Method for Approximating the Flow Stress Curves of Upsetting Test under Sticking Condition

Finite Element Modeling (FEM) is widely used in forging process for designing forming sequences, predicting material flow, and approximating the forming load. Accurate stress-strain with large strain is required. The uniaxial upsetting test with well lubricated is utilized to minimize the frictional effect. However, this condition cannot be achieved at evaluated temperature due to the temperature limitation of lubricants. The upsetting dies with a grooved pattern was proposed to overcome this problem. The measured load-stroke curve was converted to the average stress-strain by calibration curves determined by the simulation with Power’s law hardening model ( , where K =strength coefficient and n = strain-hardening coefficient) and sticking condition (m = 1). The effect of the stress-strain curve is significantly dominant to these calibration curves and needs to be considered. Therefore, this research aims to propose new calibration curves for approximation of the average stress-strain obtained by the simulation together with the statistical analysis. The effects of Power’s law model coefficients and specimen height-to-diameter ratios on the calibration curves were investigated. AISI 1045 steel and AA5052 aluminum alloys were experimentally tested to validate the approach by comparing average flow stress with results from standard frictionless tests at room temperature. The findings show that calibration curves are strongly dependent on specimen geometry, with a height-to-diameter ratio of 2 recommended for accurate determination of the average stress–strain curve.