Study On Flame-retardant Mechanism Of Spray Deposited Magnesium Alloys Under Nitrogen With Ca Element

Spray forming technology is an effective way to prepare high-performance magnesium alloys. It has high cost-effective to use nitrogen as the atomization gas in the atomization process of spray forming technology. However, magnesium well react with nitrogen at a certain temperature and bring dangerous to the spray forming process. So a further research will be taken to study the flame-retardant mechanism to ensure the production safety during spray forming magnesium alloy under nitrogen. In the present thesis, the flame-retardant mechanism of spray deposited Mg-5Ca alloy under nitrogen was studied from two parts:On the one hand, the structure and growth rhythm of surface film during atomization process were studied by SEM, EDS and XRD to search the effect of film structure on the flame-retardant mechanism. On the other hand, the generate heat and heat transfer between nitrogen and atomized droplets were studied by mathematical model to research the quick heat dissipation of flame-retardant mechanism during atomization process. Results are listed below:1. Under nitrogen atmosphere (99.5%), the surface film was continuous and dense after heating at740℃and holding on2h. And it was composed of the mixed structure of CaO and MgO, and mostly CaO、MgO and Mg3N2、Ca3N2at the first surface. However, the film under high-purity nitrogen(99.995%) was only composed of Mg3N2and Ca3N2, it was very thin and can not cover the surface entirely. The surface film in air was loose structure with cauliflower-like appearance, which was composed of mixed structure of CaO and MgO, and mostly CaO at the free surface, and the crack and deformation was very serious.2. The atomizing gas velocity, heat transfer coefficient, droplet velocity, temperature and other parameters during spray forming process were simulated by mathematical model, and the minimum heat transfer of Mg-5Ca alloy under nitrogen was determined. When droplet velocity equal to the gas velocity, the heat transfer coefficient was the minimum and the heat taken away by nitrogen was the fewest.3. The study on flame-retardant mechanism of Spray deposited Magnesium alloys under Nitrogen with Ca element indicated that:On the one hand, protective film composed of the mixed structure of oxide and nitride were formed when adding Ca element to magnesium alloys. The multilayer film was dense and filled the holes of monofilm, which prevents the Mg-5Ca alloy from burning. On the other hand, in the atomization stage, the minimum velocity of heat transfer is far greater than maximum velocity of the heat released by the reaction between magnesium and nitrogen, which avoid the accumulation of heat during spray deposition, and therefore ensure the security of spray deposited magnesium alloy with nitrogen as the atomizing gas.