The Origin of Flank Wear in Turning Ti-6Al-4V

Unlike ferrous materials, where the cementite (Fe3C) phase acts as an abrasive that contributes to flank wear on the cutting tool, most titanium (Ti) alloys possesses no significant hard phase. Thus, the origin of flank wear is unclear in machining Ti alloys. To address this question, a Ti-6Al-4V bar was turned under various conditions with uncoated carbide and polycrystalline diamond (PCD) inserts, most commonly used tool materials for machining Ti alloys. These inserts were retrieved sporadically while tuning to examine the wear patterns using a confocal microscope. To correlate the patterns with the microstructure of the original bar, the microstructure was carefully characterized using Orientation Image Microscopy™ (OIM) with electron-backscattered diffraction (EBSD). From the wear patterns, two distinct types of damage were identified: (a) microscopic and macroscopic fractures on the cutting edges and (b) scoring marks on flank faces. This paper demonstrates that both types of damage were caused primarily by the heterogeneity in hardness in the α-crystals, where the plane perpendicular to the c-axis in an α-crystal is substantially harder than any other direction in the α-crystal as well as the isotropic β-crystal. In addition to such heterogeneities, adhesion layer, ubiquitous to machining Ti alloys, detaches small fragments of the tool, which resulted in microscopic and macroscopic fractures observed on flank wear.

Numerical Modeling of Pit Growth in Microstructure

This work presents the numerical modeling of two-dimensional stable corrosion pit growth by solving the Laplace equation which defines the electric potential within the electrolyte. Microstructural features representative of a 316 stainless steel provides the matrix in which the pit grows. Real microstructural features are incorporated into the computational model. The objective is to determine the influence of the microstructure, specifically crystallographic orientation, on the shape of the pit as it grows over time. The high-resolution definition of the microstructure is obtained by the orientation image microscopy (OIM) technique and is incorporated into the finite element model through a grid-based interpolation functionality. The steel-electrolyte corrosion front movement is simulated with the help of the arbitrary Lagrangian-Eulerian (ALE) meshing technique. The front speed, or the material dissolution rate, is approximated with the use of a Butler-Volmer relationship that relates the dissolution current density to the applied overpotential. The results show that small fluctuations (5–10%) in corrosion potential due to the changing crystal orientation ahead of the corrosion front result in variations in pit shape similar to experimental observations reported in the literature.

β Microtexture Analysis in Correlation with HCP Textured Regions Observed in a Forged Near Alpha Titanium Alloy

The presence of hcp regions with grains having relatively close orientations has been reported in commercial near alpha titanium billets (IMI 834, Ti 6246, etc). The size of these textured regions (called macrozones) is significantly larger than the average grain size of the microstructure observed after thermomechanical processing. The elongated shape of these large hcp regions suggests that they are eventually related to large prior b grains that pancaked during the ingot break down process. In this contribution, Orientation Image Microscopy was used to study the relationship between the hcp local microtexture heterogeneities and the prior b orientations. Specifically, the orientations of the primary (equiaxed) ap grains and the secondary (lamellar) as colonies produced after the transformation of the b phase were discriminated from OIM maps. Furthermore, from the as inherited OIM map, it was possible to reconstruct the corresponding b OIM map over large regions. The analysis showed that the large hcp macrozones observed in the as received material are not related to corresponding bcc macrozones. However, within an hcp macrozone, various clusters of b grains with similar orientations can be found. In such coherent regions, randomly orientated b grains were also observed, which could be related to microstructural changes during deformation (continuous dynamic recrystallization) as suggested by hot deformation results.

Microstructure and Texture of a Warmed Rolled IF Steel

Steel rolling within the temperature range intermediate between hot and cold rolling represents today a very economical and technically viable operation. The present work investigates the microstructure and the texture developed in a Ti microalloyed IF steel by rolling at 400°C and 600°C. Reductions of 40% and 60% were applied to a set of as hot rolled strip specimens part of which was subsequently annealed at 800°C for 5 minutes. Both the microstructure and the texture were examined by scanning electron microscopy. The texture was analysed by Electron Back Scattering Diffraction (EBSD) and Orientation Image Microscopy (OIM), and represented via the ODF method. The plastic anisotropy was also characterised. It was found that the textures developed by warm rolling are similar to those obtained by cold rolling, the intensities being also of comparable value. The microstructure is characterised by the presence of many shear banded grains whose amount was also quantified.