| Ultrasonic-assisted friction stir welding has become one of the main welding methods in the manufacturing field at present,because of introducing ultrasonic vibration on the basis of conventional friction stir welding,which greatly improves welding efficiency and reduces weld defects.However,in the actual process of ultrasonic-assisted friction stir welding,the vibration characteristics of the ultrasonic vibration system are often affected by the external load due to the large downward pressure of the welding shaft,which leads to unsatisfactory welding effect.To solve this problem,this paper designs an ultrasonic-assisted friction stir welding vibration system with both load resistance and large amplitude output by adopting the multi-stage welding method,and through theoretical analysis and experimental tests,it is verified that the designed ultrasonic vibration system has good vibration performance and load resistance.The specific research contents and results are as follows:(1)By using analytical method and single factor analysis method,three common expressions of vibration characteristics of single horn with load are derived,and the curves of vibration characteristics of three single horns with load are drawn.The influence of load on vibration characteristics of three single horns is compared and analyzed,and it is concluded that the anti-load performance of conical horn is the best,the exponential horn is the second,and the catenary horn is the worst.(2)The influence of area ratio and processing materials on the anti-load performance of conical horn is analyzed by analytical method.On this basis,the design of multi-stage horn and the development of ultrasonic vibration system are completed,and the modal analysis is carried out by ANSYS.The results show that the anti-load performance of the vibration system is better when the area ratio of conical deformation horn is 1.8 and the processing material is 40 Cr,and the error between the resonance frequency of the vibration system and the theoretical design frequency is 3.7%.(3)The four-terminal network method is used to deduce the equivalent network of each section of the multi-stage horn,and then the equivalent network diagram of the whole multi-stage horn is established by the condition of continuous velocity at the joint end face.On this basis,through the circuit operation and the application of Kirchhoff theorem,the expressions of vibration characteristics of multi-stage horn with pure resistance and pure resistance load are derived,and the influence of load on vibration characteristics of multi-stage horn is analyzed.Theoretical analysis proves that the designed multi-stage horn has good anti-load performance and large amplitude output capability.(4)A test platform for vibration performance and anti-load performance of ultrasonic-assisted friction stir welding vibration system was set up,and the error between the actual resonance frequency and the theoretical design frequency was measured to be 3%.The output amplitude of the vibration system under different ultrasonic power supply ratios and the output amplitude and resonance frequency of the vibration system under different loads were tested,and the maximum output amplitude of the vibration system was 64 μm.When the load force was 4k N in the stable welding stage,compared with no load,The resonance frequency shifted by 513 Hz,and the amplitude decreased by 14%.At this time,the ultrasonic vibration system still has a large amplitude output of 55 μm.The experimental test proves that the designed ultrasonic vibration system meets the design requirements of both load resistance and large amplitude output.The research results of this paper have certain reference value for analyzing the influence of welding pressure on vibration characteristics of vibration system in ultrasonic-assisted friction stir welding,and are helpful to further promote and improve ultrasonic-assisted friction stir welding technology. |
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