Microstructure And Property Of Electron Beam Welded TZM Alloy Joint

Due to the good elastic modulus and strong corrosion resistance as well as excellent mechanical properties at high temperature,TZM alloy,as one of the most promising molybdenum alloys,has been widely used in aerospace,nuclear power and other industrial fields.In order to meet the requirements of TZM alloy for high temperature applications,the influences of different welding processes on the microstructure evolutions and mechanical properties of the welded joints were investigated.The mechanisms of microstructure evolutions and embrittlements of the welded joints were studied.In this paper,the influences of the precipitate formations on the mechanical properties of the TZM alloy welded joints were also investigated.The fracture mechanisms of the joints were detailedly discussed,and a new method for the microstructure regulation by alloying elements was put forward.First of all,the FEM simulations of TZM alloy butt welded joints test s were carried out to investigate influences of distinct welding parameters on the temperature fields and stress fields.As well,the ranges of the parameters for electron beam welding（EBW）process were determined.The effects of EBW parameters on the surface appearances and mechanical properties of joints were studied by using these parameters determined by FEM simulation results.The optimum process parameters were listed as follows: the focus beam of 581 m A,the welding speed of 350mm/min and welding beam of 35 m A.The tensile strength of the joint,welded under the parameters listed above,was the highest in these EBW experiments of the butt joints welded without interlayers.The mechanism of microstructure evolutions of TZM alloy welded joints was detailed studied.It was found that Mo O₂ and Ti O₂,with the relatively lower melting points,were segregated on the grain boundaries（GBs）.In addition,the precipitations of these second phase particles at GBs in the weld zone（WZ）would result in the change of coherent relationships.Thus,the stress concentrations at GBs i n WZ would cause the embrittlement of GBs and result in the formation of intergranular fracture.What’s more,particles of Zr O₂ and Mo2 C dispersedly distributed inside grains,which would increase the hardness of the WZ.As results illustrated that there was no obvious improvement on the tensile strengths of joints welded with the addition of titanium into weld metal.However,when zirconium in the WZ was maintained at an appropriate content,zirconium would preferentially react with oxygen to form Zr O₂,which was dispersed inside grains in the WZ due to its high melting point.Therefore,the purifications and stress releases of GBs in the WZ would be produced.The fracture mode of the joint was changed from a mixed fracture mode of intergrular and cleavage fracture to be a complete cleavage fracture,and the tensile strength of the joint was also improved to be 452 MPa.Tensile strength of the joint was increased with the rising of rhenium content in the WZ.The strengthen mechanisms were discussed as follows.The formation of Rerich phases near the GBs in the WZ would improve bonding strengths of GBs,leading to the suppression of the generation of intergranular fracture.On the other,the thresholds for the dislocation movement would be increased caused by the formation of low-angle GBs in grain insides at the WZ.Consequently,the formation of transgranular crack would be restrained.When the mass fraction of rhenium added into the WZ was 48.7%,the strength of the joint was the highest in this paper,reached to be 524 MPa,and the fracture generated in the heat affected zone（HAZ）.