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 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 textileand 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
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Alexander E. Shapiro is with Titanium Brazing, Inc., Columbus, OH
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 strength / Elongation, % / Young’s ModulusMPa / ksi / MPa / ksi / 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
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).
Table 3
Technical problems in magnesium brazing
Problems / Possible negative effect / Technical solutionChemical 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.
Table 4
Mechanical Properties of Magnesium Matrix Alloys and Their Composites Reinforced with SiC, SiO2, TiCor Al18B4O33 particles, and SiC, Al2O3 or graphite fibers
Matrix alloysand composites
(vol.%) / Yield strength / Tensile strength / Elongation, % / Young’s Modulus
MPa 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 9.2
AZ91C/10TiCp* / - / 214 31 / 4.0 / 52.0 7.5
AZ91C/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 7.6
ZC71 – 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 10.9
AS41 – matrix
/ 125 18 / 193 28 / 9.0 / 50.0 7.3AS41/30Al2O3 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 10.0AZ31B/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 10.7
QE22/25SiCp** / 245 36 / 325 47 / 4.0 / 73.0 10.6
Mg/10Mg2Sip *** / - / 175 25 / - / 55.0 8.0
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
Mg14Li1Al/30steelf * / - / 758 110 / - / 66.0 9.5
Mg14Li1Al/30B p** / 244 35 / - / - / 101.0 14.6
* Casting; ** Forging; *** Undirectional solidification; **** Extrusion
Footnotes: p – particles, f - fibers
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 734-752oF (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.
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.
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.
Berylliumis 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. Silvermakes 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. Tinis 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.
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 ThCl4 / 100
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.