Research On Mechanism And Control Of Liquation Cracking Of Electron Beam Welded K4169 Alloy Joint

K4169 superalloy is one kind of precipitation-strengthened Ni-Cr-Fe-based casting superalloy.It has high strength,plasticity,excellent heat corrosion resistance,radiation resistance,and other properties at medium and high temperatures.It is widely used in aerospace structural parts such as turbine discs and casings of aero-engines.However,K4169 alloy is prone to liquation cracking in the heat-affected zone during welding.In this paper,the sensitivity and mechanism of liquation cracking in the heat-affected zone of electron beam welded K4169 alloy joints under different pre-weld heat treatment conditions were studied.The liquation cracking in the heat-affected zone of the joints was controlled through pre-weld heat treatment and optimizing the welding process.The microstructure and mechanical properties of the pre-weld heat-treated joints were studied.Electron beam welding was performed on the as-cast K4169 alloy,and the weld surface was well-formed,but liquation cracking occurred in the heat-affected zone,and they were mainly concentrated in the neck of the heat-affected zone.The liquid film in the heat-affected zone of the joint is mainly derived from the liquation of the Laves phase components,and theγ/Laves eutectic structure was formed during the cooling process.At the same time,there was also the phenomenon of liquid film migration in the heat-affected zone.The liquid film migration has a significant effect on the occurrence of liquation cracking.Inhibition.A room temperature tensile test was performed on the joint.The welded joint was tensile fractured on the base material at room temperature,and the fracture mode was a ductile transgranular fracture,and the strength was equivalent to that of the base material,indicating that a small number of liquation cracks did not affect the room temperature tensile strength of the joint.The effect of pre-weld heat treatment on the susceptibility to liquation cracking of K4169 alloy electron beam welded joints has shown that the Laves phase in the matrix can be eliminated by hot isostatic pressure heat treatment before welding,although this can avoid the liquation cracking problem caused by the Laves phase liquation,as the pre-weld heat treatment Laves dissolves toward the matrix,Nb,Mo,Si,and other elements are released to the matrix,and S,P,B,and other elements segregate to the grain boundary during the heat treatment and welding thermal cycle,leading to the decreasing of the solid line of the matrix at the grain boundary and increase the wettability of the liquid film to the grain boundary.The grain boundary in the heat-affected zone segregates and liquation during the welding thermal cycle,which also produces a greater tendency for liquation cracking;the mechanical properties of the base metal before welding have a greater impact on cracking sensitivity.The pre-weld heat treatment reduced the strength of the base metal and improves its plasticity,which could relieve the joint welding stress and reduce the cracking sensitivity of the joint.Research on the influence of welding heat input on the susceptibility of electron beam welded K4169 alloy joints to liquation cracking shows that when the welding heat input is in the range of 0.136～0.244 J·mm^-1,as the welding heat input increases（the welding speed decreases）,the liquation cracking sensitivity first increased and then decreased,indicating that the increase in welding stress caused by high-speed welding and the increase in PMZ width caused by slow-speed welding will not cause a significant increase in the sensitivity of liquation cracks.The combined action of liquid film and welding stress within a certain range can induce higher crack sensitivity.At the same time,the use of higher welding heat input can significantly reduce the joint liquation crack sensitivity by reducing welding stress and promoting liquid film migration.