A Study on the Generation of a Fine-Grained Layer in Upsetting Between Flat and Conical Dies

A narrow fine-grained layer is generated in the vicinity of frictional interfaces in many manufacturing processes. The majority of previous studies on this topic is devoted to machining. The present is paper concerned with the generation of a fine-grained layer in upsetting of hollow cylinders between flat and conical dies. The layer occurs in the vicinity of the friction surface between the specimen and the conical die. The die angle varies to change the state of stress and strain near the friction surface. No lubricant is used to increase the friction stress. Based on a proposed criterion that defines the fine-grained layer, it is demonstrated that its thickness depends on the die angle. Moreover, the thickness varies along the friction surface. A general continuum mechanics model is proposed to connect the thickness of the fine-grained layer and the strain rate intensity factor. The model is based on dimensional analysis and a number of general assumptions. The strain rate intensity factor in the upsetting process is determined by an approximate method. Then, the experimental results are used to find some input parameters of the theoretical model.