High-temperature Self-propragating And Reaction-assisted Joining By Reactive Multilayer Films

In recent years,using the heat release from laser ignited self-propagating high-temperature synthesis?SHS?reaction of reactive multilayer films as a heat source to join thermal sensitive material has attracted much attention in micro/nano device manufacturing.During the SHS reaction,reactive multilayer films release the interatomic binding energy due to the decrease of entropy,result in a maximum local temperature as high as 1400°C prior to cooling to room temperature within 50 ms.The reaction velocity of SHS can be as high as 30 m/s.Moreover,SHS reactions do not inherently depend on external environments;they can be completed in vacuum,in an inert-gas environment,or even under water.Therfore,reactive multilayer films is of great significance to reduce the requirement of external heating source during the joining process,to accurately control the heat input of base material,and to obtain the precise joint of micro-nano device.In this paper,the reactive multilayer film?RNML?was used as the interlayer to assist the joining process.The study was focused on Ti/Ni and Ni/Al reactive multilayer films.Through the structure design,preparation and performance characterization,structure and thermal properties matched multilayer films were obtained.Joining of metal and nonmetal materials under different condition using multilayer films as interlayer was investigated.The influence of the atomic ratio of the multilayer film and joining parameters on microstructure and mechanical properties of joints were discussed,and interface reaction and failure mechanism of the joints were analyzed.The microstructures and reaction properties of Ti/Ni,Ti/Al,and Ni/Al multilayer films were prepared investigated.Physical vapor deposition?magnetron sputtering?was used to prepare the Ti/Ni,Ti/Al,and Ni/Al RMFs.The SHS processes and reaction speeds of all three films were investigated by high speed camera.The reaction products were determined and exothermic processes were calculated.SHS reactions of all three films can be triggered by laser ignition within a few miliseconds,the combustions were accomponied with bright flame.Ni/Al multilayers exhibit the largest heat release and the fastest reaction speed of 3 m/s,the final product was cubic NiAl.Ti/Al multilayers showed the smallest heat release and the slowest reaction speed,the final product was Al₂Ti.The phase revolution of Ti/Ni multilayers during SHS reaction was investigated.In-situ XRD showed that the final product of Ti/Ni multilayers was TiNi₃.The calculation showed that all three kinds of reactive multilayer films released enough heat to melt a solder layer.This is promising to develop very thin multilayer films for micro and nanojoining.Based on the aformentioned investigation,Ti/Ni multilayer films with three different stoichiometric compositions were used as interlayer in transient liquid phase?TLP?diffusion welding of Ti6Al4V alloys.The effect of stoichiometric compositions and pressure on joint structures and properties were investigated.The multilayers were directly deposited on the base material by magnetron sputtering.Dense and reliable joints thinner than 15?m thick were obtained.The reaction zone formed under 10-15MPa applied pressure,and it showed a layered structure.Higher pressures and higher Ni contents in the multilayers contributed to a higher shear strength.The highest shear strength value of 160 MPa was obtained by using Ti/Ni multilayers with a stoichiometry ratio of Ti/Ni at 1:3 under a bonding pressure of 15 MPa.This investigation confirmed that Ti/Ni reactive multilayer film can be used as interlayer for welding Ti6Al4V.The released heat from the SHS reaction contributed to the diffusion of the welding interface,and the temperature and pressure needed for vacuum diffusion welding of Ti6Al4V alloy are reduced.Ni/Al multilayer film was used as individual heat source for joining epoxy matrix carbon fibre reinforced plastics?CFRP?and aluminum alloys,the effect of film thickness and joining pressure on shear strength of the joints were investigated.Dense and reliable joints were obtained.Femtosecond pretreatment on the surface of CFRPs and aluminum alloys was efficient for improving the tensile strength of the joints.The highest shear strength of 9.5 MPa has been reached using Ni/Al multilayer film with the thickness of 96?m under the pressure of 5 MPa.The total reaction time was less than 100 ms.Ni/Al RMFs provided a novel method for joining CFRP and aluminum without significant power requirement and the weight of equipment transported into space.The multilayer film SHS joining method did not require high energy input and joining equipment,the heat release from the reaction can melt the filler material to finish the joining process,which provided a new idea for CFRP/aluminum alloys joining and repairing in space and/or under water.Laser direct joining of CFRPs to aluminum alloys was also investigated.The highest tensile strength of 20 MPa was obtained when using 150 W diode laser for joining with the help of 300?m PC sheets.Reactive multilayer film assisted laser welding platform was constructed.Ni/Al reactive multilayer film was used as interlayer to assist the laser joining process,and dense joints were obtained.The reaction of the Ni/Al RMF could be triggered when the temperature reached 200?.Results comfirmed that the shear strength of joints using Ni/Al multilayer films can reach to 400 MPa under 700 W laser power,which could be3 time larger than that of joints without any interlayer.Moreover,the laser power required for welding could be reduced by 10-20%when Ni/Al multilayer films were used as interlayer.Due to the heat release of the self-propragating reaction,the welding seam could be liquefied immediately during the laser welding process,which was helpful to reduce the laser energy input during the welding,reduce the heat affected zone of the Ti6Al4V,and meanwhile increase the shear strength of the joint.