preferred composition of the brazing alloy of this invention, however, has 
the following composition in weight percent: 12-14% chromium, 1-2.5% 
boron, 3-4% iron, less than about 0.03% carbon, and the balance nickel. 
The nominal, or most preferred compositions include about 13% chromium, 
about 1-2% boron, about 3.5% iron, less than about 0.03% carbon and the 
balance nickel. In the nominal compositions of the brazing alloy of this 
invention, the concentration of chromium wall range from about 
12.75%-13.25% chromium, the boron concentration will range plus or minus 
about 0.2%, the iron concentration will range from about 3-4%, with the 
balance nickel, excluding carbon and impurities. 
As described above, the brazing alloy of this invention is capable of 
filling base metal gaps in the brazed joint up to 0.02 inches and greater, 
while maintaining a fine grained and uniform secondary phase, which may 
include ultrafine secondary precipitates of nickel, chromium, iron 
borides, and the normally 1975.degree.-2050.degree. F. as-brazed 
microstructure may also include uniform alpha nickel spheres of alpha 
nickel solution at the joint feed fillet area. The resultant braze is very 
ductile, strong and provides high temperature oxidation and sulphidation 
resistance. The brazing alloy of this invention is particularly suitable 
for diffusion brazing super alloys, including gamma prime super alloys, 
because the chemistry closely matches the basic chemistry of many super 
alloys, and the preferred concentration of iron reduces the diffusion of 
boron across the braze interface. Thus, the diffusion brazing alloy of the 
present invention exhibits a substantial improvement over the prior art 
nickel base brazing alloys and provides good flow, joint filling 
properties when exposed to "superheating" temperatures of 
2175.degree.-2225.degree. F. when required to thermally process some super 
alloys at their primary and secondary solution heat treat temperatures.