Effect Of Heat Treatment Process On The Properties Of Inconel718 Alloy Electron Beam Welded Joints

Inconel718 alloy is a kind of nickel-based alloy with excellent comprehensive which is widely used as structural components in aerospace,nuclear industry,ships and other fields.The welding quality of Inconel718 alloy joints has a great influence on the quality and safety of structural components.This alloy needs to use appropriate heat treatment methods to improve the joint performance according to the performance required in the application site after welding.As a high energy beam welding,electron beam welding can reduce the number of Laves and other harmful phases in the welding of nickel-based alloys,and the fatigue and corrosion problems of welded structures are prone to occur under alternating load and environmental impact.Therefore,it is of great significance and value to study the electron beam welding process of Inconel718 alloy and the heat treatment process of welded joints,and analyze the mechanical properties,fatigue and corrosion resistance of joints.In this paper,the welding method of Inconel718 alloy by electron beam welding was studied.The influence of electron beam welding process parameters on weld formation was studied.The welding process parameters of 2.5 mm thick Inconel718 alloy were determined and optimized.The electron beam welding specimens under the optimized process parameters were treated by three different heat treatment processes.The microstructure and mechanical properties of the joints under different heat treatment processes were studied by metallographic,hardness,tensile,impact and micro shear tests.The fatigue crack growth rate and corrosion resistance of the joints under different heat treatment processes were studied by fatigue test and electrochemical test.The fracture mode and precipitates were analyzed by SEM observation and EDS analysis.The experimental results show that :The melting width increases with the increase of acceleration voltage U and decreases with the increase of welding speed V;the penetration depth increases with the increase of electron beam current I;the aspect ratio decreases with the increase of I.The optimized welding parameters of 2.5 mm thick Inconel718 alloy by electron beam butt welding are as follows : U = 70 k V,I = 16 m A,V = 1.1 m / min,f = 100 Hz.Under this process,the average tensile strength of the joint is 919 MPa,which can reach 88 % of the average strength of the base metal.Under high temperature solution and aging(1050 °C,1h / AC + 720 °C,8h / FC +620 °C,8h / AC)heat treatment process,the grain size of the base metal and the dendrite length of the weld are greatly larger.At the same time,annealing twins appear,and most of the Laves phase and δ phase in the weld are dissolved.Under solution aging(980 °C,1h /AC + 720 °C,8h / FC + 620 °C,8h / AC)heat treatment process,the grain length of the base metal is very small,and the dendrite is refined.There are a lot of Laves phase in the weld,and the needle-like δ phase is precipitated around.Under the two-stage aging(720 °C,8h / FC + 620 °C,8h / AC)heat treatment process,the grain of the base metal did not grow up,the dendrite was refined,the weld matrix was continuous microstructure,and the harmful Laves phase did not dissolve.Considering the mechanical properties,the dual-aging heat treatment process is the best heat treatment process for electron beam welded alloy joints.Under the three heat treatment processes,granular carbides always exist,the joint strength is greatly improved,and the plasticity and toughness are decreased.Under the same stress intensity factor,the fatigue crack growth rate of electron beam welding Inconel718 alloy joint is the smallest under high temperature solution aging(1050 °C,1h / AC + 720 °C,8h / FC + 620 °C,8h / AC)heat treatment process.Under the same heat treatment condition,the crack growth rate of the base metal is greater than that of the weld.Inconel718 alloy joints welded by electron beam welding have better corrosion resistance under 3.5 % Na Cl environment at room temperature and solution aging(980 °C,1 h / AC + 720 °C,8 h / FC + 620 °C,8 h / AC)heat treatment process.