This paper describes a method by means of FE code DEFORMTM-2D to simulate the formation of central bursting defects in extrusion processes; the effect of various extrusion parameters such as half die angle, reduction in area, friction factor, and strain hardening exponent on the maximum damage value is examined. The differences between various ductile fracture criteria are compared and critical damage value(CDV) of the material AA6061 is found. In addition, we get the strength coefficient(K), strain hardening exponent(n), CDV and friction factor(m) by material tests, such as uniform tensile test, notched tensile test, compression test, and ring compression test. Finally, the cold multistage extrusion experiment was conducted to verify the accuracy of the finite element simulations. From the continuous three pass extrusion experimental data, no fracture in the center of the extruded product was found. From the analytical data, it was known that the maximum damage value 1.0479 for third pass extrusion was small than critical damage value 1.068, thus, central bursting defects didn¡¦t occur in extrusion processes.

The focus of this thesis is to develop a simulation model for predicting ductile fractures during hot working at Alleima. The main fracture mechanism in these conditions is ductile fracture by void coalescence. The ductile fractures are caused by the linking of voids that appear when there is large plastic deformation near second-phase particles. The chosen method to simulate these was to use a Ductile Fracture Criterion (DFC), which builds on using FE models with a damage parameter. Two criteria were selected to be tested. The austenitic stainless-steel alloy 316L was selected as material for this work. Using the Gleeble 3500 system, hot tension and compression experiments were performed to gather data needed for the simulation models as well as inducing ductile fractures. Rupture occurred for all the hot tension samples and cracks were found for only one of the hot compression experiments. Using data from the Gleeble tests, a separate simulation model for each of the setups were created using the finite element software Marc/Mentat. A flow stress model for 316L was developed. Results from the simulations show that both selected DFCs can be used to predict ductile fractures. Particularly for hot tension. It was shown that it is important to model the temperature gradient in the sample accurately. For hot compression, it was difficult to conclude if the criterions were able to predict fracture since only one data point was available. The thesis concludes that there could be of interest with continued work using DFCs at Alleima.