WELDING RESEARCH
SUPPLEMENT TO THE WELDING JOURNAL, OCTOBER 1994
Sponsored by the American Welding Society and the Welding Research Council
Brazeability of Aluminum in Vacuum-Nitrogen
Partial-Pressure Atmosphere Brazing
Brazing process effectively minimizes volatile metal elements evaporation
BY T. HATTORI, S. SAKAI, A. SAKAMOTO AND C. FUJIWARA
ABSTRACT. In vacuum brazing, Al-10%
Si-1.5% Mg filler metal is used. The filler
metal is melted (solidus temperature: 832
K, 559°C; liquidus temperature: 864 K,
591°C), and magnesium in the filler
metal
evaporates
actively.
The
magnesium gas is the effective getter of
contaminants such as H 2 0 and 0 2 ,
which form an oxide film on the surface
of
aluminum
alloys,
lowering
brazeability. Volatile elements also
evaporate and material properties
change in high-vacuum brazing. For
example, heat exchangers made with
aluminum alloys use Al-Zn alloy for the
cathodic corrosion protection of other
aluminum alloys.
The vapor pressure of zinc in the AlZn alloy is high because zinc is a volatile
element, but Al-Zn alloy does not melt at
the brazing temperature, w h i c h is
approximately 873 K (600°C), and zinc
does not evaporate actively compared
with magnesium. However, evaporation
of voltatile elements and change in
material properties can be minimized in
vacuum-nitrogen
partial-pressure
atmosphere brazing, and Al-Zn alloy
may be used as a sacrificial alloy in
products made with aluminum alloys.
In this study, brazeability in vacuumnitrogen partial-pressure atmosphere
was investigated using T-joints with
horizontal Al-Mn or Al-Zn alloy sheet
and vertical A4004 clad A3003 alloy
brazing sheet. Specimens were brazed
over a wide range of brazing pressures
and N 2 carrier gas flow rates. The brazing
temperature and brazing time were 873
K (600°C) and 5 minutes, respectively.
Gas
contaminants
in
brazing
atmospheres were measured using a
quadruple mass spectrometer.
It was found that a higher carrier gas
flow rate gave better brazeability. Also,
many magnesium-rich swells grew at the
surface of the brazing sheets under the
low N 2 gas flow rate conditions, and the
surface of the brazing sheet was very
rough.
On the other hand, the partial
T. HATTORI, S. SAKAI, A. SAKAMOTO and C.
FUJIWARA are with Mitsubishi Heavy Industries, Ltd., Nagoya, Japan
KEY W O R D S
Aluminum Brazeability
Brazing in Vacuum
N 2 Partial Pressure
Partial-Pressure Brazing
N 2 Carrier Gas
Al-Mn/A4004 Cladding
Clad A3003 Sheet
Al-Mn or Al-Zn Sheet
Al-Si-Mg Filler Metal
AI-10%Si-1.5%Me
pressure of H 2 0 increased according to
the increase in brazing pressure, but it
did not change according to the increase
in the carrier gas flow rate at the same
brazing pressure. Therefore, brazeability
is not decided by the partial pressure of
H 2 0 in the brazing atmosphere, and it is
understood that brazeability is improved
by the higher evaporation of magnesium
from the brazing sheets according to the
increase in the carrier gas flow rate.
Introduction
The application of aluminum alloys to
machinery is spreading widely because
of its high specific strength. For this
reason, the brazing process of aluminum
alloys is very important as a highprecision fabrication technique. Highvacuum brazing with Al-Si-Mg alloy filler
metal is the most common industrial
application
because of
its high
productivity. However, volatile elements
such as zinc evaporate in vacuum
brazing and the material properties
change sometimes after brazing. For
example,
zinc
in
Al-Zn
alloys
evaporates, and the sacrificial anode
effects of Al-Zn alloys are eliminated.
The relationship between the brazing
pressure and the zinc residual ratio is
shown in Fig. 1 (Ref. 1).
To minimize the evaporation of
volatile elements such as zinc, vacuumnitrogen partial-pressure atmosphere
brazing is very effective. Reduction of the
W E L D I N G RESEARCH SUPPLEMENT I 233-s
1.0
o.a
g
3
2
c
0.6
0.4
• Material
A7072 clad A3003
• Brazing condition
873K(600°C)X5min
y
/
N
0.2
0
0~"
10~2
10'
102
104
Brazing pressure (Pa)
Fig. 1 — Relation between zinc residual
ratio and brazing pressure,
evaporation rate of elements by an inert
gas atmosphere is expressed according to
Epstein (Ref. 2), by the f o l l o w i n g
approximate formula:
Mig)
M(v)
2.7367"
IP
n+
Wm/Wgh
1+
•/ pmh
(1)
Where M(g) = grams of metal/cm 2 -s
evaporated in the gaseous atmosphere;
M(v) = grams of metal/cm 2 -s evaporated
in vacuum; T = absolute temperature; P
= pressure of inert gas atmosphere,
dyne/cm 2 ; A, = to a first approximation
the length of the heated section of the
system above the metal surface, cm; Wg
= molecular weight of inert gas; W m =
molecular weight of volatile metal; pg =
density of inert gas, g/cm 3 ; pm = density
of volatile metal vapor, g/cm 3 ; b = Van
der Waal's constant of inert gas,
cm 3 /mole.
According to Equation 1, when the
temperature and amount of the volatile
metal are kept constant, the reduction of
the metal evaporation rate is made more
effective by introducing the heavier inert
gas, and the evaporation rate is then in
inverse proportion to the inert gas
pressure.
Fromm (Ref. 3) determined that the
semi-empirical formula M(g)/M(v) = (1 +
0.012 x P)-1 (P in Pa) describes the effect
of the pressure of inert gas on the
Fig. 2 — Shape and size of specimen.
reduction of metal evaporation. The
above formula by Fromm was decided to
be at the center of the relationship
between M(g)/M(V) and atmosphere
pressure for many inert gases and metals.
In this study, the relationship between
the brazeability and the brazing
conditions in the vacuum-nitrogen
partial-pressure atmosphere brazing is
investigated. The relationship between
the brazeability and the magnesium
contents in filler metals in 1 atom N 2 gas
atmosphere
brazing
was
studied
previously (Ref. 4). However, in this
study, the effect of N 2 gas flow rate is not
mentioned.
In our study, the relationship between
brazeability and the brazing presssure,
carrier gas (N 2 ) f l o w rate and gas
contaminants in brazing atmospheres in
vacuum-nitrogen
partial
pressure
atmosphere brazing with the constant
magnesium content filler metal was
investigated. Also, in the study by
Schmatz and Winterbottom (Ref. 4), they
tell that the suitable Mg content is 0.2 ~
0.5%, and a magnesium content more
than 0.75% is not good. But in our study,
the filler metal containing 1.5% Mg clad
to A I - 1 % Mn alloy was used. This filler
metal is used in vacuum brazing.
Experimental Procedure
Materials
The chemical compositions of the
materials used in this study are
summarized in Table 1. The bare
materials are Al-Mn alloy (A3003), Al-
1 % Zn alloy, and Al-2% Zn alloy. The AlZn alloys are the most common industrial
sacrificial anode materials. The brazing
sheet is the A4004-clad A3003 alloy,
which is the traditional brazing sheet in
high-vacuum brazing. The A4004-clad
A3003 brazing sheet solidus temperature
is 832 K (559°C), and liquidus
temperature is 864 K (591 °C), brazing
temperature is 863 ~ 878 K (590° ~
605°C) (Ref. 5). The thickness of the
cladding is 10% of the sheet thickness.
The thickness of both materials together
is 0.5 mm, and the shape and size of both
materials are shown in Fig. 2. The
materials were rinsed w i t h acetone
before brazing.
Brazing Conditions
The brazing specimen is shown in Fig.
2. The reverse T-joint was brazed with a
vertical brazing sheet and the horizontal
bare material sheet. The combinations of
materials are shown in Table 2, and the
schema of the vacuum-nitrogen partialpressure atmosphere brazing furnace is
shown in Fig. 3. The brazing pressure
was controlled by a N 2 gas flow meter
and the needle valve of the evacuation
system. A 99.99% high-purity-grade N 2
was used as a carrier gas. The analysis of
N 2 gas was done by gas chromatography.
The dew point of this N 2 gas was below
203 K (-70°C), and the content of
contaminant 0 2 gas was under 5 ppm.
The brazing pressures were 67 Pa (0.5
Torr), 267 Pa (2 Torr) and 5320 Pa (40
Torr) and the N 2 gas flow rates were
changed over a wide range at the same
brazing pressure. The N 2 gas flow rate
was decided according to the mass of the
furnace and the range of the needle valve
in the vacuum pump system. Highvacuum brazing was investigated for
comparison with the vacuum-nitrogen
partial-pressure atmosphere brazing.
Before all brazing, the furnace was
evacuated to high-vacuum conditions
10 -3 Pa(l 0 -5 Torr) and the brazing furnace
was baked out at 573 K (300°C) for 10
min. The temperature of the brazing
furnace was then elevated gradually to
873 K (600°C) and held for five minutes.
After the brazing step, the electric power
was cut off, and N 2 cooling gas was
introduced into the furnace at 843 K
(570°C).
Table 1 — Chemical Compositions (mass%)
Alloys
Bare Materials
Brazing Sheet
AI-1%Mn(A3003)
AI-1%Zn
AI-2%Zn
Core
A3003
Filler
A4004
234-s I OCTOBER 1994
0.20
0.08
0.07
0.15
9.50
Fe
Cu
Mn
Mg
Cr
Zn
0.43
0.16
0.16
0.40
0.20
0.12
tr.
tr.
0.10
0.01
1.12
tr.
tr.
1.20
tr.
0.02
tr.
tr.
0.02
1.51
tr.
tr.
tr.
tr.
tr.
0.02
1.05
2.35
0.02
0.01
Al
tr.
tr.
tr.
tr.
0.01
bal
bal
bal
bal
bal
FLOWMETER
Table 2 — Material Combinations
Symbols
Brazing Sheet
Bare Materials
X
MH-1
MH-2
N
A4004-clad A3003
A3003
AI-1%Zn
AI-2%Zn
A3003
t
t
A4045-clad A3003
QUADRUPLE
MASS
SPECTROMETER
FAN
Measurement of Gas Contaminants
EFFECTIVE
HEAT
ZONE
2 0 0 W X 4 0 0 H X 6 0 0 L mm
OIL
DIFFUSION
PUMP
T h e gas c o n t a m i n a n t s in t h e b r a z i n g
atmosphere w e r e measured throughout
the b r a z i n g c y c l e . T h e gas c o n t a m i n a n t s
w e r e i n t r o d u c e d into the q u a d r u p l e mass
s p e c t r o m e t e r b y its o w n e v a c u a t i o n
s y s t e m a n d t h e a m o u n t o f t h e gas
contaminants was measured.
COOLING
GAS
PARTIAL
GAS
(B-O
MECHANICAL
BOOSTER
PUMP
Results
Fig. 3 — Schematic of vacuum-inert
gas atmosphere brazing
ROTARY
PUMP
furnace.
T h e M g contents in the surface layer
of the b r a z i n g sheets is l o w e r in t h e g o o d
brazing c o n d i t i o n c o m p a r e d w i t h the
p o o r b r a z i n g c o n d i t i o n , a c c o r d i n g t o Fig.
1 1 . T h e b r a z i n g sheet surfaces in t h e p o o r
b r a z i n g c o n d i t i o n are r o u g h , a n d t h e i r
M g c o n t e n t is r i c h . W e c o n s i d e r t h a t
swells w e r e f o r m e d because of less M g
e v a p o r a t i o n a n d f o r m a t i o n of M g O at the
<2
oo
o
Brazed ratio (%)
67Pa (0.5Torr)
cn
o
The relationship between the brazed
ratio and the b r a z i n g c o n d i t i o n s w i t h the
reverse T-joint is s h o w n in Figs. 4 - 6 . The
b r a z e d ratio is represented in Fig. 7. T h e
b r a z e d ratio is represented by the ratio of
the b r a z i n g length (55 m m ) to the b r a z e d
l e n g t h . T h e b r a z e d ratio rises to t h e high
v a l u e a c c o r d i n g to the higher N 2 gas f l o w
rate a n d reaches 1 0 0 % in t h e h i g h f l o w
rate c o n d i t i o n . T h e b r a z e d ratio in h i g h v a c u u m b r a z i n g is 1 0 0 % w i t h all
m a t e r i a l c o m b i n a t i o n s (X, M H - 1 a n d
MH-2
in
Table
2). T h e
surface
m o r p h o l o g y of the b r a z i n g sheets after
b r a z i n g is s h o w n in Table 3. T h e surface
of the b r a z i n g sheets u n d e r g o o d b r a z i n g
c o n d i t i o n s is s m o o t h and m e t a l l i c , but in
the p o o r b r a z i n g c o n d i t i o n s , it is r o u g h
and not m e t a l l i c . Scanning electron
p r o b e m i c r o g r a p h s of the cross-sections
at t h e c e n t e r o f b r a z i n g sheets after
b r a z i n g are s h o w n in Figs. 8 - 1 0 .
o
Results of the Brazing Tests
0
X
•
MH-1
•
M H - 2 A.
t
i
i
t
20
40
60
80
100
N 2 gas flow rate ( £ / m i n )
Fig. 4 — Effect of N2 gas flow rate on brazed ratio (at 67 Pa).
b r a z i n g surface, so the f l u i d i t y of the filler
metal was b a d . T h e r e l a t i o n s h i p b e t w e e n
t h e M g c o n t e n t s in t h e b r a z i n g sheet
s u r f a c e a n d t h e N 2 gas f l o w rate for
material c o m b i n a t i o n X is s h o w n in Fig.
1 1 . T h e M g c o n t e n t of the b r a z i n g sheet
surface
was
measured
by
x-ray
fluorescence spectroscopy. The M g
c o n t e n t is l o w e r as the N 2 gas f l o w rate
b e c o m e s h i g h e r . T h e results o f t h e
b r a z i n g tests u n d e r g o o d
brazing
c o n d i t i o n s are s h o w n in Table 4 .
T h e results of t h e b r a z i n g tests a r e
r e p r e s e n t e d b y t h e f i l l e t area SF, f l o w
f a c t o r K a n d leg length ratio L V / L H . A
h i g h v a l u e of SF a n d K represents g o o d
f l u i d i t y of t h e m e l t e d f i l l e r m e t a l . A
h i g h e r v a l u e of L V / L H near
1.00
represents g o o d gap f i l l i n g p r o p e r t y o f
the filler m e t a l . At l o w LV/LH v a l u e , the
f i l l e r metal f l o w s t o o h e a v i l y a n d f i l l e r
f o r m a t i o n is affected by g r a v i t a t i o n (Ref.
6). W h e n t h e f l o w factor K is greater t h a n
0 . 3 , b r a z e a b i l i t y is efficient a n d fillet size
is u n i f o r m and e x c e l l e n t b r a z i n g c a n be
d o n e at t h e b r a z i n g t e m p e r a t u r e o v e r t h e
l i q u i d u s t e m p e r a t u r e of f i l l e r metal (Ref.
7). Fillet area SF is desirable near 1.0 a n d
g o o d b r a z e a b i l i t y can be e x p e c t e d . Leg
length ratio LV/LH is d e s i r a b l e , at nearly
1.0, a l o w L V / L H v a l u e i n d i c a t e s t h a t
f i l l e r metal f l o w is t o o heavy and f i l l e t
Table 3 — Surface Morphologies of Brazing Sheets after Brazing
Brazing Condition
T.P.
Pressure Pa (Torr)
N2 Gas (t/min)
X
MH-1
MH-2
10 _ 3 (10- 5 )
—
S
S
s
267 (2)
67 (0.5)
9
R-2
R-3
R-3
20
R-2
R-1
R-1
125
S
S
S
15
S*
s*
s*
85
R-1
R-1
R-1
5320 (40)
150
R-1
R-1
R-1
220
s
s
s
S: smooth surface.
S*: smooth surface with thick oxide layer.
R-1: a little rough surface.
R-2: rough surface.
R-3: very rough surface.
W E L D I N G RESEARCH SUPPLEMENT I 235-s
atmosphere. The reactions in the highvacuum brazing process are as follows
(Refs. 8, 9):
1) Mg + 1/2 0 2 = MgO
2 ) M g + H 2 0 = MgO + H 2
3) Mg + 1/3 A l 2 0 3 = MgO + 2/3 Al.
Another reaction which is not related
in Refs. 8 and 9 is considered in the
vacuum-nitrogen
partial-pressure
atmosphere brazing. Because this
process uses N 2 carrier gas and the
partial pressure of contaminant gases
from the N 2 carrier gas system, such as
H 2 0 and C 0 2 , is at a high level
compared with the high-vacuum brazing
process.
4)2AI + 3 H 2 0 = A l 2 0 3 + 3H2
5) 3Mg + N 2 = M g 3 N 2
6) C 0 2 = C + O ,
7) 2AI + 3/2 0 2 = A l 2 0 3
Then, we considered these reactions
from the standpoint of thermochemistry.
The standard Gibbs energies of the
chemical reaction are represented as
follows:
Table 4 — Results of Brazing Tests (T.P.—X)
Pressure Pa (Torr)
N2 gas flow rate (L/min)
Fillet area SF (mm2)
Flow factor K
Leg length ratio LV/LH
Brazing conditions
Results
formation is affected by gravitation (Ref.
6). By comparison of Figs. 4 - 6 and Table
4, good brazeability can be expected
with the vacuum-nitrogen
partial
pressure atmosphere brazing compared
with vacuum brazing with a brazing
pressure of 10 :! Pa (10- 5 Torr). The test
results show that excellent brazing can
be done under a high N 2 gas flow rate
condition in vacuum-nitrogen partialpressure atmosphere brazing, the same
as in the high-vacuum brazing.
Results of Measurement Gas Contaminants
in Brazing Atmosphere and Gibbs Energy of
the Reaction in the Brazing Furnace
The baseline behavior of the brazing
furnace is the vacuum brazing condition.
For gas measurement, brazed specimens
are put in the brazing furnace. The
behavior of gas contaminants during
brazing cycles is shown in Figs. 12-14.
The behavior of gas contaminants was
measured by the mass spectrometer
under good brazing conditions (brazing
KJ-"3 (10-=)
67 (0.5)
80
1.16
0.46
1.00
0.87
0.35
0.90
267 (2)
125
1.21
0.48
0.98
5320 (40)
220
1.02
0.41
0.96
pressure: 5320 Pa (40 Torr), N 2 flow rate:
220 L/min, and high-vacuum brazing)
and under poor brazing conditions
(brazing pressure: 5320 Pa (40 Torr), N 2
flow rate: 15 L/min). The contaminants in
the vacant furnace are H 2 0 and 0 2 .
Their levels are lowered and the partial
pressure of H 2 results from the reduction
of H 2 0 by Mg gas, which evaporates
from the filler metal clad to bare material
(brazing sheets). Also, N 2 gas was not
detected in the vacuum brazing cycle. In
high-vacuum
brazing, the partial
pressure of H 2 0 is high at an early stage
in the process. The partial pressure of
H 2 0 and C 0 2 are lowered, and the
partial pressure of H 2 rises as the furnace
heats up. The rise of partial pressure of H 2
comes from the reduction of H 2 0 by Mg
gas, which evaporates from the brazing
sheets. By this behavior of the gas
contaminants, it is assumed that the Mg
evaporation and reduction of the surface
oxide film of the specimens and the
furnace wall, and Mg gets the residual
H 2 0 gas and 0 2 gas in the brazing
AG° T = A + BT logT + CT (Ref. 1 0)
where AG°T is standard Gibbs energies
of the chemical reaction (cal/mol); A, B
and C are constants of the reaction; and
T is temperature (K) at the chemical
reaction.
We can obtain the A G ° T of many
reactions from the data book (Ref. 10).
We calculated the standard Gibbs
energies (AG° T ) and Gibbs energies
(AGT) of the reactions (1) ~ (7)
1 ) M g + 1 / 2 0 , = M g O AG° T =
-1 81 6 0 0 - 7.37 T log T + 75.7 T
A G T * -134430 + RTIn P - 1 ^ •
2P-!02
2) Mg + H 2 0 = MgO + H 2 AG° T =
-1 24350 - 11.85 T log T +77.91 T
A G T * - 8 6 7 4 9 + RTIn PH 2 • P-1 Mg
P-1 H 2 0
3) Mg + 1/3 A l 2 0 3 = MgO + 2/3 Al
AG° T = -47997 - 6.04 T log T +
100
5320Pa (40Torr)
/
^
^
^
x
"
80
mM**.
100
V
— 60
0
1-
q
•0
0
3 40
ta
"TO
«
80
x
.,
N
^
m
-
^
MH-1 •
266Pa (2Torr)
60
I
0
"
I
25
N2
20
MH-2 A
I
y
I
50
75
100
gas flow rate (i>/min)
Fig. 5 — Effect of N2 gas flow rate on brazed ratio (at 267 Pa).
236-s I OCTOBER 1 9 9 4
/
/
\\
tt
l
MH-1 •
MH-2A
125
0
-
" 40
80"
120
160
N2 gas flow rate (£/min)
200
Fig. 6 — Effect of N2 gas flow rate on brazed ratio (at 5320 Pa)
44.5 T
AG T * -230990 + RT In 2 • P-1Mg
4)
3Mg
+
Nn
Mi 3 N 2
AG° T = - 1 8600 - 7.37 T log T +
757 T AG T = - 1 34430 + RT In 3 •
PMg P N2
5)2AI + 3 H 2 0 = A l 2 0 3 + 3 H 2
AG° T = -229060 - 1 7.42 T log T +
89.85 T AG T * -1 9472 + RT In 1/2 •
'H20
H2
6) C 0 2 = C + 0 2 AG° T = 94200 + 0.2
T AG T * 94375 + RT In P 0 2 • P- 1 C 0 2
7) 2AI + 3/2 0 2 = A l 2 0 3 AG° T =
- 4 0 0 8 1 0 - 3 . 9 8 T log T + 87.64 T
A G T = -325482 + R T I n 1/3- P- 1 0 2
We calculated A G ° T and AG T at a
brazing temperature 873 K (600°C) of
vacuum brazing atmosphere and
vacuum-nitrogen
partial-pressure
atmosphere, with a total pressure of 5320
Pa (40 Torr) and a N 2 gas flow rate of 220
L/min. The results of the calculations are
shown in Table 5. In this calculation,
partial pressures of gases are the values
measured by the mass spectrometer. But
partial pressures of Mg and 0 2 could not
be detected by the mass spectrometer
because the distance between the mass
spectrometer and specimens was too
large. We estimated the partial pressure
of Mg and 0 2 by the surface content of
Mg after brazing and from the literature
(Ref. 11). Also, the partial pressure of 0 2
gas was estimated by the results of
analysis of the N 2 carrier gas. The partial
pressures of Mg were estimated to be
10~5 Pa (10~7 Torr) in high-vacuum
brazing and 2 x 10 - 2 Pa (2 x 10~4 Torr) in
vacuum-nitrogen
partial-pressure
atmosphere brazing (Ref. 11). The partial
pressures of 0 2 were estimated 5 x 1 0~3
Pa (5 x 10 - 5 Torr) at high-vacuum brazing
and 2 x 10~2 Pa (2 x 10 _4 Torr) in vacuumnitrogen partial-pressure atmosphere
brazing. It is well known that the
chemical reaction is affected by the AG T
value as follows:
AGy < 0 : Reaction is advanced on
right side of the reaction formula.
^\A^Z
L
Rra7inrj
-*-
^^'^
y ^
v£
• ^ 5 * < f i l l e ;t
L f = L i + l_2
+ l_n
Fig. 7— Representation of brazed ratio, Lf/Lt.
Table 5 — Results of Calculated Gibbs Energies
AGe73K (kcal/mol)
Vacuum brazing
10_3Pa (10" 5 Torr)
No.
Reactions
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Mg + 1/20 2 = MgO
Mg + H 2 0 = MgO + H2
M g + 1/3AI203 = MgO + 2/3AI
3Mg + N2 = Mg3N2
2AI + 3H 2 0=AI 2 0 3 + 3H2
C0 2 = C + 0 2
2Al + 3/20 2 = Al 2 0 3
Controlled atmosphere brazin
5320Pa (40Torr)
-104.7
-75.9
-7.0
-124.8
-188.7
87.6
-312.6
-67.7
-54.6
-6.2
-50.3
-10.4
101.2
-285.1
Table 6 - Chemical Composition of the Filler Metal that Doesn't Include Mg (mass%)
Alloy
A4045
Si
Fe
Cu
Mn
Mg
Cr
7n
Ti
Al
10.5
0.27
0.10
0.01
0.01
tr.
0.01
0.01
bal.
AG T = 0 : Reaction is in the
equiliblium state.
AG T > 0 : Reaction is advanced on the
left side of the reaction formula or a
reverse reaction is advanced.
From Table 5, the reactions (1), (2),
(3), (4), (5), and (7) are advanced at 873
K (600°C). But, reaction (6) is advanced
on the left side and a reverse reaction is
advanced. The reaction speed cannot be
explained by the AG T value, but the
reaction speed where Mg gas is
concerned is fast because Mg is a very
active element. Besides N 2 gas, H 2 0 ,
C 0 2 and H 2 gases were detected in
vacuum-nitrogen
partial-pressure
atmosphere brazing. However, the effect
of contaminant gases ( H 2 0 ) cannot be
decided simply because the Mg
evaporation affects the reaction. In
general, contaminant gases ( H 2 0 , etc.)
are not good for obtaining good
brazeability. But if the evaporation speed
of Mg is high, damage to brazeability by
contaminant gas will be lowered. A large
change in the partial pressure of
contaminant gas was not observed in the
vacuum-nitrogen
partial-pressure
Mg Ka
SEM
Fig. 8 Scanning
electron probe
micrographs of
brazing sheet
after 10~3 Pa
vacuum
brazing.
, 100 turn ,
WELDING RESEARCH SUPPLEMENT I 237-s
SEM
Mg Ka
gas atmosphere (for example: VAW
brazing process) needs acid or alkaline
cleaning before brazing to remove the
surface oxide film on aluminum alloys. It
also needs very pure N 2 gas with a dew
point under 208 K (-65°C) and the
content of contaminant 0 2 gas must be
under 5 ppm. This process uses the filler
metal w h i c h is Al-7 ~ 13%Si alloy
containing Bi, Sb, Ba, Sr, Be, etc. These
elements are added to Al-Si filler metal to
obtain good brazeability (Refs. 12, 13).
Therefore, the brazing cost is high
compared with vacuum-nitrogen partialpressure atmosphere brazing.
On the other hand, the Mg content in
the brazing sheet surface after vacuumnitrogen partial pressure atmosphere
brazing is lower with a higher N 2 gas
flow rate. It is assumed, therefore, that the
evaporation of Mg in this brazing cycle is
accelerated by the higher N 2 gas flow
rate. It is known that the evaporation of a
volatile element is accelerated by a sharp
gradient of its partial pressure near the
evaporation surface. A high Mg
evaporation level can be expected to the
results from the high Mg partial-pressure
gradient in the boundary layer near the
melted filler metal surface at the high N 2
gas flow rate. These phenomena are
explained by the following formula (Ref.
14). A schematic of the evaporation
surface of metal is also shown in Fig. 15.
W-
i 100 jxm ,
Fig. 9 — Scanning electron probe micrographs of brazing sheets after brazing at 67 Pa.
atmosphere brazing cycle. The partial
pressure level of the contaminant gases
H 2 0 and C 0 2 in this brazing process are
higher than in high vacuum brazing.
Therefore, it is assumed that the
contaminant gases are introduced into
the brazing atmosphere with the N 2
carrier gas and outgas from the carrier gas
piping and the furnace w a l l . The
difference in the partial pressure of
contaminant gases between the good
brazing condition (brazing pressure,
5320 Pa (40 Torr); N 2 gas flow rate, 220
L/min), and the poor brazing condition
(brazing pressure, 5320 Pa (40 Torr); N 2
gas flow rate, 1 5 L/min) is very little. The
partial pressure of the contaminant gases
in vacuum-nitrogen partial-pressure
atmosphere brazing is high compared
with high-vacuum brazing, but in the
high N 2 gas flow rate condition,
contaminant gases ( H 2 0 and C0 2 ) may
be gotten by Mg w h i c h evaporates
238-s I OCTOBER 1 9 9 4
actively and doesn't decompose and
doesn't react with aluminum alloys. AG T
of the reaction ( C 0 2 = C + 0 2 ) is plus
value, and reverse reaction is advanced.
Discussion
Good brazeability in the vacuumnitrogen partial-pressure atmosphere
brazing with AI-10% Si-1.5% Mg filler
metal can be expected at the higher N 2
gas flow rate. The partial pressure of the
gas contaminants becomes higher
according to the higher brazing pressure,
and the difference in the partial pressure
of contaminant gases is very little over
the wide range of N 2 gas flow rates at the
same brazing pressure. So, good
brazeability with higher gas flow rate
cannot be explained by the partial
pressure of the contaminant gases
(especially H 2 0 gas).
Brazing in the 101.3 KPa (760 Torr) N 2
^ • A P
R-TA
+
^
RT
(2)
where W = evaporation speed of Mg
vapor from melted Al-Si-Mg filler metal,
[mol/(cm 2
D = coefficient of
diffusion of Mg vapor, ( cm 2 /s); T A =
temperature of the evaporation surface of
melted Al-Si-Mg filler metal, [873 K
(600°C)]; AP = gradient of partial
pressure of Mg vapor in the boundary
layer, (Pa/cm); S = flow rate of Mg vapor
at the surface of melted Al-Si-Mg filler
metal, (cm/s); P = partial pressure of Mg
vapor at the surface of melted Al-Si-Mg
filler metal, (Pa); R = gas constant of Mg
vapor, (cm 3 • Pa/mol • K); T =
Temperature of the brazing atmosphere,
(K).
In Formula 2, the value of (-D/RT A •
AP) is the paragraph of the diffusion
process, and the value of (SP/RT) is the
paragraph of the convection. It is known
that a paragraph of diffusion process can
be ignored in the boundary layer (Ref.
15). Therefore, W (= evaporation speed
of Mg vapor from Al-Si-Mg filler metal) is
decided by the value of (-D/RT A • AP) in
the boundary layer above the metal (AlSi-Mg alloy) surface. In the boundary
layer or laminar f i l m , the flow of
evaporated Mg gas is laminar flow.
In the value or (-D/RT A • AP), D, R and
T A are constant in this brazing process,
and W is proportional to AP. In the
boundary layer, it is known that AP is
proportional to the value of V 1 / 2 N 2 of N 2
carrier gas, w h i c h is represented by
Formulas 3 and 4).
SEM
Mg Ka
5320 Pa
(40 Torr)
APcc\/2f
(3)
15^/min
2
WxV N?
(4)
In the Formula 4, V N 2 is the velocity of
N 2 carrier gas, and V N 2 is proportional
to a flow rate of N 2 carrier gas.
Therefore, the ratio of AP at a 220 L/min
N 2 carrier gas flow rate at 5320 Pa (40
Torr) to AP at 20 l/min N 2 carrier gas
flow rate is represented by the Formula
3. Also, by Formula 4, evaporation rate
of W is proportional to N 2 gas flow rate.
We calculated the ratio of W at N 2 gas
flow rate at 220 L/min and 20 L/min. The
calculated result is shown in the
Formula 5
Wat 220 L/min
Wat 20 L/min
'220
V 20
3.32
(5)
To confirm the effect of Mg in the filler
metal on brazeability, the reverse T-joint
specimen (Fig. 2) was brazed with the
Mg-less Al-Si filler metal (A4045). The
chemical composition of the filler metal,
which does not include Mg, is shown in
Table 6. In the brazing test, the vertical
sheet is A4045-clad A3003 brazing
sheet. This clad ratio of one side of
brazing sheet is 10%. The symbol of
material combination is N as shown in
Table 2. The specimen whose materials
combination N was brazed in the
vacuum-nitrogen
partial-pressure
atmosphere brazing process. In this
brazing, the specimen w i t h materials
combination X was brazed at the same
time.
The results of brazing is shown in Fig.
16. By the Fig. 16, the brazeability of the
specimen whose material combination
1.5
100 [im
at 5320 Pa.
Fig. 10 — Scanning electron probe micrographs of brazing sheets after brazing
Fig. 12Behavior
of gas
contaminants during brazing cycle
(at
lO-iPa,
vacuum
brazing).
10~JPa(10~aTorr)
. 67Pa(0.5Torr)
I 5320Pa(40Torr)
1.0
0.5
50
100
150
200
250
N 2 gas flow rate (j2/min)
1CT 3 Pa(10 -5 Torr)
Fig. 11 — Relationship between Mg contents in brazing sheet surface
and N j gas flow rate.
W E L D I N G RESEARCH SUPPLEMENT I 239-s
„-*"••"v
e jf
10'
673K
Moo o
873K
(60OC)
\
-i873K
10! _
<u
a
«
Heating curve
p A AJl-Si-Mg
U - S i - M g melted filler
tiller imetal
/
673K
I
[400*0 ra
-
a.
473K |
izoa'c) £
273K
CL
~ ^••-•w
473K
N2 carrier
|
=
..
M
§
Q.
10
yy
y
10Q
30
100
120
b r a z i n g at
/
80 _ 5 3 2 0 P a ( 4 0 T o r r )
/
g
/
/
60
/^•—•
/
20
/
0
/ 1
I
/
'
Materia!
40
80
Nj
•
combinations
* ' NX
"
/
/
120
gas f l o w rate
160
200
(£/min)
Fig. 16 — Effects of N2 gas flow rate on
brazed ratio at 5320 Pa (material combinations are N and X).
N is n o t g o o d at a high N 2 carrier gas f l o w
rate (for e x a m p l e , N 2 gas f l o w rate is 2 2 0
L/min) in t h e 5 3 2 0 Pa (40 Torr) v a c u u m nitrogen partial-pressure atmosphere
brazing.
T h u s , it w a s c o n f i r m e d that the g o o d
effect o f 1.5% M g in the f i l l e r metal in the
vacuum-nitrogen
partial-pressure
a t m o s p h e r e b r a z i n g at a higher N 2 carrier
gas f l o w rate.
Conclusion
By s t u d y i n g the r e l a t i o n s h i p b e t w e e n
brazeability and brazing conditions
( b r a z i n g p r e s s u r e , N 2 c a r r i e r gas f l o w
r a t e , a n d p a r t i a l p r e s s u r e o f t h e gas
contained b y brazing atmosphere) in
240-s I OCTOBER 1994
H20
H2
C02
5320Pa(40Torr)
N2gas:220fl/min
30
Jh
/
Controlled atmosphere
•
A
•
10"'
60
90
B azirtg time (min)
Fig. 13 — Behavior of gas contaminants during brazing cycle (at 5320 Pa, N2 gas: 15
L/min).
I 40
'
* H2
• COj
5320Pa [40Torr)
N2gas:15fi/min
10 '
.3
boundary layer (<J)
CL
273K
[o°cl
*-^-»^___^
S io!
1
60
90
B r a z i n g ti ne ( m i n )
distance from the surface of
A l - S i - M g melted filler metal
120
Fig. 14 — Beha vior of gas contaminants during brazing cycle (at 5730 Pa, N2 gas flow
rate: 220 L/min).
vacuum-nitrogen
partial-pressure
a t m o s p h e r e b r a z i n g , it w a s f o u n d t h a t
excellent
brazing
with
Al-10%Si1 . 5 % M g f i l l e r metal c o u l d be e x p e c t e d at
a higher N 2 gas f l o w rate u n d e r the w i d e
range o f b r a z i n g pressures. This b r a z i n g
process is v e r y effective in m i n i m i z i n g
the e v a p o r a t i o n
of volatile
metal
elements ( Z n , etc.)
In v a c u u m - n i t r o g e n p a r t i a l - p r e s s u r e
a t m o s p h e r e b r a z i n g , it is u n d e r s t o o d that
M g in the filler metal evaporates a c t i v e l y
at a high N 2 gas f l o w rate a n d e v a p o r a t e d
M g gas r e d u c e the o x i d e f i l m o f b r a z i n g
s p e c i m e n s a n d gets H 2 0 i n t h e b r a z i n g
atmosphere.
By t h i s r e a c t i o n w i t h M g gas, it is
assumed that the f l u i d i t y o f m e l t e d f i l l e r
metal is raised and g o o d b r a z e a b i l i t y can
be o b t a i n e d . In v a c u u m - n i t r o g e n p a r t i a l pressure
atmosphere
brazing, the
residual ratio o f Z n in A l - Z n bare alloys
is h i g h c o m p a r e d w i t h h i g h - v a c u u m
b r a z i n g a n d t h e r e is e n o u g h f o r c a t h o d i c
protection from corrosion of products
m a d e w i t h a l u m i n u m alloys because o f
the l o w d i f f u s i o n rate o f Z n in t h e s o l i d
state in A l - Z n alloys c o m p a r e d w i t h h i g h
d i f f u s i o n rate o f M g in t h e m e l t e d A l - S i M g filler metal (Refs. 7, 16).
References
1. Hattori, T , Sakamoto, A. 1982. Pitting
corrosion property of vacuum brazed 7072
clad a l u m i n u m alloys. Welding
Journal
6l(10):339-sto342-s.
2. Seybolt, A. V., Burke, J. E. 1 9 5 3 .
Procedure in Experimental Metallurgy, John
Wiley, New York, N.Y.
3. Fromm, E. 1978. Met. Trans. A, 9A, p.
Fig. 15 — Metal vapor partial pressure, PM;
gas temperature, TC convection
velocity,
VK; as a function of the distance from the
metal surface (schematic).
1835.
4. Schmatz, D. J., Winterbottom, W. L.
1983. A fluxless process for brazing aluminum
heat exchanger in inert gas. Welding Journal
62(10): 3 1 .
5. Aluminum Brazing Flandbook, 1993.
Japanese Welding Society of Light Metal, p.
73.
6. Aluminum Brazing Handbook, 1993.
Japanese Welding Society of Light Metal, pp.
125-126.
7. Hattori, T., and Sakai, S. 1987.
Investigation on improvement of aluminum
vacuum brazing process. Technical Journal of
Mitsubishi Heavy Industries 24(5): 524-529.
8. Crever, D. K„ Ball, and J. Field, D. J. A
mechanistic study of vacuum brazing, SAE
Technical Paper 870185.
9. Terille, J. R., Cochran, C. N., Stockes, J.
J., and Haupin, W. E. 1971. Welding Journal
50(10): 833.
10. Kubaschewske, O., and Alock, C. B
1979. Metallurgical
Thermochemistry. 5th
ed. data, Pergamon Press, Oxford, U.K.
11. Aluminum Brazing Handbook, 1993
Japanese Welding Society of Light Metal, pp
54-56.
12. Fukui, T., trie, H., Taneda, M . , anc
Sugiyama, Y. 1980. Development of brazing
process of a l u m i n u m (fluxless) brazing by
VAW process, Technical Journal of Sumitomo
Light Metal Co. 21(1): 76-89.
13. Schoer, H., and Schultze, W. 1973.
Welding Journal, 52: 644.
14. Uchida, H. 1977. Wet Air and Cooling
Tower. Shoukabo, Tokyo, pp. 8 0 - 8 2 .
15. Kudo, Y. Introduction of Heat Transfer,
1970. Youkendo, Tokyo, pp. 4 5 - 9 9 .
16. Hattori, T., Sakai, S., and Sakaguchi, Y.
1986. Study on corrosion protection of
a l u m i n u m heat exchangers for car air
conditioners. Technical Journal of Mitsubishi
Heavy Industries 23(2): 1 8 7 - 1 9 2 .