Microstructure And Mechanical Properties Of Superalloy Laser Welded Joints For GH3625 By Laser Additive Manufacture

More and more advanced aerospace equipment adopts large-scale integral structural parts in order to meet their continuous pursuit of extreme lightweight requirements,and the traditional manufacturing process can not meet the processing requirements of large-scale integral structural parts.Additive manufacturing technology has become the best new technology way to deal with the above problems,but the processing capacity of additive manufacturing equipment is limited,and large-scale integral structural parts can not be additively manufactured at one time.It must be welded after adding materials in stages,so the welding of additive parts is especially necessary.Based on this,in this paper,the laser butt welding testing of GH3625superalloy by laser additive manufacturing(3D-GH3625)and 3D-GH3625/rolled GH3625 was conducted.The effects of laser power and welding speed on the microstructure and mechanical properties of the joints of 3D-GH3625 were studied,and the effects of laser power on the microstructure and mechanical properties of the joints of 3D-GH3625/rolled state GH3625 were studied.The transformation of microstructure evolution and the formation mechanism of HAZ liquefaction cracks of the two joints were analyzed.The research results show that the optimal process parameters of laser welding3D-GH3625 superalloy is P=5.3kW,V=1.8m/min,d=+2.5mm,and a great amount of Laves phase was precipitated at the grain boundary of upper layer in HAZ,resulting in significant coarsening of the grain boundary.Along the upper layer to the lower layer of the weld,the fusion zone and the intermediate zone are gradually transformed from cell crystal,the columnar crystal and the equiaxed fine crystal of the upper layer to columnar crystal of the lower layer.The number of equiaxed fine crystal growing close to the fusion line is gradually increased,and the central zone is dendritic.The particle of fine??phase is precipitated in the weld.And a number of Laves phase precipitates in the inter-dendrtic of the intermediate zone,and the morphology gradually changes from the strip of the upper layer to the particle of the lower layer.The weld grain grows preferentially along the<001>direction.The tensile strength of the welded joint is 872MPa,which is up to 98.2%of the base metal.The elongation is up to 90.7%of the base metal,the fracture surface of weld joint is approximately 45°inclined plane,and the fracture mode is the mixed fracture mode of brittle and ductile.When P>5.3kW or V<1.8m/min,the equiaxed dendrites begin to appear in the center of the weld,when P>5.0kW or V<1.8m/min,the cell crystals in the fusion zone disappear.The area of the equiaxed fine-grained area increases with increasing laser power or decreasing welding speed,and with the increase of laser power or welding speed,the tensile strength of the same kind of joints showed a trend of increasing first and then decreasing.The optimal process parameters of laser welding 3D-GH3625/rolled state GH3625is P=4.7kW,V=1.8m/min,d=+2.5mm,and a large amount of MC precipitated in the austenite grain boundary of the upper layer HAZ on the side of rolled state GH3625,a large amount of Laves phase precipitated in the austenitic grain boundary of upper layer HAZ on the side of 3D-GH3625.Along the upper layer to the lower layer of the weld,The morphology of the fusion zone on the side of rolled state GH3625 changes from columnar crystals and cell crystals in the upper layer to columnar dendrites in the lower layer.The morphology of the side of the fusion zone on the 3D-GH3625 is close to the side of the fusion zone on the rolled state GH3625,but there is an equiaxed fine grained area in the fusion zone on the side of 3D-GH3625 close to the fusion line,the area of the equiaxed fine grained zone gradually increasing with laser power increasing.The central area of the weld is transformed from columnar dendrites and equiaxed dendrites in the upper layer to columnar dendrites in the lower layer.In the dissimilar joints,??,MC,Laves phase and M₂₃C₆with small size are distributed among the dendrites of the weld,and the content of each precipitated phase is low,especially the content and volume of the Laves phase in the weld are significantly lower than the 3D-GH3625laser welding joint.The grains in the fusion zone on both sides grow preferentially in the direction of<001>,but the tendency of preferential growth of grains along the direction of<001>in the fusion zone on the side of the rolled state GH3625 is stronger.The tensile strength of the dissimilar joint is up to 895MPa,and the elongation is up to 38.3%,and the fracture mode is the mixed fracture mode of brittle and ductile.When 4.7kW<P?5.0kW,the fusion zone of the welds on both sides completed the transformation from cell crystal to cell dendrite,and with the increase of laser power,the tensile strength of the dissimilar joint decreased.When P>5.3kW,the liquefaction cracks defects appeared in the partial melting zone of the HAZ in 3D-GH3625 welding joint and the side of 3D-GH3625 in the 3D-GH3625/rolled GH3625 welding joints.The average grain size of 3D-GH3625 superalloy is relatively large.When laser welding(large laser power or welding speed),the small angle grain boundary of the HAZ is more,and the elements such as Nb and Mo segregate to the austenite grain boundaries in the partial melting zone of HAZ,promotes the formation of austenite grain boundary liquid film.In the end,under the action of welding stress,the liquefaction crack defect occurred in the partial fusion zone of the same type of joint HAZ and the partial fusion zone of the 3D-GH3625 side of the different type joint.The austenite grain boundaries of the HAZ of the rolled state GH3625 in 3D-GH3625/rolled state GH3625 welding joints are coarsening,and a large amount of MC exists in the austenite grain boundary in the partial melting zone,which pinned the grain boundaries and the liquefaction crack is restrained.