Investigations on forming of aluminum 5052 and 6061 sheet alloys at warm temperatures

In an ongoing quest to realize low-mass transportation vehicles with enhanced fuel efficiency, deformation characteristics of Al5052 and Al6061 were investigated. In the first part of this study, material behavior of Al5052 and Al6061 sheet alloys were investigated under different process (temperature and strain rate) and loading (uniaxial vs. biaxial) conditions experimentally. With the biaxial, hydraulic bulge tests, flow stress curves up to 60-70% strain levels were obtained whereas it was limited to 30% strain levels in tensile tests. The microstructure analysis showed that the change of grain size due to the effects of elevated temperatures and strain rates were not significant; therefore, it was concluded that the decrease in the flow stress at high temperature levels was mainly due to the thermally activated dislocation lines. In the second part, the effect of the temperature and the pressure on the formability was further investigated in a set of closed-die warm hydroforming experiments. The test results showed that a linearly increasing pressure profile up to similar to 20 MPa levels did not have a significant effect on the die filling ratios and thinning of the parts when a uniform temperature distribution of 300 degrees C was applied. Finally, in the third part of the study, finite element models were developed for the same closed-die hydroforming geometry using the material behavior models obtained from bulge and tensile tests. Flow stress curves obtained from tests were compared in terms of predicting the cavity filling ratios and thinning profiles from the experiments. Based on the comparison, it was revealed that flow stress curves obtained from the warm hydraulic bulge tests provided accurate predictions at high strain levels (i.e., (epsilon) over bar >0.4, when part filling is above 80%) while the flow stress curves from the tensile tests did so at low strain levels (i.e., (epsilon) over bar < 0.2, when cavity filling is below 80%). On the other hand, comparison of thinning values indicated that flow stress curves from bulge tests yielded good agreement with the experimentally measured values in general. Therefore, it can be recommended that the bulge test results should be used whenever available in order to conduct accurate numerical analyses for warm sheet hydroforming where complex geometry and loading conditions exist. (C) 2009 Elsevier Ltd. All rights reserved.