Numerical Analysis Of Linear Friction Welding Process Of Ni-based Superalloy

Nickel-based super alloy is widely used in the structure of aero-engine because of its good high temperature property.Linear friction welding is an effective method for connecting the nickel-based super alloy in the manufacture of Blisk.The temperature and stress evolution and the plastic material flow have a great influence in the quality of the joints.Thus,studying the evolution law of temperature,stress and plastic material flow of nickel-based super alloy is important to control the quality and the microstructure property of the joint.The results of welding experiment in different parameters(amplitude,friction pressure,forging pressure)suggest that the flash is closure when the heat input is high,while the flash is extruded independently for low heat input.Axial shortening increases with the increase of the amplitude and friction pressure.The residual stress of the welded joints are measured by blind hole method.The results indicate the residual stress is the lowest when the friction pressure is 400MPa and the amplitude is 2.92mm.The high speed photography and thermocouple temperature measurement are used to detect the forming of the flash and the welding thermal cycle.These experimental data provide sufficient validation for the numerical modeling of the linear friction welding process.Based on the thermo-mechanic coupling characteristic of the linear friction welding process,the numerical model is established by the finite elements simulation software Deform.According to the hot compressive data of the experimental material,the constitutive relationship of the material is established.During the numerical modeling process coulomb friction and shear friction are all taken into consideration.The correctness of the model is verified by the temperature curve of the joint,the shape of the flash and the amount of axial shortening.The calculation results of the temperature field of the linear friction welding show that the temperature field distribution of the friction interface in the initial friction stage is not uniform and alternately appear at both ends along the vibration direction.The friction interface temperature tends to be consistent during the transition phase,and the high temperature region is generated from the center.Steady friction stage interface temperature remains unchanged.At the initial stage,there is no plastic metal flow.When the friction interface material reaches the plastic state,two flow field of material with flow center will be formed on the friction interface.The plastic material of the flow area flows from the center point to the surroundings,finally the material flow Field like the shape of "X".Any point in the workpiece is forming the flash with the extrusion of the plastic material,the velocity of the material in X,Y,Z direction will change.The stress evolution of linear friction welding is not only affected by the temperature and plastic materials,but also affected by welding pressure.The stress evolution is affected by the alternating force and varies periodically with time.At the beginning of the friction,the high stress region alternately appears at both ends of the workpiece,and continuously a central high stress region is formed.The stress is gradually released from the top of the workpiece to the center.After the forging pressure is removed,the overall pressure in the joint is reduced,and the low stress area is still extended from the middle part of the workpiece to the two ends in the vibration direction.The compressive stress and tensile stress are present in different directions of the joint area during the whole welding process,and the compressive stress is always in the Z direction(the direction of the friction pressure).The stress in the X and Y directions is affected by the welding time and the welding area.With the increase of welding time,the tensile stress in the central area increases and the compressive stress decreases at both ends.Different welding process parameters have great influence on the stress variation law.During the cooling process,the higher the friction pressure is,the higher the stress level is.The effect of amplitude variation on the stress evolution is relatively low,and the stress level of the joint is the lowest when the amplitude is 2.92mm.Different pressure maintaining time mainly affects the stress change perpendicular to the vibration direction,which has little effect on the stress change in the other two directions.