Abstract:Polyimide composites have been widely used in the aerospace field due to their excellent high temperature resistance and mechanical properties. However, the complexity and high cost of autoclave forming process make it difficult to meet the rapid processing of polyimide resin matrix composites, which limits its further application. In this manuscript, the research status and development trend of polyimide matrix resin and its composites suitable for RTM technology have been reviewed, with emphasis on the domestic and foreign research of phenylethynyl terminated polyimide and its composites.It is an important developing direction to improve the temperature resistance of RTM polyimide resin and its composites while maintaining its low molding viscosity and high toughness.

Abstract:In order to improve the acoustic-vibration environment inside the fairing,the most common and efficient method is laying noise reduction materials on the inner wall of fairing to reduce the sound pressure level of noise.In this paper, the engineering application and research progress of fairing noise reduction materials and applicable frequency bands at home and abroad were summarized.Baseline properties and noise reduction principles of several main noise reduction materials were introduced in detail.On top of all, suggestions for the research and development of noise reduction materials in China were put forward.

Abstract:Woven composites inevitably suffer from internal damage caused by low-velocity impact (LVI) during service, resulting in degradation of material properties. In this paper, the damage behavior of twill woven thermoplastic composites (TWTC) at LVI was investigated by a combination of test and simulation methods. A micro, meso and macro serial multi-scale model was constructed to predict the damage behavior of TWTC at LVI, and tests were conducted under 5 and 10J impact energy to verify the correctness of the multi-scale model. The results showed that the micro, meso and macro serial multi-scale model could accurately predict the impact damage behavior of the TWTC. Furthermore, damage occurred on both the front and back of the material under the large impact energy, and the damage was mainly caused by fiber fracture. Besides, the force response curves predicted by the numerical simulation of LVI were in good agreement with the test results, and the error of the damage area of the numerical simulation was within 10%.

Abstract:In order to predict the deformation performance of composite compression struts with variable cross-section, the composite struts used in aerospace truss structure were taken as the research object in this paper. Firstly, wall panel of the composite struts was equivalent to an orthotropic plate, in approximation of an orthotropic strut, the principal direction was consistent with the axial direction of the strut, the composite strut could be approximated as an orthotropic strut. Based on the elastic deformation theory and small deformation assumption, the theoretical formula of equivalent axial compression stiffness was derived. Subsequently, accuracy of the theoretical formula of equivalent axial compression stiffness was verified by finite element method, results showed that: under conditions of different intermediate-radius and variable section-length, the deviation between values of the proposed theory and the finite element is within 3%; under the condition of different ply angles, the deviation between values of the theoretical and the finite element is within 2%.In conclusion, the theory can predict the axial deformation of composite struts with varying cross-section accurately.

Abstract:In this paper, the finite element model of drilling CFRP/TC4 stack structure was established to explore the material removal process, two-phase material and interface damage formation mechanism under different stacking sequences of CFRP/TC4 stack structure. The correctness of the model was verified by thrust force and defect type obtained from CFRP/TC4 stack drilling experiment. The results show that when the drilling sequence is CFRP→TC4, the entrance delamination dominated by tear occurs at the entrance of CFRP. There is no obvious push out delamination at the CFRP exit, but the interface delamination is formed under the continuous rotation of the drill. When the drilling sequence is TC4→CFRP, if the thickness of titanium alloy to be cut is low and the thrust force exceeds the interfacial bonding strength between CFRP layers, the interfacial phase failure occurs. The interface damage of TC4→CFRP is more serious than that of CFRP→TC4. TC4→CFRP mainly forms push out delamination with open and sliding crack propagation.

Abstract:According to the requirements of high quality machining of composite materials and the advantages of various machining technologies,the two-dimensional ultrasonic vibration composited electrochemical/electrical discharge machining (2UECM/EDM) is proposed in this paper, and surface formation mechanism is deeply studied. Benefiting from the widening of the grooves of the machined surface by the single abrasive particle motion and the levelling effect of electrochemical/electrical discharge machining, the change in the surface topography and surface roughness (Ra) of composite materials were analyzed by the grinding area ratio (δ_s).Surface formation mechanism of 2UECM/EDM was experimentally compared.The results show that single-cycle feed distance,voltage and two-dimensional ultrasonic amplitude affect the surface quality. Among them, the trend of Ra change is consistent with the feed distance of abrasive particles in a single cycle.The effect of electrochemical/electrical discharge machining is obvious at higher voltage,which leads to the exposure of reinforced particles and increases Ra.The δ_s increases significantly under axial and tangential vibration, and Ra shows an obvious turning trend near 1.8.Therefore,when greater δ_s amplitude and lower voltage are selected,the dragging and rolling of the reinforced particles during processing can significantly reduce Ra and greatly improve the surface quality of the workpiece.

Abstract:In order to investigate the structural safety of composite laminates due to impact loads in practical applications, the influence of the impactor shape on the damage mechanism of composite laminate was analyzed simultaneously by experiment and simulation.Subsequently, the variation pattern of the load-bearing capacity and energy absorption of the laminate was further revealed considering different impact energy.The results show that under same impact energy, the damage range and bearing capacity of laminates present incremental change when impacted by conical, hemispherical and flat impactors.The critical penetration energy of laminates increases along using conical, hemispherical and flat impactors. The impactor shape and the impact energy have a greater influence on the energy absorption capacity of laminates.

Abstract:In order to improve the wear resistance of aerospace aluminum alloy, especially to ensure the normal use of aluminum alloy under increasing temperature, this study designs a three-dimensional diffusion layer to enhance the bonding between coating and the substrate. Through-thickness composite coating was prepared on the surface of aluminum alloy by means of mechanical ball milling, laser texture microporous and electrodeposition, the mechanical and tribological properties of the composite coating were tested and analyzed. The results show that the electrodeposited Ni coating completely covers the texture surface of mechanical milling coating, and the microhardness of mechanical milling coating and electrodeposition coating is about 285 and 165 HV respectively, after heat treatment. Moreover,a three-dimensional diffusion layer is formed at the interface of aluminum alloy substrate, mechanical milling coating and electrodeposition coating. The tribological properties of aluminum alloy perform normal at room temperature, but show signs of failure at 300℃.For four composite coatings, the friction coefficient of N150 composite coating is the lowest at room temperature, which is about 0.7. The friction coefficient of N100, N150 and N200 composite coatings is about 0.5 at 300℃. The wear rates of the four samples are in the range of (0.9~1.6)×10^-3 mm³/(N·m) at room temperature and 300℃ with N100 and N150 composite coatings performing slightly better. The prepared composite coating can effectively protect the aluminum alloy substrate at room temperature and 300℃,and extend the applicable temperature of aluminum alloy.

Abstract:High entropy boride ceramics have broad application prospects in aerospace, automotive engine and nuclear reactor fields. In order to improve the performance of high entropy boride ceramics and expand the family of high entropy boride ceramics, (Hf_0.25Zr_0.25W_0.25Ti_0.25)B₂ high entropy boride ceramic was prepared by boron/carbothermal reduction combined with SPS(Spark Plasma Sintering) in this paper. The phase composition, microstructure and mechanical properties of the powder and ceramic were studied. The results show that after heat treatment at 1600℃,not only the high entropy phase but also W₂B₅ phase can be detected in (Hf_0.25Zr_0.25W_0.25Ti_0.25)B₂ high entropy boride powder(particle size(0.29+0.03)μm).After sintering at 2000℃,the second phase W₂B₅ decreases, and the diffraction peaks of the high entropy phase shift to a higher angle. The density of ceramic reaches 95.7%.After the introduction of WB₂, the high entropy boride ceramics exhibit excellent mechanical properties. The hardness is (21.3+1.5)GPa and fracture toughness is (3.00+0.22)MPa·m^l/2.

Abstract:In order to improve the surface quality of single crystal silicon after single-point diamond turning, this paper investigates the influence of laser-assisted turning (LAT) technology on machining quality by analyzing surface roughness, surface morphology and Raman spectroscopy of samples prepared by LAT and conventional turning (CT), respectively. Based on the orthogonal test method, the LAT process parameters of single-crystal silicon were investigated on surface roughness. The influence of each factor on surface roughness was evaluated by variance analysis and range analysis. The results show that LAT can effectively improve machined surface quality and reduce residual stress on material surface compared with CT. The contribution proportions of spindle speed, feed rate, cutting depth and pulse duty cycle to surface roughness were 17.51%,44.48%, 6.69% and 14.70%, respectively. Finally, the optimal combination of machining parameters was determined to be: spindle speed of 4000 r/min, feed rate of 2 mm/min, cutting depth of 5 μm, and pulse duty cycle of 30%, with a high-quality surface with a surface roughness (Rq) of 2.4 nm was finally obtained.

Abstract:In this paper, the simulation research about deformation after dual laser-beam bilateral synchronous welding (DLBSW) for the frame-truss tank wall structure is conducted. Firstly, the finite element model of DLBSW for the frame-truss tank wall structure is established,and verified by experimental results. Subsequently, the effect of welding sequence and direction on the deformation of frame-truss tank wall structure fabricated by DLBSW is emphatically investigated to obtain the deformation control strategy. Finally, the frame-truss tank wall structure is successfully manufactured by DLBSW with the optimized scheme. The results show that the welding deformation is greatly reduced under the welding sequence of "cross center welding". Based on the optimized welding sequence, the welding deformation can be further reduced by changing the welding direction of the middle three welds.

Abstract:In order to study the irradiation characteristics of SiGe heterojunction bipolar transistors with different emitter areas at extreme temperatures,characteristics of irradiated power SiGe heterojunction bipolar transistors (SiGe HBTs) were systematically analyzed at different temperatures in this paper; Correlation between emitter area and power SiGe HBTs'' irradiation damage of SiGe HBTs was revealed.Modeling of power SiGe HBTs device was conducted to extract the main internal parameters of the device affected by irradiation. The variation of electron density (Δ edensity), carrier recombination (Δ SRH recombination) and carrier mobility (Δ emobility) before and after irradiation were characterized, whose affecting mechanism on power SiGe HBTs irradiation was systematically analyzed. The results show proportional proton irradiation damage of emifferarea on power SiGe HBTs.SiGe HBTs have better proton irradiation resistance at extreme temperatures.which presents great potential in the field of radiation resistance and space application.

Abstract:In order to evaluate the influence of cold surface adiabatic boundary, after fully verifying the controllability and the stability of the heat radiation testing system, flexible insulation mat, rigid insulation tile and nano insulation material were used as cold surface insulation boundary respectively. The thermal radiation test of quartz lamp at 500℃ and 3 000 s was carried out, and the test results were simulated and analyzed by using iterative equivalent method. The results show that the cold surface temperature of ceramic fiber rigid insulation material is the highest when flexible insulation mat is used , while the cold surface temperature is lower and similar when rigid insulation tile and nano insulation material are used as the boundary, with maximum relative deviation of 19.0 % between the highest and lowest. Simulation results show that the contact thermal resistance caused by the cold surface insulating boundary material plays a decisive role in tests, rather than simply depending on the insulation performance of the insulating material.

Abstract:In order to address the issue of solder joint lines resulted from inadequate post-welding cooling in vacuum vapor phase welding of multi-layer printed boards,this study identified the underlying cause of solder joint lines in multi-layer boards and enhanced the post-welding cooling zone,enabling disclosure of the effects of different welding processes on surface morphology,internal structure morphology, and mechanical properties of solder joints.Experimental findings demonstrated that fast cooling led to thinner interfacial metal compound (IMC) within solder joints and a more uniform distribution of Pb in Sn. The grain diameter of IMC layer formed under fast cooling was approximately 1 µm, while shear strength reached 17.26 MPa-an increase by 43% compared to slow cooling rate - thus achieving fine crystal strengthening objectives. Furthermore, significant improvements have been observed in surface quality by fast cooling reducing solder joint lines.

Abstract:In this paper, the finite element analysis was established by ABAQUS to analyze the forming process of headless rivet pressed by 66° conical riveting die. Mechanical and metallographic analysis of the formed specimen were carried out, and the results were compared with those prepared by ordinary semi-circular head rivets. The results showed that during pressing deformation, the middle of the nail bar is thickened at the earliest forming the pier head, after which the nail bar is squeezed evenly with the hole wall gradually.With the increase of riveting displacement, the pull-out strength and shear strength of headless rivets elevate.Compared with the semi-circular head rivet formed by traditional flat riveting die, the metal material flows more evenly to the nail rod in conical riveting die, and the adiabatic shear effect on both sides of the pier head is significantly weakened.

Abstract:In order to reduce the molding defects of composite shell and optimize the winding process,the change of internal porosity of EPDM insulation under lower vulcanization molding pressures was observed by microscope. Influence of vulcanization molding pressure on the body performance and interface bonding performance of EPDM insulation was studied.The results show that, when the vulcanization molding pressure is less than 0.15 MPa, there are more pores in the insulation after vulcanization with lower density,mechanical properties and ablative properties.With the increase of molding pressure,insulation porosity gradually decreases. Insulation performance and interface bonding reach a high stable value to meet the requirements of use.