Design Of Closed-loop Control System For Axial Force Of High Rotation Speed Friction Stir Welding And Optimization Of Process Parameters

High speed friction stir welding has a wide application prospect in aluminum alloy sheet welding because of its high heat input and low top forging pressure.Thin plate is easy to deform in the welding process,which requires higher control precision.The deformation of mechanical structure leads to the actual pressure under the shaft shoulder which is not easy to control.All these will cause the change of axial pressure in the welding process.In this paper,the HRSFSW axial force control system is studied to maintain the stability of axial force in the welding stability stage.In this thesis,the HRSFSW axial shoulder pressure and axial pressure model is established by system identification method,and the model fitting degree is 80.7%.Three axial pressure control methods,PID,Fuzzy PID and Fuzzy Neural Network PID,are designed,and these three algorithms are simulated and analyzed by MATLAB.The optimal control algorithm is fuzzy neural network PID control.Fuzzy neural network PID control step response overshoot is 1.3%,the adjustment time is 0.46s,rise time is 0.22s.Based on fuzzy neural network PID algorithm,a set of FSW axial force closed-loop control equipment was designed to realize the closed-loop control of axial force.At the same time,the touch screen human-computer interaction interface was designed to monitor the change of axial force parameters in the welding process in real time and record the historical welding data.Experimental results of axial force control show that the control system has a high control accuracy of 3.05%and good stability,which can ensure that the fluctuation rate of welding axial pressure near the target value is small.Under the pressure closed-loop control,the welded joints have beautiful shape,no obvious welding defects,and excellent mechanical properties.The average tensile strength of the welded joints reaches 239 MPa,which is about82.4%of the base metal.Compared with the average tensile strength of the welded joints under the open loop control,the average tensile strength of the welded joints is increased by4.7%,which is 228.2 MPa.Based on the axial force control system,the response surface test design was carried out,and the influence of the welding process parameters on the welded joint quality under the axial force control system was analyzed.The relationship model between tensile strength and welding process parameters was established.The P value of the model was<0.0001,the significance of the model is strong,R² is 0.9883,the model fitting degree is good.The order of influence on the tensile strength of HRSFSW joint is welding speed,axial pressure and spindle speed in descending order.The interaction between welding speed and spindle speed is the most significant.The optimal process parameters predicted by the model are the spindle speed of 12060 rpm,welding speed of 846 mm·min^-1,axial pressure of 788 N,and the predicted tensile strength of 242.5MPa.The welding test is carried out according to these parameters,and the measured tensile strength of the weld is 241.8MPa,with a difference of0.7MPa.It is proved that the application of response surface method to optimize the process parameters of friction stir welding is effective.