Study On Fabrication Technology And Mechanical/Thermal Properties Of Titanium Honeycomb Sandwich Structures

Titanium honeycomb sandwich structure(THSS)has been widely used in aircraft aft fuselage,aircraft firewall,engine compartment door,engine nacelle and other structures due to its lightweight,high specific strength and stiffness,resistance of high temperature and corrosion,noise elimination,thermal insulation and other excellent performance.The core and facesheet of THSS are usually connected by brazing technology.Because Ti element is very active,intermetallic compounds are easily generated at the brazing interface,which makes the plasticity of brazing interface decrease greatly.Especially for the honeycomb thin-walled structure,the brazing interface embrittlement is more serious due to the influence of solder melting and interface reaction,which seriously affects the mechanical properties of the structure.At the same time,the problems such as insufficient machining accuracy of honeycomb core,insufficient forming precision of large area titanium THSS,the control of brazing defects,and evaluating the mechanical and thermal properties of THSS and the influence of damage defects on the mechanical and thermal properties of THSS during the design process seriously affect the use of a large number and a large area of THSS.In order to solve the above problems,the main research work carried out in this paper is as follows:(1)The interface structure and performance optimization of THSS were carried out.Through the study on the evolution law of brazing interface structure and the relationship between brazing interface structure and properties of THSS,the causes of interface brittleness were determined.On this basis,the method of low temperature vacuum diffusion brazing was proposed.The optimal brazing process of TC4/TC1 heterogeneous THSS is 875℃/240 min and the thickness of solder is 0.04 mm.The brazing interface obtained by this process has uniform structure,no intermetallic compound and wide process window.The test results of basic mechanical properties of THSS connected by this process showed that the optimized brazing process greatly improved the mechanical properties of brazing interface of the structure,and the failure modes of the structure were fracture of honeycomb core,which effectively guaranteed the comprehensive mechanical properties of the structure.(2)The forming technology of large complex curved surface THSS was studied.Based on the comparison of the processing effect of titanium honeycomb core,a filling-holding method of honeycomb core was proposed,which solved the chamfering problem during the processing of titanium thin-wall honeycomb core,and the machining accuracy of the shape surface was controlled to 0.05 mm,which met the requirements of brazing fitting of THSS.The brazing rate of large curved surface specimens obtained by manufacturing is above 95%.Based on Y model,the prediction model of the flat compression and shear modulus of THSS was derived.Based on the elastic-plastic thin-wall instability theory and considering the different instability modes of single cell wall and double cell wall,the prediction models of the flat compression and shear strength of THSS were derived.The predicted values of the model are in good agreement with the experimental results.Combined with the test results,the sensitivity analysis of the honeycomb core parameters to its performance was studied.The results showed that for a honeycomb core with a given density,using a thicker cell wall and a relatively large cell size could reduce the influence of brazing heat on the cell wall performance and obtain a honeycomb core with a stable flat compression and shear performance.(4)Based on the finite element software Abaqus,finite element analysis models contained details of honeycomb core and defects for mechanical properties of THSS were established by using the parameterized program written in Python language.The influence of defects of facesheet-core separationon,hole,impact and facesheet crack on the failure modes and mechanical properties of THSS were analyzed by the above models.The validity of the finite element model and the reliability of the predicted results are verified by the experimental data.The finite element analysis strategy and model verified by experiments can be used to evaluate the mechanical properties of THSS with no damage and defects in engineering practice.(5)By thermal insulation properties of THSS test,the effect of parameters such as the diameter,the height and the cell wall thickness of the honeycomb core on the thermal insulation performance under the steady state condition of 300 was studied;the sensitivity of geometric parameters to thermal insulation performance is analyzed;the equivalent thermal conductivity of THSS was measured at different temperatures.The research results showed that the insulation effect of THSS could be improved with the increase of the diameter and height of the honeycomb core and reduce of the cell wall thickness of the honeycomb core;the sensitivity of different geometric parameters to the thermal insulation effect of THSS is in descending order,as follows: core height,core lattice size and core wall thickness;the equivalent thermal conductivity of THSS at the same temperature increases with the increase of temperature and is about 3～5% of that of titanium alloy material.