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Brazing-magnesium:Breakthrough needed for MMC.SOLUTIONS with Effective, Practical Advice
Brazing-magnesium is performed in ways similar to those used for brazing aluminum. The processes generally used are manual torch brazing, furnace and dip brazing. Special provisions, like properly modified alloys, must be made to avoid the risk of igniting the base metal.
Conventional brazing materials and traditional brazing technologies are suitable for joining more recent high-performance cast and extruded magnesium alloys. Magnesium is selected mainly for its low density, to save on weight, like in aerospace structures. Also to minimize inertial forces with high speed moving parts. Magnesium can have useful mechanical properties up to 450 °C (840 °F). However heating for brazing may reduce the mechanical properties to those typical of the annealed condition. The brazing filler alloys must have a brazing temperature range lower than the solidus of the alloys to be brazed.
The difficulties incurred in Brazing-magnesium derive from the great chemical activity of magnesium, the highest among structural metals. Also complex oxides including magnesium oxide and magnesium hydroxide form on the metal surface upon heating in air. The density of chemical fluxes used to remove oxides is unfortunately similar to that of brazing filler metal, so that slag inclusions can remain trapped in the brazed joints. Cast parts made of magnesium metal alloyed with zinc, aluminum, zirconium or rare earth are commonly brazed. New interest in Brazing-magnesium has been promoted by the appearance of high-strength magnesium matrix composites (MMC).
These are reinforced with ceramics and graphite fibers or particles. They could be joined by Brazing-magnesium for lightweight advanced structural automotive and aerospace applications. New nontraditional reinforcing systems, having great potential to improve mechanical performance, reach strength properties comparable with some steels or titanium alloys. Their advantages for aerospace applications derive from their high strength and stiffness. They have good thermal and electrical conductivity, and resistance to space environment. Cast magnesium alloys are commonly brazed. Wrought magnesium composites of zinc containing magnesium matrixes may be joined more successfully by soldering with solders based on zinc-aluminum. That is because their recommended brazing range (up to 600 °C = 1112 °F) is lower than that of the common Magnesium Brazing Filler Metal, AWS BMg-1 per AWS A5.8 which is given as 604-627 °C (1120-1160 °F). ANSI/AWS A5.8M/A5.8:2011
There are practically only three filler metals commercially available for Brazing-magnesium: BMg-1, BMg-2a and MC3 alloy (a Japanese alloy similar to BMg-1). These standard brazing filler metals cannot be used for Brazing-magnesium matrix composites due to their low solidus limitation and to the fact that high brazing temperature adversely affects the composites macrostructure. In fact the brazing temperature of prospective new brazing filler metals for MMC should be as low as possible due to the low recrystallization temperature of the matrix and to the anisotropic structure of the composites. Experimental Brazing-magnesium alloys were developed and tested as low temperature brazing filler metals, but they still need additional research to deploy useful strength. For joining cast composites based on alloy ZK51A, whose recommended brazing temperature is 480-520 °C (896-968 °F), only low temperature brazing alloys known as P380Mg and P430Mg can be used. A few systems such as Al-Mg-Cu, Mg-Al-Ca, Mg-Li-Al-Zn, and Mg-Al-Zn-Ca were singled out as showing potential for improvement in mechanical properties in bulk and composite MMC brazed joints. They should however be tested widely before being introduced to the industry. Low temperature filler metals should be developed for furnace Brazing-magnesium matrix composites at 450–520 °C (842–968 °F). These filler metals should provide shear and tensile strengths of at least 175 MPa (25 ksi) in the brazed joints. Also for joining wrought, work-hardened and tempered magnesium alloys brazing filler metals must be developed and thoroughly tested to have low brazing temperature range of 490–520 °C (914–968 °F). The need is to avoid the significant loss of mechanical properties caused by brazing with conventional standard filler metals. Filler metals designed for brazing extruded or rolled magnesium matrix composites should have as low as possible a brazing temperature. This is not so critical for joining cast magnesium matrix composites. Low temperature filler metals having the structure of cast matrix composites reinforced with particulates can improve mechanical properties of brazed joints. If these could be designed to have low viscosity in the molten state, they could fill joint clearances of 0.1–0.25 mm (0.004–0.01 in.). Magnesium based brazing filler metals in the form of thin amorphous foil could be used if developed and made available. This form can be used to join large flat or shaped panels of magnesium matrix composites. Magnesium matrix composites are prone to stress concentration. Therefore design should provide for suitable stress distribution using tapered overlapping edges to prevent joint failure. In conclusion, a widespread application of brazed Magnesium Matrix Composites will be possible as soon as suitable low temperature Brazing-magnesium filler metals of advantageous characteristics will be fully developed and introduced.
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