Fundamental Study On A Novel Technique Of Mg Shielding Gas

Nowadays,a mixture of SF6 and bottled N2 has been the common Mg shielding gas. SF6 will be banned because of severe green house effect and cost of the mixture is high. Fundamental study on low-cost environment-friendly technique of Mg shielding gas was carried out in this dissertation. HFC-125 was selected as a substitution for SF6 from HFCs according to the principle of anti-ignition and concern of melt purity. Besides, a novel technique called as on-line generation of carrying gas that can supply a mixture of N2 and CO2 was presented to replace bottled N2. The involved theoretic and critical issues were studied as following:1) Experiments of anti-ignition for Mg melt using shielding gas of HFC-125+N2, SF6+N2 and HFC-152a+N2 were carried out respectively. The results showed that: Firstly, when the concentration of HFC125 and the flow rate of N2 were adequate, molten Mg would not burn. The minimum concentrations of HFC125 required to protect Mg melt was a function of the temperature. Higher concentration was required when the temperature was elevated, as well, for any given temperature, as the N2 flow rate increased, the HFC concentration could be reduced.Secondly, the shielding gas of HFC-125+N2 was capable of providing Mg melt effective and reliable protection continuously. Besides, Mg melt under effective shielding gas of HFC-125+N2 would not ignite when the melt surface was stirred.Thirdly, the shielding gas of HFC-125+N2 has a capability of preventing Mg melt from ignition. The capability is almost as same as that of SF6, and higher than that of HFC-152a. As a result, HFC-125 can be a substitution for SF6.2)The micro-features, as well as thickness and chemical ingredient, of the films exposed to HFC-125+N2 for different time was studied. the results showed that: The first, a dense compound protective film of MgF2, MgO and amorphous C formed by reaction between HFC-125 and Mg melt.The second, under the shielding gas of HFC-125+N2, growth of MgF2 phase at film/gas interface was the main mechanism of the film thickening. The kinetics equation of the film was as following: H= 0.3239Ln(t) + 1.2314 (1) H- thickness of film [μm], t-time [min]The third, micro cracks appeared when the film growing under the shielding gas of HFC-125+N2, but they would not lead Mg melt to ignition because a new protective film formed immediately on the surface of the exposed molten Mg..3)A novel technique of on-line generation of carrying gas was presented. Besisdes, the miniature combustion system of powered coke and air with low flow rate was studied by numerical simulation and experiments. Following results were obtained:Firstly, the effect of the four main parameters of the system on the concentration of CO2 in resultant gas was: surplus coefficient of air>size of coke powder>diameter of reactor>height of reactor, and the equation of coefficient of air surplus (α) and the concentration of CO2 (ξ, vol.% )was as following:ξ=-18.25α+39.414 (2)The effect of the four main parameters of the system on concentration of unburned powdered coke was: height of reactor>diameter of reactor>surplus coefficient of air>size of powder, and the equation of height of reactor (y, kg/m3) and concentration of unburned powdered coke (θ, kg/m3) was as following:θ=10-6y2-0.0022y+1.1995 (3)For another thing, the optimal values of the four main parameters in the system were gotten: size of coke powder100～250um, height of reactor1000mm, diameter of reactor200mm, surplus coefficient of air 1.05。4)Contrasts between carrying gas generated on-linely+ HFC-125 and bottled N2+HFC-125 by experiments of anti-ignition for Mg melt. The results showed that:Firstly, the shielding gas of carrying gas generated on-linely+HFC-125 could provide Mg melt effective and sustaining protection, and the concentration of HFC-125 could be reduced. Then the feasibility and advantage of on-line generation of carrying gas was proved.Secondly, the reason for the advantage of anti-ignition was that the carrying gas generated on-linely consisted of large content of CO2 and microcontent SO2, which both enhanced the effect of anti-ignition.Thirdly, under the shielding gas of carrying gas generated on-linely and HFC-125, the dynamics model for the surface film growing was: H= 0.3375Ln(t) + 0.6831 (4) H- thickness of film [μm], t-time [min]Above studies pave the way for low-cost environment-friendly technique of Mg shielding gas by providing the basic theory .