Abstract
In forming simulations, flow curves are cardinal inputs to predict features, such as forming forces and material flow. The laboratory-scale experiments to determine them, like compression or tensile tests, are affected by deformation heating, restricting direct flow curve determination. In principle, the current analytical and inverse methods determine flow curves from these tests, but while the analytical methods assume a simplified temperature profile, the inverse methods require a closed-form flow curve equation, which mostly cannot capture complex material behavior like multiple recrystallization cycles. Therefore, the inverse piecewise flow curve determination method “FepiM” previously developed and published by the current authors is extended by introducing a two-step procedure to obtain isothermal flow curves at elevated temperatures and different strain rates. Thereby, the flow curve is represented as tabular data instead of an equation to reproduce complex flow curve shapes while also compensating the effect of inhomogeneous temperature profiles on the flow stress. First, a flow curve at the highest temperature is determined. In the second step, using this first flow curve as a reference, the flow curves at lower temperatures are obtained via interpolation. Flow curves from conventional compression tests for aluminum and copper in the temperature range of 20–500 °C are predicted, and it is shown that these flow curves can reproduce the experimental forces with a maximum deviation of less than 1%. Therefore, the proposed new piecewise method accurately predicts isothermal flow curves for compression tests, and the method could be further extended to highly inhomogeneous methods in the future.
Highlights
Precise and reliable flow curves play a vital role in capturing relevant characteristics, like forming force, material flow, and internal stresses in metal forming simulations
A maximum error of around 7% is observed in the analytical flow curve at 400 ◦C, 0.01 s−1, and the error fluctuates as the simulation progresses, while the error in the FepiM flow curves is below 2% throughout
This paper extends the applicability of the FepiM approach developed earlier to flow curves obtained at elevated temperatures
Summary
Precise and reliable flow curves play a vital role in capturing relevant characteristics, like forming force, material flow, and internal stresses in metal forming simulations. Laboratory-scale experiments, e.g., compression tests, tensile tests, torsion tests, etc., are conducted to determine these flow curves [1]. Though these experiments are performed under controlled conditions, several aspects like friction, deformation heating, heat transfer, etc., restrict the precision when directly converting experimental data to flow curves [2]. Once validated, the novel piecewise approach may greatly improve the evaluation accuracy compared to analytical methods for highly inhomogeneous experiments like torsion tests [4] or hot tensile tests [5] beyond necking
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