Microstructure And Mechanical Roperty Of Electron Beam Welded TZM Molybdenum Alloy/30CrMnSiA Steel Joint

With the rapid development of modern industry,the use of complex structural components is becoming more and more extensive.In actual production,especially in the industrial fields such as aerospace and nuclear power which were developed vigorously by our country,the single structure is gradually being eliminated,while composite structures joined by different matels were widely used,so the welding method attracted much attention.In this paper,TZM molybdenum alloy and 30Cr Mn Si A steel were joined successfully by the vacuum electron beam welding,from the perspective of controlling energy distribution and metallurgical control,the process was optimized by direct welding and welding with vanadium filler metal,and the morphology and performance of the joints under different welding parameters were analyzed.Based on the experimental results including the microstructure evolution,phase composition and distribution of typical joints,the first calculation of the phases interface by direct welding and welding with vanadium filler metal was carried out,and the joinning mechanism between molybdenum alloy and steel was proposed.In this paper,the single-variable method was used to optimize the process of electron beam welding TZM molybdenum alloy and 30Cr Mn Si A steel,the electron beam energy distribution was controlled mainly by changing the welding beam current and the distance of beam deflection,thereby controlling the weld formation and surface morphology,which would affect the mechanical properties of the joints.The tensile strength of the joint was low with a brittle fracture mode,and the fracture loaded at the interface between the molybdenum alloy and the welding zone.The microstructures of the typical joint by direct welding molybdenum alloy and steel were analysised,the results showed that there was an obvious compound layer at the interface of molybdenum and steel.According to the characteristics of the microstructures,It was divided into layer I which was mainly composed of the Fe₂Mo intermetallic compound,and layer II which was mainly composed of Fe₂Mo and?-Fe dual phases.The center of the welding zone was mainly composed of equiaxed crystals,and the columnar crystals on both sides grew perpendicular to the fusion lines.Combined the results of nano-indentation test with the tensile fracture analysis,it could be seen that the compound layer would deteriorate the mechanical properties of the joint.Therefore,the pure vanadium with good metallurgy compatibility with molybdenum alloy and steel was used as the filler metal,and the melting ratio of the base material and filler metal in the weld was controlled by changing the thickness of the filler metal and the focused position of the beam current.When the thickness of the vanadium layer was0.8 mm,and the beam focused on the center of the vanadium layer,the optimal joint was obtained with a tensile strength of 312.6 MPa,and the tensile fracture occured in the heat affected zone on the side of the molybdenum alloy.According to the EBSD analysis,the heat affected zone was the recrystallized microstructure after rolling,and the stress was mainly concentrated at the small angle grain boundaries,which was the main reason for tensile fracture.The microstructure,element distribution and phase composition of the optimal joint for welding with vanadium filler metal were analyzed.It could be seen that the pure vanadium filler metal played a key role in metallurgical regulation and control.There was no intermetallic compound on the side of the molybdenum alloy,and the welding zone was mainly composed of(?-Fe,V)solid solution.The interface bonding mechanism between the molybdenum alloy and the weld zone of the joint without filler metal and with vanadium filler was studied.Based on the density functional method,the first-principles calculation was conducted by ultilizing the Materials Studio software.The connected mechanism between the molybdenum alloy and the welding zone was analyzed from the perspective of crystallography.Based on the previous test results,the Mo(111)/Fe₂Mo(100)interface model and the Mo(111)/V(Fe)ss(111)interface model were constructed.According to the results of the first-principles calculation,the ideal bonding energy of the Mo(111)/V(Fe)ss(111)interface was higher than that of the Mo(111)/Fe₂Mo(100)interface,so it was demonstrated that the interface bonding stregth of the joint with vanadium filler metal was higher than that of the joint without filler metal.