| Due to its low density,high specific strength,corrosion resistance and other excellent characteristics,aluminum alloy has become the ideal material for the lightweight of automobiles.Forming limit diagram(FLD)is usually used to judge the metal sheet formability and has been widely used in industry.FLD provides an important criterion for the failure and fracture of metal sheet in stamping simulation,and plays a significant role in process optimization.Hence,investigation on the forming limit diagram of aluminum alloys is of great significance to the lightweight of automobiles.At present,the research on forming limit diagram is extensive,but it still needs to be fully studied in accurately obtaining FLD and its influence factors.Studies have shown that there is influence on aluminum alloy forming limit in curvature,necking criteria,sheet thickness,temperature,speed,strain path and other aspects.In this paper,the forming limit diagram of AA5086 aluminum alloy sheet is determined by Nakazima test combined with numerical simulation,and the influence of different factors on the forming limit diagram is studied.The main contents of the dissertation are as following:(1)Uniaxial tensile experiment of AA5086 aluminum alloy sheet at different temperatures and strain rates is carried out.Parameters of Lin-Voce and Ludwick hardening equation are obtained by means of reverse analysis.The results find that a better agreement with experiment in softening behavior at high temperature is shown by Lin-Voce model,while the Ludwick model describes the hardening behavior at low temperature better.(2)Based on Nakazima experiment and Digital Image Correlation(DIC)technology,the FLDs of AA5086 aluminum alloy sheet are constructed.The effects on FLDs in these aspects of punch curvature,necking criteria,sheet thickness,temperature and speed is also investigated.It is found that the large curvature would cause the forming limit curve to shift to the upper right and a certain difference is observed in the forming limit curves determined by different necking criteria.Sheet thickness can improve the level of forming limit curve.The limit strain is positively correlated with the temperature at the same speed,while the limit strain is negaticely correlated with the temperature at the same temperature.(3)Based on the Lin-Voce and Ludwick hardening equations,the Nakazima finite element model(FE Nakazima)and M-K finite element model(FE M-K)are established,and FLDs for different plate thicknesses,temperatures,and rates are predicted and compared with the experimental results.The study shows that the hardening equation has effect on the prediction result.For FE Nakazima,the prediction result of the Lin-Voce model is slightly lower than the results of Ludwick model.For FE M-K,the predicted results in the left side by the two model are similar,but in the right side,the prediction of Lin-Voce model is higher than Ludwick model.Under normal temperature conditions,the prediction results agree not well with experimental data for both of the two hardening model in FE Nakazima.In FE M-K,the FLD constructed by the Lin-Voce model is consistent with the experimental FLD of 1.0 and 1.5 mm thickness sheets,and the FLD determined by the Ludwick model is in accordance with experimental FLD of 2.0 mm thickness sheets.In addition,the imperfection coefficient f0 in the FE M-K model shows the sensitivity to the sheet thickness,temperature,and speed.The value of f0 increases with the increasing of thickness and temperature and decreases with the increasing of speed.(4)Based on FE M-K model,the influence of strain path on forming limit curves is studied,and forming limit stress diagram with different thickness is established by theoretical derivation and numerical simulation.The research shows that the forming limit curves is sensitive to strain path,while the forming limit stress curves is not.And the sheet thickness has a positive influence on forming limit stress curves.The greater the thickness is,the higher the curve level is. |
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