Joining Of Ni/Ti And Ni/NiAl Dissimilar Metals By Rapid Thremal Explosion

As deep geotechnical and coal mining have more requirements for high-pressure bearing,corrosion-resistant and oxidation-resistant structural parts,a single material can not meet the requirements of modern industry for components.Therefore,the joining of dissimilar materials should be carried necessarily to meet the practical engineering applications.The Thermal Explosion(TE)is a novel joining method,in which the heat released through exothermic reaction is regarded as a welding heat source and the reaction product is regarded as solder.This joining technology is regarded as a prospective method owing to the advantages of time and energy-saving,relatively low temperature requirement and reducing the difference of thermal expansion coefficient of base materials.Firstly,a novel joining method(rapid thermal explosion,TE)was developed for joining dissimilar metals using Ti-Al mixture powders.The combustion temperature of Ti-Al powder mixture was measured,and the effects of process parameters on microstructure and mechanical properties of Ti/Ni joint were investigated.The growth activation energy of the diffusion layer near Ti and Ni sides were also calculated.The results show that the combustion temperature of the Ti-Al powder interlayer is1103?along with an obvious exothermic phenomenon for 20 s.The thickness of diffusion layers formed at both interfaces increases with the increasing of temperature and holding time.The growth activation energy of diffusion layers at the Ti side is79.68 k J/mol,and that at Ni side is 38.26 k J/mol,which indicate that the interfacial diffusion layer of the Ni side is easier to grow than that of Ti side during TE joining process.At the same time,the joint strength increases and then decreases with the increasing of joining temperature and holding time.The maximum shear strength of Ti/Ni joints is 34.03 MPa,which was obtained at 850?for 10 min.As the basis of above results,the Ni/Ti joint strength obtained by rapid thermal explosion was improved with the addition of Ni.The effects of Ni content on the phase composition of Ti-Al interlayer,micro-structure and mechanical properties were investigated.The results show that as the Ni content increased from 0 to 5wt.%,the initial temperature of the thermal explosion reaction increased from 667 to702.5?and then decreased to 665.2?;while the combustion temperature decreased from 1103 to 970.3?and then increased to 1084.5°C.When the content of Ni is 1wt.%,Ni particles mainly exist in the Ti-Al compound in solid solution.As the content of Ni increased to 3 wt.%,a small amount of Ni Al white phase was formed and distributed in the intermediate layer.As the addition of Ni is increased to 1wt.%,the maximum shear strength of Ti/Ni joints is 51.54 MPa and the failure mechanism of Ti/Ni joints is a typical brittle fracture,which indicates that the addition of Ni can further improve the Ni/Ti joint strength through solution strengthening.In order to further take advantage of rapid synthesis and simultaneous releasing heat of thermal explosion method,Ni Al was synthesized by rapid thermal explosion(TE)reaction,and simultaneously joined to Ni metal by taking advantage of self-exothermic reaction.During joining process,Ni-Al reaction heat acts as heat source and its reaction product acts as solder or substrate.The TE reaction behavior of Ni-Al flux,the phase composition,interfacial structure and formation mechanism of Ni/Ni Al joint were investigated.The results reveal that the maximum combustion temperature of Ni-Al compact was 1613.1?and an obvious self-exothermic peak lasted 15 s.The XRD,EDS and TEM results show that the Ni Al intermetallic compounds with high purity could be obtained.The diffusion layer with the stable layered structure is formed at Ni/Ni Al interface,which was detected as Ni₃Al.According to the growth behavior of interfacial diffusion layer,the effect of heating temperature on the growth of diffusion layer is stronger than that of hold time.According to the interfacial structure and EDS results,the Ni/Ni Al joints was formed by plastic deformation,element diffusion and the formation of the diffusion layer at Ni/Ni Al interface.This paper includes 60 figures,8 Tables and 125 references.