Research On Process And Mechanism Of The Self-propagating Joining Between C_f/Al Composites And TiAl Alloys

Carbon fiber reinforced aluminum matrix composites?C_f/Al?feature?i?low density,?ii?high specific strength and stiffness,as well as?iii?the good dimensional stability in moist and radiation environment.C_f/Al composites have been successfully applied on waveguide tube and antenna frame on satellites,as well as aircraft skin components.TiAl alloys exhibit favorable features such as high creep and oxidation resistance at elevated temperatures.As two promising aerospace materials,their successful joining can produce composites structures with combined advantages and expand their applications,which is of great practical significance.Due to their great difference in physical and chemical properties as well as the temperature sensitivity of C_f/Al composites,conventional welding methods are unable to achieve sound joints.In this paper,the self-propagating high-temperature synthesis joining was adopted.The exothermic systems were optimized by theoretical calculations and experiments.Through the optimization of the interlayer composition and the design of the composite interlayer,the interfacial reactions and the joint microstructure were controlled and the plastic deformation ability of the joint increased,thereby realizing the high-strength bonding.Based on the principle of closed system,the adiabatic temperature of the three kinds of exothermic systems,which are thermite system,alloying system and ceramic phase formation system,were calculated.According to the basic principle of the joining interlayer,the Al-CuO,Al-Fe₂O₃,Ni-Al and Ti-Al-B systems were selected as the representatives.By calculating the effect of the type and content of the diluent on the adiabatic temperature of the reaction systems,the optimal adiabatic temperature of the four systems were respectively determined to be T_ad=2327 K,T_ad=2327 K,1800 K<T_ad<2767 K,T_ad=1912 K.The T-Jump?Temperature-Jump?experiment indicated that the explosion phenomenon was observed in the combustion process of Al-CuO and Al-Fe₂O₃ systems,which went against the densification of the joint.The spectrum data revealed that the formation of O₂ because of the thermal decompositions of CuO,Fe₂O₃ during the combustion process led to the explosion.The CuO crystal has a higher average bonding energy than Fe₂O₃.The oxygen ions have a higher immigration rate in CuO crystal.During the combustion,the CuO can rapidly release more O₂,which makes the explosion of Al-CuO system more violent.The DSC and T-Jump experiments of Ni-Al,Ti-Al-B systems indicate that the ignition temperatures of the Ni-Al,Ti-Al-B systems at slow-heating rate were respectively⁸00 K and⁸50 K,which were below the melting point of Al.At high-heating rate,the ignition temperatures of the reaction systems were over 1000 K,which were much higher than the melting point of Al.The T-Jump experiment was performed on the nano Al powder,and the hollow Al₂O₃ structure was observed in the samples,which showed that during the high heating rate process the oxide shell of Al particles would shortly store the Al liquid.The contact of Al and other components was temporarily cut off.After the oxide shell broke,the Al liquid contacted with other components and made the ignition mode transfer from solid-solid diffusion mode to liquid-solid mode.Due to the transformation of ignition mode,the effective activation energies of Ni-Al,Ti-Al-B systems at high heating rate were lower than that at slow heating rate.The reaction mechanism of the Ti-Al-B system was analyzed by thermodynamic calculation and quenching test,which proved the feasibility of in-situ synthesis of TiB₂/Al composites.The exothermic reaction of Ti-Al-B system started from the initiation reaction of Ti and Al,forming the Ti-Al melt.The B atoms dissolved in the Ti-Al melt,which enhanced the activity of Ti,B atoms.The Ti,B atoms bonded with each other in the Ti-Al melt.The TiB₂ precipitated out and formed the mixture of Al and TiB₂.When the volume fraction of TiB₂ in the ideal products ranged in 29.3%-53.5%,the Ti-Al-B interlayer had high physical chemical compatibility with the substrates.In the self-propagating joining of C_f/Al composites and TiAl alloys,there was a quenching effect of substrates on the combustion of the interlayer.The reaction products of Ti-Al-B interlayer in the joint were the quenching microstructure of Ti_0.36Al_0.64+TiB₂+Al B₂ with a low density.The represent joint structure was?TiAl alloys?/TiAl₃+Ti/Ti_0.36Al_0.64+TiB₂+Al B₂/Ti₂Al₃+Ti/?C_f/Al composites?.The C_f/Al composites and TiAl alloys were joined using Ni-Al interlayer.The Ni-Al reaction products had a high melting point and poor affinity to carbon fibers,thus forming many pores and cracks in the joint.With the addition of strong carbide-forming element Ti and Zr,the interlayer products transformed from Ni Al to the eutectic of Ni Al+Ni-Al-Ti,Ni Al+Ni-Al-Zr,which increased the liquid in the interlayer products and the joint density.Ti,Zr accumulated around the carbon fibers,forming a 200-300 nm thick Ti-C and Zr-C reaction layer.The bonding between the interlayer products and the carbon fibers was enhanced,which is the key to the improvement of the joining quality.By the comprehensive balance of joint defects,interfacial microstructure and mechanical property,the optimal parameter was achieved.When the joining pressure was 4 MPa,the optimal Zr content was 5 wt.%and the corresponding joint shear strength was 60 MPa.To eliminate pores and relieve the joint residual stress,the composite interlayer was design to introduce a ductile phase and control the joint microstructure.Adding AgCu alloy powders to the Ni-Al-Zr interlayer didn't lead to a mass formation of Ag?s,s?phase,but decreased the interlayer exothermicity and increased the cooling rate at the interface,thus decreasing the joint density.The AgCu/Ni-Al-Zr/AgCu composite interlayer was designed by applying the AgCu foils.The exothermicity of the interlayer was not affected.Ag?s,s?was produced and eliminate the pores in the joint.But the diffusion of Zr towards the substrates was impeded,which weakened the bonding between the carbon fibers and the interlayer products.The TiZrNiCu/AgCu/Ni-Al-Zr/AgCu/TiZrNiCu composite interlayer was further designed by applying the TiZrNiCu foils.Using this composite interlayer,Ti,Zr enriched at the substrate/interlayer interfaces and formed a⁴00 nm thick?Ti,Zr?C reaction layer around the carbon fibers.The affinity between carbon fibers and interlayer products was enhanced.Block Ag?s,s?dispersively distributed in the joint and formed the soft-hard mixed structure with Ni Al,Ni-Al-Zr+Al-Cu-Ti.The plastic deformation ability of the joint seam was improved.The optimal parameter was 4MPa of joining pressure and 100?m of AgCu thickness.The maximum joint shear strength was 72 MPa,which was 118.7%of the joint shear strength using the Ni-Al-Zr interlayer.