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/mK, and

exhibit excellent ability in shielding electromagnetic interrupt. Low density ~1.75 g/cm3 in the combination

with relatively high tensile strength 33-42 ksi (228-290 MPa), heat resistance up to 840 oF (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

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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 Tensile strengthElongati

on, %

Young’s Modulus

MPa ksi MPa ksi GPa 103 ksiM1A 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

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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).

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Table 3

Technical problems in magnesium brazing

Problems Possible negative effect Technical solution

Chemical activity of base metal.Fast growth of oxide film [MgO + Mg(OH)2]

Difficult wetting by brazing filler metal

Halide brazing fluxes.Vacuum brazing at <10-5 Torr.Brazing in dry argon.

Low solidus of base metals.Brazing temperature is close to solidus.

Changes in structure and shape of brazed parts

Short holding time at brazing temperature.Low-temperature filler metals are needed!

Structural changes in base metal due to brazing heating

Loss of strength of base metal for 20-30%

Brazing at lower temperature.Heat treatment of brazed parts.

Possible atmospheric corrosion of base metals and joints

Corrosion of brazed articles Postbraze chromate or phospate coating, or electrolytic oxidation

Density of molten filler metal is less than that of the brazing flux

Inserts of flux residues in the brazed joint

Selection of flux/filler metal combination.Watchful control of process parameters.

Erosion of base metal by liquid filler metal

Loss of fatigue, inpact, or creep strength

Short holding time at brazing temperature.Watchful control of process parameters.

Susceptibility of Mg-matrix composites to stress concentration

Failure of base metal in the overlapping area

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 fiber-

reinforced 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.

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Table 4Mechanical Properties of Magnesium Matrix Alloys and Their Composites Reinforced with SiC,

SiO2, TiC or Al18B4O33 particles, and SiC, Al2O3 or graphite fibers

Matrix alloysand composites

(vol.%)

Yield strength Tensile strengthElongation,

%

Young’s ModulusMPa ksi MPa ksi GPa 103 ksi

AZ91C – matrix 145 21 225 33 7.2 42.7 6.2AZ91C/15SiCp * 178 26 218 32 1.1 57.0 9.2AZ91C/10TiCp * - 214 31 4.0 52.0 7.5AZ91C/30Al2O3 f * 230 33 280 40 1.8 64.0 9.3AZ91/30Graphite f * - 350 51 - 70.0 10.1Mg1Al/60Graphite f - 1470 213 - 155.0 22.4ZK51A – matrix 131 19 221 32 7.3 43.2 6.3ZK51A/15SiCp * 162 23 210 30 1.8 52.4 7.6ZC71 – matrix 340 49 360 52 5.0 -ZC71/12SiCp ** 397 57 453 66 1.0 -AM100 – matrix 110 16 230 33 2.0 45.0 6.5AM100/20Al2O3 f * - 220 32 1.5 75.0 10.9AS41 – matrix 125 18 193 28 9.0 50.0 7.3AS41/30Al2O3 f * 240 35 270 39 1.0 78.0 11.3AZ31B - 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 10.0AZ31B/20SiC f **** 417 60 447 65 0.9 100.0 14.5QE22 – matrix 180 26 250 36 4.5 46.0 6.7QE22/30Al2O3 f * 250 36 300 43 1.6 74.0 10.7QE22/25SiCp** 245 36 325 47 4.0 73.0 10.6Mg/10Mg2Sip *** - 175 25 - 55.0 8.0Mg/10Mg2Nip *** 117 17 202 29 3.6 -AZ91/10Al18B4O33* 266 38 480 70 1.0 78.0 11.3MB15/30Al18B4O33* 230 33 303 44 0.5 76.0 11.0Mg14Li1Al/30steelf * - 758 110 - 66.0 9.5Mg14Li1Al/30B p** 244 35 - - 101.0 14.6

* Casting; ** Forging; *** Undirectional solidification; **** ExtrusionFootnotes: p – particles, f - fibers

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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 450-

480 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 Al18B4O33 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 734-752 oF

(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.

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 die-

cast magnesium alloys.

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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-8Zn-

0.2Mn) 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

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.

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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-Al-

based 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, %

Alloying element

Metal or master-alloy

Apparent liquid

solubility, %Ag, Al, Au, Ba,

Bi, Cd, Cu, Ga,

Ge, In, Li, Ni,

Pd, Pb

Metal 100

Rare Earths

(Mishmetal),

Sb, Sn, Sr,

Th, Zn

Metal 100

Beryllium Al-Be 0.01 Silicon Ferrosilicon (95% of Si)

100

Calcium Ca-20Mg 100 Sodium Metal 0.1

Chromium Metal powder 0.04 Tantalum Metal powder 0.015

Cobalt Metal 5.0 Tellurium Metal 0.2

Iron Metal powder 0.1 Thorium Metal or ThF4

or ThCl4100

Manganese Al-Mn or MnCl2

5.0 Titanium Metal or TiCl4 ≥1.0

Molybdenum Metal powder or MoCl4

≥1.0 Tungsten Metal powder ≥0.2

Niobium Metal powder 0 Vanadium Metal powder or VCl4

≥0.02

Phosphorus Fe2P 0.01 Yttrium Mishmetal 100

Potassium Metal 0.02 Zirconium Metal 0.95

Rhodium Metal 0.5

Impurities such as iron, nickel, and copper should be controlled in the parts-per-million range in Mg-

based 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.

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-

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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 Filler metal Brazing technique

Shear strength

Tensile strengthTesting

temperature Refe- rence

MPa ksi MPa ksi oCM1A Mg-12Al-11Cd-

4NiBrazing with flux F390Mg

190 27.5 20 33

AA8009Al-25Mg-3.5Cu

Vacuum brazing

- - 122-136 17.7-19.7 2032- - 93 13.5 260

Al-32Mg-2Cu-1Si - - 87 11.2 20AZ91C/15SiCp Mg-12Al-2Ca Vacuum

brazing- - 180-193 26-28 20 36- - 58-70 8.4-10.1 200

AZ31B Zn-3Mg-1Al Ultrasound assisted soldering

- - 50-68 7.2-9.8 20 41,42AM50 Zn-3Mg-1Al - - 46-82 6.7-11.9 20

M1A Silver film Brazing in dry argon

68.6 10.0 - - 20 33,47M1A Ga-4Mg-4Cd-4Zn 58.8 8.5 - - 20

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.

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).

9

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 Al-

based filler metal should be as fast as possible (485oC, 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 AZ91/15SiCp 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 Mg-

Al-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

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.

10

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-8Zn-

0.6Ca 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 350-

400oC (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 449oC, 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) Mg-33Al-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 Mg17Al12 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 Al2Y 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 Al2O3 particulates.

Experiments with composite Mg-based filler metals were recently started and shell be accomplished in

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 418 oC (785oF)

11

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 1040oF

(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 Mg2Cu 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-11Sn-

4Mg-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.

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.

12

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 Method of Application

Ferric nitrate bright pickle

Chromic acid, 1.5 lb (0.68 kg)

Ferric nitrate, 5.33 oz (0.15 kg)

Potassium fluoride, 0.5 oz (0.014 kg)

Water to make one gallon (3.8 L)

Temperature 60–100°F (16–38°C)

0.25 to 3 min immersion followed by cold and hot water rinses and air dry

Chrome pickle

Sodium dichromate, 1.5 lb (0.68 kg)

Conc. nitric acid, 24 fl. oz. (710 mL)

Water to make one gallon (3.8 L)

Temperature 70–90°F (21–32°C)

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)

Postbraze cleaner (A) Sodium dichromate, 0.5 lb (0.23 kg)

Water to make one gallon (3.8 L)

Temperature 180–212°F (82–100°C)

2 hour immersion in boiling bath followed by cold and hot water rinses and air dry

Postbraze cleaner (B) (1 step) 2-3% solution of Na2CO3

(2 step) Chromium anhydride 150 g/L

Nitric acid 240 g/L

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

13

Temperature 68-86oF (20-30oC) (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 magnesium-

based alloy, galvanic corrosion of brazed joints is minimized.

Table 11

Anticorrosive conversion coatings

Treatment Composition Method of Application

Cr-Mn

Alkali NaOH 2 min immersion at

55oC (130oF)

Chemical cleaning CrO3, 0.22-0.44 lb (100-200 g)

Water, 1L

Na2Cr2O7 2H2O, 0.22lb (100 g)

1-15 min immersion in

boiling bath

Conversion

coating

MnSO4 5H2O, 0.11lb (50 g)

MgSO4 7H2O, 0.11lb (50 g)

Water, 1L

5-20 min immersion at

35-55oC (95-130oF)

Cr-Al

Alkali NaOH 2 min immersion at

55oC (130oF)

Chemical cleaning CrO3, 0.22-0.44 lb (100-200 g)

Water, 1L

Na2Cr2O72H2O, 0.22lb (100 g)

1-15 min immersion in

boiling bath

Conversion

coating

NaF, 0.033 lb (15 g)

Na2Cr2O7 2H2O, 0.4 lb (180 g)

Al2SO4 14H2O, 0.022 lb (10 g)

HNO3 70%, 125 mL

Water, 1L

2 min immersion at

35-55oC (95-130oF)

14

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 3-

baking 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 (CeCl2) 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.

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 cross-

section 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.

15

TYPICAL APPLICATIONS OF MAGNESIUM ALLOYS AND THEIR BRAZED JOINTS

The use of magnesium alloys in car design is continuously expanded, and now includes also ultra-

lightweight 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)

MaterialIn year 2000) Predicted (In year 2009)

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

Mass saving over steel in the car production and cost of lightening (data on year 2000) (Ref. 11)

Material Steel AluminumAlloy 6061

Aluminum matrix

composite 6061/15SiCp

Magnesium alloy AZ80

Magnesium matrix

composite ZC71/12SiCp

Cost per unit weight * 1 3.4 4.8 7.5 10.6Mass for equal stiffness, kg 1 0.5 0.5 0.4 0.4

16

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 equal stiffness, kg

0 0.5 0.5 0.6 0.6

Mass saving over steel for equal strength, kg

0 0.7 0.7 0.8 0.8

Cost for 1kg mass saving for equal stiffness *

1 3.4 4.3 4.8 6.2

Cost for 1kg mass saving for equal strength *

1 1.4 1.8 2.3 2.8

* in relative values of steel price

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 fiber-

reinforced 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/cm3 (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 creep-

resistant 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.

Fig. 2 shows a dip brazed M1A magnesium alloy microwave antenna (Ref. 6).

17

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-Al-

Zn-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

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 Al2O3 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.).

18

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 Cr-

based 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 heat-

sinking 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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22

Table 5Composition and Physical Properties of Commercial Brazing Filler Metal

Designation

Nominal composition, wt.%

(Balance Mg) Specific Gravity Temperatures

AWS A5.8 ASTM

or UNS Al Zn Mn Cu Be Ni Others g/cm3 lbs/in3

Solidus

°F oC

Liquidus

°F oC

Brazing Range

°F oC

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 - - 0.008 - 0.3 2.10 0.076 770 410 1049 565 1058-1103 570-595

MC3* UNS

M11920

8.3-9.7 1.6-2.4 0-0.1 0-0.25 0.0005 0-0.01 <0.3Si 1.83 0.066 830 443 1110 599 1120-1140 605-615

*Standard Japanese filler metal

23

Table 6Compositions and Physical Properties of Low Temperature Brazing Filler Metals

Designation

Nominal composition, wt.%

(Balance Mg) Specific Gravity Temperatures

Al Zn Mn Be Others g/cm3 lbs/in3

Solidus

°F oC

Liquidus

°F oC

Brazing Range

°F oC

GA432

P430Mg

P380Mg

P435Mg

P398Mg*

2 55 - - -

0.7-1.0 13-15 0.1-0.5 - 0.3

2.0-2.5 23-25 0.1-0.5 - 0.3

25-27 1-1.5 0.1-0.3 - -

21-22 0.2-0.5 0.1-0.3 - Cd 25-26

4.7 0.169

2.7 0.097

3.0 0.107

2.1 0.076

3.7 0.133

626 330

716 380

644 340

815 435

750 398

680 360

806 430

716 380

968 520

779 415

925-940 495-505

1020-1040 550-560

896-932 480-500

968-1040 520-560

806-932 430-500

* Filler metal P398Mg is used for correction defects of magnesium cast

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Table 9

Compositions and Operational Temperatures of Magnesium Brazing Fluxes

Flux Flux Composition, wt.% Temperatures

KCl LiCl NaCl NaF LiF CaCl2

CdCl2 ZnF2 ZnCl2 CryoliteCarnalite 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. - - - - - - 842 450 842-1202 450-650

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Table 1

Compositions and physical properties of brazeable magnesium alloysASTM Alloy Designation

Available Forms

Nominal Composition, %

(Balance Mg)

Density Temperatures

Al Mn Zn Zr Si Cu Others g/cm3 lbs/in3Solidus Liquidus Brazing Range

(recommended)°C oF °C oF °C oF

M1A E, S - 1.5 - - - - - 1.77 0.064 648 1198 650 1202 580-620 1076-1148

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 - - Ag 2.5,REM 2.0

1.83 0.066 535 995 640 1185 490-520 914-968

QH21A C - - 0.7 - - Ag 2.5REM 2.0,

Th 1.0

1.84 0.067 535 995 640 1185 490-520 914-968

HK31A C - - 0.3 0.7 - - 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

26

* E = Extruded shapes and structural sections. S = Sheets and plates. C = Castings.

** 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

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