High Entropy Alloy Interlayer Design And Welding Parameters Optimization For Diffusion Bonding Of TC4 Titanium Alloy To 316L Stainless Steel

The titanium alloy/stainless steel composite component can give full play to the complementary advantages of the two materials in performance and economy,realize the lightweight of high-end equipment manufacturing,the integration of material structure and function,and has a broad application prospect in high-tech fields such as aerospace.However,due to the poor metallurgical compatibility and great difference in thermal properties between titanium alloy and stainless steel,the titanium alloy/stainless steel joints are prone to form hard and brittle Ti-Fe intermetallic compounds and produce large residual stresses,which seriously deteriorate the joint properties and become a bottleneck restricting the engineering application of titanium alloy/stainless steel heterogeneous metal composite components.Diffusion bonding with interlayer is an ideal method to realize the connecting of the two metals.However,currently widely used strategy of mutli-interlayer is more complex and not conducive to industrial application.Because of its innovative design concept,high entropy alloy（HEA）provides a new strategy for the design of interlayer materials,which is expected to break through the bottleneck of high-performance welding manufacturing.Therefore to carry out the design of high entropy alloy interlayer and research on diffusion bonding process of titanium alloy/stainless steel,to protect the titanium alloy/stainless steel composite component reliable application in the field of high-end equipment manufacturing,has important theoretical meaning and engineering value.At the same time,it can provide theoretical basis and experimental support for the development of newly interlayer for high performance welding of other heterogeneous metals.In view of the problem that the interface of titanium/steel heterometal welding is prone to form hard and brittle intermetallic compounds leading to poor joint strength and toughness.Combined with the new design strategy of high entropy alloy materials,the TC4 titanium alloy/316L stainless steel typical heterogeneous metal composition was taken as the research object and the high entropy alloy interlayer was designed and prepared,meanwhile the experimental study of titanium/steel heterogeneous metal vacuum diffusion bonding with the addition of high entropy interlayer was carried out.Using electron probe microscope analyzer（EPMA,JXA-8530F Plus,Japan）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）analysis method,and universal testing machine（DNS-100）,the evolution of interface microstructure between interlayer and base metal with process parameters was systematically studied,the diffusion behavior of interface elements under the action of high entropy system was deeply discussed,and the influence mechanism of high entropy alloy composition on the growth behavior of intermetallic compounds was elucidated.The main conclusions are as follows:Based on pseudo-binary design strategy,Al Ni（Co Cr Ni）Cu HEA（Al Co Cr Cu Ni₂）was designed by using Co Cr Ni（FCC）phase as one component and Al Ni（B2）phase with stable structure as the second component,and adding Cu with equal atomic ratio to release stress of bonded joint.The effects of bonding temperature on microstructure evolution,element diffusion behavior and mechanical properties of TC4/316L bonded joints with Al Co Cr Cu Ni₂interlayer were revealed.The sluggish diffusion effect of the high entropy alloy interlayer effectively suppressed the formation and growth of intermetallic compounds on the TC4 side interface,and the sound bonding of TC4 titanium alloy/316L stainless steel was achieved by using Al Co Cr Cu Ni₂ high entropy alloy interlayer.With the increase of bonding temperature,the compressive shear strength of the joint gradually increases,and reaches the maximum value of 214 MPa when the process parameter is 1010 ^oC/60 min.TC4/HEA diffusion zone consists of Ti₂Ni,Ti（Co,Ni）,Al（Co,Ni）₂Ti,Cr-rich phase and discontinuous precipitates zone near FCC phase of Al Co Cr Cu Ni₂ HEA interlayer.The solution diffusion zone is formed near the316L matrix,and some needle-like Al-rich phase is precipitated,which results in nailing effect and improves the shear strength of the joint.With the increase of temperature,the thickness of diffusion zone increases gradually,and the growth activation energy of TC4/HEA diffusion zone is 162 k J/mol by growth dynamics calculation.Due to the difference in crystal structure and chemical composition between B2 and FCC phases in high entropy alloy,the growth activation energy of B2/316L diffusion zone（126 k J/mol）is lower than that of FCC/316L diffusion zone（234 k J/mol）.Furthermore,the effects of different composition of high entropy alloy interlayer and bonding time on the microstructure and mechanical properties of joint were obtained.By adjusting the content of Al and Ni,Al_xCo Cr Cu Ni_1+x（x=0.8,1.0,1.2）high entropy alloy with three different components were designed and prepared,and characterized by EPMA and XRD.The results show that the interlayer of Al_0.8Co Cr Cu Ni_1.8,Al_1.0Co Cr Cu Ni_2.0and Al_1.2Co Cr Cu Ni_2.2 high entropy alloys all have FCC+B2 dual-phase solid solution structure.With the increase of Al and Ni content and the extension of bonding time,the joint strength increases.At 1010 ^oC/90 min,the joint with Al_1.2Co Cr Cu Ni_2.2（x=1.2）interlayer reaches the maximum shear strength of 240 MPa.With the increase of the bonding time,the Ti₂Ni phase on the TC4 side is liquefied and squeezed out of the interface,and the thickness decreases obviously,meanwhile,the Cr-rich phase is dispersed in the reaction layer III（Al（Co,Ni）₂Ti）.All joints fracture at HEA/316L interface,presenting typical cleavage fracture characteristics.Kirkendal voids formed at the interface tend to cause crack initiation,making the side interface a weak bond of the joint.