ORNL-1771 - Oak Ridge National Laboratory

ANP QUARTERLY PROGRESS REPORr
Development of Brazing Alloys for
Use in Fabricating Radiatars
K. W. Reber
P. Patriarca
G. M. Slaughter
Metallurgy Division
J. M. Cisar
Aircraft Reactor Engineering Division
Several alloys were investigated for brazing
copper fins clad with Inconel, type 310 stainless
steel, or type 430 stainless steel in order to es-
tablish an optimum combination for fabrication of a
sodium-to-air radiatoi. The evaluation of these
alloys was based on metallographic examination of
tube-to-fin specimens before and after exposure to
static air for periods up to 600 hr. The alloys
tested are listed in Table 7.3,
TABLE 7.3. BRAZING ALLOYS FOR
HIGH-CONDUCTIVITY CLAD COPPER FBNS
Saazing
Composition
Alloy Name Temperature
(wt %I
. . . . . . . . . .
~~ ~....~
(Or=)
__ ..... . .-~.
Low-melting-point 80 Ni, 6 Fe, 5 Cr, 1920
Nicrobraz (LMNB) 5 Si, 3 B, 1 C
Coast Metals alloy 89 Ni, 5 Si, 4 €3, 1840
52 2 Fe
Electroless nickel 88 Ni, 12 P 1800
Ni-P-Cr 80 Ni, 10 P, 10 Cr 1800
A microsection is shown in Fig. 7.6 of an lnconelclad
copper fin (2 mils of lnconel on each side of
6-mil-thick copper) ioined to 3 /,6-in.-OD, 0.025-in.-
wall Inconel tubing with 88% Ni-12% P alloy and
then exposed to static air at 1500°F for 600 hr.
Solution of the tube wall during the brazing cycle
was negligible, but rather severe solution of the
fin lip occurred. It is evident, however, that the
oxidation resistance of the joint was not affected.
Examinations of specimens brazed with the other
alloys listed in Table 7.3 revealed similar results,
that is, minimumsolution of tube walls and ex-
cellent oxidation resistance of the braze joints.
The effects of diffusion, however, were in evi-
dence after each test. Color changes in the copper
core, along with the formation of voids, indicate
that the use of Inconel-clad copper in a radiator
would not be advisable, since the void formation
118
and the change in core chemistry from copper to a
copper-nickel alloy would seriously reduce the heat
transfer efficiency of the fin. Other claddings, such
as types 310 and 430 stainless steel, have been
found to be relatively immune to diffusion effects,
however, and Q number of brazed joints were evalu-
ated to determine the optimum alloy for brazing each
of these materials to Inconel tubing.
A joint of type-310 stainless-steel-clad copper
brazed to lnconel tubing with the 88% Ni-12% P
alloy is shown in Fig. 7.7. It may be seen that fin
dilution and intergranular penetration of the resulting
brazing alloy into the tube wall occurred. This
penetration, undetected in the case of Inconel-clad
copper, appears to be due to the formation of a
complex eutectic resulting from the presence of an
iron-base cladding material rather than from a
nickel-base material. Siini Iar intergranular pene-
tration was found when the 88% Ni-12% P alloy
UNCLASSIFIED
0.02
Fig, 7.6, Inconel-Clad Copper Fins Brazed to
Inconel Tubing with 88% Ni-12% P Alloy and
Exposed to %tic Air at 1500° for BOO hr. Note
dilution of fin, hole formation in copper due to
diffusion, and the excellent oxidation resistance
of the bronze joint. As polished. 50X. Reduced
1.5%.
.