Research On Axial Hydro-forging Of 6063 Aluminum Alloy Tube With Variable-diameter

Tube hydroforming technology is an advanced light-weight forming technology,which is widely used in the manufacturing of hollow components with variable cross-sections in the fields of automobile and aerospace.However,thinning and non-uniform thickness distribution are unavoidable during hydroforming process because of the expansion deformation of the process,which would lead to fatigue failure of components in engineering application.In order to solve these problems,the axial hydro-forging process was proposed,in which the axial compression deformation was combined with hydrforming.Its core idea is that the compression deformation was generated by applying an axial displacement to decrease the length of the tube under the supporting of internal pressure,and the circumferential rigid constraint was applied using the rigid die to increase the thickness.In this paper,6063 aluminum alloy was studied as the base materials and an experimental setup was designed and assembled for axial hydro-forging.The theoretical analysis,the finite element simulation and the experiment were used to nvestigated systematically the plastic deformation behavior,the thickness distribution as well as wrinkling mechanism of aluminum alloy tubes,so as to provide theoretical gui dance for the application of the axial hydro-forging process.The deformation sequence and the mechanism of decrease of thinning ratio in axial hydro-forging process were revealed through analyzing the effective stress of different deformation zones using the plane strain yield criterion.In hydro-forging stage,the stress state of different deformation zones demonstrates the effective stress always has the biggest value at the maximum diameter zone and has its lowest value at the guide zone,thus,the deformation sequence of the tube in hydro-forging stage is that maximum diameter zone > transition zone > guide zone.This demonstrates the plastic deformation occurs firstly in the thinning zone of the tube and then extends towards the other zones.The theoretical analysis proves that the thinning ratio of the tube can be decreased by the axial hydro-forging process.The typical defects and failure modes of the tubes during the axial hydro-forming sequence were given through experiments.For corner wrinkle appeared at the corner between the guide zone and transition zone in corner filling stage,an axisymmetric mechanical model was established to reveal the mechanism of wrinkling behavior and analyzed the relationship between the corner wrinkle and corner filling internal pressure.In corner filling stage,when the horizontal force cause by the bending deformation of the corner zone is greater than that under the internal pressure and friction force,corner wrinkle will be emerged.For sidewall wrinkle appeared at the maximum diameter zone,a critical wrinkling model of the tube was builded under the constraint between internal pressure and the rigid die.The effects of the parameters(such as tube radius,wall thickness,the ratio of tube diameter to thickness,and the mechanical properties of the tube)on the critical wrinkling internal pressure are analyzed.The critical wrinkling loading path for hydro-forging stage was achived through theoretical analysis and finite element simulation and verified by the experiments.When the internal pressure is greater than the critical wrinkling internal pressure,the wrinkle will be suppressed and a stable axial compression deformation can be realized.The regularity of thickness distribution during the axial hydr-forging sequence was investigated.The thinning ratio can be significantly reduced by axial compression deformation in hydro-forging stage,and the maximum thinning ratio oftube with variable-diameter with expansion ratio of 30% can be reduced to less than 6%.Meanwhile,the serrated thickness distribution caused by useful wrinkle was eliminated and the uniformity of the thickness distribution was improved by axial compression deformation in hydro-forging stage.Furthermore,the reason for the unaltered thinning ratio at the transition zone and barrelling defect appeared at the end of the maximum diameter zone in hydro-forging stage were ravealed through mechanical analysis and finite element simulation.The normal concentrated force can significantly decrease the effective stress in the corner segment of the transition zone by increasing the absolute value of the radial stress under the triaxial compressive stress state,which make the condition of the plastic deformation in the corner segment more difficult to satisfy,and the material will not flow into the corner segment and the transition zone.Then the material in the end of the maximum diameter zone undergoes inhomogeneous deformation and lead to the barrelling defect.Based on the analysis,a new improvement approach was proposed to abate the defects.The mechanical condition of the transition zone is changed by improving corner radius,and then the thickness of the whole deformation zone will be uniformly increased during the hydro-forging stage.An experimental setup for the multistage axial hydro-forging process was developed to forming the double-stepped tube with large expansion ratio.During the process,the formability of the tube can be effectively increased by the increase of the thickness usin g axial compression deformation,and a double-stepped tube with maximum expansion ratio of 70% was obtained by the multistage axial hydro-forging process.The mechanism of the improvement of the corner filling in axial hydro-forging process was revealed.In coener filling process,the unfilled corner segment was bent to fill the die corner under the internal pressure and axial compression force,and then relative limiting corner radius Rlim/t can be less than 1.Moreover,the change rule of the profile shape during the axial hydro-forging process were analyzed by the finite element simulation and experimental investigation.The results show that this process can decrease the springback and improve the dimensional accuracy of the tube.In hydro-forging stage,the axial stress distribution at the wave crest and trough of the tube is changed by the axial compression deformation and the tensile stress of the inner layer of the wave crest and the outer layer of the wave trough is changed into compressive stress.The difference of the axial stress along the radial direction of the tube is decreased,then the bending moment is reduced and the springback is reduced.