Friction Assessment in Hot Forging Operations Using Large-Scale Ring Compression Test

Abstract The ring compression test is the most extended experiment for friction characterization in metal forming processes. However, these experiments are commonly carried out under laboratory environments with small specimens and highly controlled conditions, which may not accurately represent the industrial forging operations. For instance, a small size specimen does not provide enough surface area for the oxide scale formation and heat transfer, and these phenomena are critical in industrial processes. Then, friction characterization results are more accurate by using large-size specimens. In this paper, friction was determined by reverse engineering, using the commercial finite element (FE) code QForm, which is a commercial code for specific metal forming applications. Finite element models were adjusted considering both the final ring dimensions as well as the load-stroke during the entire experiment. The design of experiments (DOE) matrix includes different factors and levels such as temperature, deformation speed, and high reduction. The temperatures were 900, 1100, and 1200 °C, the deformation speeds were 20, 80, and 150 mm/s, and the degree of deformations were 40, 60, and 80% in high reduction. The experiments were conducted to determine the friction in an actual crankshaft’s forging process. To determine that, ring specimens were done in a servo-hydraulic press, using compression plates made with the same material, thermal treatment, and surface coating as actual forging dies used in the crankshaft pre-forming operation. The temperature effect was higher than the deformation speed; however, these two factors have a strong interaction effect. On the other hand, from the FE models’ results, it was confirmed that the inner diameter reduction is highly sensitive to the coefficient of friction. However, the load-stroke behavior is less sensitive to friction changes, although it varies when different friction models are used.