Brazing Magnesium Alloys and Magnesium Matrix Composites

Brazing Magnesium Alloys and Magnesium Matrix Composites A favorable strength-to-weight ratio makes magnesium a desirable material for automotive and aerospace parts Methods, filler metals, and fluxes suitable for brazing of cast and extruded magnesium-based alloys were developed in the 1960s and 1970s. Since that time, the furnace, torch, and dip brazing processes have been successfully employed without considerable changes. New interest in brazing magnesium has been recently aroused due to the expansion of use of magnesium alloys in the 1990s 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 almost tripled last decade, and the world production capacity reached 515,000 tons per year in 2002 (Ref. 1). Both the increased production of magnesium and applications of new high-performance magnesium alloys have posed a scientific and technical challenge to the brazing engineering community.      This article summarizes the experience in joining of cast, extruded, and rolled magnesium alloys, evaluates the potential of conventional brazing technologies for improving mechanical properties and corrosion resistance of joints, and discusses new developments in response to industrial demand for joining 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 metals. 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 interruption. Low density, ~1.75 g/cm3, in combination with A relatively a high tensile strength  of 33–42 ksi (228–290 MPa), heat resistance up to 840°F (450°C), and oxidation resistance up to 930°F (500°C) make magnesium alloys attractive for various structures in the automotive and, especially, aerospace industries, as well as in textile and printing machines where lightweight magnesium parts are used to minimize inertial forces  at high speed (Ref. 2). Moreover, magnesium alloys are recyclable, which minimizes their environmental impact. However, the surface of magnesium alloys should be protected because they corrode easily when exposed to atmosphere. There has been a significant growth in the production and applications of structural magnesium alloys over the past two decades. The demand is driven primarily by the 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 the  standard titanium Alloy Ti-6Al-4V, hardened aluminum Alloy AA7075, or  AISI 4340 steel. The specific strength of extruded magnesium Alloy AZ31B is higher than 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 Tables 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.     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 loses about 35% of elongation, 22% of yield strength, and 8% of tensile strength after brazing at 595°C (1102°F) for 1–2 min (Refs. 5, 6). This significant loss of mechanical properties is the main motivation for developing and implementing low-melting brazing filler metals.     Torch brazing reduces base metal properties locally in areas heated for brazing, whereas 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. However, the reduction in aluminum results in a slight decrease in strength for AM alloys (Ref. 7). Alloys AS41, AS21, and AE42 are employed for applications requiring long-term exposure at temperatures above 250°F (120°C) and 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 was observed for Alloy AZ31 fabricated by casting, extrusion, and rolling (Ref. 8). The strength weakening is accompanied by a remarkable increase in ductility. The elongation increased from 21.5% to 66.5% as the test temperature changed from RT to 482°F (250°C). Magnesium brazing is not simple due to the high chemical activity among the structural metals. A complex oxide film containing magnesium oxide and magnesium hydroxide is formed on the surface of the base metal when heated in air. This chemically stable film is not reduced either in conventional active gaseous atmospheres nor in vacuum up to 10–5 mm-Hg (10–5 Torr). Additionally, magnesium hydroxide decomposes into hydrogen and water during the heating at 572°–752°F (300°–400°C), which further hinders the brazing process (Ref. 9). The density of magnesium filler metals is less than the salt systems used in brazing fluxes, which often results in the appearance of slag inclusions in the joints. Also, magnesium has a high negative value for electrode potential (–2.38 V), which hinders the deposition of a reliable electrolytic or chemical coating, which in turn could improve wetting of the  molten brazing filler metal or protect against 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 ultralightweight structural materials joined by brazing. They 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 parts are particularly useful 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 for the design of zero thermal expansion structures, which provide dimensional stability over a wide temperature range and accurate pointing angles for reflectors and antennas (Ref. 10).     The application of lightweight metal composites in the automotive industry is expanding due to efforts to make more- fuel-efficient cars. Mechanical properties of brazeable composites are presented in Table 4 (Refs. 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 4 shows the main advantages of MMC are the increase of Young’s modulus, higher strength at elevated temperatures, and lower CTE. Improved creep resistance in alloys with  ceramic fiber reinforcement is also impressive. For example, the creep rate at 392°F (200°C) 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 1.13 x 10–9 s–1. That is six times lower than the creep rate of a cast matrix alloy (Ref. 22). Promising mechanical properties were also achieved for direct powder forged composites. That allows making of near-net shape products.   Some magnesium matrix composites exhibited impressive increases in mechanical performance in contrast with nonreinforced matrix alloys. For example, the composite consisting of Mg-14Li-1Al matrix and 30 vol-% of steel fibers has a tensile strength 600–700 MPa (87–123 ksi) at room temperature and 450–480 MPa (65–69 ksi) at 200°C (392°F), while the matrix alloy exhibits only 144 MPa (21 ksi) at room temperature, and 14 MPa (2 ksi) at 200°C (Ref. 26). Advanced Mg-based materials have great potential to improve mechanical performance. New nontraditional reinforcing systems reach strength characteristics 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) and very good rigidity characterized with Young’s modulus 11 Mpsi (76 GPa) and  0.5% elongation (Ref. 21). An increase in volume fraction of the reinforcing component can result in drastic change of mechanical properties. The Swiss company EMPA  recently reported 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 also have potential for high-damping 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 the 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 other cast or forged composites can be performed by placing filler metal GA432 or P380Mg between the brazed parts and heating to 734°–752°F (390°–400°C) thoroughly controlling 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 a composition close to BMg-1. All three alloys are suitable for torch, furnace, or dip brazing.     If torch or dip brazing are done at lower temperature, filler metals shown in Table 6 (Refs. 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) affect the properties of magnesium-based filler metal somewhat similar to their effects on die-cast magnesium alloys. Aluminum increases room-temperature strength and hardness, and improves fluidity. However, excessive aluminum  causes a decrease in ductility due to formation of brittle intermetallic phases. Also, aluminum widens the 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 also prevents  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 strength. 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 resistance to hot cracking, yet retains exceptional fluidity (Ref. 29). Small additions of manganese do not affect mechanical properties, but they do produce a beneficial effect in the control of corrosion, especially in saltwater. The filler metals are alloyed with 0.1–0.5 wt-% Mn to improve corrosion resistance. In the presence of aluminum, the solubility of Mn in solid solutions of magnesium alloys is less than 0.3 wt-%. Cadmium is the only  metal with a crystal lattice compatible with magnesium, but most importantly, cadmium forms solid solutions with magnesium at any concentration. 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 the 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. However, silicon affects corrosion resistance in the presence of iron impurities. Silver makes possible age hardening, which results in higher strength for both cast and wrought magnesium alloys. Rare-earth metal additions of 2–4 wt-% (for example, mishmetal containing 55 wt-% Ce, 20 wt-% La, 20 wt-% Nd, and 5 wt-% 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 when added in amounts up to 4 wt-% and up to 0.7 wt-%, respectively. Also, zirconium is an effective grain refiner in magnesium alloys because lattice parameters of a-Zr are very close to those of magnesium. But, zirconium is not used in alloys containing both Al and Mn, since they form intermetallics with zirconium and remove it from solid solutions. The corrosion rate increased abruptly with the addition of >1 wt-% Ca. The negative effect of Ca can be distinguished by adding zinc or rare-earth metals. Recent investigations demonstrated  a positive effect of calcium on creep resistance in magnesium alloys. Calcium is not recommended for magnesium alloys to be welded due to cracking, but it is harmless for brazeable alloys. Strontium up to 2 wt-% improves fluidity of Mg-Al-Mn alloys without affecting corrosion resistance (Ref. 30). Lithium is the only alloying metal that decreases the 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 a decrease in strength. Tin is added to magnesium in combination with aluminum to improve ductility and reduce tendency to hot cracking. Thorium in the amount of 1–3 wt-% is very effective for improving creep resistance in magnesium alloys, especially in combination with REM.     The elements Si, Ge, Pb, Sb, and Bi of the IVA and VA groups form stable intermetallic phases with magnesium (Ref. 31), and they 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 of alloying metals in the melt. Liquid solubility of alloying metals in magnesium is shown in Table 7.     Impurities such as iron, nickel, and copper should be controlled in the parts-per-million range in Mg-based filler metals to minimize their effects on mechanical properties and corrosion resistance. The upper limit of Ni or Fe in magnesium alloys should be 0.005% for maximum corrosion resistance. However, there is some addition of copper in Al-based filler metals used for joining magnesium alloys (Ref. 32). In this case, special attention should be paid to corrosion protection of brazed joints by metallic coatings or polymer paint coats.     Filler metals BMg-1, MC3, P430Mg, and P380Mg allow electrolytic oxidation as a finishing treatment for corrosion protection of brazed parts.     Filler metals BMg-1 and BMg-2a are usually hot-formed by heating to between 500° and 600°F (260° 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 0.04–0.06 in. (1–1.5 mm) 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  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.     The brazed joints had a shear strength of approximately 27.5 ksi (190 MPa) with the Mg-12Al-11Cd-4Ni filler metal (Ref. 33), which has a melting range of 1040°–1076°F (560°–580°C), when brazing was performed with the flux F390Mg (Table 9). 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°–1022°F (300°–550°C) for brazing times ranging from 0 to 110 min, but all of them actively reacted with the cast alloy substrate, which 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 fillers was not reported. It is doubtful that binary Mg-Zn  alloys are suitable as brazing filler metals because of susceptibility to hot cracking, which is well known from magnesium die casting experience.     Pure 15-µm-thick aluminum foil is suitable as a filler metal for microspot brazing of extruded Alloy AZ31 at a current of 500–800 A, but the strength of such joints was inadequate (Ref. 34). Promising Alloy Systems Alongside traditional filler metals, several recent new alloy systems can be considered promising for improving the mechanical properties of brazed joints. Among them, filler metal Al-25Mg-3.5Cu in the form of melt-quenched ribbons showed promise with a solidus temperature of 840°F (448°C), a liquidus of 864°F (462°C), and tensile strengths of 17.7–19.7 ksi (122–136 MPa) at RT and up to 13.5 ksi (93 MPa) at 500°F (260°C) (Ref. 32). Partial substitution of copper for silicon in Al-32Mg-2Cu-1Si resulted in a significant decrease in tensile strength to 11.2 ksi (87 MPa) at room temperature. The thermal cycle when vacuum brazing with the Al-based filler metal should be as fast as possible (485°C, 1 min) to prevent the formation of thick brittle intermetallic layers on the interface. Postbraze heat treatment for 24 h at 482°F (250°C) was used for precipitation strengthening of the brazed joints. Structures brazed with this filler metal should be protected against moisture corrosion due to the presence of copper in the joint composition. Creep-resistant alloys Mg-Al-Ca-Sn and Mg-Al-Ca-Zn were recently developed (Ref. 35), and they showed yield strengths of 27.5–29.4 ksi (190–203 MPa), ultimate tensile strengths of 34.8–36.2 ksi (240–250 MPa), and elongations of 3–5% at room temperature. The minimum creep rate was less than 0.9 x 10–9 s–1 at 392°F (200°C) under loading of 8 ksi (55 MPa). Similar improvement of creep resistance was also measured for the Ca-added Mg-Al-Mn Alloy AM60B. It showed at least 10 times lower creep rate at 392°F (200°C) at the load of 13 ksi (90 MPa) than Ca-free cast alloy (Ref. 30). This study confirmed the positive effect of a relatively large addition (1–3 wt-%) of calcium in magnesium alloys, despite the traditional point of view. The brazing filler metal Mg-12Al-2Ca-0.8Zn (designed according to  the above-mentioned cast alloys) has a melting range of 818° –1050°F (437°–565°C) and tensile strengths of 26–28 ksi (180–193 MPa) at room temperature, and 8.4–10.1 ksi (58–70 MPa) at 200°C (Ref. 36) for joints brazed on magnesium matrix composite AZ91/15SiCp. 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 nonequilibrium microstructure comprised of  solid solution grains, Mg-Al eutectic, and intermetallics that were crystallized in the forms of both relatively big crystals (g-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°–838°F (440°–448°C), but the tensile strength of the brazed joints was only 1.6–2 ksi (11–14 MPa) at 392°F (200°C). The quest for low brazing temperature is caused by the necessity to perform brazing as close as possible to the  recrystallization temperature of the matrix alloys in magnesium composites to avoid excessive residual stresses in the composite after cooling (Ref. 58). Magnesium matrix composites manufactured by extrusion or rolling are characterized by pronounced deformation and anisotropy of mechanical properties. Also, the recrystallization temperature of the magnesium matrix is only about 150°C at critical deformation of ≤10%. Therefore, decreasing the brazing temperature is very important for the reliability of the  brazed joints.     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. There is extensive experience    in industry to manufacture various ternary or quaternary alloys in the 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) with a liquidus of about 600°C can be used as a filler metal like BMg-1, but with a 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 also would be higher than BMg-1 (Ref. 37). A little increase of Zn may decrease  the melting point of the projected filler metal by 30°–40°C without significant loss of strength.     Also, tests of the low-melting brazing filler AZ88 (Mg-8Al-8Zn-0.2Mn) indicate  that it can be reasonably expected to prevent overheating of work-hardened and tempered base metals. The Alloy AZ88 has a  liquidus temperature of 520°C (968°F). This means that brazing with AZ88 filler metal can be done at 530°–550°C (986°–1022°F). Rods or strips of this cast alloy can be manufactured by warm rolling at 350°–400°C (662°–752°F).     New prospective filler metals based on  a In-Mg-Al-Zn system were developed by T. Watanabe et al. (Refs. 38–40). The best alloy of this system In-34.5Mg-0.8Al-0.2Zn exhibits liquidus at 476°C, brazing temperature at 490°C, a hardness of 110 HV, and tensile strength of the brazed joints comparable with the strength of 0.9-mm (0.036-in.) -thick foil of extruded Alloy AZ31B base metal. Addition of zinc, up to 6.4 wt-%, results in a decrease in the melting point to 449°C and, also, a considerable decrease in tensile strength. Magnesium alloys that require low operational temperatures can be successfully joined with ZnMg3Al (Zn-3Mg-1Al), which has a melting range of 642°–752°F (338°–400°C), and Mg48Zn43Al9 (Mg-43Zn-9Al), which has a melting range of 644°–660°F (340°–348°C). Ultrasonic assisted soldering with ZnMg3Al filler metal provided high-tensile strength in the 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 joints soldered with Mg48Zn43Al9 showed lower strength in the range of 1.5–3.8 ksi (10–26 MPa) but better corrosion resistance than ZnMg3Al (Refs. 41, 42).     Several nonstandard brazing filler metals and solders with the joining temperature in the range of 662°–887°F (350°–475°C) were offered for joining magnesium matrix composites reinforced with graphite fibers (Ref. 43). These alloys have the following compositions: 1) Mg-32Al-2Zn with a liquidus temperature 425°C and brazing temperature >450°C, 2) Mg-39Li-2Zn with the liquidus of 325°C and soldering at >350°C, 3) Mg-48Ag-2Zn with the liquidus of 450°C and brazing at >475°C, and 4) Mg-33Al-33Li with the liquidus of 300°C and soldering at >325°C. Also, 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 cannot be dissolved into the a-Mg matrix, and the content of aluminum in the matrix of Mg-9Al-1Zn-1Y alloy is decreased.     Substantially improved mechanical properties can be expected in joints  brazed with 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 will be finished in the near future to respond to strength requirements of new high-strength base materials such as magnesium matrix composites. Filler metal matrix reinforced with fine ceramic particles can increase yield strength in brazed joints  by at least 20% and creep strength by 50–70% (Ref. 45). The Mg-Al-Li system,  which has a eutectic Mg-36.4Al-6.6Li (wt-%) composition at 418°C (785°F), looks like a possible candidate in the liquid phase to prepare and test for composite brazing filler metals. There are also other low-melting Mg-based alloys that might have good plasticity in solid state.     Another alloy of this system Mg-8Li-5Al-1Zn is a filler metal with a melting point around 1040°F (560°C). This alloy demonstrates an unusually high tensile strength of 220 MPa (32 ksi) after age hardening (Ref. 46). Supposedly, the strength can be further increased by adding a small amount of zirconium.     Promising results were obtained by brazing magnesium alloys using a transient liquid phase (TLP) technique with nickel, copper, and silver layers as filler metals (Ref. 47). Brazing with a nickel interlayer 0.1mm thick was carried out at 1004°F (540°C) for 5 min. A multicomponent structure was formed in the joint  comprised of intermetallics Mg2Ni, MgNi2, and eutectic Mg-Mg2Ni. Decreasing of the nickel layer thickness from 0.1 to 0.02 mm resulted in a three times strength gain. The liquid phase appears at 950°F (510°C) in 3–4 s after starting the TLP contact reaction in the magnesium-copper system. In 15 s, the intermetallic layer Mg2Cu formed at the interface. After TLP brazing for 5 min, the crystallized joint consisted of an intermetallic phase Mg2Cu at the copper side,  eutectic Mg+Mg2Cu, and pure Mg at the magnesium side (Ref. 48). Coating of Ni, Ag, or Cu films ~20 microns thick deposited on the base metal by vacuum evaporation provided the best strength of brazed joints of magnesium alloys. Supposedly, TLP brazing is also effective for joining magnesium matrix composites. A method of brazing and soldering magnesium alloys with gallium-based pastes was developed by I. Y. Markova (Refs. 33, 47). Compositions of tested gallium solders are Ga-4Mg-4Cd-4Zn and Ga-26Zn-11Sn-4Mg-4Cd. Soldering/brazing temperatures were in the range of 150°–600°C (302°–1112°F) for joining M1A parts in dry argon. Maximum joint shear strength was 58.8 MPa (8.5 ksi). As with all Mg-Ga alloys, these joints are susceptible to atmospheric corrosion and need to be protected by phosphate of chromate coatings. Fluxes American Welding Society (AWS)  brazing fluxes, Type FB2-A (ANSI/AWS A5.31-92) are used when brazing magnesium alloys. Because of their corrosive nature, 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 (Refs. 9, 27, 38). So called “contact-reactive” fluxes also can be effective due to their ability to deposit a thin zinc film that promotes filler metal wetting of the fluxed magnesium surface (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, which makes quality brazing impossible. Fluxes are used in dry powder form for 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 since it retards the flow of filler metal. Flux pastes are dried by heating at 350°–400°F (177°–204°C) for 5 to 15 min 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 a preserving oil coating, chromate coated surface, or acid pickled surface (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 (Refs. 6, 9). 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°–176°F (60°–80°C) (Ref. 27). Interaction between any of the mentioned surface preparations and brazing should be less than 5 h.     T. Watanabe and H. Adachi investigated surface treatment of Alloy AZ31B by pickling in hydrofluoric acid before brazing (Ref. 49). Such pickling produced a thin protective film of MgF2 on the alloy surface, which improved wetting with In-30Mg-4Zn-1Al brazing filler metal at 480°–500°C. Postbraze Cleaning and Corrosion Resistance Regardless of the brazing process,  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 1–2 min in chrome pickle, followed by a 1–2 h boil in either A or B postbraze flux remover 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.     If necessary, corrosion resistance of brazed joints or an entire brazement can be increased with phosphate coating (Ref. 51) chemically deposited by rubbing the surface or immersing in a 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. A protective coating 2.5–3 µm thick is deposited after 1–5 min at 68°–95°F (20°–35°C). 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 epoxy-based primer, acrylic or polyester color base, and  acrylic top clear paint is the main finishing process for the purpose of better corrosion resistance under a severe corrosive environment. The completely finished corrosion protection, which includes three coats and three baking paint films on conversion coated surfaces, is so effective that it meets automotive industry regulations for magnesium parts (Ref. 50). Nonchromate conversion coating may be deposited on brazed magnesium articles with a water solution of 20 g/L cerium chloride (CeCl2) or 10 g/L vanadium oxide (V2O5) at pH = 6–8 and 40°C for 5 min (Ref. 52). Also, a sealing treatment at 60°C for 10 s 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 a solution containing MgO at 9 g/L, CrO3 at 45 g/L, and H2SO4 at 1.5 g/L. This solution provides the deposition of a lightgold colored oxide film. To prepare this composition, magnesium oxide is mixed with a small amount of the acidic water solution up to a paste state, then CrO3 and water are added. Stirring the resulting mixture continues until there is a full dissolution of MgO (Ref. 53). The surface to be coated is prepared with sandpaper, degreased with acetone or the like, and dried. The oxidative solution is deposited by fabric or cotton tampon, with a processing time of 2–3 min. Wet residues should be removed from the brazed joint. Design of Brazed Structures Designing brazed joints for magnesium alloys could be the subject for an entire article by itself. Only two specific considerations for magnesium joint brazing design are mentioned here. First, magnesium alloys have low yield strength. This means that overlapping value equal to two thicknesses of the base metal is usually sufficient to provide the shear strength of brazed joints. On the other hand, it is reasonable to make the overlap as thick as possible to increase total thickness, and consequently, strength of the brazed structure. In each case, the designer has to make the appropriate decision in accordance with operational conditions of the brazed structure. Second, magnesium matrix composites are susceptible to stress concentration due to a big difference between the mechanical properties of hard fibers and the soft matrix. In order to prevent failure from stress concentration in the overlap, the brazed joint design should distribute 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 equalizing stresses along the brazed joint. Typical Applications for Magnesium Alloys and Their Brazed Joints The use of magnesium alloys in car design is expanding, and now includes 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 housings, and transmission housings. A magnesium instrument panel for General Motors’ vans saves 5.9 kg per piece compared to an aluminum welded tubular design. The Volkswagen-Kaefer has about 20 kg (44 lbs) of magnesium alloys and composites (Ref. 54). The automotive industry target is to achieve a 45% weight saving in 2009 relatively to the average car weight in 1990. Aluminum- and magnesiumbased parts must be substantially increased according to published forecasts (Refs. 11, 55, 56) (Table 12). A comparison of production costs (Table 13) shows the cost of lightweight advanced materials going down in the near future, although they will remain higher than the cost of traditional steel or aluminum (Ref. 11). Material-handling equipment and commercial applications include parts for magnesium dockboards, grain shovels, gravity conveyors, luggage, computer housings, digital camera housings, electrical conductors, and hand-held tools. 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). 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 space mirror material. These materials and efficient joining techniques to produce low-cost mirrors are sought for space deployed optical systems. For example, the Swiss company EMPA manufactured parts of the Hubble Space Telescope from a MgAl1/T300 composite reinforced with 60 vol-% graphite fibers. The Young’s modulus of the composite is 155 GPa (22.4 Mpsi) at density 1.8 g/cm3 (Ref. 25). Brazing is the technique for joining all the composite structures. New creep-resistant cast magnesium alloys have a great potential for aerospace applications, and there will be a focus on development of suitable creep-resistant brazing filler metals to respond to the 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, and camcorders (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. Brazed composite structures will also have application in missiles and in aircraft, both civilian and military. Figure 2 shows a dip brazed M1A magnesium alloy microwave antenna (Ref. 6). Conclusions 1) Conventional brazing materials and traditional brazing technologies are suitable for joining the new high- performance cast and extruded magnesium alloys that came 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 the negative effect of high brazing temperature on the macrostructure of the composites. New brazing filler metals need to be developed for furnace brazing of magnesium matrix composites at 842°–968°F (450°–520°C). These prospective filler metals should provide shear and tensile strengths of at least 25 ksi (175 MPa) in the brazed joints. 2) Brazing filler metals having operational temperatures in the range of 490°–520°C (914°–968°F) need to be developed and comprehensively tested for joining wrought, work-hardened and tempered magnesium alloys. Low-melting brazing filler metals will avoid the significant loss of mechanical properties encountered when brazing with conventional standard filler metals. 3) Promising new filler metals such as Al-Mg-Cu, Mg-Al-Ca, Mg-Li-Al-Zn, and Mg-Al-Zn-Ca systems, which improve mechanical properties in both bulk and composite magnesium brazed joints, should be tested widely and introduced to industry. 4) Filler metals designed for brazing extruded (or rolled) magnesium matrix composites should have a brazing temperature as low as possible due to the low recrystallization temperature of the matrix and anisotropic structure of the composites. The brazing temperature is not as critical for joining cast magnesium matrix composites. 5) Filler metals having the structure of cast matrix composites reinforced with SiC, TiC, or Al2O3 particulates can substantially improve mechanical properties of brazed joints. Composite filler metals should have low viscosity in the molten state in the range of 842°–968°F (450°–520°C) to fill joint clearances of 0.1–0.25 mm (0.004–0.01 in.). 6) The strengthening effect of thorium, yttrium, and/or zirconium additions in brazing filler metals should be investigated, since they successfully raise tensile strength and creep resistance in cast magnesium alloys. 7) Melt spinning technology should be developed to manufacture Mg-based brazing filler metals in the form of thin amorphous foil. This form can be used to join large flat or shaped panels of magnesium matrix composites, including ultrastrength Th- or Zr-alloyed matrix composites reinforced by SiC or graphite fibers. 8) Anticorrosive chemical treatment, including the deposition of phosphate coatings or conversion Cr-based coatings, is recommended to improve corrosion resistance of brazed joints and increase the service life of magnesium alloy parts 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 joints of such dissimilar base materials should be tested. The successful developments of new, reliable, cost-effective brazing technologies will open up many commercial applications. 10) Magnesium matrix composites are susceptible to stress concentration. 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SHAPIRO (ashapiro@titanium-brazing.com) is with Titanium Brazing, Inc., Columbus, Ohio.

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