| CrMnFeCoNi was the first-developed multi-component equi-atomic high-entropy alloy(HEA)and charactered high working hardening rate,plasticity,fracture toughness,corrosion resistance,toughness and strength at low temperature,good processability,and high anti-irradiation performance,which makes it a potential structural material with good performance.To further develop its potential application,the weldability of CrMnFeCoNi HEA need to be studied deeply,especially the connection mechanism of it with other tradition alloys.This study adopted 50Ag-Cu and 72Ag-Cu as filler metal and systematically studied the process of vacuum brazing,microstructure,performance,and connection mechanism of the dissimilar joint of CrMnFeCoNi HEA and 304 stainless steel(304 SS).In the microstructure observation of the dissimilar joint brazed by 50Ag-Cu filler metal,the inter-granular penetration was detected in each joint,and the structure of those joints were consisted of CrMnFeCoNi HEA,penetration zone,filler metal layer and 304SS.The micro-hardness at penetration zone was higher than those at filler metal and CrMnFeCoNi HEA without penetration.The ultimate tensile strength of the dissimilar joint of as-rolled CrMnFeCoNi HEA was 434.7 MPa.The researches indicated that the fundamental reason for inter-granular penetration was that it was difficult to maintain equilibrium between the solid-liquid interface energy and the grain boundary energy at the grain boundary.The difference in chemical compositions of 304SS and CrMnFeCoNi HEA,especially the difference of Mn content,was the main reason why the penetration occurs only at the CrMnFeCoNi HEA side.The strong diffusion of Mn atom could reduce the solid-liquid interface energy and promote the permeation behavior.The penetration level of liquid filler metal depended on the diffusion level of Cu atoms in the base metal,and a large number of Cu atoms in the grains and grain boundaries of base metal in the penetration area was also the main reason for the high micro-hardness in this area.The inter-granular penetration promoted the migration,including migration induced by the flow of filler metal and the difference in chemical potential,between the base metal and the filler metal.The calculation results of penetration coefficients revealed that the highest value of penetration coefficient could reach 1.1×10-9 m2/s when the brazing temperature was 890?and the holding time was1 min.The chemical potentials of Cu and Mn calculated by pseudo-ternary systems indicated that those two kinds of atoms had conditions for diffusion in the penetration phase and the base metal.The difference in the chemical potential of Mn in penetration phases could reach 238.60?248.84 k J/mol and that of Cu in base metal could reach56.80?59.19 k J/mol,which was main reason why Mn atoms showed a stronger ability of diffusion than Cu atoms.Inter-granular penetration was affected by the heating input and the state of base metal.The increasing heating input provided more energy for inter-granular penetration,making the process of penetration severe.The penetration level in base metal could be restrained to some extent when the base material was annealed before brazing.Inter-granular penetration was the main factor affecting the performance of the joint.In order to promote the joint strength of the CrMnFeCoNi HEA/304 SS dissimilar joint,the penetration was regulated by reducing the content of Cu in filler metal and annealing CrMnFeCoNi HEA before brazing.The experiment firstly obtained the dissimilar joint by using 72Ag-Cu filler metal with a thickness of 0.1 mm.The inter-granular penetration was maintained because the elemental types of filler metal did not be changed,but the penetration level was declined clearly.The chemical potentials of Cu and Mn calculated by pseudo-ternary systems indicated that the difference in chemical potential of Cu in penetration phase was improved to 123.73?136.73 k J/mol.However,the decrease in the content of Cu in filler metal declined the diffusion level of Cu atoms in base metal,which leaded that the penetration coefficient of 72Ag-Cu at 880?(5.8×10-11 m2/s)was lower than that of 50Ag-Cu at 870?(2.1×10-10 m2/s).The difference in chemical potential of Mn in penetration phase decreased to 206.23?227.87 k J/mol,indicating that the diffusion ability of Mn in penetration phase was limited as well.The tests on the mechanical properties showed that penetration zone became a transition zone between filler metal with low micro-hardness and base metal with high micro-hardness.The improvement of the dissimilar joint was limited because there still existed stress at the penetration zone and the strength of 72Ag-Cu filler metal was weaker than 50Ag-Cu filler metal,which made the ultimate tensile strength of the joint reached 459.7 MPa.Before brazing,CrMnFeCoNi HEA were annealed at a series of temperatures for 1h and 72Ag-Cu filler metal was used to join the annealed CrMnFeCoNi HEA and 304 SS at certain brazing parameters.The researches indicated that the penetration was gradually inhibited following with the increasing annealing temperature,the decreasing density of grain boundaries and the release of internal stress,and the limit penetration was detected in the joint of annealed at 1200?CrMnFeCoNi HEA.The mechanical tests indicated that the width of transition zone in micro-hardness narrowed with decreasing inter-granular penetration;the tensile strength of the joint of annealed at 900?CrMnFeCoNi HEA reached 515.7 MPa and was higher than that of the joint of annealed at 1200?(316.2 MPa),revealing that inter-granular penetration did not always hurt the tensile performance of the joint.The stress at penetration zone was mainly induced by the inhibition of penetrating filler metal on the grain growth in penetration zone.The direct inter-migration between base metal and filler metal was extremely limited,and the atoms of base metal in filler metal mainly came from dissolved base metal because.It was necessary to consider welding stress,atomic migration and solid-liquid contact area when regulating inter-granular penetration.Meanwhile,the performance of the CrMnFeCoNi HEA/304SS dissimilar joint can be further improved by changing the composition of filler metal and modifying the CrMnFeCoNi HEA surface. |
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