Microstructure Evolution And High-Temperature Deformation Behavior Of Laser Welded Joint Between Ni-Based Single Crystal And Polycrystalline Superalloy

High power fiber laser welding has significant advantages on dissimilar welding of superalloys.However,it will inevitably result in microstructure changes and even cracks in the connection position,which seriously affects the strength and toughness of the welded joint.Moreover,as an important part of blisk,the welded joint will bear the combined effect of high temperature and complex stress,which brings more severe challenges to the service safety of joint.The single crystal superalloy of DD407 and polycrystalline superalloy of IN718 were butt welded by continuous fiber laser system.The evolution of microstructure,microhardness and composition segregation of the welded joints fabricated under the different welding parameters as-welded(AW)and after post weld standard heat treatment(PWHT)were investigated,to study the effects of temperature and strain rates on the tensile properties and deformation behavior of the welded joint.The results show that the microstructures of fusion zone(FZ)consist of planar crystal,cellular crystal,columnar crystal and equiax crystal under the optimized welding parameters.The difference of the dendrite microstructures between the two sides of the weld centerline is very obvious.The microstructure in the FZ of DD407 is mainly single crystal and that of IN718 is mainly polycrystal.And a large number of Laves phase and carbide particles are distributed in the interdendritic region in the FZ of the welded joint.There is an obvious resolution phenomenon in the heat-affected zone(HAZ)of DD407,but a large number of secondary ?' phases are precipitated in the ? matrix.In terms of the AW joint,the microhardness of the FZ is low and there exists obvious micro-segregation.After PWHT,the micro-segregation significantly increases in the FZ,which is much more than those of both base metals(BMs)of DD407 and IN718 alloys.There exists local hardening zone in the HAZ of DD407 and narrow softening zone and grain boundary liquation phenomenon in the HAZ of IN718.At the strain rate of 10-3s-1,the mechanical properties of the welded joint are obviously influenced by the polycrystalline alloy.With the increase of temperature,the yield strength and tensile strength of the joints are gradually decreasing,and the elongation first rises and then drops.At the same time,the fracture positions of the joints have changed.When the temperature is lower than 580?,all of the joints are fractured in the BM of DD407.The main deformation mode of the welded joint includes the multi-slips of dislocation in the BM and FZ of single crystal alloy,and the dislocation slip and grain-boundary sliding in the BM of polycrystalline alloy.When the temperature is higher than 620?,all of the joints are fractured in the BM of IN718.And with the increase of temperature,the deformation mode of the multi-slips of dislocation in the BM and FZ of polycrystalline alloy transforms to grain-boundary sliding in the BM of polycrystalline alloy,which the degree of plastic deformation in the FZ gradually decreases.At the strain rate of 0.926s-1,the tensile strength and elongation of the joint first rise and then drop.The high strain rate inhibits the coordination deformation between the FZ and the BM,and reduces the strength and plasticity of the welded joint.Compared with the low strain rate,the HAZ of IN718 is prone to result in intergranular cracking,and the fracture position becomes more complex.The welded joint fractures in the side of the single crystal BM at the room temperature,and fractures in the side of the poly crystalline BMs at the temperatures between 500? and 620?.And at the 720?,the welded joint fractures near the FZ.