Cold rotary forging is an innovative incremental metal forming process whose main characteristic is that the workpiece is only partially in contact with a conical tool, reducing therefore the required forging loads. However, in spite of many benefits of such a process, wide industrial implementation of rotary forging is not possible without proper understanding of material behaviour. In the present work, the capability of rotary forging process was explored for the manufacturing of flared cylindrical parts by cold forming. Another main aim was to assess the cold formability of high-strength materials for aerospace applications (martensitic stainless steels) under incremental processes. In order to understand the impact of rotary forging on the final properties of formed components, microstructural and mechanical analysis were performed. Microstructural and hardness analysis were conducted on both axial and transverse sections along the cold formed flange in order to study the grain flow orientation and strain distribution. In a similar fashion, mechanical test specimens were machined from different positions and orientations along the rotary forged component. Further analysis was performed on the components in the as-treated condition in order to understand the response of cold-worked Jethete M152 components to subsequent heat treatments.
Microstructural and hardness analysis clearly reveals a strong grain reorientation and strain localization around “pickup“ defects (material attached to the upper tool) observed on the flange top surface, close to the flange edge. These results suggest that an excessive deformation is localized during the early stages of the flange formation. Another characteristic feature found in the rotary forged parts is the presence of a buckling phenomenon which appears in later stages of the rotary forging process. Strain hardening along with the increasing flange length requires higher levels of forging loads to keep forming the flange. This results into a significant accumulation of compressive stresses in the transition region between the flange and the straight region. Gradually the resultant compressive force exceeds the critical buckling load, leading to the occurrence of the buckling phenomenon. This latter issue determines the limit of the cold flaring process. This can help to determine the maximum length of the flange part, achievable in this process, which is of great importance for the design of these manufacturing technologies. From the mechanical testing results, large differences were found as a function of both position and orientation (axial, transverse) throughout the rotary forged components (anisotropic properties). Concerning the impact of heat treatments on cold-worked components, no differences were found in the as-treated condition, in terms of microstructural and mechanical properties between regions with a large difference in strain distribution. These results denote the normalizing effect of conventional hardening treatments on cold-worked Jethete M152 components, restoring the homogenous and isotropic properties across the whole component.
Distribution of Microstructure and Vickers Hardness in Spur Bevel Gear Formed by Cold Rotary Forging