Development Of Vibration System For Ultrasonic Friction Stir Welding Of Aluminum Alloy And Research On Ultrasonic Mechanism

With the increasing performance of aluminum alloy,it has been widely used in rail transit,aerospace,ship and other fields,mainly for manufacturing lightweight structural parts.These aluminum alloy structural parts put forward higher requirements for weld quality and welding efficiency when welding.However,traditional welding methods such as MIG and FSW have been difficult to meet the requirements of green,high quality and efficient welding.By applying ultrasonic vibration to conventional friction stir welding,ultrasonic friction stir welding technology has significant advantages in improving weld quality and welding efficiency,prolonging the life of stirring head,etc.it is one of the most important methods to realize green,high-quality and efficient welding of aluminum alloy structural parts.In this paper,the mechanism of ultrasonic acoustic softening in ultrasonic friction stir welding is analyzed,the vibration system of ultrasonic friction stir welding is developed,the welding test platform is built,the change rules of welding temperature,axial pressure,stirring torque and weld surface integrity are studied.The main research content and results are summarized as follows:(1)According to the characteristics and needs of the ultrasonic friction stir welding process,combining with the maximum current search and phase-locked tracking technique of voltage and current phase information.A hybrid frequency tracking strategy suitable for rotary ultrasonic machining is proposed,and the frequency tracking program was compiled.The component modules of ultrasonic generator were designed,and the prototype preparation of ultrasonic power was completed.The output performance test results show that the prototype has good power output capability.According to the requirements of ultrasonic vibration system,the energy-transmission system and ultrasonic horn are designed,and the vibration performance is tested.The ultrasonic vibration system is integrated in X53k milling machine to complete the construction of welding test platform.(2)Through single factor comparative test,the variation rule of process parameters(including stirring speed and welding speed)on peak temperature of weld centerline,axial force and stirring torque under the two welding methods was studied.Using the same process parameters,the peak temperature of weld centerline,axial force and stirring torque under the influence of ultrasonic acoustic softening effect are lower than the measured values of friction stir welding with the same parameters,and the average decreases are 4.52%,12.59%and14.28%,respectively.Among them the peak temperature of weld centerline increases with the increase of the stirring speed,and decreases with the increase of the welding speed,while the axial force and stirring torque increase with the increase of welding speed and decrease with the increase of stirring speed.(3)The variation laws of surface roughness,surface morphology and micro-hardness of joint surface in the surface integrity indexes of ultrasonic friction stir welding and friction stir welding were compared and studied.The results show that:The application of ultrasonic vibration can significantly improve the quality of the weld surface integrity,and the average value of the weld surface roughness Ra at the initial and stable stages of welding decreases by56.61%and 55.45%,respectively.The weld is well formed without defects such as grooves and holes,the surface texture of the weld is even and delicate,and the average interval between the peaks and troughs drops by more than 80%.According to the"W"type hardness curve from the center line of the weld to both sides of the weld,the lowest point of the micro-hardness is located at the junction of the heat affected zone and the thermo-mechanically affected zone on the advancing side 5.2mm away from the weld centerline,and The lowest hardness value increased by 13.8HV_0.5 to 64.7HV_0.5,with an increase of 27.11%.