Study On The Influence Of Grain Size And Strain Rate On The Mechanical Properties And Deformation Behavior Of Copper

The manufacturing requirements of micro-parts have been increasing in recent years.The development of traditional quasi-static micro-plastic forming processes has been more mature.Dynamic micro-plastic forming processes such as laser forming and electromagnetic forming have been emerged.The size effect and strain rate effect of the mechanical properties and deformation behavior of material will be more significant as feature size decreases and loading rate increases,which will further affect the implementation of forming process and the forming quality of the micro-parts.In this paper,T2 copper was used as the experimental material.Micro-tensile and micro-compression tests under quasi-static conditions and split Hopkinson pressure bars(SHPB)test at high strain rate were conducted.The influence of feature size,grain size and strain rate on the mechanical properties of T2 copper was analyzed.The material constitutive model considering grain size effect was established based on the true stress-strain curves obtained from the above three tests.The applicability of the constitutive model was verified by quasi-static micro-bending and laser shock micro-bulging experiments.The main conclusions of this paper are as follows:(1)The quasi-static micro-tensile test was conducted using copper foil/plate with a thickness of 100?500 ?m.The influence of the initial grain size and foil/plate thickness on true stress-strain curve and mechanical properties such as yield strength and elongation was studied.The result shows that as initial grain size decreases,the elongation increases and a size effect of "the smaller.the stronger" is shown on the yield strength and flow stress.The relationship between initial grain size and flow stress obeys the Hall-Petch rule.As foil/plate thickness decreases,the elongation decreases;a size effect of "the smaller,the stronger" is shown on the yield strength and a size effect of "the smaller,the weaker" is shown on the flow stress.However,the flow stress will increase when foil/plate thickness is reduced to only one or two grains along the thickness direction.(2)The quasi-static micro-compression test was conducted using a micro cylinder copper specimen.The influence of initial grain size on true stress-strain curve was studied.The result shows that as the initial grain size decreases,a size effect of "the smaller,the stronger" is shown on the yield strength and flow stress.The relationship between initial grain size and flow stress obeys the Hall-Petch rule.(3)SHPB test with strain rates between 1500 and 4000 s-1 was carried out.The influence of strain rate and initial grain size on true stress-strain curve of copper materials was investigated.The result shows that the surface morphology of specimen after SHPB test turns rougher with the increase of initial grain size and strain rate.The flow stress at the same strain increases with the decrease of initial grain size.The relationship between the initial grain size and flow stress obeys the Hall-Petch rule.The true stress-strain curve is not sensitive to the strain rate in the range of 1500 and 4000 s-1 but improves significantly compared with that in quasi-static micro-compression test.The improvement of the flow stress decreases with the increase of the strain until it is stable.The ultimate improvement,which can reach 70?78%.increases with the increase of the initial grain size.(4)Two constitutive models in the form of power function and exponential function were established according to the true stress-strain curves obtained by quasi?static micro-tension,quasi-static micro-compression and SHPB test.The initial grain size factor was introduced to both two models to consider the grain size effect.The result shows that the constitutive model in power function form is in good agreement with the results of quasi-static micro-tension and SHPB test,but not suitable for quasi-static micro-compression test.The constitutive model in the form of exponential function has a good applicability for quasi-static micro-tensile,quasi-static micro-compression and SHPB test.and it has a closer correspondence with the test results than the constitutive model in power function form.(5)The applicability of the constitutive model established in this paper is verified by quasi-static micro-bending and laser shock micro-bulging experiments.The grain size effect in quasi-static micro-bending and laser shock micro-bulging process is studied combined with FEA technology.The result shows that for the quasi-static micro-bending process,there is only a little difference between the calculated results using constitutive models in the power function form and exponential function form.Both models can accurately predict the bending height under different initial grain sizes.For the laser shock micro-bulging process,the calculated results using constitutive models in the power function form and exponential function form are both in the experimental result range.Both two models can predict the bulging height under different initial grain sizes accurately,and the calculated results using constitutive model in exponential function form are closer to the averages of experimental results.