Investigation of the protective gas atmospheres used for melting process of magnesium alloys

Magnesium alloys have been used and preferred in many sectors, especially in automotive, aerospace and electronic industries with its appropriate characteristics like lightness and strength. High pressure die casting of magnesium alloys has been the fastest grown up and the most globally developed section in magnesium industry. High pressure die casting has a unique ability to transform the molten magnesium into an accurately dimensioned form in the shortest possible cycle time without any problem. However, there isn't any manufacturer engaged in pressure die casting of magnesium alloys products in Turkey. Due to the lack of experience and knowledge in this area, our industry is keeping away from this sector thus can not be benefited from this metal that is widely used around the world by importation. Molten Mg alloys behave differently than molten Aluminium. A thin film which is porous, forms on the melt surface and oxidize increase. This formation allows oksijen entering inside and leads to ignition under the oxide layer. Control of these rapid oxidation reactions is very important for the safe and efficient production of magnesium. Molten magnesium can be protected by protective sand (flux) or protective gas (fluxless). Usage of protective sands causes releasing of corrosive HCl and MgCl2 smoke. This smoke reduces the corrosion resistance of casting and by creating slurry inside melt, leads to mass loss. Therefore, it affects cost of production adversely. In order to protect magnesium melt, colourless and non-toxic SF6 gas is preferred. Though SO2 and BF3 gases show the same protective behaviour, their usage is limited because of toxicity and they are more corrosive than SF6 gas. SF6 gas can be used as mixture with gases such as CO2 or dry air and its various concentrations cause different effects. Addition of small amount of Be in the Mg alloys protect the alloy from burning. It increases oxide temperature and burning resistance of magnesium. During melting and casting of magnesium, HFC-134a is trying to use as a protective gas mixtures instead of SF6 because SF6 has a high global warming potantial. HFC-134a has a globol warming potansial 18 times lower than SF6. Using nitrogen as a carrier gas, in the SF6 and HFC-134a gas mixtures as a protective gas, we will try to understand protective properties of these gases. The optimum protective gas parameters of the die casting magnesium alloys can be determined precisely by the experimental works. Molten magnesium is exposed to SF6 and HFC-134a with nitrogen as carrier gas. The formation of protective film and thickness of the film on the molten magnesium was investigated. Taguchi Lg orthogonal experimental design was used when determining the protective film parameters. Control factors are respectively; molten Mg casting temperature, amount of gas in the protective gas mixture, exposure time and the protective gas mixture flow rate. Film thickness is chosen as the response factor. The film thickness was measured by the optical microscope and with these values S/N ratios were calculated by the MİNİTAB 16 program. "Larger is better" option was used for the calculation. In order to Taguchi analyse, optimum parametres for the formation of thickest protective film at the temparature of 660°C, with the 60 minutes exposure time, a volume of 0.25 % SF6 gas mixture and 600 l/h flow rate. For HFC-134a these values do not change dramatically, optimum parametres for the formation of thickest protective film at the temparature of 660°C, with the 60 minutes exposure time, a volume of 0.30 % HFC-134a gas mixture and 600l/h flow rate. The most important goal is to give the obtained information as knowledge to the literature and to the industry. This MSc thesis and the project will be reached its purposes only when it could guide to the researchers and the manufacturers working in the field of the die casting magnesium alloys.