Cutting of tantalum: Why it is so difficult and what can be done about it

Tantalum has long drawn the ire of machinist, being particularly difficult to cut. Often referred to as being 'gummy,' cutting of tantalum is characterized by very thick chips, large cutting forces, and a poor surface finish on the machined surface. These unfavorable attributes of the cutting have usually been attributed to bcc tantalum's high strain-hardening capacity, relative softness, and low thermal conductivity; and a small shear plane angle. Here, we show using in situ high-speed imaging at low speeds, and ex situ chip morphology observations at higher commercial cutting speeds (625 mm/s), that the gummy nature of Ta in cutting, including the large forces and thick chips, is actually due to the prevalence of a highly unsteady plastic flow - sinuous flow - characterized by large-amplitude folding and extensive redundant deformation. The sinuous flow and the associated folding are much more amplified and extreme in tantalum (chip thickness ratio, 30-50), and with different morphology, than that observed in fcc Cu and Al (chip thickness ratio, 10-20), wherein this flow mode was first uncovered. The in situ observations are reinforced by force measurements, and chip morphology and cut-surface characterization. The observations also suggest that the sinuous flow, in the same genre of unsteady mesoscale flow modes such as shear banding and segmented flow, is quite prevalent in cutting of highly strain-hardening metal alloys. By application of a surface-adsorbing (SA) medium, e.g., permanent marker ink, to the initial workpiece surface, we show that sinuous flow can be disrupted and replaced by a more energetically favorable flow mode - segmented flow - with thin chips and >70% reduction in the cutting force. This flow disruption is mediated by a local ductile-to-brittle transition in the deformation zone, due to the action of the SA medium - a mechanochemical (MC) effect in large-strain deformation of metals. Equally importantly, the MC effect and underlying segmented flow are beneficial also for machined surface quality - producing nearly an order of magnitude improvement in the surface finish, creating a surface with minimal residual plastic strain, and reducing level of material pull-out. Thus, by use of the SA medium and triggering the MC effect, a promising new opportunity is demonstrated for improving the machinability of Ta by ameliorating its gumminess. The results could enhance the viability of Ta for applications such as gun barrel liners - applications for which it has been hitherto considered but discarded due to its poor machining characteristics.