Study On Microstructure And Fatigue Crack Propagation Of GH4169 Superalloy Electron Beam Welded Joints

In this paper,GH4169 nickel-based superalloy was used as material,and the fatigue crack propagation of welded joint was studied by electron beam welding.The microstructures of welded joints under different electron beam,scanning waveform and heat treatment were analyzed by metallographic microscope(OM).The tensile deformation process of weld was analyzed by scanning electron microscopy(SEM)and three-dimensional speckle dynamic strain gauge(XTDIC).The fatigue crack growth rate of weld was analyzed by fatigue crack tester.The main results of the study are as follows:Beam 29 mA,can get the best shape of the weld,forming a beautiful no obvious defects,joints cross-section was wide and narrow on the nail shape.The electron beam flow increases,the penetration depth increases significantly,and the aspect ratio increases.The addition of the scanning waveform is easy to cause the weld surface is not welded,the joint cross section in the lower part of the bending phenomenon,was asymmetric.The heat-affected zone of the welded joint is small and the upper part is ?-phase grown to the heat source.At the same time,with the formation of secondary dendrites,the growth pattern of the central tissue grows from columnar growth to columnar growth until the equiaxed grains grow.Crystal arm.The electron beam has little effect on the microstructure evolution,and the scanning waveform changes the ? phase orientation distribution and reduces the number of secondary dendrites,the grain refinement of the weld after welding,and the decrease of the number of long dendrites.The hardness of the joint is high and the weld is low.The electron beam flow increases and the weld hardness increases.Adding a scan waveform does not increase weld hardness.Heat treatment is to enhance the joint hardness of the most effective means of heat treatment after the parent metal hardness increased by 96 HV,weld hardness increased by 111.5HV.Under the optimum process parameters,the tensile strength of the welds reached 96.3% of the base metal.After the heat treatment,the tensile strength of the base metal and the weld was increased by 281 MPa and 275 MPa,and the elongation was 0.36 and 0.31.DIC test results show that the heat treatment base metal and weld deformation fracture in different ways.Heat treatment The base material sample center forms the maximum strain field without offset.The center of the grain elongation serious deformation,crack initiation in the center,toward the expansion of the last break occurred.The initial maximum stress field of the heat treatment weld is distributed in the upper left corner of the specimen,then shifted to the right side of the sample,the overall strain field decreases from right to left,the right grain is elongated and deformed first,The other side of the expansion of the final break.Heat treatment weld fracture more heat treatment base material fracture dimple shallow,plastic worse.After welding heat treatment weld fracture fracture failure,porosity and other defects.GH4169 alloy base material,the best parameters of the weld,S-scan weld and circular scanning weld fatigue crack propagation in line with the Paris formula.The crack resistance of different welds is better than that of the base metal,and the optimal parameters are the best.It is estimated that the crack initiation threshold of the base metal and the different welds is better than that of the different welds.Adding a circular scan waveform helps to improve the crack initiation capability of the weld.The fatigue crack propagation fracture of GH4169 alloy is divided into crack initiation region,crack propagation zone and transient fault zone.At the beginning of the expansion there is a small step of cleavage,quasi-cleavage fracture.In the middle of the expansion there is a significant fatigue band formation,accompanied by tearing edges.The extension of the fast zone is the tearing ribs,holes and secondary cracks composed of microporous aggregated dimple fracture.The crack source of the base material is sprouted in the middle area,and the crack source is mainly produced in the near surface inclusions and welding defects.Optimum parameters There are significant secondary cracks in the weld fatigue zone,which helps to release the crack tip energy and improve the crack propagation resistance.There are more defects in the crack propagation fracture of the circular scanning weld,the crack propagation rate is fast and the fatigue band is very shallow.