Formability Evaluation And Parameters Optimization Of 6061 Aluminum Alloy Tubes

With the good properties such as better ductility,high specific strength,corrosion resistance,excellent welding performance and the outstanding resistance to stress corrosion ability,aluminum-magnesium-silicon 6061 aluminum alloy has played an increasingly important role in the areas of aerospace,nuclear power engineering,transportation and automotive industry.In order to study the hydroforming properties of 6061 aluminum alloy tube based on tube hydroforming technology,uniaxial tensile experiments were adopted to obtain basic mechanical properties of 6061 aluminum alloy tube,and test system of tube hydroforming properties was designed and developed in this paper,which could collect the real-time bulging height and bulging pressure during the forming procedure.Experiments were conducted to study the influence law of bulging length,pressurization rate,and initial thickness and heat-treat condition on tube hydroforming properties.Then,the initial strain strengthening coefficient and initial strain strengthening exponent can be parsed from the parabolic profile model with the bulging height and bulging pressure collected.At last,parameters adaptive optimization software was developed in Visual Basic 6.0,which enhanced the accuracy of parameters by the combination of computer programming software and the finite element simulation software(DEFORM-3D).Firstly,analytical model of the bulging tube was established according to parabolic profile model,which presents the mathematical relationship between the important parameters,such as: initial inner and outer diameters,bulging length,pressure,thickness of tube during bulging process and so on.The equivalent stress and equivalent strain were analyzed out based on the plane stress condition with the anisotropic yield criterion.After using logarithmic function to the equivalent stress and equivalent strain relationship referring to Hollomon formula,parameters expressions were obtained for the initial strain strengthening coefficient and initial strain strengthening exponent.Secondly,the mechanical property parameters of ultra thin-walled tubes of 6061-O-state,W-state,and thin-walled tubes of 6061-O-state aluminum alloy were obtained by uniaxial tensile test.It was found that elongation of thin-walled tubes of 6061-O-state aluminum alloy was better than the other two.Then,the free bulging die system was designed and its strength was verified according to the data.In addition,the hydraulic control system was designed to load pressure,feed horizontal punches,and collect the real-time internal pressure and the real-time internal bulging circumference,and with the security measures of differential pressure and overload protection.Then,the influence laws of different bulging spacing,compression speed,initial wall thickness and pipe heat treatment state on bulging experiment were studied and experiments were conducted.Experiment results showed that with the increase of bulging spacing,the bulging height curves and the burst pressure curves decreased generally;There were a rising trend in the bulging height curve and a decline trend in the burst pressure curve,along with the enlargement of pressurize rate;The bulging height curves of O-state tube overall higher than that of W-state tube,and the burst pressure curves lower than that of W-state of tubing;Thin-walled tube bulging height higher than the super thin-wall tube significantly,but its burst pressure curve bellowed the super thin-wall tube burst pressure curve.Experimental results exhibited a good regularity,which will provide important guidelines for mold design and process development in fields of aerospace,nuclear power engineering,automotive industry and so on.Finally,adaptive optimization system for tube forming parameters was developed in Visual Basic 6.0,combining with finite element simulation technology.In this system,initial K and n were worked out and inputted into pre-process for simulation.DEFORM-3D provided different ductile fracture criteria,and calculated the material damage until the damage reached the set value according to the ductile fracture criteria.The post-process documents captured by the system automatically.Then,initial K and n were amended through the comparison of simulation and experiment P-H curves.Parameters accuracy has been enhanced and the simulation results have been optimized after iterations.