Abstract:Al-Li alloy has the characteristics of low density， high specific modulus， high specific strength， good fatigue resistance and corrosion resistance， and is considered to be one of the most promising structural materials in the aerospace field. This article reviews the development history of Al-Li alloy， introduces the composition， structure and weldability of the new generation of 2195 Al-Li alloy， and analyzes the main problems in the welding process. Meanwhile， this paper also summarizes the research work completed by domestic and foreign institutions to improve the structure and mechanical properties of the joint by optimizing the welding wire and arc welding crafts of 2195 Al-Li alloy. It looks forward to the development trend of domestic Al-Li alloy arc welding technology.

Abstract:Silica aerogels have the characteristics of low density， high porosity， high specific surface area and low thermal conductivity， but they have some defects such as low strength and poor toughness. On this basis， flexible silica aerogels show better mechanical properties and applicability by improving the preparation process of traditional silica aerogels. This paper reviews the latest research progress of flexible silica aerogels from home and abroad and compares the performance parameters of different types of aerogel. The commonly used preparation methods， such as precursor modification， polymer modification and fiber reinforcement， are introduced emphatically. And the precursor modification method is easier to realize the design of microstructure and properties of aerogel materials by selecting and combining the types of silicon sources， which is considered as the primary method for preparing flexible silica aerogels and the research hotspot at present. In addition， its applications in the fields of thermal insulation， oil-water separation， and high efficiency sound insulation are summarized and the development of flexible silica aerogel is prospected in this article.

Abstract:In order to study the mechanical behavior and constitutive model of 5083P-0 aluminum alloy at high strain rate， the RPL100 material testing machine and the split Hopkinson pressure bar （SHPB） were used for quasi-static experiments and impact dynamic compression experiments with strain rates ranging from 950 to 3 000 s^-1.The results show that with the increase of strain rate during compression， the yield strength， strength limit and flow stress of 5083P-0 aluminum alloy increase， and the strain hardening rate decreases. It has strain strengthening effect， obvious positive strain rate effect and thermal softening effect. The plastic deformation mechanism of the material is analyzed， it is found that the sliding system leads to large plastic deformation of the material. At the same time， due to the adiabatic temperature rise， the material shows the competition between strain hardening and thermal softening. Based on the Johnson-Cook model， the variability term is improved， and the adiabatic temperature rise is introduced. The improved Johnson-Cook model can better describe the strain rate effect of the material and accurately predict its flow stress. Finally， the new strain rate is used to supplement the experiment， and the rationality of the model is verified by comparison.

Abstract:In order to investigate the mechanical properties of the three-dimensional （3D） random fibrous （RF） materials under the service condition and improve the safe service performance and structural life of the thermal protection system （TPS）， a 3D finite element model of 3D RF materials with morphology characteristics was developed based on the finite element modeling software. The macro-mechanical properties of 3D RF materials related with geometric parameters （fiber direction， fiber length and diameter） in the TTT （Through-the-thickness） and IP （in-plane） directions was studied and the evolution law of mechanical properties of 3D RF materials in both directions was uncovered. The results show that the higher rate of sloped fibers in the material is， the higher TTT strength is and the lower IP strength is. As the fiber length increased， the compressive strength of the material increases until the fiber length reaches 0.9 mm. When the fiber diameter decreases， the compressive strength decreases. These results can be applied as the theoretical basis for the material production of 3D RF materials.

Abstract:The influence of the dinitrogen tetroxide （N₂O₄） content， temperature and pressure on the diffusion and adsorption behavior of N₂O₄ in fluoroether rubber was investigated by employing the full atomic molecular dynamics simulation and grand canonical Monte Carlo simulation. The results show that the free volume of the system increases with the N₂O₄ content， which enhances their diffusion coefficient. In addition， the free volume of the system becomes smaller with increasing the external pressure which reduces the diffusion coefficient. Furthermore， the high temperature increases not only the free volume but also the velocity of atoms. As a result， the diffusion coefficient of N₂O₄ is increased which exhibits an Arrhenius-like behavior. Moreover， the solubility coefficient of N₂O₄ in fluoroether rubber decreases rapidly with the increase of temperature. Therefore， the permeability coefficient of N₂O₄ in fluoroether rubber first increases slightly and then decreases with the increase of temperature.

Abstract:Most high temperature absorbing materials belong to non-magnetic loss type， and it is difficult to achieve ideal absorbing properties in single layer. In order to solve this problem and optimize the coating thickness， a multi-objective optimization model of multi-layer high-temperature absorbing coating was established by differential evolution algorithm. The reflectivity RL<-10 dB frequency bandwidth and total coating thickness in the range of 8.2 to 12.4 GHz were taken as the optimization objectives. In this paper， three kinds of high-temperature absorbing materials were set up. And the result shows that it is difficult for single-layer coating to achieve the ideal absorbing performance. In the single-objective optimization， the frequency bandwidth is taken as the optimization objective， and it obtains the effective bandwidth is 3.2 GHz， and the wave absorption performance is improved. In the multi-objective optimization， the total coating thickness is optimized at the same time. Compared with the single objective optimization， the coating thickness d decreases by 30% while maintains good absorbing performance. The results show that the model can optimize the absorbing performance and reduce the thickness to the maximum. The model established in this paper is suitable for many kinds of materials， not limited to the three kinds of materials， and can achieve the ideal optimization results.

Abstract:Based on the theory of magnetohydrodynamics， the mathematical models of different current densities for TIG （tungsten inert gas） arc with external longitudinal magnetic field were established in the rotating cylindrical coordinate system. The flow field and electromagnetic field were coupled by fluent user-defined function， and the results show that the temperature field of TIG arc under longitudinal magnetic field presents the shape of hollow bell， and the distribution is bimodal， but a zone of low temperature occurs near the anode. With the increasing of current density， zone of "low temperature cavity" becomes larger near the anode surface. In addition， effect of longitudinal magnetic field on the pressure of anode surface is also distributed in double peak， and the peak pressure decreases by about 70% compared with that without magnetic field. And the dropping become sharper with the increasing of current density.

Abstract:The Ti₃C₂T_x MXene/WPU composite double-layer films was prepared by the alternate suction filtration， and the microscopic morphology was characterized by scanning electron microscope （SEM） and transmission electron microscope （TEM），and the crystal structure was tested by X-ray diffractometer （XRD）.And electromagnetic shielding performance was tested by vector network.The results show that a few layers of Ti₃C₂T_x can be prepared by ultrasonic centrifugation，the composite double-layer films have high flexibility，excellent conductivity and electromagnetic shielding performance， the surface resistance of the composite double-layer film is 3.57 Ω. The electromagnetic shielding results show that the shielding performance of the composite film of MWPU_3：1 is 37.9 dB. In X-band and K-band，MWPU_3：1 composite film has better performance， and the composite films is absorbent electromagnetic shielding material.

Abstract:Three kinds of C/C composites with different pore structures were infiltrated by silicon. SEM， XRD，micro-CT and mercury intrusion porosimetry methods were adopted to characterize the microstructure， phases and pore-structure changes before and after the infiltration.The apparent porosity of all infiltrated samples was below 2%，indicating high densification degree of them.During the infiltration process，the carbon fibers in the C/C composites can be protected by resin carbon which formed by the pyrolysis of high-char-yied resin or PyC fabricated by chemical vapor depostion method，and the bending strength increases by 1.5 and 2.5 times， respectively.While the carbon fibers in the composites are not protected by the matrix， the bending strength of the samples decreases by 41%.

Abstract:High-porosity carbon fiber reinforced carbon matrix （C/C） composites， using three dimensional needle-punched carbon fiber preforms， were prepared via the impregnation and high-temperature pyrolysis of resin containing graphite filler. The C/C composites were transformed into C/SiC composites by the reactive melt infiltration method. The phase composition， microstructure and mechanical properties of C/SiC composites were investigated. The results show that the small pores among fiber bundles are efficiently filled by resin， while the large pores in the short-cut web region are filled by flake graphite particles. The yield of carbon is evidently increased after one cycle of impregnation and high-temperature pyrolysis of resin. As-prepared C/SiC composites are composed of C， SiC and a little residual Si. The internal carbon fibers are well reserved， since the Si is not impregnated into the carbon fiber bundles. The bonding strength of fiber/matrix interface and friction force during fiber pullout can be increased due to the introducing of graphite filler， so that the C/SiC composites are strengthened. The toughness of composites is also improved， since the cracks can deflect along the flake graphite. The average flexural strength of C/SiC composites reaches to 118 MPa with a maximum strain of 1.0%， and the composites exhibit pseudo-plastic fracture characteristic.

Abstract:The effects of cured temperature， insulation platform and isolation material on the porosity and mechanical properties of vacuum formed CCF300/BA9913 resin matrix carbon composites were studied. The results show that CCF300/BA9913 get better mechanical properties and heat resistance after curing at 125 ℃ for 2 h.The porosity decreases and mechanical properties of the cured CCF300/BA9913 increases by adding 0.5 h insulation platform near the temperature （85 ℃） for lowest viscosity.The porous polytetrafluoroethylene film and semi permeable film are used to improve the air permeability during curing process and increase the mechanical properties of cured CCF300/BA9913 composite.The 0°tensile strength，0°compressive strength，0°flexural strength and interlaminar shear strength are 1 896 MPa，1 387 MPa，1 668 MPa，89.0 MPa.

Abstract:The porous C/C body of pyrolytic carbon and asphaltic carbon was respectively introduced into the porous C/C body by needling and fine woven puncture structure. The C/C-ZrC composites were prepared by reacting melt infiltration （RMI） process with molten metal Zr. The effects of preform structure and matrix carbon type on the microstructure and mechanical properties of C/C-ZrC composites were studied. The results show that the material is only composed of C and ZrC phases after melting， the porosity is between 5% to 10%， and the mass fraction of ZrC is between 53% to 63%. The C/C body with needled structure has better melting effect， and the C/C-ZrC composite with the highest density reaches 3.21 g/cm³. The ZrC mass fraction reaches 62.8%. The pyrolysis carbon matrix has a better protective effect on the fiber during the melting and infiltration process， and the bending strength of C/C-ZrC composites reaches 148 MPa and the compression strength reaches 408 MPa.

Abstract:To study the influence of the placement process of the flexible FSS panels on the passband insertion loss of transmission window， several panels with different defects were designed and fabricated. The free-space transmission/reflection method was used to study the passband insertion loss of the panels at different incident angles. The results show the aperture makes the passband insertion loss increase by 0.3 dB， when incident angel is between 0° and 60°.The drape makes the passband insertion loss increase by 0.77 dB when incident angel is 0° and by 1.55 dB when incident angel is 60°； the dissymmetry make the passband insertion loss increase by 0.3 dB when incident angel is 0° and by 1.05 dB when incident angel is 60°. The drape has the greatest damage to the periodic structure， so it increases the passband insertion loss more significantly， and the passband insertion loss increases more obviously at large incident angle. The integrity of the flexible FSS panels should be avoided damage as far as possible in the placement process.

Abstract:The composite tube of small fixed wing unmanned aerial vehicle is a kind of carbon fiber composite thin-walled laminated tube structure with large aspect ratio， In addition to high internal and external quality and the requirements of unmanned aerial vehicle structure， the internal dimension shall also meet the requirements of subsequent assembly dimension control. Therefore， it is difficult to meet the above requirements by using the traditional prepreg roll forming method. In order to meet the above requirements and obtain a product with simple molding process and good molding quality， a thermal expansion core mold suitable for prepreg molding was prepared by using acrylate rubber and metal core mold， and the process molding of thin-walled laminated circular tube structure products with large length diameter ratio was realized by vacuum bag pressing method with the core mold and metal core mold. The test results show that the thermal expansion and compression of prepreg can be realized by using acrylate rubber as thermal expansion material. The product not only has good internal and external quality， but also meets the requirements of UAV body structure， and its internal dimension also meets the requirements of assembly dimension tolerance control. It is a kind of thin-wall lamination for carbon fiber composites with large aspect ratio and a good forming method for manufacturing round tube.

Abstract:Refill friction stir spot welding test was carried out on 2219+5A06 aluminum alloy. The effect of technological parameters such as the rotation speed of the stirring head， the downward pressure-withdrawal speed and the indentation amount on the mechanical properties of the spot welded joint was studied. Shear tensile resistance test was carried out on spot welded joints with different parameters. The results show that the rotation speed of the stirring head and the downward pressure-withdrawal speed have a greater impact on the mechanical properties of the spot welded joint， the indentation amount has little effect on the mechanical properties. The test analyzes the microstructure of the spot welded joint， and weld joints can be divided into three parts： NZ（nugget zone），VTMAZ（vertical thermomechanical affected zone） and HTMAZ（horizontal thermomechanical affected zone）. The NZ and VTMAZ are small equiaxed grains， The HTMAZ forms a joint surface parallel to the plate，the VTMAZ and the hook defect at the root of the solder joint is a weak area of the mechanical properties of the solder joint.

Abstract:The interface debonding of the double-base propellant rubber coatings reduces the working performance of the propellant and makes a potential safety hazard to its operation. To solve this problem， a set of tap detection systems composed of signal processing components， automated control components and intelligent diagnostic components was designed and developed， aiming to obtain a more comprehensive acquisition of devices under test through the combination of automated control and intelligent tap diagnosis technology， and it improves the accuracy and reliability of detection. The results show the tap detection system can identify the debonding defects of the rubber coating， and when the tap detection resolution is between 3 to 10 mm， the detection accuracy of debonding defects is more than 87.5%. When the number of neurons in the hidden layer of the back propagation neural network is setted to 6 or 7， the fault recognition effect is good， and the fault diagnosis rate of K-means clustering algorithm to the tap detection data is more than 90%. In summary， the tap detection system has high detection resolution and accuracy， so it can realize the objective evaluation to the bonding quality of the double-base propellant coatings.

Abstract:Taking a spacecraft composite material integrated rocker arm as an example， the molding process plan is studied for its fully enclosed， nonlinear， variable cross-section， variable wall thickness and slender hollow thin-walled structure. By using the soluble mandrel as the layering mandrel， the problems of layering and demoulding are solved. The airbag pressurizing method is selected to solve the problems of pressurizing and forming. The results show that after 4 high and low temperature cycle tests between -65 ℃ and 60 ℃， there are no defects such as delamination or cracks in the rocker structure. The prepared composite rocker successfully passes the appraisal level mechanical test， and the mechanical properties meet the design requirements. The combination of airbag pressurization and soluble mandrel technology has achieved the goal of low-cost and short-period preparation of the main bearing components of spacecraft composite materials， which has certain reference significance for the development of similar fully enclosed special-shaped composite material structures.