Process And Numerical Simulation Of Temperature Field Of Friction Stir Welding With Auxiliary Heating

Friction stir welding(FSW),as a solid phase welding method,has been widely used in the connection of light alloy materials due to its environmental protection and high joint quality.Friction stir welding(FSW)is a complex thermo mechanical coupling process,and its temperature field has three-dimensional asymmetry.The weld bottom temperature of friction stir welding is low and the heat dissipation is fast,which will affect the weld forming and lead to a narrow process window.In this paper,the heating system is preset at the bottom of the weld to heat the plate.By heating the plate,the forming temperature at the bottom of the weld can be increased to promote the forming of the weld.In this thesis,the weld forming characteristics of conventional FSW process and P-FSW(preheating-friction stir welding)process with low heat input parameters are compared.The mechanical properties and microstructure of weld,welding force,thermal cycle curve and temperature field were analyzed.The P-FSW process has lower surface roughness and beautiful surface than the conventional FSW process,which can effectively bridge the defects such as holes and tunnels in the weld and promote the weld forming;Under the conventional FSW process,the hardness of the cross section is the shape of "W",and the grain size of the nugget is the smallest and the hardness is the highest,while the TMAZ are slightly lower.Under the P-FSW process,the hardness distribution is more average,and the hardness value is smaller than that of the conventional FSW process.The tensile strength and elongation of the weld under the P-FSW process are higher,reaching 89.1% and 62.5% of the base metal,respectively.The weld fracture of conventional FSW process is at the defect,while the fracture of P-FSW process is in the TMAZ,And the fracture dimples are dense,which is ductile fracture.The welding force of P-FSW process accords with the characteristics of general friction stir welding force curve.The axial force and forward resistance of P-FSW process are slightly lower than those of conventional process,and the lateral force is slightly higher than that of conventional process.The force curve of P-FSW process is also more stable than that of conventional process,indicating that the material fluidity is better than that of conventional process.The temperature field of friction stir welding presents "three-dimensional asymmetric nonlinear gradient characteristics".The weld bottom temperature of the P-FSW process is 30?higher than that of the conventional FSW process.The surface and middle temperature of weld is 80 ? higher than that of conventional FSW process.The high temperature residence time of weld feature points in P-FSW process is 2 ? 3 times of that in FSW process.The classical model can effectively predict the peak temperature of the weld stable section,but it still has some shortcomings in the aspects of the high temperature retention time and the "lag effect" of the peak temperature.The classical model is improved by adding the modeling of backing plate in the geometric model,introducing the instantaneous axial force and instantaneous volume into the heat source model,cutting the welding process in detail,and introducing the heat flow distribution coefficient on the shaft shoulder surface,so that the heat generation of the heat source is more accurate.The improved model is more accurate for the simulation of the temperature field of this test.The "lag effect" of the peak temperature and the phenomenon of heat accumulation,high temperature residence time and the overall characteristics of the curve are all in accordance with the measured data,and the model can also be applied to other welding process parameters under this condition.