Study On Microstructure And High Temperature Mechanical Properties Of CMT Welded Butt Joints Of Inconel 718 Sheet

Inconel 718 nickel-based alloy is a typical precipitation hardening superalloy.Due to its ultra-high temperature strength,high oxidation resistance,fatigue and corrosion properties,it is widely used in gas turbines,ships,molten salt reactors and aviation Aerospace.In this paper,Inconel 718 nickel-based superalloy is used as the main research material,ERNiFeCr-2 is used as the welding filler material,and CMT(cold metal transfer)method is used for welding the Inconel 718 nickel-based superalloy to reduce the welding line energy,speed up the cooling rate,reduce the influence of Nb element segregation and improve the welding efficiency,and realize automatic production.The morphology of CMT butt joint,microstructure and high temperature mechanical properties were systematically analyzed.The metallographic microscope(OM)and scanning electron microscope(SEM)were used to observe the microstructure of the joint and fractures.The energy dispersive spectrometer(EDS)analyzed the element distribution of the joint and fractures.The Gleeble-3500 thermal simulation testing machine was applied to analyze the high-temperature mechanical properties of the welded joint.The influence law of joint morphology and microstructure is expounded.In order to further explore the effect of the low dilution rate of CMT on the interface structure and mechanical properties of the fusion line.Hence,Inconel 718/SUS 316(Ni/Fe)dissimilar metals were welded by CMT process.The results showed that:The surface of Inconel 718 weld obtained by proper CMT welding process was evenly formed without obvious welding defects.When the wire feed speed(welding current,unification parameter)was the same,the weld width,reinforcement and cross-sectional area decrease with increasing welding speed,and there was no obvious change trend in wetting angle.When the welding current was 45 A,the welding speed was increased from 4 mm/s to 5 mm/s,and the weld width,reinforcement,cross-sectional area and wetting angle were correspondingly reduced from 6.63 mm to 6.29 mm,1.81 mm reduced to 1.22 mm,12.05 mm~2 reduced to 9.92 mm~2,52°reduced to 45°,respectively;and when the welding current was 58 A,the welding speed increased from 5 mm/s to 6mm/s,the weld width,reinforcement and cross-sectional area were correspondingly reduced from 6.79 mm to 4.81 mm,1.79 mm to 1.36 mm,12.39 mm~2 to 11.38 mm~2,and the wetting angle increased from 47°to 56°,respectively.There is obvious microsegregation between the dendrites in the weld,the spacing between secondary dendrite is small(9.1?12.5?m);there is a clear PMZ(partial melting zone)on the side of the Inconel 718 base material,and as the welding line energy(94?128J/mm)increases the HAZ(heat-affected zone)range tends to increase from 1.15 to 1.35mm,the HAZ grains tend to grow from 11?m to 15?m,and the segregation rate in weld center tends to decrease from 39.79%to 32.22%.The high-temperature tensile tests showed that the Inconel 718 joints fractured in HAZ and fusion line.Under the same welding current and at the temperature of 650°C,as the welding speed increases,the tensile strength of Inconel 718 joints showed a decreasing trend(45 A:833?804 MPa;58A:765?699 MPa).Especially when the test temperature was 750°C,the fracture was located at the fusion line.The surface showed the characteristics of solidification crack,and the tensile strength of the joint decreased significantly,and its value was between 524MPa and 618 MPa.Inconel 718/SUS 316(Ni/Fe)joints did not show Inconel 718 side fracture,in which the tensile strength and elongation of the joint fractured in the Ni-Fe dissimilar material fusion line interface were significantly lower than the SUS 316 side HAZ fracture joint,when welding current was 130 A,and tested at 650?,the tensile strength of the joint increased with the increase of the welding speed(481?535 MPa),the fusion line interface width of Ni-Fe dissimilar materials is approximately 4?m,and the fracture is mainly Fe element.It indicated that no adequate element diffusion at the fusion line interface.The fusion line interface of the Ni-Fe exhibits the characteristics of solidification cracks,and the tensile strength of the joint is greatly reduced(317?338 MPa).