Submicrocristalline Structure and Dynamic Recovery of Cold Flowformed ELI Grade Ti-6Al-4V

Abstract

Flowforming is a means to produce seamless tubes by plastic deformation at room temperature. It consists in reducing the thickness of a tubular part mounted on a mandrel by deforming it using several rollers translating along the tube axis, while the tube is rotating along its axis. Thanks to the high compressive stresses, and to the incremental nature of the deformation process, flowforming can lead to a high thickness reduction and thus to high elongation of the deformed tubes. Ti-6Al-4V (Extra Low Interstitial grade) tubes have been deformed by cold flowforming, with a thickness reduction ratio higher than 60%, and their microstructures have been investigated using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Based on EBSD data, a post-processing analysis has been performed in order to study the texture of the flowformed parts. Optical Microscopy showed that the material could be deformed without displaying flow instability such as adiabatic shear banding, despite the fact that it has been processes out of the stable processing maps (high strain rate and low temperature). It also evidenced a major deformation along the tube axis accompanied with a slight twist due to torsion stress. EBSD analysis indicated the occurrence of continuous dynamic recrystallization, which is rarely reported in the α-β domain of such alloys. The recovery/ recrystallization effects resulted in a submicrocrystalline equiaxed structure, which is consistent with that previously reported for Ti-6Al-4V subjected to severe plastic deformation (SPD). The texture of the hexagonal α-phase appeared to be similar to that obtained on extruded Ti-6Al-4V, with a basal component perpendicular to the tube axis.

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

“…Init. tube While several materials are widely used for cold flowforming, such as aluminium alloys [1,2] and low carbon steel [3,4], a few authors [5,6] have reported successful flowforming of Ti-6Al-4V alloy (also known as Ti64). In [5], authors have produced tubes utilizing different means (β extrusion, α-β extrusion, rotary piercing and flowforming) and extensively studied the mechanical properties of those tubes.…”

“…They have concluded that the flowforming process was the best candidate to improve yield stress, ultimate tensile stress and fatigue properties together without any remarkable decrease in ductility. Those improvements are related to the grain refinement and the dynamic recovery/recrystallization phenomena occurring during flowforming [5,6].…”