Research On Microstructures And Mechanism Of Self-propagating Joining Between TC4 And C_f/Al Using Al-Ni-Ti

TC4 alloy and C_f/Al composites are lightweight and high-strength materials.They have broad applications in the aerospace field because of their excellent performance.The successful joining of these two materials is of great significance for the realization of lightweight spacecraft.For the purpose of avoding the performance degradation of C_f/Al composites,TC4 alloy and C_f/Al composites are joined by laser-induced self-propagating high-temperature sysnthesis joining method based on the welding characteristics of C_f/Al composites in this thesis.The thermodynamics and kinetics of the interlayer are analyzed.The self-propagating reaction mechanism of Al-Ni-Ti is studied according to the quenching experiment results.The typical microstructure of the joint and the influence of process parameters on the microstructure of the joint are studied by relevant analytical methods.The formation mechanism of the joint is elaborated based on the joint microstructure analysis results.According to the requirements of the laser-induced self-propagating high-temperature systhesis joining for the performance of the interlayer,Al-Ni-Ti interlayer is selected to join these two materials.The thermodynamic calculation results show that Al-Ni system has higher themodynamic stability and higher exothermic performance.Only Al+Ni reaction could be self-sustaining in all possible reactions because its adiabatic temperature exceeds 1800 K.The apparent activation energy of the main exothermic reaction calculated by the Kissinger equation is 157.4k J·mol-1 and the pre-exponential factor is 1.23×107.The apparent activation energy calculated by Ozawa equation is 162.1k J·mol-1.The interlayer exothermic performance is significantly effected by the addition of Ti.The Al-Ni-Ti system undergoes complex exothermic reactions after the main exothermic reaction occurs,which is beneficial to maintain the red heat state of the intelayer.The laser-induced self-propagating reactions in Al-Ni-Ti system begin with the melting of Al particle.The molten Al wraps Ni and Ti particles though capillary effect.Ni diffuses into Al and reacts with Al to form Al/Ni Al3 eutectic dispersed in Al.Ni Al3 comes into being around Ni particles with the reaction processing The released heat promotes the remaining Ni particles to participate in the reaction,forming a Ni Al3/Ni2Al3/Ni3 Al ring structure.At the same time,the outer Ni Al3 dissolves Ti particles so that Ti participates in the self-propagation reactions.During the product homogenization stage,the ring phase gradually transforms into Ni Al phase,and more Ti reacts with the ring structure and eventually forms Al2 Ni Ti eutectic.The laser-induced self-propagating joining process requires the interlayer to have good heat release performance.In the joining process,both base material side are applied certain pressure and heat insulation materials to ensure that heat acts more on the base material.The layer needs to have a certain burning rate to avoid quenching of the interlayer due to heat dissipation from the base material.When the Al-Ni interlayer with better exothermic performance is used to join the two materials,more pores appear in the joint.The possible microstructure of the joint is TC4/?3-Al3 Ni Ti2+Ni-rich Ni Al/Ni Al/Ni-rich Ni Al+Ni2Al3+Al/Ni Al3 eutectic/C_f/Al.The pores mainly come from the pores in the original compact and the volume contraction between Al-Ni due to the Kirkendall effect.The self-propagating joint abtained by Al-Ni-Ti interlayer has a better joining quality and the pores in the joint are effectively eliminated.The microstructure of this joint is TC4/?3-Al3 Ni Ti2+Ni-rich Ni Al/Ni Al+Al2Ni Ti/Ni-rich Ni Al+Ni2Al3+Ni Al3/C_f/Al.The addition of an appropriate amount of Ti can improve the wettability around the carbon fiber and improve the mechanical properties of the joint.While the addition of excessive Ti would weaken the exothermic performance of the interlayer and seriously deteriorate the joining quality of the joint.Larger or smaller interlayer densities are not beneficial to self-propagating joining.The use of nano-scale powder particles would promote the densification of the joint and self-propagating high-temperature systhesis joining.