Research On Forming Limit Of Aluminum Alloy Sheets Based On Different Stress-strain Curve Models

In recent years,with the continuous advancement of industrial lightweight,the requirement for aluminum alloys in industrial production has continued to increase.Therefore,it is urgent to achieve accurate prediction of the forming limit for aluminum alloys.Improving the forming performance of aluminum alloy effectively and obtaining its forming limit diagram(FLD)accurately have always been the research focus in the field of metal plastic forming.In the current research of FLD,the fitting curve of stress-strain data is generally used to approximate the true stress-strain relationship of the material.However,the fitting curve cannot strictly reflect the true stress-strain relationship of the material,which affects the prediction accuracy of the sheet forming limit and cannot provide a reliable reference for industrial production.In this paper,the power exponent fitting stress-strain curve model,the cubic polynomial fitting stress-strain curve model,and the stress-strain original measurement data model are constructed on the basis experimental results.Combinating the theoretical derivation,numerical simulation with the experimental verification,the effects of different stress-strain curve models on the forming limit of aluminum alloy sheets are systematically studied.The main research contents and conclusions of this paper are as follows:1.The lightweight aluminum alloys 6016-T4 and 7075-T6 are identified as the research objects.Based on tensile experiments of the materials,the power exponential fitting stress-strain curve model,the cubic polynomial fitting stress-strain curve model,and the stress-strain original measurement data model are constructed.Based on the theoretical theories such as plastic stress-strain relationship,yield criterion and instability criterion,the prediction methods for forming limit of aluminum alloy sheet are studied systematically.2.Combining with Marciniak-Kuczynski(M-K)theory,the theoretical formulas of forming limit under different stress-strain curve model are deduced,and the theoretical forming limit curves(FLC)of aluminum alloy sheet are obtained through iterative calculation of Matlab program.Moreover,the influence of groove angle on sheet forming limit is studied.The researchs show that the stress-strain curve model has a prominent effect on the theoretical prediction results for forming limit of aluminum alloy sheet,and this effect is mainly reflected in the plane stress state and its vicinity on the left FLD.3.Based on the Dynaform numerical simulation software,the finite element numerical simulations of the sheet forming limits are carried out by the comprehensive judgment method combining the maximum load criterion and the strain path transition criterion,and the numerical simulation results of sheet forming limit under different stress-strain curve model are obtained.The researchs show that the stress-strain curve model has a certain effect on the numerical simulation prediction results for forming limit of aluminum alloy sheet,and this effect is mainly reflected in the plane stress state and its vicinity on the left FLD.4.By the Nakazima hemispherical convex bulging test and ARGUS optical non-contact strain measurement,the ultimate strain data and the necking critical region of the FLD are obtained.Comparing the theoretical predictions,the numerical simulation predictions,with the experimental results of the forming limit,the results show that the FLC prediction results of theoretical calculation and numerical simulation based on the stress-strain original measurement data model have the best agreement with the experimental results,while the FLC prediction results of theoretical calculation and numerical simulation based on the other two fitting curve models are all exceeded the necking critical area obtained by experiments to varying degrees.The FLD prediction method based on the stress-strain original measurement data model can achieve accurate prediction of sheet forming limit,which is more reliable and superior.