Study On Plastic Softening Mechanism And Forming Properties Of Ultrasonic-assisted Incremental Sheet Forming Of Aluminum Alloy Sheets

As a flexible dieless forming technology,incremental sheet forming is suitable for small batch production and has been applied in aeropace,automotive manufacturing and medical field.Compared with the traditional stamping forming technology,the incremental sheet forming process is dieless,flexible,low-energy-consumption and has broad development prospects in the manufacturing field of complex thin-walled parts.However,its own characteristics also make the sheet lack of supporting,which is easy to cause uneven thickness and poor precision,limiting the industrial application of this technology.Ultrasonic vibration has been used for various plastic processses,and has the effect of improving the plastic deformation behavior of the material and reducing the forming force.To this end,the ultrasonic vibration energy was introduced into the incremental sheet forming,and the plastic softening mechanism of it and its influence on the forming performance were studied.Firstly,based on the ultrasonic vibration stress superposition effect mechanism,the influence mechanism of the cyclic loading and unloading of materials under the ultrasonic vibration field on the incremental forming process is explored.On the basis of simulating the incremental sheet forming process by the ANSYS/LS-DYNA software,the finite element simulation of the ultrasonic assisted incremental forming process was carried out.Among them,the sinusoidal displacement was superimposed to explore the effect of the stress superposition effect of ultrasonic vibration on the forming process.A series of ultrasonic-assisted point forming experiments were carried out.The softening effect caused by the stress superposition effect and its proportion in the ultrasonic softening effect were identified by comparison experiments and simulation results.When the forming depth is 5 mm,the forming force reduction rate caused by the ultrasonic softening effect is 37.07%,and the forming force reduction rate due to the stress superposition effect is 19.24%.By comparing the simulation and experimental results under different process parameters,using the material with high yield strength,large ultrasonic amplitude and large tool head diameter can make the sheet absorb more ultrasonic energy,thus effectively improving the stress superposition effect.The reduction rate of the forming force caused by the stress superposition effect increaess and the reduction rate of the total forming force is also significantly increased.Secondly,based on the acoustic softening effect mechanism of ultrasonic vibration,the influence mechanism of ultrasonic vibration on the mechanical properties of the material is explored,and the material theoretical model is constructed,which further improves the prediction accuracy of finite element simulation for ultrasonic assisted progressive forming process.Based on the material plasticity model framework,the thermal activation process and the dislocation evolution process were adjusted to consider the acoustic softening effect of ultrasonic vibration,and the material constitutive model under ultrasonic vibration field was constructed.The relevant parameters were obtained using neural network algorithms and based on experimental data.The stress-strain curve after applying ultrasonic vibration not only has a significant decrease,but also has the low slope.When the ultrasonic assisted incremental forming process was simulated,using the improved constitutive model can effectively reduce the simulation error.The overall error of the forming force with the depth variation curve is kept below 20%,which can effectively improve the prediction accuracy of the simulation.The stress-strain results obtained by finite element simulation was used to investigate the plastic deformation behavior of the material under ultrasonic vibration field.It is found that the application of ultrasonic vibration can effectively reduce the equivalent stress of the material,but the equivalent strain increases due to element distortion and sheet thinning.Finally,the coupling influence between the process parameters and the ultrasonic vibration on the forming force during the processing of the cone shape and the forming performance of the formed part after the processing is further explored.The influence of the ultrasonic vibration on the variation of the forming force in the whole and a single forming step was analized.It has been found that the application of ultrasonic vibration can effectively reduce the axial forming force and the transverse forming force during the incremental forming process,and reducing effect of the axial forming force is more significant.At the same time,the axial force reduction rate curve shows a certain periodicity.After that,the influence of the coupling of processing parameters and ultrasound on the ultrasonic softening effect was explored.It was found that the influence of the thickness on the ultrasonic softening effect was significant,mainly related to the load.Finally,the appropriate process parameters were selected to analyze the influence of ultrasonic vibration on hardness thickness and geometric accuracy.It is found that the application of ultrasonic vibration can effectively reduce the microhardness of the contact surface of the formed part and improve the thickness thinning of the formed part during processing.The thickness value can be increased by up to 2.35%.At the same time,the excessive bending deformation of the deformation transition region can be improved,and the geometric error is reduced,but when the forming angle is large,the sidewall of the forming part has a concave phenomenon,and the application of the vibration increases the concave deformation.In this paper,the influence mechanism of ultrasonic vibration stress superposition effect and acoustic softening effect on the incremental forming process is explored.In the finite element simulation,the superimposed sinusoidal displacement and the improved constitutive model were respectively applied,which further improves the prediction accuracy of the ultrasonic-assisted incremental form ing process.By forming the pyramid shape,the influence mechanism of the coupling effect between different processing parameters and ultrasonic vibration on the forming force during the machining process and the hardness,thickness and geometric precision of the formed part after machining is further explored.