| Cold extrusion process has been used widely in the production of the automotive parts.However,conventional cold extrusion method may be not suitable for manufacturing the difficult-to-deform parts because of their large dimension,complex shape,high resistance to deformation and low formability.In order to change this situation and further develop the cold extrusion process,upon the background of National Foundation of Natural Sciences“Research for surface effect and formation mechanism on the metal forming process with high frequency chattering”,finite element analysis and surface effect study for the cold extrusion process with electric–hydraulic chattering were carried out.And an optimized electric-hydraulic condition,which can be used in cold extrusion process,was proposed to provide the automobile industry with technical support and services.In this paper,the main research works and achievements are as follows:(1)The mechanics analysis of the technology for the conventional and electric-hydraulic chattering backward extrusion process was performed.By using the principal stress method,the force equilibrium equations were built.The non-local friction theory presented by Oden was used in stead of the classical Column friction model.The calculation formulas for the friction force and extrusion load were given.The relation between frictional coefficient and sliding velocity proposed by Kpare?bck?? and the contact and separation mechanism were applied to quantify the effect of the electric-hydraulic chattering on the frictional coefficient,friction force and extrusion load.(2)A commercial software Deform-3D was employed to analyse the conventional and electric-hydraulic chattering backward extrusion process.The different chattering was applied on the ejector and container.The results showed that the higher instantaneous velocity and direction change of material flow were observed during the backward extrusion process with chattering.When the chattering velocity is less than the extrusion one,the extrusion load and friction force are similar to that of the conventional extrusion process.When the chattering velocity is more than the extrusion one,the extrusion load and friction force are reduced by the chattering.The smoother strain distribution was attained in comparison with that from the conventional extrusion method.(3)To carry out the electric-hydraulic chattering process,a special experimental setup was designed and prepared.Aluminum(6061)specimens were used in the backward extrusion tests.A comparison of the load requirement,material flow,surface finish and hardness of extrusion part between the conventional and electric-hydraulic chattering backward extrusion processes was given.The experimental results showed that an extrusion load reduction of 8.22% was detected in the electric-hydraulic chattering extrusion process if the ejector and container were subjected to the chattering with a frequency of 200 Hz and amplitude of 0.012 mm.The hardness of the extrude cup bottom and surface was increased from 191 HV and 204 HV to 427 HV and 387 HV.Moreover,a clear boundary of metal material flow line can be observed in the extrude cup bottom.A significant improvement in the surface quality and higher hardness of the deformed parts were attained.(4)In order to determine the frictional coefficient of the cold extrusion process with electric-hydraulic chattering,a tool for the double cup extrusion process was built.Cup height ratio for different electric-hydraulic chattering condition was obtained.Then based on the calibration curve of the frictional coefficient from the finite element analysis,the frictional coefficient between the billet and extrusion tools was given.In case of the conventional cold extrusion process,the frictional coefficient was about 0.167.In case of the electric-hydraulic chattering extrusion process with frequency of 100 Hz,200Hz and amplitude of 0.012 mm,the frictional coefficients dropped to 0.045 and 0.038 respectively.(5)Finite element analysis and experimental study for the cold extrusion process with electric-hydraulic chattering on the cup part were carried out.The experimental results showed that the maximum extrusion load was decreased by 5.1%,the surface roughness of Ra was reduced from 0.755?m to 0.503?m when electric-hydraulic chattering with oil pressure of 8MPa and vibration frequency of 200 Hz was superimposed on the lower die.The effect of the oil pressure was more significant than the vibration frequency on the reduction of the extrusion load,and was weaken after the chattering velocity reaching a higher value.The effective stress distribution of the deformation zone close to the container was shifted upward and downward due to the chattering.A significant improvement in the surface finish of the formed parts was observed using the chattering method.Moreover,the hardness distribution was uniform in comparison with that produced by the conventional extrusion process. |
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