Diffusion Welding Of Cu And 304 Stainless Steel/Ti Based On High Entropy Alloy Interlayer

In this dissertation,copper and 304 stainless steel/titanium and copper were bonded in this work by utilizing vacuum solid-state diffusion method with an interlayer of Co Cr Fe Mn Ni high entropy alloy?HEA?because micro-voids and micro-cracks can propagate into the bond region of 304 stainless steel/titanium and copper,and diffusion reaction mechanism and properties was investigated by using scanning electron microscopy,X-ray diffraction,energy disperse spectroscopy and micro hardness test.The effects of different process parameters on the diffusion reaction mechanism and mechanical properties of the welded joint,the law of the diffusion of elements on the formation of the interface phase,and the feasibility of the application of high entropy alloy in the field of welding as the intermediate layer of dissimilar metal connection are explored.By adjusting the process parameters and selecting three different welding temperatures,304SS/Cu and Ti/Cu were directly diffused and connected,and each element had obvious mutual diffusion,and no intermetallic compound was formed at the interface.However,due to the small solid solubility of the two in the solid state,the joint is easy to produce internal stress,resulting in poor mechanical properties.A variety of intermetallic compounds?such as Cr₂Ti?Mn₂Ti and Ni₃Ti?are formed at the Ti/Cu interface.With the increase of welding temperature,the thickness and quantity of reaction layer are increasing,and the distribution of reaction layer is zigzag.Cu/HEA/304SS diffusion bonding test was carried out with Co Cr Fe Mn Ni high entropy alloy as interlayer.At the interface of diffusion,the elements have obvious mutual diffusion and form a continuous concentration gradient.The higher the temperature,the smaller the concentration gradient of the elements at the interface,the less holes and cracks at the diffusion interface,and the better the interfacial bonding effect.However,the effect of joint time on joint interface and properties is not obvious.Therefore,the most reasonable process is determined as follows:the connection time is 120min,the connection temperature is 850?,and the connection pressure is 10MPa.Cu/HEA/Ti diffusion bonding test was carried out with Co Cr Fe Mn Nihigh entropy alloy as interlayer,and the obvious mutual diffusion of all elements occurred at the diffusion bonding interface.A continuous concentration gradient is formed at the Cu/HEA interface,while elements at the HEA/Ti interface diffuse to form multiple platforms,and?+?Ti widmanstatten structure is formed at the Ti side.The higher the temperature,the smaller the concentration gradient of the elements at the cu/HEA interface,the less holes and cracks at the diffusion interface,and the better the interfacial bonding effect.A variety of intermetallic compounds?Cr₂Ti?Mn₂Ti and Ni₃Ti?are formed at the HEA/Ti interface.With the increase of temperature,the thickness of reaction layer increases,and different layers with different scales are formed.The different reaction layers are zigzag distribution,and the welding interface is slightly deformed.The longer the holding time is,the more sufficient the diffusion of joint elements is and the thicker the diffusion layer is.However,the effect of joint time on the interface and properties of joint is not obvious.Therefore,the most reasonable process is determined as follows:the connection time is 90min,the best connection temperature is 900?,and the connection pressure is 10MPa.The element distribution of welded joints at different temperatures is studied.The energy spectrum data is fitted by MATLAB.The average diffusion coefficient of Cu and Fe in HEA is calculated by Boltzmann-Matano method.The the diffusion equation of diffusion coefficient of Cu and Fe at any temperature is obtained.The main factors affecting the thickness of reaction layer are welding temperature and holding time.The better welding joint can be obtained by properly adjusting the Cu/HEA/Ti welding temperature and holding time.The parameters?,?and VEC are calculated and predicted.It is found that the parameters predict the phases of Cu/HEA diffusion interface and HEA/304SS diffusion interface more accurately.The criterion is modified to reduce the formation range of solid solution.The modified criterion can better predict the phase selection rule of HEA/Ti welding interface.