<p>Tantalum,
niobium, stainless steels, and nickel are corrosion-resistant metals that have
become critical in many industrial sectors. Due to the demanding environments
and temperatures in which they operate, few materials can serve as substitutes.
The advantages of these materials are offset by the difficulties in their
machining. Belonging to a group of metals and alloys often referred to as
‘gummy’, their poor machinability or gumminess is manifest as thick chip
formation, large cutting forces, and poor finish on cut surface. Hence,
machining costs can be prohibitive, and applications limited. The gumminess has
been attributed broadly to their high strain-hardening capacity.</p>
<p>To examine why
these metals are difficult to machine, we used direct <i>in situ</i> observations of the cutting process with a high-speed
imaging system, complemented by force measurements. The observations showed that chip formation
occurred by repeated large-amplitude folding of the material – sinuous flow –
with vortex-like components and extensive redundant deformation. The folding
was particularly severe in Ta and Nb. Although Ta and Nb displayed a higher
rate of fold nucleation than the Ni and stainless steel, the flow dynamics
underlying chip formation across the metals was the same – sinuous flow
nucleated by a plastic (buckling-type) flow instability on the workpiece
surface just ahead of the advancing tool. The large strains and energy
dissipation associated with sinuous flow is the reason for the poor machinability
of these metals. </p>
<p>Prior work with
Cu and Al has shown that sinuous flow can be disrupted and replaced by an
energetically more favorable (segmented) flow mode, characterized by
quasi-periodic fracture, when suitable chemical media are applied to the
initial workpiece surface – a mechanochemical effect. The segmented flow is
beneficial for machining processes since it involves much smaller forces and
plastic strains. It has been hypothesized that the chemical media influence the
flow through their adsorption onto the workpiece surface, thereby altering the
surface energy and/or surface stress, and effecting a local embrittlement
(ductile-to-brittle transition). </p>
<p>We demonstrate
similar media (mechanochemical) effects and segmented flow development in cutting
of the corrosion-resistant metals, with significant benefits for their
machining. These benefits include > 35 percent reduction in the cutting
force/energy, and an order of magnitude improvement in cut surface quality
(finish, tears and residual strain). Importantly, the experiments with the
corrosion-resistant metals provide strong evidence that it is indeed adsorption
– not corrosion, as in case of hydrogen
embrittlement – that underpins the mechanochemical effect. The
experiments used chemical agents well-known for their strong adsorption to
metal surfaces, namely green corrosion inhibitors (e.g., plant extracts,
propolis) and other natural organic molecules (e.g., dyes, antibacterial drugs,
cow’s milk). Lastly, the suitability and application of the mechanochemical effect
at industrial cutting speeds is explored in turning experiments with these
corrosion-resistant metals. Collectively, our observations, measurements, and
analysis show that the gumminess of metals in cutting is due to sinuous flow;
the gumminess can be eliminated by use of chemical media; and adsorption is the
key to engendering the mechanochemical effect. Implications of the results for
industrial processes ranging from machining to particle comminution, and for sustainable
manufacturing are discussed.</p>
<br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12563921 |
| Date | 25 June 2020 |
| Creators | Jason Marion Davis (9027656) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Exploring_the_Role_of_Surface-Adsorbing_Media_in_Cutting_of_Corrosion-Resistant_Metals/12563921 |
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