Grain size control method for enhancing high-temperature durability of Al–Cu–Mg–Ag alloy

Reducing the grain boundary density (obtaining larger grain size) is beneficial for improving high-temperature durability of the heat-resistant aluminum alloys, and thus for extending their service life. In this paper, a grain size control method was proposed to enhance high-temperature durability of an Al–Cu–Mg–Ag alloy. A relationship formula between grain size, holding time, and annealing temperature was obtained through theoretical analysis. The recrystallization curve of a deformed Al–Cu–Mg–Ag alloy under different heating rates was determined through three-point bending thermal relaxation experiments. Based on the above relationship formula, the recrystallization kinetics parameters were obtained. Subsequently, based on actual processing conditions, the annealing temperature with the maximum grain size under a given annealing time was calculated. After the annealed sample underwent solution-aging (T6) treatment, the grain size increased by more than 60% compared with that of the directly solution-aging treated sample. This method can increase the duration by more than 60% at conditions of 210 °C/190 MPa without significantly reducing the room-temperature mechanical properties

The crash behaviour of hot stamped components – the effect of tailoring conditions

It is known that tailoring a hot stamping part, to achieve locally graded properties, can improve the crash behavior. Depending on the role of the structural part (carrying either bending or axial crash load), the best position for the local regions with lower strength and higher ductility can be different. The distribution of these local regions and their mechanical properties affects the crash behavior of the part in each loading case and therefore can be effectively designed to improve the crash performance. To investigate these effects and examine the improvement possibilities, a numerical thermalmechanical-metallurgical model of a hot stamping process and a representative side impact crash model were created and analysed. The hot stamping model was used to predict the consequent phase fractions and mechanical properties of tailored hot stamping parts produced with different tailoring scenarios. In the metallurgical model, a modified phase transformation model based on Scheil’s additive principle was incorporated. The geometry and mesh of the stamped part was exported to a crash numerical model with a 3-point bending configuration. A constitutive model was used to define the plastic behavior of the stamped part corresponding to different hardness values. Various possibilities in locally positioning the high strength or high ductility zones of material were examined. The results show that the positioning of the soft zones has a more significant effect on the crash performance than the variation in their mechanical properties of these soft zones

Sensitivity of the final properties of tailored hot stamping components to the process and material parameters

The final mechanical properties of hot stamped components are affected by many process and material parameters due to the multidisciplinary nature of this thermal-mechanical-metallurgical process. The phase transformation, which depends on the temperature field and history, determines the final microstructure and consequently the final mechanical properties. Tailored hot stamping parts - where the cooling rates are locally chosen to achieve structures with graded properties - has been increasingly adopted in the automotive industry. In this case, the robustness of final part properties is more critical than in the conventional hot stamping parts, where the part is fully quenched. In this study, a wide range of input parameters in a generalized hot stamping model have been investigated, examining the effect on the temperature history and resulting final material properties. A generic thermo-mechanical finite element model of hot stamping was created and a modified phase transformation model, based on Scheil\u27s additive principle, has been applied. The comparison between modeling and experiments shows that the modified phase transformation model coupled with the incubation time provides higher accuracy on the simulation of transformation kinetics history. The robustness of four conditions relevant to tailored hot stamping was investigated: heated tooling (with low and high tool conductance), air cooling, and conventional hot stamping. The results show the high robustness of the conventional hot stamping compared to tailored hot stamping, with respect to the stamped component\u27s final material properties (i.e. phase fraction and hardness). Furthermore, tailored hot stamping showed higher robustness when low conductivity tools are used relative to high conductivity tools