Flow forming: a review of research methodologies, prediction models and their applications

After years of largely academic interest and niche applications, a new generation of high duty CNC machines is enabling the flow forming process find increasing application in aerospace, automotive and defense industries. The versatility of digital control has made it economically viable to deliver weight and cost savings for small to medium batch sizes while simultaneously improving quality and material proprieties.
To better understand the capabilities of flow forming this review surveys the reported research over last fifty years and summarizes both theoretical models and experimental investigations. Where possible the contributions of different researchers are described and assessed in terms of the accuracy of their predictive capabilities. In some cases practice has preceded the development of theory for example: the ratio of circumferential to axial contact is widely used as a defect prediction parameter, even if the process' failure mechanism is still not fully understood. In other areas, such as forming forces and powers, the literature provides a clear rational based on experimentally validated analytical models.
In addition to summarizing current knowledge the review also identifies gaps in current literature where more research is required. For example: the evolution of stress/strain tensors during a flow forming process has not been reported due to the high computational cost and a lack of consensus on the most appropriate finite elements modeling approach to adopt. Similarly while the final microstructure of a formed part is often evaluated models of its development (during the series of plastic deformations created by a flow forming process) have not been reported. Likewise residual stress and final material proprieties, such as corrosion behaviour, have been not studied numerically or experimentally. It is also noted that the impact of tool paths (e.g. their geometry and topology) has not been deeply explored. Lastly the authors note, the surprising observation, that only a few researchers have reported the experimental optimization and characterization of flow forming process parameters using a 'Design of Experiments' methodology.