Characterization of Inconel 625 Superalloy Braze Joints using 4
Different Brazes
Ryan Kraft, and Rajiv Asthana, University of Wisconsin-Stout
Objective and Significance
Experimental Procedure
Our research investigates how the process of brazing modifies the original
braze composition and braze metallurgy in the vicinity of joined interfaces
and how braze alloys modulate the structure, composition and metallurgical
adhesion to substrates.
1. Cut Inconel on high-speed precision saw, lay braze foil at mating surface,
apply a load (0.30-0.40 N) during brazing.
Developing and demonstrating methods to braze high-temperature alloys are
the practical goals of our broader research effort.
Background
Inconel 625 (melting point: 1350°C) is a general purpose nickel-chromiummolybdenum alloy in the family of Inconel alloys that were developed for
elevated temperature use. Inconel 625 has good corrosion and oxidation
resistance which makes it suitable for harsh environment applications.
Inconel alloys are used in a wide variety of applications, from gas turbine
blades and heat exchangers to NASCAR exhaust systems and X-15 rocket
planes.
In applications, Inconel must be joined to itself or to other alloys to make
complex assemblies .
2. Heat under vacuum (~10-6 torr) to 15-20C above braze TL. After 5 (or 30)
min. soak, slowly cool.
4. Mount joints in epoxy, grind, polish and examine joint microstructure
using optical microscopy (OM) and scanning electron microscopy.
KHN = 252
5. Measure Knoop hardness (Buehler Micromet 2001 Unit, 100 g load, 10 s).
KHN = 415
100 μm
KHN = 278
Composition and Properties of Brazes
Braze
MBF-20[b]
Inconel
The microstructure of the Inconel MBF-20
braze joint show many layers within the
reaction layer, similar to that of MFB-30. The
hardness of the braze was lower than that of
the substrate’s.
3. Heat treat joints in air at two-thirds of absolute braze melting
temperature for 45 min. completed at UW-Stout.
MBF-30[b]
Braze
Inconel/Inconel Joint with MBF-30 Braze
(R2, 1045 C, 5 min)
TiCuSil[a]
CusilABA[a]
Comp., %
TL, K TS, K
YS,
MPa
---
CTE, x10- K, W/m.K %El
6 K-1
----
Ni-4.61Si-2.8B0.02Fe-0.02Co
Ni-6.48Cr-3.13Fe4.38Si-3.13B
1257
1327
E,
GPa
--
1242
1297
--
68.8Ag-26.7Cu4.5Ti
63Ag-35.3Cu1.75Ti
1173
1053
85
292
18.5
219
28
1088
1053
83
271
18.5
180
42
--
--
Joint Microstructure vs.
Knoop Hardness Profile
Inconel/Inconel Joint with Ticusil Braze
(R19, 920 C, 5 min)
KHN = 335
Reaction Layer
-KHN = 121
KHN = 660
100 μm
[a]Morgan
Advanced Ceramics, [b]Honeywell, Inc., TL: liquidus temperature, TS:
solidus temperature, E: Young’s modulus, YS: yield strength, CTE: coefficient of
thermal expansion, K: thermal conductivity, El: percent elongation.
Inconel
Sample and Identification
Research Approach
•Demonstrate vacuum brazing of Inconel 625 using four brazes of different
compositions .
•Characterize joint microstructure using optical microscopy and scanning
electron microscopy, and hardness distribution using the Knoop test.
•Heat treat vacuum-brazed joints in air at two-thirds of the absolute melting
point of braze to simulate service conditions and characterize
microstructure and hardness.
•Heat treat unbonded braze foils and compare through-thickness braze
microstructure, composition, and hardness with microstructure,
composition, and hardness of braze foils in joints.
•Analyze metallurgical changes that occur in brazing and subsequent heat
treatment, and compare and contrast effectiveness of different braze
compositions to join Inconel 625.
Sample ID
Substrates
Braze alloy
R2
Inconel/inconel
MBF-20
Braze
Temperature(°C)
1045
R4
Inconel/inconel
MBF-30
Inconel/Inconel Joint with Cusil-ABA Braze
(R9, 835 C, 5 min)
Time(min) Weight(g)
5
40
1045
5
Inconel/inconel
Cusil-ABA
835
5
40
R19
Inconel/inconel
Ticusil
920
5
110
KHN = 335
KHN = 87
Results and Data
100 μm
Inconel/Inconel Joint with MBF-20 Braze
(R4, 1045 C, 5 min)
The microstructure of the
MBF-20/Inconel braze joint
contained multiple layers
within the reaction layer.
The hardness of the inner
brazed region was very
high, while the other layers
were softer than the
surrounding Inconel
substrate.
KHN = 321
KHN = 501
The Cusil ABA/Inconel
reaction layer was very
narrow. Although the
region is visible no
hardness data could be
obtained.
40
R9
KHN = 1197
Cu-Ag System
(base alloy for Cusil-ABA and Ticusil)
Joint Microstructure
Reaction Layer
Binary Phase Diagrams of Major Braze
Constituents
Ni-Cr System
(base alloy for MBF-20)
Knoop Hardness profile
across braze joint
It appears that the
microstructure of the
Ticusil/Inconel interface
contains a reaction layer
extending from the brazed
region into the Inconel
substrate along the
Inconel’s grain boundaries.
The reaction layer was
much harder than the braze
region or Inconel substrate.
Knoop Hardness profile
across braze joint
Joint Microstructure showing
eutectic structure
Continuing Research
-Complete microstructure characterization using scanning electron microscopy and elemental
analysis of brazed joints using energy dispersive spectroscopy
-Complete polishing, microscopy, elemental analysis and microhardness of unbonded braze foils
(heat treated and non-heat treated). Compare and contrast with the behavior of heat treated
and non heat treated braze foils within joints.
-Measure shear strength of joint.
100μm
KHN = 281
Joint Microstructure vs. Knoop
Hardness Profile
Acknowledgement
• NASA Glenn Research Center, Ceramics Branch, for the materials used in the study and for use
of high vacuum brazing furnace.
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