Studies On Fire Behavior And Oxidation/nitridation At High Temperature Of Three Typical Magnesium Alloys

With the outstanding mechanical property,relative low density and cost,magnesium alloy materials have been widely applied in all fields and every walk of life.Along with the current dramatically increasing of demanding for weight deduction in the field of aviation and automobile,magnesium alloy has been one of the best materials to solve the problem of weight loss.However,the reaction activity of metallic magnesium is high,and it is likely to be reacted with oxygen,nitrogen or carbon dioxide,which lead to the combustion in machining,smelting and using process easily.Therefore,the application of magnesium alloy materials in the field of aviation has been restricted greatly.In this work,three typical brand kinds of magnesium alloy materials have been used to investigate their oxidation and nitridation behaviors under high temperature.Furthermore,the standard instrument in the field of fire science,cone calorimeter has been applied to analyze the fire behavior of magnesium alloy materials,which provides the basic data supporting of application,research and development.The main research work including:The thermogravimetric-differential scanning calorimetry(TG-DSC)has been applied to measure the kinetic parameters in oxidation/nitridation of magnesium alloy powder with the variation of heating rate under the different environmental atmosphere.The result of TG indicates the reaction process is similar with that of pure magnesium with the existence of oxygen,and it can be divided into three periods.The iso-conversional method has been used to calculate the activation energy of three types of magnesium alloy materials under the atmosphere of air,the result shows the activation energy of magnesium alloy materials is higher than that of pure magnesium,and that of ZE10 magnesium alloy material is the highest in three kinds of samples.With the condition of pure nitrogen,the increasing process of samples weight can be simplified as a peak of weight increment.The peak shape of AZ31 magnesium alloy material is sharp,and the starting temperature of it is not postponed to the high-temperature region obviously with the increasing of heating rate.While comparing to the other two kinds of magnesium alloy materials,their process of weight gain with the atmosphere of pure nitrogen is evident slower than that of AZ31.According to the method used to calculate the thickness of oxidation film based on TG data,the activation energy of three samples in nitrogen has been calculated and the generation process of nitrid layer is analyzed.The result shows the nitrid layer of AZ31 keeps a status of increasing thickness and density in the whole nitridation process.As for the others,their results are the same as that of pure magnesium,and their nitrid layers show the progressive failure and regrowth.The inerting effect of nitrogen mainly reflects in three aspects,and they are delaying the initial temperature of reaction,decreasing the mass of reactant in the first stage of vigorous oxidation,and reducing the reaction exotherm,which is consistent with the previous research.One of the standard devices in the field of fire science,cone calorimeter has been applied to investigate the fire behavior of magnesium alloy materials.In the same time,the veracity of oxygen consumption measurement is analyzed.The TG experimental result in the last caption is used to explain some data in the cone calorimeter experiments partly.The external radiation ignition process of three types of magnesium alloy materials at atmospheric pressure can be divided into three stages.The first one is the fracture of alloy surface and side.Secondly,the magnesium tunnel generated on the surface can lead the magnesium alloy inside to outside,so that the ignition and sharply combustion happen with the contact of air.Thirdly,the growth of massive MgO results in a loose surface,then the combustion process keeps in a steady status.The DSC data is used to calibrate the reaction heat of magnesium alloy.According to the calculation,the heat value that AZ31 and WE43 released from the consumption of 1 kg oxygen can be replaced by the formation heat of MgO,while that of ZE10 should be replaced by the DSC result,since it is less than the formation heat of MgO.On the basis of some assumption in the temperature-rise period,when comparing the cone calorimeter experimental results with TG ones,it shows they coincide with each other,which indicates the data of these two experimental methods have mutual corroboration.The influence of low-oxygen and low-pressure environment to fire behavior of magnesium alloy is analyzed.In general,the low pressure causes the obvious decrease of ignition time and slight reduction of severity in combustion.The ignition temperature drops with the decreasing of atmospheric pressure,which is matched with the previous research.When the environmental pressure goes down,the peak value of mass increasing rate also debases in magnesium alloy combustion,which shows the exponential relationship with pressure.The exponential coefficient of WE43 is the lowest,and it is approximately 0.5.As for the others,they are over 1,which verifies the applicability of previous k-n empirical model to the mass change in magnesium alloy combustion,and also investigates the change of coefficient k in the model with the variation of pressure.The result shows the change of magnesium alloy materials coefficient is reasonable agreement with theoretical values.Besides,according to the phenomena in magnesium alloy combustion,the parameters which influence the time of ignition are analyzed,whose time of surface fracture is the main factor of affecting the ignition time.The kindling model of magnesium alloy is constructed and the linear variety of ignition time along with pressure is derived.The result of the theoretical study is almost the same as that of the experiment.