Numerical Modeling and Experimental Validation of Reverse Flow-Forming on 15CDV6 Seamless Tubes for Aerospace Application

Abstract

Flow-forming is an unconventional incremental forming process that involves pressing and elongating a preform kept on a rotating mandrel with the aid of rollers. A 3D thermo-mechanical model is developed to simulate the reverse flow-forming of 15CDV6 Steel. This process can produce seamless cylindrical tubes in a single step with a high strength-to-weight ratio, reducing the need for additional manufacturing processes. Flow-forming also has an inherent advantage of producing tubes with almost uniform thickness distribution. The numerical model is validated by comparing the thickness distribution of the simulated formed tubes with those obtained from the experiments. This study also investigates the effect of input variables, like, axial stagger, percentage reduction, and feed ratio, on state variables like stresses, strains, equivalent plastic strains, and the measurable output parameters. The measurable output responses, like spring-back, diametral growth, ovality, and thickness, are predicted from the numerical results. Stress and strain distribution at relevant tube regions, such as the outer circumference, inner circumference, and roller contact zone, are also investigated. In addition, a significant contribution of this work is investigation of the strain-rate history of the process, which has direct implications towards the final tube geometry and structural integrity. The results have indicated that a higher percentage reduction leads to significant plastic deformation thereby increasing spring-back and ovality. Lower feed rates are found to result in a larger area of the tube material (locally) engaging in contact with the rollers, thereby leading to a higher ovality and diametral growth. In contrast, a higher axial stagger promotes larger spacing, greater support, and a controlled deformation, resulting in lower diametral growth and spring-back. Equivalent plastic strain is a crucial numerically-measurable output for material deformation, as it indicates structural integrity and also plays a significant role in crack formation. Notably, increasing the feed ratio was found to reduce the equivalent plastic strain since the amount of local contact with the rollers is reduced. However, increasing the percentage reduction was found to increase it globally. The Taguchi method is employed to develop the design of experiments and perform optimization of the process parameters.