Investigation On Hot Formability Of 5A90 Al-Li Alloy Sheet And Its Experimental Verification

Aluminum-lithium alloys have been widely used in the aerospace industry due to their high specific stiffness and strength,low density,good corrosion resistance and weldability.And with the increasing attention on environment,the desire of replacing traditional cast iron and steel with lightweight aluminum and magnesium alloys has become more and more significant to reduce fuel consumption and environmental pollution.However,compared with traditional steel materials,Al-Li alloys have poor plasticity at room temperature,and thus can not be used to produce complex-shaped parts.It has been found that the formability of the alloy could be effectively improved at elevated temperature without changing alloying constituent.Therefore,the rheological behavior and formability of 5A90 Al-Li alloy sheet under thermal condition were studied by experiments and numerical simulation in this work.The main work and conclusions are as follows:(1)Firstly,the rheological behavior of 5A90 Al-li alloy was studied by uniaxial tensile test at different temperatures(25?,150 ?,200 ?,250 0C,300 ?,350 ?,400?,425? and 450?)and strain rates(0.00025 s-1,0.0025 s-1 and 0.01 s-1).It was found that the flow stress decreased with increasing temperature and with decreasing strain rate.The tension tests were conducted under different temperatures(25 0C,200?,300? and 400 ?)and directions(0,45 and 90 degrees to rolling direction)to analyze the anisotropic behavior.The strong anisotropy was observed and would decrease with increasing temperature,and it was considered to be isotropic at 200 ?.Based on above analysis,two constitutive models(strain compensated Arrhenius model and modified Johnson-Cook model)were used to describe and predict stress-strain curves for 5A90 alloy.(2)Based on the FE simulation of Nakaziam test,the effects of friction coefficient on the failure position,strain path and failure time of the sheet were compared and analyzed.And according to the experimental and simulated results,the friction coefficient under the experimental condition was predicted.(3)The setup and sample for Nakazima test were designed and manufactured according to ISO 12004:2008.The Nakazima experiment was carried out based on the EC600 sheet forming machine.At the same time,the full strain field during the deformation process was calculated by the ARAMIS non-contact strain measuring system.And forming limit diagrams of 5A90 alloy at room temperature,150? and 250? were obtained.(4)Based on the simulation of Nakazima experiment,four kinds of necking criterion were selected to analyze the effect of necking criteria on the limit strain at 250 ?.By comparing the predicted results with experiments,it was found that the maximum punch force criterion was the most suitable criterion for 5A90 alloy.The built constitutive models in Chapter 2 were embedded into ABAQUS for simulation by the UHARD subroutine and forming limit diagrams at 350 ? and 450 ? were predicted.