Evaluation of dry-in-place lubricants for cold forging by using an optimal steady combined forward and backward extrusion testing method

This study evaluated dry-in-place lubricants used for cold forging. A group of isothermal compression tests with a strain rate $(\dot{\epsilon })$ range of 0.001–1 s⁻¹ and temperature (T) range of 30–400 °C were completed. The flow stress (σ) curves of annealed steel S45C were obtained, and a corresponding Hensel–Spittel model was developed to support finite element (FE) simulation. The sensitivity of the steady combined forward and backward extrusion (SCFBE) test proposed in another study was improved by approximately 20% after it was optimized using the results of the FE simulations. Key parameters were identified, and the calibration curves after optimization were obtained. On the basis of the optimized test, a friction testing setup with a heating system was developed, in which the die temperature could be adjusted from room temperature (RT) to 230 °C. Three dry-in-place lubricants and conventional phosphating lubricant were tested, and the friction factors (m), forming loads, and ejection loads were measured. The surface features of the specimens after testing were also investigated. According to the testing results, of the three tested dry-in-place lubricants, the mica type was the best. In addition, the optimized friction testing design was verified as effective.