BRAZING OF MAGNESIUM ALLOYS AND MAGNESIUM MATRIX COMPOSITES FOR AUTOMOTIVE
AND AEROSPACE APPLICATIONS
Alexander E. Shapiro
INTRODUCTION
Methods, filler metals, and fluxes suitable for brazing of cast and extruded magnesium-based alloys
were well developed in 1960-s and 1970-s. Since that time, the furnace, torch, and dip brazing processes
are successfully employed without considerable changes. New interest to brazing magnesium has been
recently aroused due to the expansion of use of magnesium alloys in 1990-s, and especially, due to an
appearance of high-strength magnesium matrix composites as lightweight advanced structural materials
for automotive and aerospace. Magnesium alloys are considered as possible replacements for aluminum,
plastics, and steels, primarily because of their higher ductility, greater toughness, and better castability.
Production of magnesium was almost tripled last decade, and the world production capacity reached 515
thousand tons per year in 2002, including 250 thousand t/year in the Western World, 150 thousand t/year
in China, and 65 thousand t/year in countries of former Soviet Union (Ref. 1). Both big volume of
magnesium production and applications of new high-performance magnesium alloys that came up in the
world market cause a scientific and technical challenge to the brazing engineering community.
This paper (a) summarizes the experience in joining of cast, extruded, and rolled magnesium alloys,
(b) evaluates the potential of conventional brazing technologies for improving mechanical properties and
corrosion resistance of joints, and (c) discusses new developments to be done to response industrial
demand in joining of new advanced cast or rolled magnesium alloys and magnesium matrix composites
reinforced with carbon or ceramic fibers and particles.
CHARACTERIZATION AND BRAZEABILITY OF BASE METALS
Magnesium is the lightest and one of the cheapest structural metal. Magnesium alloys are
environmentally friendly, lighter than aluminum (only 2/3 of aluminum and 1/3
of titanium specific
weights), better in heat dissipation and heat transfer due to high thermal conductivity of 51 W/mK, and
exhibit excellent ability in shielding electromagnetic interrupt. Low density ~1.75 g/cm 3 in the combination
with relatively high tensile strength 33-42 ksi (228-290 MPa), heat resistance up to 840oF (450oC), and
oxidation resistance up to 930oF (500oC) make magnesium alloys attractive for application in various
structures in automotive, and especially, aerospace industry, as well as in textile and printing machines
where lightweight magnesium parts are used to minimize inertial forces when they operate at high speed
(Ref. 2). Moreover, the use of magnesium can minimize the negative impact on the environment because
_______________________
Alexander E. Shapiro is with Titanium Brazing, Inc., Columbus, OH
ashapiro@titanium-brazing.com
1
magnesium alloys are recyclable. However, the surface of magnesium alloys should be protected
because they corrode easily when exposed to atmosphere.
A significant growth in the production and applications of structural magnesium alloys has been seen
past two decades. The demand is driven primarily by automotive and aerospace industries to reduce
weight and fuel consumption (Ref. 3). Conventional magnesium alloys are strength-competitive not only
with aluminum alloys but also with steels and titanium alloys. For example, a specific tensile strength (a
ratio of the strength to density) of hardened cast magnesium alloy HK31A is the same as this of standard
titanium alloy Ti-6Al-4V, hardened aluminum alloy AA7075, or steel AISI 4340. The specific strength of
extruded magnesium alloy AZ31B is higher than this of aluminum alloys AA6061 and AA3003, or carbon
steel AISI 1015 (Ref. 4).
Compositions, physical properties, and typical mechanical properties of brazeable magnesium alloys
are presented in Table 1 and 2. Because of their low solidus temperatures, some magnesium alloys
cannot be brazed with commercial brazing filler metals BMg-1 and BMg-2a and require the application of
other filler metals of the Mg-Al-Zn system having lower brazing temperature range.
Table 2
Typical mechanical properties of brazeable magnesium alloys
at room temperature
ASTM Alloy
Designation
Yield strength
Young’s Modulus
Tensile strength
MPa
ksi
MPa
ksi
Elongati
on, %
GPa
103 ksi
M1A
138
20
234
34
9.0
45.0
6.5
AZ31B
170
25
260
38
15.0
45.0
6.5
AZ61A
205
30
305
44
16.0
45.0
6.5
AZ63A
130
19
275
40
6.0-12.0
45.0
6.5
AZ91C
145
21
225
33
6.0
42.7
6.2
AS41A
140
20
215
31
6.0 - 9.0
45.0
6.5
AM100
110
16
230
33
2.0
45.0
6.5
ZE10A
179
26
255
37
12.0
45.0
6.5
ZK21A
228
33
290
42
10.0
45.0
6.5
ZK51A
131
19
205
30
3.5
43.2
6.3
ZK60A
285
41
350
51
11.0
45.0
6.5
QE22A
195
28
260
38
3.0 - 4.5
46.0
6.7
QH21A
186
27
241
35
2.0
46.0
6.7
HK31A
112
16
225
33
9.5
46.0
6.7
ZC71
342
47
360
52
3.0 – 5.0
43.5
6.4
2
The temperatures involved in brazing reduce the properties of work-hardened (tempered)
magnesium sheet alloys to the annealed temper level. For example, the extruded and tempered alloy
AZ31B looses about 35% of elongation, 22% of yield strength, and 8% of tensile strength after brazing at
595oC (1102oF) for 1-2 min (Ref. 5, 6). A significant loss of mechanical properties is the main motivation
to develop and implement low-melting brazing filler metals.
Torch brazing reduces properties of base metals only locally, in those areas heated for brazing;
furnace and dip brazing reduce properties of the entire structure. The properties of cast alloys or of
annealed sheet alloys are not greatly affected by the heat of brazing.
Magnesium alloys with reduced aluminum content AM60, AM50, and AM20 are suitable for
applications requiring improved fracture toughness and ability to absorb energy without failure. However,
the reduced amounts of aluminum result in slight decrease in strength of AM alloys (Ref. 7). Alloys AS41,
AS21, and AE42 can be employed for applications involving with long term exposure at temperatures
over 250oF (120oC) and requiring creep resistance.
Mechanical properties (especially plasticity) of magnesium alloys depend on the fabrication
parameters and the testing temperature. For example, a considerable change in mechanical properties of
the alloy AZ31 fabricated by casting, extrusion, and rolling was indicated (Ref. 8). The strength weakening
is accompanied with a remarcable increase in ductility: the elongation is increased from 21.5% to 66.5%
as the test temperature is changed from RT to 482oF (250oC).
Brazing of magnesium is not simple process due to the highest chemical activity among all structural
metals. Complex oxide film containing magnesium oxide and magnesium hydroxide is formed on the
surface of base metal at heating in air. This chemically-stable film is not reduced neither in conventional
active gaseous atmospheres nor in vacuum up to 10-5 mm-Hg (10-5 Torr). Additionally, magnesium
hydroxide is decomposed to hydrogen and water during the heating at 572-752oF (300-400oC), that
further hinders the brazing process (Ref. 9).
Density of magnesium filler metals is less than density of salt systems used as brazing fluxes that
often results in appearance of slag inclusions in the joints. Also, magnesium has high negative value of
the electrode potential (-2.38 V) that hinders deposition of reliable electrolytic or chemical coatings that
could improve wetting by molten brazing filler metals or protect against the flux corrosion. Risk factors and
methods of preventing defects in magnesium brazing are presented in Table 3.
Magnesium matrix composites (MMC) reinforced with ceramics and graphite fibers or particles
present a new class of ultra-lightweight structural materials joined by brazing. These base metals are
ideal for aerospace applications owing to their high strength and stiffness, good thermal and electrical
conductivity, and resistance to space environment. Continuously reinforced, thin-walled metal matrix
parts are particularly used in spacecrafts as stiff, dimensionally-stable structural members. Thinner parts
permit more efficient design resulting in reduced weight and increased payload. Also, continuous fiber
reinforcement allows design of zero thermal expansion structures to obtain dimensional stability over wide
temperature range and accurate pointing angles for reflectors and antennae (Ref. 10).
3
Table 3
Technical problems in magnesium brazing
Problems
Possible negative effect
Chemical activity of base metal.
Fast growth of oxide film
[MgO + Mg(OH)2]
Low solidus of base metals.
Brazing temperature is close to
solidus.
Difficult wetting by brazing filler
metal
Structural changes in base metal
due to brazing heating
Possible atmospheric corrosion
of base metals and joints
Density of molten filler metal is
less than that of the brazing flux
Loss of strength of base metal for
20-30%
Corrosion of brazed articles
Erosion of base metal by liquid
filler metal
Loss of fatigue, inpact, or creep
strength
Susceptibility of Mg-matrix
composites to stress
concentration
Failure of base metal in the
overlapping area
Changes in structure and shape
of brazed parts
Inserts of flux residues in the
brazed joint
Technical solution
Halide brazing fluxes.
Vacuum brazing at <10-5 Torr.
Brazing in dry argon.
Short holding time at brazing
temperature.
Low-temperature filler metals are
needed!
Brazing at lower temperature.
Heat treatment of brazed parts.
Postbraze chromate or phospate
coating, or electrolytic oxidation
Selection of flux/filler metal
combination.
Watchful control of process
parameters.
Short holding time at brazing
temperature.
Watchful control of process
parameters.
Stress-distributive design of
brazed joints.
New high-strength filler metals
are needed!
Application of such lightweight metal composites in automotive industry is being also expanded year
after year due to efforts directed to make more fuel-efficient cars. Mechanical properties of brazeable
composites are presented in Table 4 (Ref. 2,8,11-26) in comparison with the matrix alloys. Magnesium
matrix composites are manufactured by casting or infiltration of reinforcing ceramic powders or fibers
followed by extruding, hot rolling, or forging.
The strengthening effect in particle-reinforced composites is smaller than in continuous fiberreinforced materials but the properties are more isotropic (Ref. 14). Table 3 demonstrates that the main
advantages of MMC are the increase of Young’s modulus, higher strength at elevated temperatures, and
the lower CTE. Improvement in creep resistance of alloys with the ceramic fiber reinforcement is also
impressive. For example, the creep rate at 392oF (200oC) and 8.7 ksi (60 MPa) loading of the composite
QE22/20Al2O3f based on Zr- and REM-alloyed magnesium matrix reinforced with alumina Saffil fibers is
1.13x10-9 s-1 that is 6 times lower than the creep rate of cast matrix alloy (Ref. 22). Promising mechanical
properties were achieved also for direct powder forged composites that allows making a near-net shape
products.
4
Table 4
Mechanical Properties of Magnesium Matrix Alloys and Their Composites Reinforced with SiC,
SiO2, TiC or Al18B4O33 particles, and SiC, Al2O3 or graphite fibers
Matrix alloys
and composites
(vol.%)
AZ91C – matrix
AZ91C/15SiCp *
AZ91C/10TiCp *
AZ91C/30Al2O3 f *
Yield strength
42.7
6.2
218
214
280
350
32
31
40
51
1.1
4.0
1.8
-
57.0
52.0
64.0
70.0
9.2
7.5
9.3
10.1
1470
221
210
360
453
230
213
32
30
52
66
33
7.3
1.8
5.0
1.0
2.0
155.0
43.2
52.4
22.4
6.3
7.6
45.0
6.5
220
32
1.5
75.0
10.9
MPa
ksi
145
21
225
178
26
230
33
-
Mg1Al/60Graphite f
ZK51A – matrix
ZK51A/15SiCp *
-
AM100 – matrix
103 ksi
ksi
AZ91/30Graphite f *
ZC71 – matrix
ZC71/12SiCp **
GPa
33
Elongation,
%
7.2
MPa
-
131
162
340
397
110
19
23
49
57
16
-
AM100/20Al2O3 f *
Young’s Modulus
Tensile strength
-
AS41 – matrix
125
18
193
28
9.0
50.0
7.3
AS41/30Al2O3 f *
240
35
270
39
1.0
78.0
11.3
AZ31B - matrix
170
25
260
38
15
-
AZ31B/4SiO2 p**
229
33
314
45
4.4
-
AZ31B/8SiO2 p**
260
38
330
48
6.0
-
AZ31B/10SiC f ****
314
46
368
53
1.6
69.0
10.0
AZ31B/20SiC f ****
417
60
447
65
0.9
100.0
14.5
QE22 – matrix
180
26
250
36
4.5
46.0
6.7
QE22/30Al2O3 f *
250
36
300
43
1.6
74.0
10.7
QE22/25SiCp**
245
36
325
47
4.0
73.0
10.6
175
25
-
55.0
8.0
-
Mg/10Mg2Sip ***
Mg/10Mg2Nip ***
117
17
202
29
3.6
AZ91/10Al18B4O33*
266
38
480
70
1.0
78.0
11.3
MB15/30Al18B4O33*
230
33
303
44
0.5
76.0
11.0
758
110
-
66.0
9.5
-
101.0
14.6
-
Mg14Li1Al/30steelf *
Mg14Li1Al/30B p**
244
35
-
-
* Casting; ** Forging; *** Undirectional solidification; **** Extrusion
Footnotes: p – particles, f - fibers
5
Some magnesium matrix composites exhibit impressive increase in mechanical performance in
contrast with non-reinforced matrix alloys. For example, the composite consisting of Mg-14Li-1Al matrix
and 30 vol.% of steel fibers has tensile strength 600-700 MPa (87-123 ksi) at room temperature and 450480 MPa (65-69 ksi) at 200oC (392oF), while the matrix alloy exhibits only 144 MPa (21 ksi)at room
temperature, and 14 MPa (2 ksi) at 200oC (Ref. 26).
The advanced Mg-based materials have great potential to improve mechanical performance in the
near future. New non-traditional reinforcing systems allow to reach strength characteristics of magnesium
matrix composites comparable with some steels or titanium alloys. For instance, the squeeze-casting
composite of the matrix AZ91D alloy reinforced with 10 vol.% of Al 18B4O33 particles exhibits a tensile
strength 480 MPa (70 ksi) (Ref. 20). Even the low-alloyed magnesium matrix MB15 reinforced with 30
vol.% of Al18B4O33 whiskers demonstrates a yield strength of 230 MPa (33 ksi) at very good rigidity
characterized with Young’s modulus 11 Mpsi (76 GPa) and elongation 0.5% (Ref. 21). An increase of
volume fraction of the reinforcing component can result in drastic change of mechanical properties. The
Switzerland company EMPA reported recently about the super-strength composite MgAl1/T300
containing 60 vol.% of graphite fibers (Ref. 25). This material exhibited tensile strength of 213 ksi (1470
MPa) and Young’s modulus 22.4 Mpsi (155 GPa).
Magnesium matrix composites are also prospective as high-damping materials used to reduce
mechanical vibrations. For example, undirectional solidification of Mg-2Si alloy yields Mg/Mg2Si
composite structure with a mechanical strength as high as the industrial cast alloy AZ63 but with a
damping capacity 100 times higher (Ref. 19). A similar Mg-10Ni alloy with Mg/Mg2Ni structure provides a
damping capacity 40 times higher than that of AZ63 cast. Moreover, Mg-2Si alloy reinforced with long
carbon fibers has a Young’s modulus of ~200 GPa with a damping capacity of 0.01 for strain amplitude of
10-5.
Due to low solidus limitation of the matrix, only low-temperature filler metals such as P380Mg and
P430Mg can be used for joining casting composites based on ZK51A and QE22A matrix alloys, or forged
composites based on ZK60A and ZC71 matrix alloys. Joining of other casting or forged composites can
be performed by placing filler metal GA432 or P380Mg between brazed parts and heating to 734-752oF
(390-400oC) with thorough control of temperature. Joining of wrought magnesium composites based on
Mg-Zn matrixes is preferably carried out by soldering with Zn-Al solders.
FILLER METALS
There are only three filler metals commercially available for brazing magnesium: BMg-1, BMg-2a
(their ASTM designations are AZ92A and AZ125, respectively) and MC3 alloy. The nominal compositions
and physical properties of these alloys are shown in Table 5. The standard filler metal MC3, used in
Japan, has the composition close to BMg-1. All three alloys are suitable for torch, furnace, or dip brazing
processes.
If torch or dip brazing are to be done at lower temperature, other filler metals showed in Table 6 (Ref.
5,9,27) can be used with appropriate testing of mechanical and corrosion properties of brazed joints.
6
Alloying elements such as Al, Zn, Mn, Be, Si, Zr, Ca, Ag, Th, Y, and Rare-earth metals (REM) have
effect on properties of magnesium-based filler metal somewhat similar to their effect on properties of diecast magnesium alloys.
Aluminum increases room temperature strength and hardness, and improves fluidity. However,
excessive aluminum amounts cause a decrease in ductility due to formation of brittle intermetallic phases.
Also, aluminum widens solidus-liquidus range. Zinc generally improves fluidity and strength of
magnesium alloys through solid-solution strengthening; but high levels of >2 wt.% of Zn can cause hot
cracking (Ref. 28). Zinc is also useful to prevent corrosion caused by Fe or Ni impurities in magnesium
alloys. Magnesium filler metals containing zinc in combination with zirconium or rare-earths can be
precipitation-hardened to increase the strength. However, zinc may not deteriorate hot cracking
resistance in combination with aluminum and manganese. For example, the cast alloy AZ88 (Mg-8Al-8Zn0.2Mn) exhibits sufficient resistant to hot cracking, yet retaining exceptional fluidity (Ref. 29). Small
additions of manganese do not affect mechanical properties, but they do produce beneficial effect in the
control of corrosion, especially in saltwater. The filler metals are alloyed with 0.1-0.5 wt.% of Mn to
improve corrosion resistance. In presence of aluminum, the solubility of Mn in solid solutions of
magnesium alloys is less than 0.3 wt.%. Cadmium is the only one metal which crystal lattice is fully
compatiblewith magnesium, but the most important
fact is that cadmium forms solid solutions with
magnesium at any concentrations.
Beryllium is added in amounts of <0.002 wt.% to suppress excessive oxidation of molten metal and
to reduce risk of ignition during the torch brazing. Silicon improves fluidity of magnesium alloys in molten
state. Also, silicon is present in some alloys such as AS21 and AS41 to improve creep strength due to
formation of the reinforcing Mg2Si phase. The same effect of silicon can be expected in the filler metal
compositions that should be checked in future developments. However, silicon affects corrosion
resistance in presence of iron impurity. Silver makes possible age hardening that results in higher
strength both casting and wrought magnesium alloys. Rare-earth metal additions in amount of 2-4 wt.%
(for example, mishmetal containing 55 wt.% of Ce, 20 wt.% of La, 20 wt.% of Nd, and 5 wt.% of Pr)
produce stable grain-boundary precipitates that improve creep strength. Yttrium has high solubility in
magnesium – up to 12.4 wt.%. Yttrium and zirconium additions promote creep resistance of cast
magnesium alloys being added in the amounts up to 4 wt.% and up to 0.7 wt.%, respectively. Also,
zirconium is effective grain refiner in magnesium alloys because lattice parameters of α-Zr are very close
to those of magnesium. But, zirconium is not used in alloys containing both Al and Mn, which form
intermetallics with zirconium and remove it from solid solutions.
The corrosion rate increased abruptly with the addition of >1 wt.% of calcium. The negative effect of
Ca can be distinguished by adding zinc or rare-earth metals. Recent investigations demonstrated positive
effect of calcium on creep resistance of magnesium alloys. Calcium is not recommended for magnesium
alloys to be welded due to cracking but it is harmless for brazable alloys.
Strontium up to 2 wt.%
improves fluidity of Mg-Al-Mn alloys without affecting corrosion resistance (Ref. 30). Lithium is the only
one alloying metal that decrease density of magnesium alloys. Solubility of Li in solid magnesium
solutions is as high as 5.5 wt.%, and lithium can be added up to this amount to improve ductility of the
alloys, but it may cause decreasing of strength. Tin is added to magnesium in combination with aluminum
7
to improve ductility and reduce tendency to hot cracking. Thorium in amount of 1-3 wt.% is very effective
to improve creep resistance of magnesium alloys, especially in combination with REM.
The following elements of IVA and VA groups: Si, Ge, Pb, Sb, and Bi form stable intermetallic phases
with magnesium (Ref. 31) and can be used as alloying components for precipitation strengthening Mg-Albased filler metals.
Preparation of brazing filler metals always includes melting of magnesium followed by dissolution
alloying metals in the melt. Liquid solubility of alloying metals in magnesium is shown in Table 7 .
Table 7
Liquid solubility of alloying elements in magnesium melt
Alloying
element
Metal or
master-alloy
Apparent
liquid
solubility, %
Ag, Al, Au, Ba,
Bi, Cd, Cu, Ga,
Alloying
element
Metal or
master-alloy
Apparent
liquid
solubility, %
Metal
100
Ferrosilicon
(95% of Si)
Metal
100
Rare Earths
Metal
100
(Mishmetal),
Sb, Sn, Sr,
Ge, In, Li, Ni,
Th, Zn
Pd, Pb
Beryllium
Al-Be
0.01
Silicon
Calcium
Ca-20Mg
100
Sodium
Metal powder
0.04
Tantalum
Metal powder
0.015
Metal
5.0
Tellurium
Metal
0.2
Iron
Metal powder
0.1
Thorium
100
Manganese
Al-Mn or
MnCl2
Metal powder
or MoCl4
Metal powder
5.0
Titanium
Metal or ThF4
or ThCl4
Metal or TiCl4
≥1.0
≥1.0
Tungsten
Metal powder
≥0.2
0
Vanadium
≥0.02
Phosphorus
Fe2P
0.01
Yttrium
Metal powder
or VCl4
Mishmetal
Potassium
Metal
0.02
Zirconium
Metal
0.95
Rhodium
Metal
0.5
Chromium
Cobalt
Molybdenum
Niobium
0.1
100
Impurities such as iron, nickel, and copper should be controlled in the parts-per-million range in Mgbased filler metals to prevent their negligible effects on mechanical properties and corrosion resistance.
Upper limit of Ni or Fe in magnesium alloys should be 0.005% for maximum corrosion resistance.
However, some addition of copper is admitted in Al-based filler metals that can be used for joining
magnesium alloys (Ref. 32). In this case, special attention should be paid to corrosion protection of
brazed joints by conversion metallic coatings and polymer paint coats.
8
Four filler metals: BMg-1, MC3, P430Mg, and P380Mg allow electrolytic oxidation as finishing
treatment for corrosion protection of brazed articles.
Filler metals BMg-1 and BMg-2a are usually hot-
formed by heating to between 500°F and 600F (260°C and 316C) to fit the joint profile. The filler metals
can be formed over a heated steel mandrel to the desired contour.
All filler metals, especially alloys containing aluminum over 9 wt.%, are characterized by considerable
erosion of the base metal during brazing. The depth of erosion may reach 1-1.5 mm (0.04-0.06 inch) if the
brazing is carried out with the BMg-1, BMg-2a, and especially P435Mg or P398Mg. Therefore, both filler
metals P435Mg and P398Mg are not suitable for joining thin-wall structures.
Typical tensile strength of brazed joints of magnesium alloys is in the range of 12-17 ksi (82-117 MPa)
depending on design, filler metal, and thickness of the joint. Some strength data are shown in Table 8.
Table 8
Strength of joints of magnesium alloys and matrix composites brazed with some non-commercial
filler metals
Base metal
M1A
Filler metal
Mg-12Al-11Cd4Ni
Al-25Mg-3.5Cu
AA8009
AZ91C/15SiCp
Al-32Mg-2Cu-1Si
Mg-12Al-2Ca
AZ31B
AM50
Zn-3Mg-1Al
Zn-3Mg-1Al
M1A
M1A
Silver film
Ga-4Mg-4Cd-4Zn
Brazing
technique
Brazing with
flux F390Mg
Vacuum
brazing
Vacuum
brazing
Ultrasound
assisted
soldering
Brazing in
dry argon
Shear
strength
MPa
ksi
190
27.5
Tensile strength
MPa
ksi
Testing
temperature
Reference
oC
20
-
-
122-136
93
87
180-193
58-70
50-68
46-82
17.7-19.7
13.5
11.2
26-28
8.4-10.1
7.2-9.8
6.7-11.9
20
260
20
20
200
20
20
68.6
58.8
10.0
8.5
-
-
20
20
33
32
36
41,42
33,47
Higher shear strength of brazed joints about 27.5 ksi (190 MPa) was reported for a Mg-12Al-11Cd-4Ni
filler metal (Ref. 33) having the melting range of 1040-1076oF (560-580oC) when brazing was performed
with the flux F390Mg (Table 8).
Simple binary Mg-Zn and Al-Zn systems were tested as filler metals for brazing cast AZ91A alloy in
argon (Ref. 34). All binary Mg-Zn filler metals such as Mg-42Zn, Mg-51Zn, Mg-63Zn, and Mg-92Zn
exhibited poor spreading along the base metal surface in the temperature range 572-1022oF (300-550oC)
for brazing time varied from 0 to 110 min, but all of them actively reacted with the cast alloy substrate that
resulted in the formation of a reaction layer at the interface and erosion of the base metal. Strength of
brazed joints made with those binary filler was not reported. I doubt that binary Mg-Zn alloys are suitable
as brazing filler metals because of susceptibility to hot cracking that is well known from magnesium die
casting experience.
9
The pure aluminum foil 15 m thick is suitable as filler metal for micro-spot brazing of extruded alloy
AZ31 at electric current 500-800 A, but strength of such joints was inadequate (Ref. 34).
Alongside with traditional filler metals, there are several new alloy systems appeared last years that
can be considered as promising filler metals able to improve mechanical properties of brazed joints.
Among them, a filler metal Al-25Mg-3.5Cu in the form of melt-quenched ribbons showed solidus
temperature 840oF (448oC) and liquidus of 864oF (462oC) provides tensile strength of brazed joints over
17.7-19.7 ksi (122-136 MPa) at RT and up to 13.5 ksi (93 MPa) at 500oF (260oC) (Ref. 32). Partial
substitution of copper for silicon in the filler metal Al-32Mg-2cu-1Si resulted in significant decrease of
tensile strength to 11.2 ksi (87 MPa) at room temperature. Thermal cycle of vacuum brazing with the Albased filler metal should be as fast as possible (485 oC, 1 min) to prevent the formation of thick brittle
intermetallic layers on the interface. Postbraze heat treatment for 24 h at 482 oF (250oC) was used for
precipitation strengthening of brazed joints. Brazing structures manufactured with this filler metals should
be reliably protected against moisture corrosion due to presence of copper in the joint composition.
Creep-resistant alloys of Mg-Al-Ca-Sn and Mg-Al-Ca-Zn systems were recently developed (Ref. 35)
and showed yield strength 27.5-29.4 ksi (190-203 MPa), ultimate tensile strength 34.8-36.2 ksi (240-250
MPa), and elongation 3-5% at room temperature. The minimum creep rate was less than .9x10 -9 s-1 at
392oF (200oC) under loading of 8 ksi (55MPa). Similar improvement of creep resistance was also
measured for the Ca-added Mg-Al-Mn alloy AM60B that showed at least 10 times lower creep rate at
392oF (200oC) at the load of 13 ksi (90 MPa) than Ca-free cast alloy (Ref. 30). This studies confirmed
positive effect of relatively big addition (1-3 wt.%) of calcium in magnesium alloys despite a traditional
point of view. The brazing filler metal Mg-12Al-2Ca (designed according to above mentioned cast alloys)
has melting range of 818-1050oF (437-565oC) and exhibited tensile strength 26-28 ksi (180-193 MPa) of
brazed joints of magnesium matrix composite AZ91/15SiC p at room temperature, and 8.4-10.1 ksi (58-70
MPa) at 200oC (Ref. 36). Metallography of brazed joints (Fig. 1) demonstrated perfect fluidity of the MgAl-Ca filler metal, formation of smooth fillets, active interaction with the base metal, but non-equilibrium
microstructure comprising solid solution grains, Mg-Al eutectic, and intermetallics that were crystallized in
the forms of both relatively big crystals (γ-Al3Mg4) and a dispersed phase (supposedly CaMg2 and Al4Ca).
The low melting Ca-bearing filler metal Mg-(32-35)Al-2Ca showed near eutectic melting in the narrow
temperature range of 824-838oF (440-448oC) but the tensile strength of the brazed joints only 1.6-2 ksi
(11-14 MPa) at 392oF (200oC). The quest for low brazing temperature is caused by the necessity to
perform brazing as close as possible to the temperature of recrystallization of matrix alloys of magnesium
composites in order to avoid exsessive residual stresses in the composite structure after cooling (Ref.
58). Magnesium matrix composites manufactured by extrusion or rolling are characterized by pronounced
texture of deformation and anisotropy of mechanical properties, and besides, the recrystallization
temperature of magnesium matrix is only about 150oC at critical deformation of ≤10%. Therefore,
decreasing of brazing temperature is so important from the viewpoint of reliability of brazed joints of
magnesium matrix composites.
A liquid-quenching technique, such as melt spinning, opens an opportunity to manufacture Mg-based
brazing filler metals in the form of amorphous or partly amorphous foils and ribbons. Thin foils may be
very attractive for brazing large flat panels made from magnesium matrix composites. A solid experience
10
has already accumulated in the industry to manufacture various ternary or quaternary alloys in
amorphous state, e.g., Mg-12Zn-3Ce or Mg-5Al-5Zn-5Nd (Ref. 2), that can be used not only as brazing
filler metals but also may improve corrosion resistance of brazed joints.
A new cast alloy ZAC 8506 (Mg-4.7Al-8Zn-0.6Ca) can be used as a filler metal with the liquidus about
600oC
like BMg-1, but with significantly higher ultimate tensile strength of 32 ksi (219 MPa) at modest
elongation of 5% at room temperature. Creep strength of the filler metal based on the Mg-4.7Al-8Zn0.6Ca alloy would be also higher than BMg-1 (Ref. 37). A little increase of Zn content may decrease
melting point of the projected filler metal by 30-40oC without significant loss of strength.
Also, testing of cast alloy AZ88 (Mg-8Al-8Zn-0.2Mn) as low-melting brazing filler metal looks
reasonable to prevent overheating of work-hardened and tempered base metals. The alloy AZ88 has
liquidus temperature of 520oC (968oF). This means that brazing with AZ88 as filler metal can be done at
530-550oC (986-1022oF). Rods or strips of this cast alloy can be manufactured by warm rolling at 350400oC (662-752oF).
New prospective filler metals based on the system In-Mg-Al-Zn were developed by T. Watanabe et al.
(Ref. 38, 39, 40). The best alloy of this system In-34.5Mg-0.8Al-0.2Zn exhibits liquidus at 476oC, brazing
temperature 490oC, hardness 110 HV, and tensile strength of brazed joints comparable with the strength
of 0.9 mm (0.036 inch) thick foil of the base metal - extruded alloy AZ31B. Addition of zinc up to 6.4 wt.%
results in the decrease of melting point to 449 oC, and also, in considerable decrease of tensile strength.
Magnesium alloys that require low operational temperature can be successfully joined by solders
ZnMg3Al (Zn-3Mg-1Al) having melting range of 642-752oF (338-400oC) and Mg48Zn43Al9 (Mg-43Zn-9Al)
having melting range of 644-660oF (340-348oC). Ultrasonic assisted soldering with the ZnMg3Al filler
metal provided high tensile strength of joints of AZ31 and AM50 base metals: 7.2-9.8 ksi (50-68 MPa) and
6.7-11.9 ksi (46-82 MPa), respectively. The solder Mg48Zn43Al9 showed lower strength of joints in the
range of 1.5-3.8 ksi (10-26 MPa) but better corrosion resistance than the alloy ZnMg3Al (Ref. 41, 42).
Several non-standard brazing filler metals and solders with the joining temperature in the range of
662-887oF (350-475o) were offered for joining magnesium matrix composites reinforced with graphite
fibers (Ref. 43). These alloys have the following compositions: (a) Mg-32Al-2Zn with the liquidus
temperature 425oC and brazing temperature >450oC, (b) Mg-39Li-2Zn with the liquidus of 325oC and
soldering at >350oC, (c) Mg-48Ag-2Zn with the liquidus of 450oC and brazing at >475oC, and (d) Mg33Al-33Li with the liquidus of 300oC and soldering at >325oC.
Besides, the strength of the filler metal BMg-1 can be improved by adding ~1 wt.% of yttrium and age
hardening of the brazed joint. The grain size of the Mg-9Al-1Zn (BMg-1) alloy decreases and a new
phase Al2Y which has a higher melting point than Mg 17Al12 is formed by addition of yttrium (Ref. 44). The
hardness of the alloy containing yttrium is higher than that of Mg-9Al-1Zn alloy after a solid solution
treatment. The age hardening process is delayed by yttrium owing to the fact that Al 2Y can not be
dissolved into the -Mg matrix, and the content of aluminum in the matrix of Mg-9Al-1Zn-1Y alloy is
decreased.
We can also expect a substantial progress in mechanical properties of brazed joints made by using
filler metals having the structure of cast matrix composites reinforced by SiC, TiC, or Al 2O3 particulates.
Experiments with composite Mg-based filler metals were recently started and shell be accomplished in
11
the nearest future to response on strength requirements that are coming up with new high-strength base
materials such as magnesium matrix composites. Reinforcing of filler metal matrix with fine ceramic
particles can increase yield strength of brazed joints at least by 20% and creep strength by 50-70% (Ref.
45). The system Mg-Al-Li having eutectic of the Mg-36.4Al-6.6Li (wt.%) composition at 418oC (785oF)
looks like a possible candidate of the liquid phase for preparing and testing of composite brazing filler
metals, as well as other low-melting Mg-based alloys that might have good plasticity in solid state.
Another one alloy of this system Mg-8Li-5Al-1Zn is ready filler metal with melting point around 1040 oF
(560oC). This alloy demonstrates unusually high tensile strength of 220 MPa (32 ksi) after age hardening
(Ref. 46). Supposedly, the strength can be further increased by adding small amount of zirconium.
Promising results were obtained by brazing magnesium alloys using transient liquid phase (TLP)
technique with nickel, copper, and silver layers as filler metals (Ref. 47). Brazing with the nickel interlayer
0.1 mm thick is carried out at 1004oF (540oC) for 5 min. A multi-component structure is being formed in
the joint that comprises intermetallics Mg2Ni, MgNi2, and eutectic Mg-Mg2Ni. Decreasing of the nickel
layer thickness from 0.1 mm to 0.02 mm resulted in the strength gain by three times. The liquid phase
appears at 950oF (510oC) in 3-4 seconds after starting the TLP contact reaction in the system of
magnesium-copper. In 15 seconds, the intermetallic layer of Mg2Cu is formed at the interface. After the
TLP brazing for 5 min, the crystallized joint consists of: (a) an intermetallic phase Mg 2Cu at the copper
side, (b) eutectic Mg+Mg2Cu, and (c) pure Mg at the magnesium side (Ref. 48). Coating of Ni, Ag, or Cu
films ~20 micron thick deposited on the base metal by vacuum evaporation provided the best strength of
brazed joints of magnesium alloys. Supposedly, the TLP brazing can be also effective for joining
magnesium matrix composites.
Method of brazing and soldering of magnesium alloys by gallium-based pastes was developed by I.Y.
Markova (Ref. 33,47). Compositions of tested gallium solders are Ga-4Mg-4Cd-4Zn and Ga-26Zn-11Sn4Mg-4Cd, soldering/brazing temperature was varied in the range of 150-600oC (302-1112oF) for joining
parts of M1A alloy in dry argon. Maximal shear strength of joints was 58.8 MPa (8.5 ksi). As all Mg-Ga
alloys, these joints are susceptible to atmospheric corrosion and need to be protected by phosphate of
chromate coatings.
FLUXES
AWS brazing fluxes, Type FB2-A (ANSI/AWS A5.31-92) are used when brazing magnesium alloys.
Because of the corrosive nature of the flux, complete removal of flux residues is extremely important if
good corrosion resistance is to be maintained in brazed joints.
Fluxes are based on halide salts of alkali- and alkali-earth metals with LiCl and/or NaF as active
components. A few practically used brazing fluxes are shown in Table 9 (Ref. 9,27,38). So called
“contact-reactive” fluxes also can be effective due to their ability to deposit thin zinc film that promotes
wetting of fluxed magnesium surface by molten filler metal (Ref. 9).
The flux must be completely dried before torch brazing (sometimes by additional heating and
grinding) to avoid the formation of magnesium hydroxide on the brazed surface that makes quality
brazing impossible. All fluxes are used in the form of dry powder (for the furnace brazing) or paste with an
alcohol binder for torch brazing.
12
The best results are obtained when dry powdered flux is sprinkled on the faying surfaces of the joint.
Flux should not be mixed with water that retards the flow of filler metal. Flux pastes are dried by heating at
350°F to 400F (177°C to 204C) for 5 to 15 minutes in drying ovens or circulating air furnaces. Flame
drying is not desirable because it leaves a heavy soot deposit.
PRECLEANING AND SURFACE PREPARATION
Magnesium alloys are usually supplied with preserving oil coating, chromate coated surfaces, or acid
pickled surfaces (Ref. 5). The assemblies to be brazed should be thoroughly clean and free from burrs.
Oil, dirt, and grease should be removed either with hot alkaline cleaning baths or by vapor or solvent
degreasing. Surface films, such as chromate conversion coatings or oxides, must be removed by
mechanical or chemical means immediately prior to brazing.
In mechanical cleaning, abrading with aluminum oxide cloth or steel wool has proved very
satisfactory. Chemical cleaning should consist of a 5-10 min dip in a hot alkaline cleaner followed by a 2
min dip in the ferric nitrate bright pickle solution that is described in Table 10 (Ref. 6, 9). Besides, the
preserving oil coatings can be removed by boiling in 1% aquatic solution of soda for 20-30 min followed
by rinsing with warm water and drying at 140-176oF (60-80oC) (Ref. 27). An interaction between any of
mentioned surface preparation and brazing should be less than 5 h.
T. Watanabe and H. Adachi investigated a positive effect of surface treatment of alloy AZ31B by
pickling in hydrofluoric acid before brazing (Ref. 49). Such pickling produces a thin protective film of MgF 2
on the alloy surface that improves wetting by In-30Mg-4Zn-1Al brazing filler metal at 480-500oC.
Table 10
Chemical treatment solutions
Treatment
Composition
Chromic acid, 1.5 lb (0.68 kg)
Ferric nitrate bright pickle
Ferric nitrate, 5.33 oz (0.15 kg)
Method of Application
0.25 to 3 min immersion followed
by cold and hot water rinses and
air dry
Potassium fluoride, 0.5 oz (0.014 kg)
Water to make one gallon (3.8 L)
Temperature 60–100°F (16–38°C)
Sodium dichromate, 1.5 lb (0.68 kg)
Chrome pickle
Conc. nitric acid, 24 fl. oz. (710 mL)
Water to make one gallon (3.8 L)
1 min to 2 min immersion, hold in
air 5 s, followed by cold and hot
water rinses and air or forced dry
250°F max (121°C max)
Temperature 70–90°F (21–32°C)
Postbraze cleaner (A)
Sodium dichromate, 0.5 lb (0.23 kg)
Water to make one gallon (3.8 L)
2 hour immersion in boiling bath
followed by cold and hot water
rinses and air dry
Temperature 180–212°F (82–100°C)
13
Postbraze cleaner (B)
(1 step) 2-3% solution of Na2CO3
(2 step) Chromium anhydride 150 g/L
Nitric acid 240 g/L
Temperature 68-86oF (20-30oC)
30-60 min immersion in boiling
bath followed by cold water rinse.
1-3 min immersion in the bath
followed by cold and hot water
rinses, and air dry at 140-158oF
(60-70oC) in drying chamber
POSTBRAZE CLEANING AND CORROSION RESISTANCE
Regardless of the brazing process used, the complete removal of all flux residues is of the utmost
importance. The brazement should be rinsed thoroughly in flowing hot water to remove flux. A stiff-bristled
brush should be used to scrub the surface and speed up the cleaning process (Ref. 6).
The brazement should then be immersed for one to two minutes in chrome pickle, followed by a 1-2
hour boil in one of postbraze flux remover cleaners A or B described in Table 10.
The corrosion resistance of brazed joints depends primarily on the thoroughness of flux removal and
the adequacy of joint design to prevent flux entrapment. Since the brazing filler metal is a magnesiumbased alloy, galvanic corrosion of brazed joints is minimized.
Table 11
Anticorrosive conversion coatings
Treatment
Alkali
Composition
NaOH
Method of Application
2 min immersion at
55oC (130oF)
Cr-Mn
Chemical cleaning
CrO3, 0.22-0.44 lb (100-200 g)
1-15
min
Water, 1L
boiling bath
immersion
in
Na2Cr2O7 2H2O, 0.22lb (100 g)
Conversion
MnSO4 5H2O, 0.11lb (50 g)
5-20 min immersion at
coating
MgSO4 7H2O, 0.11lb (50 g)
35-55oC (95-130oF)
Water, 1L
Alkali
NaOH
2 min immersion at
55oC (130oF)
Cr-Al
Chemical cleaning
CrO3, 0.22-0.44 lb (100-200 g)
1-15
min
Water, 1L
boiling bath
immersion
in
Na2Cr2O72H2O, 0.22lb (100 g)
14
Conversion
NaF, 0.033 lb (15 g)
2 min immersion at
coating
Na2Cr2O7 2H2O, 0.4 lb (180 g)
35-55oC (95-130oF)
Al2SO4 14H2O, 0.022 lb (10 g)
HNO3 70%, 125 mL
Water, 1L
If necessary, corrosion resistant of brazed joints or an entire brazed article can be increased with
phosphate coating (Ref. 51) chemically deposited by rubbing the surface or immersing in the water
solution containing NaH2PO4 40-100 g/L, NH4H2PO4 120-180 g/L, (NH4)2SO4 5-20 g/L, and Mg(OH)2 5-15
g/L. The protective coating 2.5-3 m thick is deposited for 1-5 min at 68-95oF (20-35oC).
Corrosion resistance of magnesium alloys and brazed joints can be improved by Cr-Mn or Cr-Al
conversion coatings deposited after chrome pickling by dipping in one of the solutions presented in Table
11 (Ref. 50). Painting with (a) epoxy-based primer, (b) acrylic or polyester color base, and (c) acrylic top
clear paint is principal finishing process having the purposes of better corrosion resistance under the
severe corrosive environment. The completely finished corrosion protection which includes 3-coat and 3baking paint films on conversion coated surfaces is so effective that meets automotive industry
regulations to magnesium parts (Ref. 50). Non-chromate conversion coating may be deposited on brazed
magnesium articles from the water solution of 20 g/L cerium chloride (CeCl 2) or 10 g/L vanadium oxide
(V2O5) at pH = 6-8 and 40 oC for 5 min (Ref. 52).
Also, sealing treatment at 60oC for 10 sec is recommended after the chemical conversion to provide
excellent corrosion resistance. The sealing agent is selected from vinyl-silane, glycidoxy-silane, and
mercapto-silane, or a titanium coupling agent.
Local protection of brazed joints can be carried out by treating with the solution containing MgO 9 g/L,
CrO3 45 g/L, and H2SO4 1.5 g/L. This solution provides deposition of light-gold colored oxide film. To
prepare this composition, magnesium oxide is mixed with small amount of the acidic water solution up to
a paste state, then CrO3 and remained water are added. Stirring of the resulting mixture continues up to
full dissolution of MgO (Ref. 53).
The surface to be coated is prepared with a sand paper, degreased by acetone or the like, and dried.
The oxidative solution is deposited by fabric or cotton tampon, processing time is 2-3 min. Wet residues
should be removed from the brazed joint.
DESIGN OF BRAZED STRUCTURES
Design of brazed joints of magnesium alloys needs detail consideration in a special article. We have
to make here only two notes about main specifics of magnesium brazing design.
Firstly, magnesium alloys have low yield strength. On the one hand, this means that overlapping value
equal to two thickness of base metal is usually sufficient to provide the shear strength of brazed joints. On
the other hand, it is reasonable to make overlapping as big as possible to increase total thickness, and
consequently, strength of the brazed structure. In each case, the designer has to make appropriate
decision in accordance with operational conditions of the brazed structure.
15
Secondly, magnesium matrix composites are susceptible to stress concentration due to big difference
between mechanical properties of hard fibers and soft matrix. In order to prevent their failure caused by
stress concentration in overlapping, the design of the brazed joints should provide distribution of stresses
outside the overlapping edges. This can be done by changing the edge shape or local change of crosssection of the base metal part. Sometimes, so called “false” stress concentrators just in the overlapping
area may be effective for leveling average grade of stresses along the brazed joint.
TYPICAL APPLICATIONS OF MAGNESIUM ALLOYS AND THEIR BRAZED JOINTS
The use of magnesium alloys in car design is continuously expanded, and now includes also ultralightweight matrix composites. Typical automotive applications are engine blocks, cylinder liners,
pushrods, valve spring retainers, instrument panels, clutch and brake pedal support brackets, steering
column lock housing, and transmission housings. The design of magnesium instrument panel for the
General Motor vans allowed to save 5.9 kg per piece against aluminum welded tubular design. The
Volkswagen-Kaefer comprises now about 20 kg (44 lbs) of magnesium alloys and composites (Ref. 54).
The target of Automotive industry is to achieve a 45% weight saving in 2009 relatively to the average
car weight in 1990. Obviously, the part of aluminum- and magnesium-based materials must be
substantially increased according to published forecast (Ref. 11,55,56), Table 12. A comparison of
production costs (Table 13) shows that the cost of lightweight advanced materials will go down in the
nearest future though will remain higher than the cost of traditional steel or aluminum.
Material-handling equipment and commercial applications include parts of magnesium dockboards,
grain shovels, and gravity conveyors, luggage, computer housings, digital camera housings, electrical
conductors, and hand-held tools.
Table 12
The change in structure of materials in a typical family sedan
in North America (Ref. 11)
In year 2000)
Predicted (In year 2009)
Material
Mass, kg
Portion, %
Mass, kg
Portion, %
Steel
648
54
92
16
Cast iron
132
11
23
4
Aluminum
96
8
138
24
Magnesium
2.4
0.2
87
15
Plastics
96
8
132
23
Fluids/Lubricants
72
6
35
6
Rubber
48
4
23
4
Glass
36
3
29
5
Others
69.6
5.8
17
3
Total
1200 kg
100%
576 kg
100%
Table 13
16
Mass saving over steel in the car production and cost of lightening
(data on year 2000) (Ref. 11)
Steel
Aluminum
Alloy 6061
Cost per unit weight *
1
3.4
Aluminum
matrix
composite
6061/15SiCp
4.8
7.5
Magnesium
matrix
composite
ZC71/12SiCp
10.6
Mass for equal stiffness, kg
1
0.5
0.5
0.4
0.4
Cost for equal stiffness *
1
1.7
2.3
2.9
3.9
Mass for equal strength, kg
1
0.3
0.3
0.2
0.2
Cost for equal strength *
1
1.0
1.3
1.6
2.2
Mass saving over steel for
0
equal stiffness, kg
Mass saving over steel for
0
equal strength, kg
Cost for 1kg mass saving
1
for equal stiffness *
Cost for 1kg mass saving
1
for equal strength *
* in relative values of steel price
0.5
0.5
0.6
0.6
0.7
0.7
0.8
0.8
3.4
4.3
4.8
6.2
1.4
1.8
2.3
2.8
Material
Magnesium
alloy AZ80
In the aerospace industry, lightweight and stiff magnesium alloys are employed in various units and
devices, for example, aircraft transmission systems and their auxiliary components, gear housings, rotor
housings, and generator housings in cold areas of engines (Ref. 57). For example, helicopter
transmission housings are manufactured from forged alloys AZ80 and ZK60 (Ref. 55). Graphite fiberreinforced magnesium matrix composites offer the best combination of low specific weight (target is 5 kg
per m2 of the base plate), low coefficient of thermal expansion 2 x 10-6 K-1 , high specific stiffness, and
high thermal conductivity of any known space mirror material. This materials and efficient joining
techniques to produce low cost mirrors are sought for space deployed optical systems. For example, the
Swiss company EMPA reported about the manufacture of parts of Hubble Space Telescope from a
MgAl1/T300 composite reinforced with 60 vol.% of graphite fibers, whereby the Young’s modulus of the
composite is 155 GPa (22.4 Mpsi) at density 1.8 g/cm 3 (Ref. 25). Brazing is the valuable technique for
joining of all composite structures. New creep-resistant cast magnesium alloys have a great potential for
aerospace applications, and brazing community shall focus R & D on development of suitable creepresistant brazing filler metals to response future needs of the Aerospace industry.
In audio-, video-, computer-, and communication equipment plastics are being replaced by magnesium
alloys that have advantages in strength, heat sink, and service life. Consequently, thin magnesium net
shapes are used now in many models of cellular phones, laptop computers, camcorders, etc. (Ref. 37).
Joining of magnesium matrix composites reinforced with carbon or ceramic fibers and particles is
possible only by brazing. These high-tech materials are widely utilized for automobile parts, for large
spacecraft panels, space based telescopes, space based optical systems, and space stations. The
composite brazed structures will also have application in missiles and in aircraft, both civilian and military.
17
Fig. 2 shows a dip brazed M1A magnesium alloy microwave antenna (Ref. 6).
Fig. 1. Structure of brazed joint of AZ91C/15SiCp composite material made
by Mg-12Al-2Ca filler metal
Fig.1 Dip brazed M1A microwave antenna
CONCLUSIONS
1.
Conventional standard brazing materials and traditional brazing technologies are suitable for joining
new high-performance cast and extruded magnesium alloys that came up on the market since 1990.
But standard brazing filler metals BMg-1, BMg-2a, and MC3 cannot be recommended for joining
magnesium matrix composites due to negative effect of high brazing temperature on macrostructure
of the composites. New brazing filler metals need to be developed for furnace brazing of magnesium
matrix composites at 842-968oF (450-520oC). These prospective filler metals should provide shear
and tensile strength of brazed joints at least 25 ksi (175 MPa).
2.
Brazing filler metals having operational temperatures in the range of 490-520oC (914-968oF) need to
be developed and comprehensively tested for joining wrought, work-hardened and tempered
magnesium alloys. Low-melt brazing filler metals will allow to avoid significant loss of mechanical
properties of base metals caused by brazing with conventional standard filler metals.
3.
New filler metals promising to improve mechanical properties of brazed joints both bulk and
composite magnesium alloys, such as alloy systems of Al-Mg-Cu, Mg-Al-Ca, Mg-Li-Al-Zn, and Mg-AlZn-Ca, should be tested widely and introduced in the industry.
4.
Filler metals designed for brazing extruded (or rolled) magnesium matrix composites should have as
low as possible brazing temperature due to low recrystallization temperature of the matrix and
18
anisotropic structure of the composites. Brazing temperature is not so critical for joining cast
magnesium matrix composites.
5.
Substantial progress in mechanical properties of brazed joints can be reached with filler metal having
the structure of cast matrix composites reinforced by SiC, TiC, or Al 2O3 particulates. Composite filler
metals should have low viscosity in molten state in the range of 842-968oF (450-520oC) to fill capillary
brazing gaps of 0.1-0.25 mm (0.004-0.01 in.).
6.
It is necessary to check a strengthening effect of thorium, yttrium, and /or zirconium additions in
brazing filler metals, just as this effect was successfully demonstrated in raising tensile strength and
creep resistance of cast magnesium alloys.
7.
Melt spinning technology should be developed to manufacture Mg-based brazing filler metals in the
form of thin amorphous foil that is necessary for joining large flat or shaped panels of magnesium
matrix composites, including ultra-strength Th- or Zr-alloyed matrix reinforced by SiC or graphite
fibers.
8.
Anti-corrosive chemical treatment including the deposition of phosphate coatings or conversion Crbased coatings is recommended to improve corrosion resistance of brazed joints and increase
service life of joined magnesium alloy articles working at environmental impact in automotive and
aerospace applications.
9.
Brazing of ceramics (especially silicon carbide and silicon nitride) to magnesium alloys needs to be
investigated and mechanical properties of brazed joint of such dissimilar base materials should be
tested. The successful developments of new reliable, cost-effective brazing technologies will open up
commercial applications in automotive, aircrafts, avionics, and electronic packages, as well as heatsinking and lightweight hybrid metal-ceramic structures.
10. Magnesium matrix composites are susceptible to stress concentration. In order to prevent their failure
due stress concentration caused by overlapping, the design of the brazed joints should provide
distribution of stresses outside the overlapping edges.
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19
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22
Table 5
Composition and Physical Properties of Commercial Brazing Filler Metal
Nominal composition, wt.%
Designation
AWS A5.8
Specific Gravity
(Balance Mg)
Solidus
ASTM
or UNS
Temperatures
Al
Zn
Mn
Cu
Be
Ni
Others
g/cm3
lbs/in3
°F
o
Liquidus
C
°F
o
Brazing Range
C
°F
o
C
BMg-1
AZ92A
8.3-9.7 1.7-2.3 0.15-0.5 0.05 0.0002-0.0008 0.005 0.3
1.83
0.066
830
443
1110
599
1080-1140
582-616
BMg-2a
AZ125A
11-13 4.5-5.5
-
2.10
0.076
770
410
1049
565
1058-1103
570-595
MC3*
UNS
8.3-9.7 1.6-2.4
0-0.1
1.83
0.066
830
443
1110
599
1120-1140
605-615
0-0.25
0.008
0.0005
-
0.3
0-0.01 <0.3Si
M11920
*Standard Japanese filler metal
23
Table 6
Compositions and Physical Properties of Low Temperature Brazing Filler Metals
Nominal composition, wt.%
Designation
Specific Gravity
(Balance Mg)
Temperatures
Solidus
°F
o
626
330
680
360
925-940
495-505
0.097
716
380
806
430
1020-1040
550-560
3.0
0.107
644
340
716
380
896-932
480-500
-
2.1
0.076
815
435
968
520
968-1040
520-560
Cd 25-26
3.7
0.133
750
398
779
415
806-932
430-500
°F
4.7
0.169
0.3
2.7
-
0.3
0.1-0.3
-
0.1-0.3
-
Zn
Mn
Be
Others
2
55
-
-
-
P430Mg
0.7-1.0
13-15
0.1-0.5
-
P380Mg
2.0-2.5
23-25
0.1-0.5
P435Mg
25-27
1-1.5
P398Mg*
21-22
0.2-0.5
GA432
Brazing Range
C
lbs/in3
Al
g/cm3
o
Liquidus
C
°F
o
C
* Filler metal P398Mg is used for correction defects of magnesium cast
24
Table 9
Compositions and Operational Temperatures of Magnesium Brazing Fluxes
Flux Composition, wt.%
Temperatures
Flux
KCl
LiCl
NaCl
NaF
LiF
CaCl2
CdCl2
ZnF2
ZnCl2
Cryolite
Carnalite
ZnO
Melting
°F
°C
Brazing Activity
°F
°C
F380Mg
Bal.
37
10
10
-
-
-
-
-
0.5
-
-
716
380
716-1110
380-600
F530Mg
Bal.
23
21
3.5
10
-
-
-
-
-
-
-
986
530
1000-1110
540-600
F540Mg
Bal.
23
26
6
-
-
-
-
-
-
-
-
1000 540
1004-1202
540-650
F390Mg
Bal.
30
-
-
-
-
15
10
10
-
-
-
734
390
788-1112
420-600
F535Mg
Bal.
-
12
4
-
30
-
-
-
-
-
-
995
535
1004-1202
540-650
F400Mg
-
-
-
-
-
-
-
-
-
8
89
3
752 400
797-1148
415-620
F450Mg
-
9
15
-
-
Bal.
-
-
-
-
-
-
842
842-1202
450-650
450
25
Table 1
Compositions and physical properties of brazeable magnesium alloys
ASTM
Alloy
Designat
ion
Available
Forms
Nominal Composition, %
(Balance Mg)
Density
Temperatures
Solidus
Al
Mn
Zn
Zr
Si
Cu
Others
g/cm3
Liquidus
F
1198
°C
650
F
1202
Brazing Range
(recommended)
o
°C
F
580-620
1076-1148
lbs/in3
o
o
M1A
E, S
-
1.5
-
-
-
-
-
1.77
0.064
°C
648
AZ31B
E, S
3.0
0.5
1.0
-
≤0.3
≤0.1
Ni ≤0.3
1.78
0.064
566
1050
632
1070
500-560
932-1040
AZ61A
E
6.5
0.3
1.0
-
≤0.1
-
-
1.80
0.065
525
977
610
1130
495-505
925-940
AZ63A
C
6.0
0.25
3.0
-
≤0.3
≤0.25
-
1.82
0.066
455
860
610
1130
430-450
806-842
AZ91C
C
8.7
0.2
0.7
-
≤0.3
≤0.1
Ni ≤0.3
1.81
0.065
468
875
598
1108
430-460
806-860
AS41A
C
4.3
0.4
0.12
-
1.0
0.06
-
1.78
0.064
570
1058
620
1148
500-560
932-1040
AM100
C
10.0
0.2
≤0.3
-
≤0.3
≤0.1
-
1.81
0.065
465
870
595
1102
430-460
806-860
ZE10A
S
-
-
1.2
-
-
-
REM 0.17
1.76
0.063
593
1100
646
1195
560-590
1040-1094
ZK21A
E
-
-
2.3
0.6
-
-
-
1.79
0.064
626
1159
642
1187
580-620
1076-1148
ZK51A
C
-
-
4.6
0.7
-
≤0.1
-
1.83
0.066
550
1022
640
1185
500-540
932-1004
ZK60A
E, S
-
-
5.5
0.4
-
-
-
2.06
0.074
520
970
635
1175
495-505
925-940
QE22A
C
-
-
0.7
-
-
1.83
0.066
535
995
640
1185
490-520
914-968
QH21A
C
-
-
0.7
-
-
1.84
0.067
535
995
640
1185
490-520
914-968
HK31A
C
-
-
0.3
0.7
-
-
Ag 2.5,
REM 2.0
Ag 2.5
REM 2.0,
Th 1.0
Th 3.3
1.85
0.067
642
1187
648
1200
580-620
1076-1148
ZC71
E, S
-
0.5
6.5
-
-
1.25
-
2.15
0.080
420
788
630
1166
390-410
734-770
*
E = Extruded shapes and structural sections.
S = Sheets and plates.
C = Castings.
26
** Coefficient of thermal expansion of all magnesium alloys is in the range of (25-27) x 10-6 m/m∙oC [(14-15) ) x 10-6 in/in
27
28