Experimental Study Of Forming Limit Diagram Of 2198 Al-Li Alloy Sheet At Warm State

2198 aluminum-lithium(Al-Li) alloy is the third generation aluminum-lithium alloy, which has high specific strength and stiffness, light quality, good corrosion and fatigue resistance, and appropriate ductility, and thus has received extensive attention of the aviation industry. However, the deformation resistance of 2198 Al-Li alloy is sensitive to temperature and strain rate, and it has poor formability at room temperature. This leads to difficulty of deformation. Therefore, the investigations of deformation formability of 2198 Al-Li alloy sheets under different temperatures and establishing its forming limit diagram are very important for new aviation materials research and application.Forming limit diagram(FLD) or Forming limit curves(FLC) is the most simple and intuitive method to estimate and evaluate the formability of sheet metal, and also can be used to predict and judge the success or failure of stamping process, to assist for the determination of stamping process parameters. So it is a very effective tool to solve the problem of sheet metal stamping. The discretion of the forming limit curve location reflects the limit deformation capacity of material in a variety of stress state, which has higher position, the material formability is better. In recent years, FLDs were used widely to evaluate the comprehensive properties of the first and second generation Al-Li alloy sheets by domestic and foreign scholars, but the research about FLD for the third generation Al-Li alloy sheets is little. And thus, in this work, experimental investigations of FLD were conducted for the third generation 2198 Al-Li alloy sheets at elevated temperatures.The hot sheet formability test machine(BCS-50AR) developed by BUAA and the grid measurement analysis system(VIALUX Guto Grid) were used to determine the FLDs of 2198 Al-Li alloy sheets at the forming temperature range of 25?~350?, and the influence of forming temperature, strain rate and stress state on the formability of 2198 Al-Li alloy were analyzed; the microstructure morphology and changes after deformation of 2198 Al-Li alloy sheet at the corresponding temperature by analysis of metallographic(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM).Results show that, the forming temperature has a great influence on the FLDs of 2198 Al-Li alloy, the height of FLD curve increases with the increase of forming temperature. The punch speed has little influence on the formability of 2198 Al-Li alloy at 25?, yet the influence at the temperature between 300? to 350? is rather huge; The stress state has great influence on the formability of 2198 Al-Li alloy at 25?, while it has less influence at the temperature range of 300~350?.The microstructure analysis results show that, the bulging fracture of 2198 Al-Li alloy sheets at 25? has obvious characteristics of the layered fracture, and also has a little dimples; while at the temperature of 350?, there is a large number of holes, and with the increasing of deformation time, the size of the holes tends to increase, and a dynamic recrystallization appears in deformation process of 2198 Al-Li alloy sheets.Base on the experimental of Forming limit diagram and the M-K theory, a successful prediction of FLD for 2198 Al-Li alloy sheet at elevated temperature was conducted by using Swift yield criterion and Hill yield criterion. And the most ideal exponent parameters were obtained by comparison. A good theoretical computation model for 2198 Al-Li alloy sheet at different temperatures was acquired, and to provide theoretical support for the establishment of 2198 Al-Li alloy sheet in the actual production process specification.