【作者】 游国强；

【导师】 龙思远；

【作者基本信息】 重庆大学 ， 材料加工工程， 2007， 博士

【摘要】 目前,镁合金熔体的高温阻燃保护介质主要是由反应性气体SF6和载流气体N2混合而成的保护气体。本文在深入分析镁熔体氧化燃烧与阻燃机制的基础上,围绕现有气体保护成本高和SF6污染环境的问题展开低成本载流气体技术和SF6替代气体基础研究。从阻燃保护原理和保障熔体冶金质量的角度出发,本文在HFCs类气体中优选HFC-125作替代SF6的重点研究对象;针对外购瓶装N2作为载流气体导致的高成本问题,提出了将空气中的O2转化为CO2、从而获得N2+CO2廉价载流气体的“在线发生”技术线路。本文在上述两个方面,重点进行了以下理论和技术基础研究:1)用HFC-125+N2对镁熔体进行的阻燃保护实验以及与SF6+N2和HFC-152a+N2的熔体保护实验对比,发现:第一、在每个实验温度,对于不同的载流气体流量,总存在一个最低HFC-125浓度,当大于此浓度时镁熔体不会燃烧,这个最低浓度值随载流气体流量增大而降低,随温度升高而增大。第二、HFC-125+N2气氛可持续为镁熔体提供有效阻燃保护,并且熔体表面搅动不会对保护效果产生实质影响。第三、以N2为载流气体时,HFC-125能为镁熔体提供有效阻燃保护所需的最低浓度和体积流量与SF6相近,但强于HFC-152a。因此,可以用HFC-125替代SF6。2)对HFC-125气氛下不同存在时间的表面膜形态观察、化学成分检测和膜厚度估测表明:第一、HFC-125与镁熔体反应生成由MgF2、无定型C和MgO组成、连续致密的复合表面膜,并阐明了保护气体与熔体的交互作用和成膜阻燃机理。第二、在HFC-125+N2气氛下,MgF2相在膜/气体界面上的生长是表面膜主要生长机制,MgF2相含量增加导致膜不断生长变厚,厚度与成膜时间呈近似对数关系: H= 0.3239Ln(t) + 1.2314 (1)式中:H―膜厚度[mm],t―时间[min]。第三、发现了在HFC -125+N2保护气氛下,镁熔池表面膜在增厚的过程中会产生小尺度微裂纹,但随即在暴露镁熔体表面形成的保护膜有效抑制了镁熔体的燃烧的现象,维持了气氛的阻燃效果;并阐述了其机理。3)提出了载流气体“在线发生”的技术线路,并对载流气体“在线发生”涉及的微小、非高速“空气-炭粉”两相流燃烧系统进行了数值模拟和实验验证,取得以下成果:第一、确定了在线发生系统的“空气过剩系数”、“颗粒大小”、“反应器直径”、“反应区高度”等主要工艺参数对“炭粉-空气”两相流燃烧产物气体的CO2浓度有下述影响规律:对产物气体中CO2浓度的影响程度:空气过剩系数>颗粒大小>反应器直径>反应器高度,其中,空气过剩系数的影响规律可表示为:ξ=-18.25α+39.414 (2)式中:ξ―产物气体中CO2浓度[vol.%],α―空气过剩系数对炭粉燃烬程度的影响程度:反应器高度>反应器直径>空气过剩系数>颗粒大小,其中,反应器高度的影响规律可表示为:θ=10-6y2-0.0022y+1.1995 (3)式中:θ―为残留碳浓度[kg/m3],y―反应器高度[mm]第二、在线发生系统工艺参数最优化值:炭粒大小100～250um、反应器高度1000mm、反应器直径200mm、空气过剩系数1.05。4)用“在线发生”的载流气体+HFC-125与瓶装N2+HFC-125进行对比实验和检测结果表明:第一、“在线发生”载流气体+HFC-125的混合气体可为镁熔体提供有效的和持续的阻燃保护,其阻燃保护能力比瓶装N2+HFC-125气体稍强;证实了载流气体“在线发生”技术线路的可行性和优越性。第二、用在线发生载流气体能镁熔体提供更佳阻燃保护的机理是:在线发生载流气体中含有大量CO2和微量SO2,强化了HFC-125的阻燃效果.第三、“在线发生”载流气体+HFC-125的气氛下,表面膜厚度与存在时间之间呈近对数关系(如下式),与N2+HFC-125气氛下的膜相似。H= 0.3375Ln(t) + 0.6831 (4)式中:H―膜厚度[mm],t―时间[min]上述研究工作的成果为环境友好、低成本的镁熔体气体保护技术开发奠定了理论基础和提供了技术原型。更多还原

【Abstract】 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 .更多还原