Effects of flow forming parameters on dimensional accuracy in Cr-Mo-V steel tubes

Flow forming is a near-net shape forming process used to produce a range of tubular components. Advantages of the process include increased mechanical properties, grain refinement and high production rates. The cost effectiveness of the process stems from a reduction in input weight and a reduction in final machining time as compared to machine from solid routes. Careful selection of flow forming parameters, such as feed rate and spindle speed, is needed to ensure that the dimensional requirements of component design are met. The main purpose of this work was to explore the effects of machine parameters on geometrical outputs of flow formed trial parts in Cr-Mo-V steel, which is used in aerospace applications. Cr-Mo-V steel was flow formed in the annealed and the hardened and tempered conditions to assess the formability of the material across a range of input hardness. Results including inner diameter growth, formed wall thickness and material sectional hardness are presented. Forming trials were conducted at the Advanced Forming Research Centre on a WF STR600/3 flow former, which is equipped with force sensors on all three of the forming axes. The required forming loads are a significant aspect of managing tool life in an industrial setting, therefore the roller loads generated during forming have been studied. Experiments showed that there is a clear link between machine parameters and geometrical outputs. Slower feed rates and faster spindle speeds resulted in larger inner diameter growth and reduced wall thicknesses. Increased spindle speeds also caused a significant reduction in forming load. The hardness of the material was found to be proportional to the thickness reduction imposed on the trial parts in the annealed condition. Overall, it was observed that varying parameters in flow forming produced clear trends in the outputs, which can be used to predict tolerances in the design of components.