The Study Of Diffusion Welding Of High Entropy Alloy With Aluminum,Copper And Stainless Steel

CoCrFeMnNi High Entropy Alloy(HEA), as a single face-centered cubic(fcc) structure,can dissolve some kinds nonferrous metals into its structure and keep the original fcc structure in a certain extent and its diffusion coefficients is significantly lower compared with that of the conventional alloys, has enormous potentiality to be the diffusion barrier interlayer materials or directly as the welding materials in the field of dissimilar metals welding. The essential problem with the welding of dissimilar material joints is the intermetallic phase(IMP) formation which may detrimental to the mechanical strength and ductility of the joint,and the huge difference of physical and chemical performance between dissimilar metals may generate some defects such as porosity and microfissures formed which may result in bad mechanical properties of the joint. In order to solve these problems, we use vacuum solid-state diffusion welding technology to study the joints of HEA/Al, HEA/Cu and HEA/304 stainless steel, respectively. The influence of bonding temperature on the interfacial behavior and the joints mechanical properties was investigated by Scanning electron microscopy, Energy dispersive spectrometer, micro-hardness test and tensile test.Aluminum has been successfully welded to Co Cr FeMnNi HEA using vacuum solid-state diffusion welding technique at 540-600 ℃. The tensile Strength increased firstly and then decreased as the temperature rose, and reach its maximum value 105 MPa at the 570 ℃.Through EDS, XRD analyses and combine with micro-hardness test, it is found a new intermetallic compounds(IMCs) phase named Al79.5(Cr MnFeCo Ni)20.5 formed in the interface.The morphology of the IMCs changed from isle shape at 540℃and 555℃ to a continues layer at 570℃. The thickness of the IMCs layer increase as the bonding temperature rise. The maximum thickness of IMCs layer at 570℃ is 0.91 um with no defects. As the thickness of IMCs layer increase there are some microfissures formed in the layer. All the joints fracture occurred in the IMCs layer, the fracture is almost plastic fracture at 540℃. There is some brittle fracture pattern appeared in the fracture surface at 555℃ and the fracture is almost brittle fracture at 585℃. It may draw such conclusion:if there is a very thin layer formed in the interface, there will be a little influence on the tensile strength of the joint; if there is a fairly thick layer formed in the interface, there will be a significant detrimental to the mechanical strength of the joint.A good bonding between the CoCrFeMnNi HEA and pure Cu is obtained at 750-850℃by vacuum solid-state diffusion welding. The diffusion coefficient of Cu in CoCrFeMnNi HEA is calculated and analyzed by fick`s second law. The results show that the diffusion rate of Cu in HEA side is faster than that of HEA elements in Cu side and,as increasing with the temperature, the reduce extent of the HEA elements concentration distribution in thediffusion area agree with the sequence of elements in the order of Mn>Cr>Fe>Co>Ni.Theoretical calculations show that the average diffusion coefficient of Cu in HEA is significantly lower than those in the reference stainless steel. After diffusion bonding, a reaction layer consisted with FCC solid solution is formed in the interface region of all the joints, and there is no intermetallic compounds appeared. All the joints fracture occurred only in Cu sides far away from the interface, the tensile strength and strain all decreased as the bonding temperature increased. In the bonding temperature of 750℃, the tensile strength and strain reach its maximum value of 224 MPa and 33%, respectively.A sound bonding between CoCrFeMnNi HEA and 304 stainless steel is achieved at900℃-1000℃ by vacuum diffusion welding. The results show that, there are lots of micro voids exist in the interface when the bonding temperature is low, the quantity of micro voids decrease as the bonding temperature increase. After diffusion reaction, a reaction layer consisted of FCC solid solution formed in the interface and no intermetallic compound appeared. All the joints fracture occurred in HEA sides far away from the interface. Tensile strength and strain rose as the bonding temperature increased, it may be connected with the amount of precipitated-phase. Tensile strength and strain, at the bonding temperature of1000℃, reach its maximum value 585 MPa and 50%, respectively.