On the Damage Criteria and their Critical Values for Flowforming of ELI Grade Ti64

Cold flowforming is a chipless forming process that deforms tubular parts by reducing theirouter diameter and thickness while increasing their length. It consists of a rotating mandrel and oneor more rollers that are translated along the tube axis, thus plastically deforming it. Flowforming ofTi-6Al-4V (also known as Ti64) is of great interest for improving the mechanical properties of thematerial, such as yield stress and fatigue strength. However this alloy is known to have poor ductilityat room temperature. Therefore, flowforming of Ti64 without failure or crack is a great challenge. Inthis present paper, the authors have attempted to predict the different failure modes occurring duringflowforming. An experimental machine has been built at the Center forMaterial Forming (CEMEF) inorder to monitor the force on the single roller, the torque on the mandrel and the actual rotation speedof the roller as well. Numerous flowforming tests have been performed using different processingparameters, such as working depth, roller feed and initial geometry, in order to investigate the criticalvalues which lead to the failure of the flowformed tube. In addition, numerical simulations of theprocess have been performed using the FORGE FEM solver. The results of the simulations have beenused to evaluate the relevance of usual failure criteria (Crockford-Latham, Rice-Tracey and Oyane).