Study On Materials Deformation Behaviors And Ultrasonic Acting Mechanisms Of Ultrasonic Vibration Assisted Mg/Al Alloys Metal Forming

Ultrasonic vibration assisted plastic forming technology is a novel metal processing method in which a vibration of high frequency is superimposed on the workpiece or the tooling which can influence greatly on the plastic deformation behaviours of metals.Compared to the conventional metal forming process,ultrasonic vibration assisted metal forming process can reduce the forming load and the friction between the workpiece and the tooling,resulting in the improved plastic deformation capacity and surface quality.Many studies regarding on the propagation of the ultrasonic vibration in plastic deformation and the effects of ultrasonic vibration on the materials' performances had been conducted.However,the focus was mainly on the change of deformation force caused by ultrasonic vibration.The systematic studies on the impact of ultrasonic vibration on materials' deformation behaviors and mechanisms was still lacked,and the constitutive relationship was not established quantificationally and accurately.Thus,comprehensive investigations about the physical mechanism of ultrasonic vibration in the metal forming process,and constitutive modeling under ultrasonic vibration,have important theoretical and practical significances for promoting the engineering application of this promising technology.In this study,the action mechanisms of the ultrasonic vibration on the deformation behaviors of A2Z31 magnesium alloy and 6063 aluminum alloy are studied systematically via numerical simulations and process experiments.The influence of ultrasonic vibration condition on softening effect,hardening effect,deformation characteristics,friction condition,surface quality,failure mode and the microstructure are investigated in detail.Based on the experimental and numerical simulation results,the constitutive equations of the materials under ultrasonic vibration are constructed by using phenomenological model and crystal plasticity model respectively,which can describe the effect of ultrasonic vibration on the stress-strain relationship quantificationally and accurately.The main contents and conclusions in this study are as follows:The tensile tests were conducted for AZ31 magnesium alloy and 6063 aluminum alloy under static and vibrated loading cases.The stress-strain relationship,area reduction and elongation ratios,fracture modes of tensile specimens,microstructure and microhardness under different vibration conditions were analyzed,in order to study the effect of the ultrasonic vibration on microstructure and performance of these two alloys under tensile deformation.In addition,the influence of vibration on the dislocation evolution and motion in the forming process was investigated.The conventional upsetting(C-upsetting)of cylinder and ultrasonic vibration assisted upsetting(UV-upsetting)experiments of AZ31 magnesium alloy and 6063 aluminium alloy were conducted.Different deformation speeds,vibration frequencies and amplitudes were considered to investigate their influence on the softening effect and hardening effect.Then the ultrasonic vibration-assisted ring upsetting process of Al alloy with different lubricants was performed to explore the effect of ultrasonic vibration on the interfacial friction between the sample and the toolhead.The change in friction coefficient was analyzed according to the profile dimensions of the compressed specimens under different cases.The surface quality was also investigated by analyzing the variation of surface hardness,roughness,and topography.The quantitative relationship between surface effect and vibration parameters was obtained,and the"surface effect",variation with deformation process were revealed.The influence of vibration amplitude and frequency on the yield strength,hardening coefficient and exponent were analyzed quantitatively on the basis of the stress-strain curves obtained from ultrasonic vibration assisted upsetting experiments at room temperature.The constitutive equation of AZ31 magnesium alloy and 6063 aluminium alloy under ultrasonic vibration were constructed based on the Perzyna empirical model and Johnson-Cook model respectively,gaining the stress-strain relationship affected by the ultrasonic vibration quantitatively and accurately.Vibrated tensile and upsetting processes of AZ31 magnesium alloy and 6063 aluminium alloy were simulated using ABAQUS based on the experimental stress-strain curve,taking into account the volume effect and surface effect.The influence of ultrasonic vibration parameters on the physical field distributions,such as material flow,strain,deformation nonuniformity,stress inside and on contact surface was analyzed.The functionary mechanism of the ultrasonic vibration on the material flow and forming properties were revealed.Based on the theory of thermally activated dislocation motion and the mechanism of dislocation damping,a crystal plasticity constitutive model of materials under ultrasonic vibration was established,which can reveal the internal mechanism of the softening effect.The effects of evolution of dislocation density,stress,strain,ultrasonic vibration energy on the dislocation motion and proliferation were considered in this model.The model coupled the stress superposition,acoustic softening and acoustic hardening of ultrasonic vibration,the physical mechanism of acoustic softening and acoustic hardening is revealed from the microscopic scale.The effect of ultrasonic vibration on riveting process was analyzed via experiment and numerical simulation.By comparing the load-displacement curves under different experimental conditions,the influence of ultrasonic vibration on riveting force was analyzed.The dimension of riveted nail rod was measured and the influence of ultrasonic vibration on riveting interference was analyzed.The influence of ultrasonic vibration on the material flow and microstructures in the head and rod during riveting process was also investigated.