Full Temporal Domain Test Of Solidification Parameters On The Melt-cast Explosives And Cause Of Internal Defects

In view of the current situation that TNT-based melt-cast explosives cannot meet the requirements of sustainable development of munitions,the DNAN with low toxicity,low sensitivity and similar process method was proposed to replace TNT.Same as TNT,DNANbased melt-cast explosives also faced the need of increasing the content of solid phase highenergy components to improve their detonation performance.However,the increase of solid content will lead to a high viscosity,which will reduce its processability and charging quality.How to increase the solid content to improve the power,while ensuring the charging quality of melt-cast explosive had become a hot research topic,which had great academic and engineering significance.Rheological property was the key index to evaluate the technological properties of melt-cast explosive.Study on its influence law under various factors can lay a foundation for charge technological design.The temperature and strain fields in the casting process,as the most important physical parameters affecting the explosive charging,were the bridges between the process parameters and the charging quality,and the involvement of FBG in the field of energetic materials.The numerical simulation can be used as an effective supplement to the experimental research,and can provide support for simplifying the design process reasonably.In this paper,a series of systematic researchs were carried out.On this basis,the solidification rule and quality cause of high solid content DNAN-based melt-cast explosives were analyzed from the perspective of solid particles.DNAN-based suspension with 50%～80% solid content of multi-component was studied.The fluid characteristics and viscoelasticity were measured with a rotary rheometer.The effects of solid content,temperature and cooling on the viscosity and storage modulus of the suspension were investigated.It was found that the shear viscosity of 50%～80% DNAN-based suspensions become thinner with shear,and the change of viscosity and shear rate can be described by Ostwald-de Waele equation.As the increase of solid content,and the viscosity of the DNAN-based suspension increase the dependence on temperature,and the viscosity relationship with temperature can be expressed by the Arrhenius equation.With the solid content increases,the shear strain at the end of the linear viscoelastic region decreases,while the yield stress increases.The viscosity and storage modulus of suspensions are increased exponentially during the solid-liquid transition,but the values vary with the solid content.Based on the sensing principle of fiber Bragg grating,the temperature and strain distributions of 80% DNAN-based melt-cast explosives under natural cooling,continuous cooling,isothermal cooling and step cooling were measured in full temporal domain by using the self-made casting charge device and the constructed multi-point grid distribution fiber Bragg grating test system.The results showed that the latent heat release process of 80%DNAN-base explosives have a gentle and long temperature plateau and a small temperature rise.The phase transition temperature is a range,which is related to the cooling rate,vacuum degree and riser holding time.Under different cooling conditions,80% DNAN-based garins have larger tensile strain in the radial direction of its 2/3 height,up to 2626με.By analyzing the temperature and strain results under different cooling conditions,it was concluded that the stepped cooling method is the optimal cooling condition.The physical parameters(temperature,strain distribution)monitored by the FBG testing system and their relationship with the processing parameters showed that the testing system have high sensitivity and precision,and can predict the charging quality in full temporal domain to some extent.According to the experimental solidification system,a discrete sequential solidification model was established,and the optimal charging process was simulated by Pro CAST software.The results showed that the deviation between simulated and measured temperature is less than6 ℃,which can better reflect the measured results and predict the qualities.On this basis,the sequential solidification modeling method of moving boundary was established.Through orthogonal test and extreme value analysis,the relationship between four process variables and three quality indexes of 80% DNAN-based melt-cast explosive was analyzed,and the main process factors affecting the shrinkage volume,crack sensitivity index and maximum equivalent force are the water inlet rate and the riser holding time.Taking into account the essential characteristics,production energy consumption and thermal safety of 80% DNANbase explosives,the optimum process parameters are: 150 min of riser insulation time,60 ℃ of mould initial temperature,60 ℃ of water bath temperature and about 0.03mm/s of water entering rate.The solidification law of 80% DNAN-base melt-cast explosive was analyzed under different cooling conditions.The results showed that the solidification method of DNAN-base explosive in the axial direction tends to solidify layer by layer under the step cooling condition,while it tends to solidify in the radial direction.The evolution of the interface front and solidification interface of high solid content DNAN-base melt-cast explosive is essentially caused by the diffusion theory and the thermodynamic interaction of particles with liquid-solid interface.Increasing the cooling speed and reducing the vacuum can effectively increase the undercooling of the suspension,decrease the grain size and improve the microscopic morphology.The influence of the solid phase composition on the crystallization process of the suspension was discussed,and it was found that the crystallization mechanism is mainly affected by factors such as the varying degree of subcooling and the attraction,cutting and obstruction of solid phase particles at different stage.The crystals formed under sequential solidification conditions are equiaxed regions with a single structure.The main type and cause of defects in charging process of 80% DNAN-based explosive is gas cavity caused by entrapment of unescaped air and thermal cracking caused by interdendritic gas cavity on grain surface.Proper vacuum condition,riser holding time and cooling rate are the key measures to improve the charging quality of 80% DNAN-base melt-cast explosive.