Study On The Effect Of Pre-strain On 6063 Aluminium Alloy Tensile Mechanical Properties

Cold working is a common reinforcing method for metal materials.Torsion has multiple advantages over other cold-working methods.For example,it can strengthen materials more sufficiently and evenly without causing changes to the size and shape.A torsional cold-working is proposed as a major pre-strain method in this thesis to study the effect of different pre-strain treatment methods and parameters on tensile mechanical properties of 6063 aluminum alloy.The mechanism of this effect is also studied.The thesis is organized as follows:Firstly,original mechanical properties of 6063 aluminum alloy T1/T5 samples were tested through tension tests,torsion tests and compressive tests.We designed special test samples and derived various mechanical properties including yield strength,ultimate tensile strength,percentage total elongation,percentage reduction of area.These experiment results will be used to evaluate the improvement of mechanical properties of the pre-strained test samples.Furthermore,experiment parameters used in subsequent tension tests were determined based on the above experimental results.Secondly,different pre-strain treatments,including torsional pre-strain in various degrees,compressive pre-strain in various compressional displacements,compressive-torsion combination pre-strain and torsion-compressive combination pre-strain were imposed to several T1/T5 test pieces.Mechanical property changes were measured by subsequent tension tests.It was shown that the strength of test pieces improved significantly,but plasticity decreased compared to non-treated samples.The more torsion is applied,the more significantly the strength of test pieces improved and the more violently the plasticity declined.It was also shown that among combination pre-strain treated on test pieces,pre-torsion treatment played the most significant role in influencing test pieces’ mechanical properties.No significant differences between compress-torsion strain path and torsion-compress strain path were observed in impacting the mechanical properties of test pieces.Thirdly,the effect of plastic torsional deformation on mechanical properties of 6063 aluminum alloy were investigated through theoretical analysis and hardness tests.It was shown that the formation of yield strength gradient within pre-torsion test pieces is the main reason for the improvement of mechanical properties.Different torsional angles result in different gradient structures formed within pre-torsion test samples.The larger the torsional angle was imposed,the more improvement in strength was observed.Hardness tests also proved the existence of gradient structures.In the end,a finite element model was constructed to simulate the tension tests imposed to the test pieces to verify the findings.In the model,the yield gradient structure of pre-torsion test pieces was simulated by gradually changing yield strength of materials from inside to outside.The simulation results showed that the variation tendency of mechanical properties correlate with torsional angle,which agrees with that of experimental results.Hence,it is concluded that the formation of gradient structure within cylindrical test pieces after torsional deformation is the main reason of changes in mechanical properties of test pieces.