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Magnesium alloys with improved combustibility

Магниевые сплавы с улучшенной стойкостью к возгоранию
Since the beginning of the last century it was discovered that the addition of calcium to magnesium alloys increases their resistance to oxidation in the molten state and causes grain refinement in a cast structure. However, no attention were paid to the fact that calcium is a useful addition to the magnesium alloys, so calcium not often used in industry. There are no industrial Mg-based alloys with calcium. This is due both to availability of affordable magnesium alloys with rare-earth metals and some difficulties in its use. It was observed that the small addition of calcium (in an amount of more than 0.3%) decreases weldability and casting properties in сommon magnesium alloys, calcium supplements a 0.5% increase brittleness and reduced mechanical properties. In alloys of Mg-Al-Ca system calcium makes heat treating more difficult: it prevents the process of dissolution of aluminum magnesium. Increasing of calcium content in magnesium alloy above a certain limit may cause some reduction of corrosion resistance of castings. Flux melting technology is not compatible with calcium due to its reaction with components of traditional fluxes, especially MgCl2. As a result of this facts such alloying component of magnesium alloys as calcium was almost forgotten . However, recently interest in magnesium alloys with calcium as an alloying element or conditioning agent was renewed. Such systems as Mg-Al-Ca, Mg-Al-Ca-Mn, Mg-Al-Zn-Ca, Mg-Al-Zn-Mn-Ca are most promising for the application of calcium. It is an element that can increase the resistance of the liquid magnesium alloy from oxidation. Furthermore, calcium may serve as an effective grain refiner in the casting , improving its mechanical properties. Were performed experiments of smelting alloy samples with calcium. The charging materials was ML5pch GOST 2856-79 and calcium additive in an amount of from 0.1 to 1% by weight of Ca . Samples was obtained by fluxless melting technilogy. Even if alloy contain more than 0.1 % calcium increasing of ignition point was observed. Were obtained samples by sand and chill casting . Grinding of structure was noted in samples containes more than 0,1 %. The best results were achieved with the introduction of 0.2-0.3 % Ca , further increasing the calcium content didn’t lead to a significant improvement of the structure . The main effect of calcium is increasing of the temperature of ignition and protective properties of the surface oxide film, as compared with the alloy ML5pch (MAG2). This allows make heat treatment of the hardening cast in air environment without reducing of their mechanical properties.


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