Abstract:With the further increasement of vehicle speed and the demand for reusable vehicles,hypersonic film cooling technology has become a hot issue in the development of aerospace technology.The research on active film cooling technology of hypersonic vehicle is very important to solve the problem of thermal protection and break through the bottleneck of thermal protection technology.In this paper,the principle,classification and mechanism of active cooling thermal protection technology are studied firstly.Then,from both experimental research and numerical simulation point of view,the research status of two-dimensional slot film cooling technology,discrete film hole technology and hypersonic reverse jet technology,as well as the factors affecting the film cooling effect are reviewed and analyzed.At last,the research directions such as heat-resistant material development,material preparation process,experimental study of multi film hole characteristics, experimental verification of reverse jet film hole cooling technology for hypersonic film cooling technology are put forward.

Abstract:Recent progress of structure design,material, wear failure and service life of stirring tool for friction stir welding of aluminum alloy,magnesium alloy,copper alloy,titanium alloy,iron and steel alloys is summarized.The advantages and disadvantages of structure design and material properties of various stirring tool are compared,and life-cycle are analyzed.At last,the research and development trend of stirring tool is evaluated.

Abstract:The elastic modulus of Al/Al₂O₃ FGM with mesoscopic pore was studied by using the method of asymptotic homogenization.The feasibility of the asymptotic homogenization method to predict the elastic modulus of Al/Al₂O₃ FGM with mesoscopic pores was verified by comparison with the experiment.In the case of ignoring the number of pores and other minor factors，the prediction formula of the elastic modulus of Al/Al₂O₃ FGM with mesoscopic pores was obtained by fitting,which is related to porosity and Al₂O₃ volume fraction.The results show that the mesoscopic pores can not be ignored in the prediction of the elastic modulus of Al/Al₂O₃ FGM. Under the same porosity,the number of pores has little influence on the elastic modulus of Al/Al₂O₃ FGM,while the size and location of pores have great influence on the elastic modulus.The elastic modulus of Al/Al₂O₃ FGM with random pores is lower than that of Al/Al₂O₃ FGM with uniform pores under various volume fractions.Porosity also has an obvious influence on the elastic modulus of Al/Al₂O₃ FGM.With the increase of porosity,the elastic modulus of materials gradually decreases,and the greater the volume fraction of Al₂O₃,the greater the decrease.

Abstract:In order to solve the damage to GFRP composite during the riveting process of multi-layer consisting of GFRP composite and aluminum alloy,the dynamic damage analysis of GFRP composite have been simulated by ABAQUS on the effects of diameter of pre-drilled hole and riveting squeeze of Φ4 mm 2A10 aluminum alloy rivet.On the basis of the simulation the experimental investigations on multi-layer consisting of GFRP composite and aluminum alloy were performed.The non-destructive test and shear test of the specimens were conducted.Results of the analysis indicate that the adequate process parameters of Φ4.2 mm pre-drilled hole and (18.5±0.2) kN riveting force can achieve the requirements of low damage to GFRP composite.

Abstract:Aiming at the problems of high cutting force,high cutting temperature and low surface quality when carbide tool was used to milling AISI304 stainless steel with high speed internal cooling.Based on the response surface center composite test design method,the high-speed internal cooling milling test was carried out, and the second-order regression forecast model of each milling force component was established and verified by experiments. The surface quality after dry milling and internal cooling milling was compared.The influence of milling parameters on the milling force components was analyzed,and the milling parameters were optimized with minimum milling force components as the goal.The results indicate that the error between the predicted value and the experimental value of the feed force and the radial force are 4.77% and 6.16%,respectively.The Ra value of internal cooling milling is 0.193 to 0.327 µm.The influence on milling components is milling depth>rotation speed>feed rate, the feed force first increases and then decreases with increasing milling depth and speed,the radial force increases gradually, the interaction between milling depth and speed have a significant influence on feed force and radial force.The optimal milling parameter combination are that speed is 11 643.63 r/min,milling depth is 1 mm,and feed rate is 0.08 mm/r.

Abstract:In order to explore the NC machining method of spiral groove winding grid structure on conical surface,the parameter equation of the cone equal pitch spiral line was studied.Through coordinate transformation,the machining tool positions of spiral groove were deduced.Finally,the NC machining of the spiral groove with equal pitch on the surface of the cone with a constant blade angle was realized on the three-axis linkage machine bed.The research results show that the verticality of the side wall and bottom of the spiral groove reaches 0.02 mm,the surface roughness reaches Ra 1.6 μm ,and the depth of the groove is 6 ~ 6.03 mm.Compared with the five-axis processing,it is a remarkable improvement.In addition,the processing efficiency is doubled and the cost is reduced by 68%.This method should be promoted and applied in engineering.

Abstract:Defects such as edge,fuzzing and tearing could be produced on aramid fiber composite(AFRP)during milling,which seriously affect the assembly and use of materials.In order to solve the above problems,orthogonal experiments were carried out on the tool structure,spindle speed,and feed speed.The effects of milling parameters and tool structure on the milling surface quality of aramid fiber composites were studied from the aspects of milling force,surface roughness and fracture morphology of fibers.The experimental results show that the spindle speed and feed speed have significant effects on the surface roughness and milling force.The surface roughness of burr tool is 28% lower than that of conventional four-edge end milling tools, and the width of rough edge is smaller, the surface quality of milling is better,and it is more suitable for AFRP milling.

Abstract:ZrB₂-SiC-B₄C ultra-high temperature ceramic coatings were prepared on C_f/SiC substrate surface by slurry painting-sintering method.The effects of the mass fraction of powder filler,diluent in slurry and the sintering temperature on the microstructure,composition and phases of ZrB₂-SiC-B₄C coating were studied.The results indicate that a dense and well-bonded ZrB₂-SiC-B₄C coating can be obtained on C_f/SiC substrate surface when the mass ratio of powder to resin is 1∶1,the mass ratio of diluent to resin is 1∶2,and the sintering temperature is 1 500 ℃.This coating has uniform phases,low surface roughness of Ra<1 μm,low porosity of around 4.2%,and high average tensile shear strength of about 5.4 MPa.After isothermal oxidation at 1 500 ℃ for 30 h,the weight loss of ZrB₂-SiC-B₄C coated C_f/SiC composite is around 10.7%.An intact ZrO₂-SiO₂ film is formed during the oxidation,which provides the effective anti-oxidation protection.Thus,the ZrB₂-SiC-B₄C coated C_f/SiC composite can be a candidate material for applications in the high temperature combustion flow environment.

Abstract:Ti48Al2Cr2Nb alloys and ZrB₂-SiC ceramics were brazed with Cu_41.83Ti_30.21Zr_19.76Ni_8.19 (at.%) amorphous alloy filler.The microstructure and mechanical properties of the joint were analyzed by scanning electron microscopy,energy spectrometer,X-ray diffraction and testing machine.The results show that the typical joint interface structure of TiAl alloy and ZrB₂-SiC ceramic brazed joint is:TiAl/Ti₂Al/AlCuTi/(Ti,Zr)₂(Cu,Ni)+TiB+TiCu/Ti₅Si₃/ZS.When the brazing temperaure is 910 ℃,the width of the reaction layer near the ZrB₂-SiC side increases gradually with the increase of the holding time,and the TiB and TiCu aggregates in the diffused part of the joint grow.The joint shear strength increases first and then decreases with the increase of holding time.When the brazing temperature is 910 ℃ with holding time of 20 min, the joint shear strength reaches the maximum value of 187 MPa.Through the fracture analysis of each process,it is found that the joints are all broken on the ceramic side,and the fracture mode is brittle fracture.

Abstract:Because SiO₂ fiber reinforced polyimide composites are heterogeneous anisotropic materials,there are many serious issues in the traditional milling process,such as severe tool wear,large cutting force and low processing efficiency.In this paper,SiO₂ fiber reinforced polyimide composites are processed by cryogenic cooling milling.And the comparison between cryogenic cooling milling and traditional milling is also carried out.In addition,the machined surface morphology,roughness,cutting force and tool wear were analyzed.The results show that,the surface roughness decreases with the increase of spindle speed and decreases first and then increases with the increase of cutting depth under the two working conditions.Compared with traditional milling,cryogenic cooling milling at different cutting speeds can effectively inhibit the fiber fuzzing phenomenon during traditional milling with low-speed and the ablation defects of adhesives during conventional milling with high-speed. The main reasons for improving machining quality are the increase of cutting force,the change of fiber chip breaking mode and the decrease of cutting heat caused by ultra-low temperature cooling.

Abstract:Automatic fiber placement (AFP) technology could manufacture large composite components effectively,but there are different angle embedded defects due to the same cross-section at the end of the tow in the process of placement.In order to solve the related problems,according to the laying sequence of [(90°/0°)₅/90°] and [(0°/90°)₅/0°],the gap defects or overlapping defects with different end angles of the tow were set in the 0° and 90° laying sequence respectively.The experimental results show that the difference of the composite components is obvious when the defects are embedded in different angles.In the direction of 90° fiber layer,when 90° void defect and 90° overlapping defect are embedded, the tensile strength of the specimen is the highest,and the tensile strength ratio is 90.89% and 90.11%,respectively.In the direction of 0° fiber layer,the tensile strength of the specimen is the highest when embedded with ± 30° gap defect and 30° overlapping defect,and the tensile strength ratio is 28.48% and 50.71% respectively.

Abstract:SiO₂ fiber reinforced polyimide composites were heterogeneous anisotropic materials,the processing properties were highly dependent on the fiber orientation angle which is between the direction of fiber layers and the direction of processing feed.In this paper,the effects of fiber orientation angle on surface morphology,surface roughness,milling force and tool wear were studied through a series of experiments of dry cutting and ultra-low temperature cooling milling with different fiber orientation angles.The results show that the types of shear stress and styles of cutting chips are different under different fiber orientation angles.The milling surface roughness is fine when the fiber orientation angle is acute.When the fiber orientation angle increases to 90°,the cutting surface quality decreases and the cutting force increases.In the same milling time, the tool wear is serious in dry cutting,and the falling off area of coating is about 70% of the measured area. But compared to dry cutting,in ultra-low temperature cooling milling,the coating is not seriously damaged,the tool is still in a stable wear stage, and the tool durability is better than that under dry cutting condition.

Abstract:In order to obtain the aging mechanism of a certain kind of sealing fluororubber under high-low temperature cyclic conditions,the fluororubber seal ring was subjected to an accelerated aging test of high-low temperature conditions in oil medium.The changes of compression set,mechanical properties,microstructure, molecular structure and dynamic mechanical properties under the condition of temperature-changing oil medium were studied.The dissociation energy of several important chemical bonds of fluororubber molecules was calculated. The test results show that the maximum tensile strength and elongation at break of special fluororubber decrease with the increase of the number of cycles,and the compression set rate of fluororubber samples stored in oil medium increases gradually in the early stage of aging.The compression set rate of fluororubber samples slightly restored in the late stage of aging,as the aging time increases,the side groups of the fluororubber molecules decompose and the main chain flexibility increases.

Abstract:In order to investigate the tension-tension fatigue behavior of C/SiC composite fasteners,the fatigue life under different stress levels was obtained when the stress ratio was 0.1 and the loading frequency was 10 Hz. Using the fracture analysis and microstructure observation method,the meso-mechanism of fatigue failure of C/SiC composite fasteners was also analyzed.The results indicate that there are two types of failure modes for the tension-tension fatigue behavior including thread breaking and thread pulling off.An empirical two-parameter power exponent form of formula characterizing the fatigue life is proposed,and fatigue life for the two failure modes are similar.The fatigue limit for C/SiC composite fasteners is 65% to 70% of tensile strength,which means that if the maximum fatigue stress is more than 70% σ_max,the damage of C/SiC composite material significantly increases with cyclic number.

Abstract:The microstructure and properties of TIG repairing welding joints of 2219 alloy with different bevel angle were studied against the bright line image in the X-ray film.After X-ray detection,SEM observation,energy spectrum analysis,and tensile test,it was found that,the blackness of bright line image in the X-ray film decreased gradually,with the increase of the α bevel angle at the range of 0° to 20°.In addition,the α(Al)+θ non-equilibrium eutectic structure with high copper content,had a stronger effect on X-ray scattering and absorption,which was the cause of bright line image in the X-ray film.Meanwhile,the electromagnetic stirring nearby the α bevel angle was weaker,and the boundary layer width in front of the solid-liquid interface was wider,than that of beside the β bevel angle.Also the temperature gradient and the degree of constitutional supercooling in the boundary layer was larger than that of alongside the β bevel angle,both of which promoted the growth of the dendrites beside the α bevel angle.When the β bevel angle was for 20°,the tensile strength and elongation of the joint gradually increased with the increase of α bevel angle,in the range of 0° to 20°,and the excellent room temperature mechanical property was obtained at 20°.The bright line image in the X-ray film of the repair welding joint could not be treated as the welding defect.

Abstract:The low conductivity of carbon fiber composite generated clutter signals which have a bad effect on the communication equipment mounted on carbon fiber composites of helicopter.In order to improve the conductivity of connection structure of carbon composite helicopter,the composite beam of helicopter was taken as the research object.Through increasing the connection performance between the communication equipment interfaces and the composite beam structure and the conductivity of helicopter tail beam itself,the conductivity of carbon fiber composite was improved efficiently.The results from several experiments and verifications of installation show that the conductive material of carbon fiber composite should be continuous,and there is a significant linear correlation between the length of the conductive material and the resistance value.The pretreatment on the communication equipment interfaces can increase the conductivity of carbon fiber composites.The steps of the pretreatment are as follows:(1)Copper meshes should be placed along the conductive direction continuously to keep the continuity of conductive structure；（2）During the conductive connection with other parts，the copper sheet used for connection need to be pretreated and welded with the copper mesh to obtain a stable conductive interface；（3）The conductive effect is related to the length of conductive path and the quality of pretreatment，which is basically linear relationship.

Abstract:In order to enhance the machining precision of titanium alloy thin-walled cylinders,two Ti-6Al-4V thin-walled cylinders with different structures were prepared by using hot isostatic pressing technology.The influences of the fixture,poured slurry and cylinder structure on the cylindrical surface roughness and cylindrical precision were studied.The results show that hot isostatic pressing Ti-6Al-4V thin-walled cylinders after machining can reach the accuracy requirements,poured slurry can reduce elastic spring back to improve outer ring precision of the thin-walled cylinders;the inner ring and thick end of the cylinder structure can enhance stiffness to reduce the cylindrical surface roughness effectively;while big fixture layout error can make a significant influence on roundness and coaxiality,the large error remarkably decreases the outer ring precision.

Abstract:By exposing high silica/phenolic premix at a relative humidly (RH) of 85％ for 24 h,the composite was prepared by hot pressing the premix,and then it was evaluated by non-destructive testing and mechanical property measurement.Experimental results show that there are a large amount of internal defect in composite materials,including 37.4% Class II region and 49.8% Class Ⅲ region.In addition,mechanical properties of the composite decrease dramatically,showing 55% decrease in tensile strength,60% decrease in compressive strength and 15% decrease in shear strength.The formation of the internal defects can be mainly ascribe to two aspects:(1)The volatilization of part of water absorption in the premix forms pores in the composite during the curing process,forming Class II region;(2)The rest water absorption by fiber weakens the interfacial adhesion between fiber and resin matrix and causes the debonding of the interface in serious conditions, resulting in the formation of the Class Ⅲ region.These reasons lead to decreased mechanical properties of composite.