Welding of magnesium and its alloys.
Magnesium in its pure form due to the low corrosion resistance and durability for the manufacture of structures unsuitable. The technique used for this purpose magnesium alloys, because of its doping with some elements greatly improves its mechanical properties without increasing weight.
Main alloying elements: manganese, aluminum, zinc, and supplements - zirconium and cerium. Tensile strength alloy grades MAI, MA8, mostly manganese-doped (1,3-f-2,5%), reaching 21-23 kgf/mm2 at 10% elongation, and the conditional yield point 11.9 kgf/mm2. Tensile strength alloy grades MA2, MA21, MH, M5, more complexly (up to 7-9% A1 to 1,5% Zn, 0.8% Mn), up to 26-30 kgf/mm2, yield 14-15 kgf/mm2, elongation 5-8%. Rolled Alloys of this type are used in the annealed condition.
Magnesium alloys ML4, ML5, and others (the letter A indicates that the alloy casting) is used to produce castings. Eliminate welding defects in casting. These alloys have a higher tendency to form hot cracks in the seams, pores and shrinkage porosity. Alloys based on magnesium is actively oxidized in air. Magnesium oxide film of their own on the metal surface is loose and unstable. Therefore, the surface of magnesium alloys artificially protect the film from the salts of chromic acid. By UCA-zannoy reason prior to welding the edges and adjacent to the surface of the base metal (the width to 30 mm) by etching or by mechanical means carefully remove the protective film, oxides and other contaminants. After welding, the surface of the weld re-applied protective film.
Magnesium alloys are used in aircraft, rocket, ship, for the manufacture of various capacities for kerosene, mineral oil, for the manufacture of various magazines, etc.
Features of magnesium
1. The resulting welding refractory oxide film of magnesium MgO (mp = 2500 deg. C) makes it difficult to weld. For its failure to apply the flux, or use the effect of cathode sputtering during welding Tungsten inert gas-shielded (alternating current).
2. Tendency to crack the appearance of crystallization associated with the possibility of the formation of fusible eutectics: MgCu (Tm = 485 deg. C); MgAl (Tm = 436 deg. C); MgNi (Tm = 508Q degrees. C). Therefore, the beginning and end of the welds must be placed on pin bars. The sequence of fusion: long-welded joints and seams with a large cross-section should be shorter weld seams and joints with a smaller cross section.
3. The tendency of alloys, particularly those containing manganese, a metal grain growth in heat-affected zone does not allow for significant overheating of the metal (eg, cluster in one spot welds in multilayer welding without a break to cool the metal, etc.).
4. The increased absorption of the active gas liquid metal generates a tendency to the formation of pores, which also requires a reliable weld zone from the surrounding air.
5. The high coefficient of linear expansion of magnesium alloys leads to a significant warping of welded structures.
Main method of welding of magnesium alloys - tungsten arc welding in a protective atmosphere of inert gases. Such methods of welding, as gas, coal and metal electrode coated with the use of which necessarily fluoride-chloride fluxes of different compositions (eg, flux composition of 34% KF and 66% LiCl or composition of 40% LiCl, 20% CaF2 and 40% NaCl) , is now rarely used in industry. Flux residues and slag on the surface of the joints causing corrosion of magnesium alloys, so that after welding their remains should be washed off with hot water. welding in inert gas (argon high and first grades of high purity helium) tungsten electrode ittrirovannym lantanirovannym or perform on alternating current. Filler wire composition is close to the base metal or has impurities (eg, cerium), providing a more ductile weld metal. Welding is feasible for all types of connections. When assembling the necessary close fit edges. Metal thickness of 3 mm with no weld bevel edges of sheets with a thickness of 3-6 mm is required V-shaped cutting and a thickness exceeding 6 mm - X-shaped with blunt 1.5-2 mm. to prevent contact with the metal oxide film on the back side edges of the weld should be conducted with full penetration of edges on the pads of the metals with low thermal conductivity (usually from high-alloy steel). They also serve to protect the back of the seam. From this perspective, lap, corner and T-joints less technical. For manual welding metal thickness of 3 mm diameter tungsten electrode is used 2-3 mm, the current Isv = (30 - 40) / dw in the argon flow rate 9.7 l / min. Automatic welding is possible for a metal thickness of 1 mm or more tungsten electrode diameter of 2-6 mm on the welding current Isv = (40 - 75) / dw in the distribution of argon during 6-10 l / min. The diameter of filler rods 1.5 - 3 mm. To reduce the heat to be welded at high speed. length of the arc support the minimum (1.0-1.5 mm), as in this case, a vigorous destruction of the oxide film by cathode sputtering, and improved protection of weld zone by inert gas. The main and filler metals before welding must be thoroughly cleaned of the oxide film and contamination by mechanical or chemical means. When TIG welding, weld strength as compared with the base metal up to 85-90% in arc welding carbon and a metal electrode of 70-80% and 60-75% of the gas only.
See also:
Welding of Aluminum
Titanium
Welding Magnesium
Welding of Titanium
Copper
Welding of Copper
Refractory Metals
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