Numerical Analysis of Thermo-mechanical Coupling Process in Linear Friction Welding of Ni-based Superalloy

Author:Geng Pei Zuo

Supervisor:zou zeng da qin guo liang


Degree Year:2019





As one of solid state welding technologies with excellent quality and high efficiency,linear friction welding(LFW)is mainly applied to welding of the same or dissimilar materials with noncircular section,and it is the key manufacturing technology of blisk in aeroengine at abroad.At present,with the development aeroengneering in China,LFW process was explored for the manufacturing of blisk,which is in urgent need of the support of basic theoretical research.By being taken LFW of nickel-based superalloy as the objective,based on the material mechanical and thermophysical properties and the physical movitation in welding process,a transient mathematic model is founded by using the coupled thermo-mechanical finite element method,which accords with the physical nature of LFW.In the numerical model,the transient friction heat is calcaluated from the friction shear force based on the modified Coulomb friction model,and friction heat and plastic deformation work are used to describe the total heat input in LFW.Based on the strain-compensated Arrhenius constitutive model established by thermal simulation test and the plastic/plastic friction pair model,a transient mathematical model is established to study the physical nature of LFW.The reliability and accuracy of the developed numerical model of LFW have been verified,and the results indicate that the developed model is reliable and accurate,which could be used to study the thermo-mechanical coupling behavior in LFW process.Based on the developed LFW model,the characteristics of the heat generation,thermal process,stress/strain field and plastic flow in and close to the friction interface during LFW of Ni-based superalloy GH4169 are studied.Owing to the periodic alternating friction force,both interfacial heat flux from friction motion and volumetric heat generation rate from plastic deformation show the periodic fluctuation characteristic.During one oscillation cycle,the friction heat reaches the peak value and plastic defomation heat is minimum at the alignment moment,while the plastic deformation heat reaches the peak and the friction heat is zero at the moment of maximum amplitude.At the welding parameters of 400 MPa friction pressure,25 Hz frequency and 2.9 mm amplitude,the peak value of heat flow rate on the central zone of friction interface reaches up to 6.0±0.5 MW/m2 at equilibrium phase.During one oscillation cycle,the plastic deformation work fluctuates slightly at the center of the friction interface,ranging from 0 to 160 MW/m3,and there are two peaks,while the plastic deformation heat fluctuates greatly near the edge of the oscillatory direction,ranging from 0 to 320 MW/m3,with only one peak.The total heat generation power at equilibrium phase is about 1.6 kW,and the powers from friction and plastic work are about 1.45 kW and 150 W,respectively.During equilibrium phase,the total heat power generated by friction and plastic work reaches the steady-state.The percentage of plastic deformation heat to the total heat input is about 4.8%Being affected by the periodic frictional heat generation,the interfacial temperature also shows a periodic fluctuation characteristic.At the given welding parameters,the fluctuated amplitude of peak temperature at equilibrium stage is approximately 30℃ and the maximum of peak temperature reaches up to 1230℃.As friction pressure increases from 200 MPa to 500 MPa,the peak temperature of interface decreases from 1280℃ to 1202℃.As frequency increases from 30 Hz to 50 Hz,the peak temperature of interface increases from 1241℃ to 1281℃ and as amplitude increases from 2.5 mm to 3.7 mm,the peak temperature of interface increases from 1211℃ to 1247℃.The degree of non-uniformity temperature distribution at the interface can be reduced at high oscillation frequence,while large amplitude can intensify the uneven degree of temperature distribution.Plastic strain of friction interface at initial phase belongs to the pure shear deformation mode.Then,the compressive strain initially occurs at the permanent contact interface at transition phase while the shear deformation mode predominates the strain of friction interface.As the process reaches the equilibrium phase,the plastic strain belongs to the hybrid mode of compressive and shear deformation modes.The distribution of internal stress field on both sides of joint interface varies periodically and alternately.At the equilibrium phase,the largest fluctuated amplitude of interfacial stress along the direction parallel to oscillation direction appears at the edge of interface.The maximum and minimum stresses are 220 MPa and 62 MPa,respectively.The fluctuated amplitude near the center is smaller.The maximum stress is 146 MPa and the minimum is 126 MPa.Plastic flow velocity component along the oscillation direction exhibits an obvious periodic fluctuation.and the higher value appears the zone close to the edge of interface.During process,the extrusion of interfacial material mainly occurs as the amplitude being maximized and the change of friction force direction hindles the material extrusion as the amplitude being decreased from maximum.The maximum instantaneous plastic flow velocity at equilibrium phase approximates to 40 mm/s.The overall level of flow velocity increases with the decrease of friction pressure and the increase of oscillatory parameters.The amount of deformation to remove the original friction interface increases at high friction pressure.While high frequency or amplitude make the extrusion out of material at the original interface in a shorter time with less workpiece deformation.The characteristics and physical nature of thermo-mechanical coupling during LFW process were studied by numerical simulation,which not only can provide the theoretical foundation for the establishment and optimization of process parameters window,the control of the joint microstructure and performance and the prevention of weld defects,but also can improve the development of the manufacturing technology of blisk of aero engine in China.