Research On Ignition Behavior And Fire-retardant Coating Of Magnesium Alloys

Magnesium（Mg）alloys are potential lightweight structure materials in aviation industry.However,the aircraft components have strict requirements on the ignitability of manufacturing materials.In view of the high activeness of Mg and the oxide film with poor compactness of Mg alloys at high temperature,the ignitability of Mg alloy components is a key factor for the application in aircrafts.The existing studies on the ignition behavior of Mg alloys mainly focus on the influence of composition on ignition temperature,and few discussion on the ignition kinetics and essential mechanism of ignition.In addition,the current research on ignition-proof of Mg alloy components focus on alloying methods,and no report on the fire-retardant coating of Mg alloys.The fire-retardant coatings can avoid the contradiction between ignition-proof and mechanical,workability and corrosion resistance caused by alloying methods.Therefore,the ignition behavior and fire-retardant coatings of Mg alloys are systematically studied,which have important theoretical and practical significance to expand the application of Mg alloys.In this thesis,the ignition kinetics of Mg alloys have been investigated and the role of oxide film and second phase in the ignition process have been analyzed.In addition,the influence of heat source intensity on the ignition behavior of Mg alloys has been studied.The quantitative relationship between heat source intensity vs ignition time and a mathematical model of ignition for Mg alloys have been established,which supplementing the ignition mechanism of Mg alloys.In the aspect of fire-retardant coatings,a MAO/CeO2-AlPO4 composite coating and a dual layered structure intumescent coating have been fabricated by composition and structure design.The relationships between composition and structure of coatings and ignition resistance have been explored,and the ignition-proof mechanism is revealed.In terms of the ignition behavior of Mg alloys.①Three reaction stages of ZM5 and ZM6 are determined by thermal analysis kinetic method:protective oxidation stage,non-protective accelerated oxidation stage and ignition stage,and ZM6 has a wider protective oxidation interval and a higher ignition temperature than ZM5.②The MgO oxide film of ZM5 is not protective,while the Nd2O3/MgO mixed oxide film of ZM6 is protective.③The low melting point phase Mg17Al12 in ZM5 alloy is preferentially melted to promote the burning interface to move forward,which has obvious directionality.The Mg12Nd phase in ZM6 forms a protective layer after melting,and will not become the ignition kernel.The burning path has no directionality,and the surface enrichment effect of Nd element can inhibit the burning.④The ignition time of Mg alloys decrease sharply with the increase of heat source intensity,and the ignition time of ZM6 is significantly longer than that of ZM5.However,with the increase of heat source intensity,the gap between the ignition time of ZM5 alloy and ZM6 alloy decrease sharply,which is directly caused by high reaction rate and high concentration of Mg vapor.The sequence of ignition time of the MAO samples prepared by aluminate,silicate and phosphate electrolyte is P-MAO>Si-MAO>Al-MAO.The MAO coatings have self-extinguishing property and can inhibit the burning.The microarc oxidation technology and slurry method are combined to fabricate a MAO/CeO2-AlPO4 composite coating with high ignition resistance,which extending the ignition time of ZM5 alloy with a thickness of 2 mm from less than 2 min to 20 min.The excellent ignition resistance is achieved through synergetic combination increased densification,structural strength and thermal expansion coefficient match of the composite coating.A dual-layered intumescent coating with a thickness of 1 mm developed can protect ZM5 alloy from igniting after being exposed to 1090℃ flame for 15 min,and still has considerable heat insulation ability.The low-melting glass powder（GP）and basalt scales（BS）have synergistic effect on enhancing the fire retardancy and smoke suppression of intumescent coating.Compared with the coating without BS and GP,the peak heat release rate and total heat release rate of the coating with BS and GP were reduced by 34%and 21%,respectively,and the total smoke release rate,peak smoke generation rate and CO generation rate were reduced by 28%,26%and 29%in the cone calorimeter test,respectively.Both BS and GP can melt and form a lava-like ceramic char layer with other inorganic fillers in the coating,which can serve as a robust fire barrier for inner char layer,and block the mass and heat transfer.Furthermore,BS and GP can react with the decomposition products of ammonium polyphosphate at high temperature to generate SiP₂O7 and BPO4,which can enhance the thermal stability of the outer layer.