Formation mechanisms and rules of typical types of folding defects during die forging

Folding defect is one of the most important forging defects, which can deteriorate the surface quality, mechanical properties, and material utilization of forged parts. In this work, the common folding defects in die forging were classified into three typical types: local-loading-type folding, confluence-type folding, and bending-type folding. Three simple Eigen experiments were employed to analyze the formation mechanisms and rules of three typical types of folding accordingly. By FE simulation, the formation mechanisms of three typical types of folding defects were analyzed based on the evolution of the velocity field. In addition, the effects of geometric and forging parameters on each type of folding defect were investigated by uniform experiment design. Especially, a quantitative folding index was applied to uniformly evaluate the formation possibility and severity of folding defect for any samples whether producing folding or not. It is found that the local-loading-type folding is produced by the formation and collapse of a step, which is sensitive to the reduction amount but insensitive to the feed amount. The confluence-type folding is formed due to the local lack of material, which is sensitive to the reduction degree and insensitive to the rib width and billet height in this work. The bending-type folding is mainly formed by the geometric instability of workpiece during the forming process, which is sensitive to the reduction degree and insensitive to the geometric parameters of billet.