Warm forming process for an AA5754 train window panel

A warm forming process is designed for AA5754 to overcome low room temperature formability. The solution includes increased working temperature and is demonstrated with a railway vehicle component. A Finite Element (FE) based methodology was adopted to design the process taking into account also the starting condition of the alloy. In fact, the component's dent resistance can be enhanced if the yield point is increased accordingly: the stamping process was thus designed considering the blank in both the H111 (annealed and slightly hardened) and H32 (strain-hardened and stabilized) conditions that were preliminarily characterized. Tensile and formability tests were carried out at different temperature and strain rate levels, thus providing the data to be implemented within the FE model (Abaqus/CAE): the stamping was at first simulated at room temperature to evaluate the blank critical regions. Subsequently, the warm forming process was designed by means of an uncoupled thermo-mechanical approach. Thermal simulations were run to properly design the heating strategy and achieve an optimal temperature distribution over the blank deformation zone (according to the results of the material characterization). Such a distribution was then imported as a boundary condition into the mechanical step (Abaqus/Explicit) to determine the optimal process parameters and obtain a sound component (strain severity was monitored implementing an FLD-based damage criterion). The simulation model was validated experimentally with stamping trials to fabricate a sound component using the optimized heating strategy and punch stroke profile.

Development of Measurement Equipment and Experimental and Numerical Simulation Studies for Warm Forming Limits of High-Strength Steel

This paper describes the research and development of a set of measurement equipment for the warm forming limits of high-strength steel based on the Nakazima bulging test method and the digital image correlation method. The equipment could provide an argon shield and a water-cooling atmosphere, as well as two heating options: heating the specimen, dies, and environment to the test temperature simultaneously or heating the specimen to the test temperature at a higher speed than that for the dies and environment. The equipment was applied to measure the forming limit curves for high-strength DP600 steel at room temperature and at the temperature of 300 °C to verify its performance. The DYNAFORM software was then applied for the digital simulation of the bulging test method. A new limit-strain-fitting method was proposed to eliminate the impact of the distorted grid on the digital simulation process. The change trend of the forming limit curve acquired in the test had sound consistency with the test results.

Influence of temperature on the formability of an aluminum alloy

Warm forming is widely used as increasing the temperature is a solution to improve the formability of aluminum alloys. The stress (or strain) state is one of the most important factors affecting the formability of metals. In warm forming, the temperature and strain rate also play an important role on the deformation and fracture behavior. Figuring out the relationship between formability, temperature, strain rate and stress state is of great importance for providing more understanding of ductile fracture in warm forming conditions. Therefore, the objective of this work is to investigate the influence of temperature on the ductile fracture of a 6000 series aluminum alloy sheet metal under different stress states. Dogbone specimens, notched tensile specimens with different radius, tensile specimens with a central hole and shear specimens are used to cover a wide range of stress states. The hybrid experimental-numerical approach is used to identify the fracture strain and the corresponding stress state parameters (i.e. stress triaxiality and Lode parameter). To this end, fracture tests are carried out at 200°C using a tensile machine to determine the instant of fracture. Numerical simulations of the tensile tests are performed in 3D with the finite element code Abaqus to predict the strain field and calculate the evolution of the stress state. To accurately model the material behavior the positive strain rate sensitivity in the flow stress response at elevated temperature is considered. The results show a strong dependency of the ductile fracture on the temperature, strain rate and stress state.