| CFRP/aluminum stacks have been widely used in the field of aviation manufacturing because of their excellent mechanical properties,such as lightweight and structural strength.In the process of large aircraft manufacturing and assembling,1.5 to 2 million connecting holes need to be drilled in the components.According to the statistics,70% of aircraft body fatigue failure originates from structural connections,and more than 80% of fatigue cracks occurs at the connection holes.Therefore,improvement of hole drilling efficiency and processing quality is the key to ensure aircraft performance and reliability.Compared with manual hole drilling,robotic drilling has the advantages of high flexibility,high quality consistency and high phase accuracy.In recent years,the use of robots to machine holes in aircraft parts has attracted much attention in aviation manufacturing enterprises.However,due to the weak rigidity of industrial robots and the difficult processing of stacks materials,the robotic drilling system is prone to processing instability,which seriously restricts further improvement of drilling quality and efficiency.This study proposes a new method that combines rotary ultrasonic machining technology with robotic drilling(RRUD)to achieve high efficiency,high quality and high stability drilling.The main research contents and innovations include the following aspects:1)A modeling method for whole process of thrust force of the CFRP/aluminum stacks during robotic rotary ultrasonic drilling is proposed.Based on the drilling characteristics of robotic drilling and CFRP/ aluminum stacks,the effect of weak stiffness of robot and drilling position of CFRP/ aluminum stacks on the axial force is analyzed.Through the kinematic characteristics analysis of the tool tip during rotary ultrasonic drilling process,the effect mechanism of cutting edge trajectory change on the cutting layer thickness is investigated,and a theoretical calculation model is established.At the same time,the force models of the main cutting edge and chisel edge when cutting CFRP and aluminum are established based on the micro-element method.Based on the processing characteristics of each stage in stacks drilling,the axial force prediction models for different drilling positions are respectively constructed.Thrust force prediction for whole process in RRUD of CFRP/aluminum stacks is realized.Finally,the drilling process of stacks is analyzed through RRUD experiments,the variation law of axial force in each stage of drilling is clarified and the accuracy of the theoretical model is verified.The results of drilling verification experiments show that the average prediction error of thrust force in the whole process of RRUD of CFRP/aluminum stacks is 10.59%,and the thrust force model has high prediction accuracy.At the same time,the prediction errors of thrust force in stage 2 and stage 4 of stable drilling CFRP and aluminum are only 7.42% and 8.01%.The whole process model has high prediction accuracy,which lays a foundation for the study of CFRP hole wall damage,aluminum hole wall residual stress,exit burr and cutting parameter optimization.2)The suppression mechanism of ultrasonic vibration on drilling chatter of weak stiffness robot is revealed.From the perspective of RRUD processing stability analysis,in accordance with the chatter problem of weak stiffness for robotic drilling system,a dynamic model of RRUD system based on robotic modal characteristics,circumferential variation characteristics of axial cutting thickness in RRUD process and material properties is established.Combined with the frequency domain analytical method and the stability domain lobe expansion analysis,the suppression mechanism of ultrasonic vibration on robotic drilling chatter is revealed.At the same time,the drilling verification experiment is carried out for the stability lobe diagram.The simulation results show that the introduction of rotary ultrasonic vibration plays an important role in suppressing the chatter of robotic drilling.Compared with robotic conventional drilling(RCD)without ultrasonic,the stability region is effectively expanded when RRUD is used,and the expansion degree increases gradually with the increase of ultrasonic amplitude.The verification experiment results show that under the same drilling parameters,the processing state in RRUD is more stable than that in RCD,the aperture deviation after RRUD processing is significantly better than RCD processing,and the maximum reduction of aperture deviation is more than 60% due to the introduction of ultrasonic vibration.3)A manufacturing method of hole wall compressive stress based on ultrasonic vibration is proposed.Firstly,based on the research on the thrust force of RRUD of stacks,the press mechanism of twist drill bit on the hole wall is analyzed,and the theoretical analytical models of cutting force and cutting heat for cutting edge fillet are constructed.Secondly,according to the elastoplastic mechanics,stress release and equilibrium,a theoretical model of residual stress considering the joint influence of drilling temperature and cutting force is established.Finally,the accuracy and rationality of the model are verified by the experiment of residual stress in robotic drilling with or without ultrasonic vibration.The experimental results show that the residual stress of the hole wall changes from tensile stress to compressive stress under the action of ultrasonic vibration,which is caused by the high-frequency press and impact effect of ultrasonic vibration on the hole wall.The ultrasonic amplitude has a significant effect on the residual stress,when the ultrasonic amplitude changes from 4μm to 14 μm,the increase of residual compressive stress on the hole wall reaches173.1%.The analysis of residual compressive stress provides theoretical support for the subsequent study of riveting fatigue life of stacks.4)The improvement mechanism of ultrasonic vibration on hole wall quality and exit burr is revealed.Firstly,based on the influence mechanism of ultrasonic vibration on fiber cutting angle,the removal mechanism of high frequency ultrasonic vibration on CFRP material and the improvement mechanism of machining defects are clarified.Secondly,combined with the burr formation theory,thin plate deformation theory and energy conservation law,theoretical prediction model of burr height including robot drilling parameters,ultrasonic machining parameters and material properties is constructed and the improvement effect of ultrasonic vibration on the exit burr of stacks is proposed.At the same time,the numerical simulation method is used to dynamically simulate the burr formation process of robotic drilling.The stress distribution characteristics of burr formation are consistent with the burr formation mechanism,which verifies the rationality of the theoretical prediction method of burr height.Finally,a comparative experimental study with and without ultrasonic robot drilling is carried out.The results show that ultrasonic vibration is helpful to improve the scratch area and depth of CFRP hole wall,as well as the fragmentation and delamination of hole exit edge.The suppression effect of fiber cutting angle change caused by ultrasonic vibration on machining defects is verified.The experimental results of burr height show that the variation trend of the theoretical prediction value of burr height model is in good agreement with the experimental value,and the average relative error is only 6.3%.The introduction of ultrasonic vibration in robotic drilling can effectively reduce the burr height,and the maximum reduction of burr height reaches 41.2%.5)The application verification research of robotic rotary ultrasonic drilling and fatigue life of stacks are carried out.Firstly,prediction of riveting fatigue life of laminated connectors is studied by comprehensively considering the influence of riveting residual stress and drilling residual stress.Secondly,the optimization of machining parameters is conducted according to the prediction models of cutting force,stability region,burr height and residual stress.At the same time,based on the optimal parameter combination,the tensile fatigue experiment of riveted joints of CFRP/aluminum stacks is carried out to verify the improvement effect of RRUD on the service performance of CFRP/aluminum stacks.The fatigue test results show that the hole wall during RRUD has stronger static bearing capacity compared with conventional drilling,and the effect of ultrasonic vibration increases the static tensile strength of riveted stacks by 12.7%.In terms of tensile fatigue strength,the fatigue strength of single hole riveted connectors increases by more than 68%,and the fatigue strength of 4 hole riveted connectors arranged according to aviation design standards increases by more than 86%.Finally,the RRUD process is carried out on typical aviation parts,and the practical application of RRUD process in aviation manufacturing enterprises is realized.The application verification research of robotic rotary ultrasonic drilling lays a foundation for the application of robotic rotary ultrasonic machining technology in the field of aviation manufacturing. |
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