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Consequently in this paper, the cold rotary forging and conventional forging process of a cylindrical workpiece is simulated by the elastic-plastic dynamic explicit FE method under the ABAQUS software environment. Through simulation, the deformation characteristics and mechanism of cold rotary forging are given and the difference between cold rotary forging and conventional forging is clarified in detail. The research results provide useful theoretical and experimental guidelines for the cold rotary forging process.
Results and discussion
Metal flow analysis
Fig. 7 shows the comparison of mesh deformation between conventional forging and cold rotary forging. It can be seen from Fig. 7(b) that in conventional forging, the metal only flows along two directions. One is the axial flow, resulting in thinning in the axial height of the cylindrical workpiece, and the other is the radial flow, leading to the expansion in diameter of the cylindrical workpiece. Obviously, metal flow does not exist in the circumferential direction. That is, because of the symmetry of geometry and boundary conditions, the deformation of the cylindrical workpiece exhibits the characteristic of axial symmetrical deformation and thus conventional forging can be simplified as a 2D deformation process. In cold rotary forging, besides the axial and radial flow, the metal also flows along the circumferential direction, as shown in Fig.7 (c). Therefore, cold rotary forging is an asymmetrical deformation process and thereby the 3D analytical model has to be proposed to investigate the forming process.
Contact area between the dies and cylindrical workpiece
In conventional forging, the workpiece contacts with the upper and lower die completely at any time of the process. During the cold rotary forging process, the workpiece contacts the dies only partially because the upper die is a conical die. Furthermore, the contact area has been shifting continuously due to the oscillation of the upper die. Thus, the contact area exhibits a much complex and changeable geometry shape and thereby has an essential effect on the cold rotary forging process. Fig. 8 shows the contact area comparison between cold rotary forging and conventional forging. In conventional forging, the contact area between the upper die and workpiece is identical with that between the lower die and workpiece. It is clear that the contact area increases quasi-linearly from a certain value to the maximum value. Moreover, the curve is very smooth with time, indicating that the cylindrical workpiece is in a steady deformation state. Different from conventional forging, the contact area between the dies and workpiece in cold rotary forging is obviously different. The two curves can be pided into three different stages. At the beginning of cold rotary forging, the upper die begins to contact the workpiece, so the contact area between the upper die and workpiece increases rapidly from zero to a certain value while the contact area between the lower die and workpiece decreases significantly. As the forming process continues, the cylindrical workpiece has entered the steady deformation state; thus the contact area increases slowly up to the maximum value. At the end of the process, the lower die stops the axial feed while the upper die still oscillates, resulting in the sharp decrease in the contact area. Furthermore, it can be found that the two curves have been oscillating with time, indicating that cold rotary forging is a complex dynamic contact, highly nonlinear and non-steady-state deformation process. It can be also observed that the contact area between the upper die and workpiece is always smaller than that between the lower die and workpiece, thus leading to the higher axial unit pressure on the metal near the upper die. Under this circumstance, the metal near the upper die is easier to satisfy the yield condition to be involved in the plastic deformation state. From the above analysis, it can be concluded that the contact pattern, the shape and size of contact area between cold rotary forging and conventional forging are obviously different, thus resulting in the different deformationcharacteristics between them.