CRANE, Ind. –
A Naval Surface Warfare Center, Crane Division (NSWC Crane) mechanical engineer (ME) received an international award for analyzing the cutting of tantalum, a rare metal that is highly corrosion-resistant. Dr. Jason Davis, an ME at NSWC Crane, led the U.S.-Japanese research team from the Center for Materials Processing and Tribology at Purdue University, IN, USA. The award was announced in September from the Belgium-based Tantalum-Niobium International Study Center (TIC).
The team received the 2021 Anders Gustaf Ekeberg Tantalum Prize (Ekeberg Prize) for its paper “Cutting of tantalum: Why it is so difficult and what can be done about it” published in the journal International Journal of Machine Tools and Manufacture.
Dr. Davis, who was the lead author of the research paper and lead researcher, says this recognition is meaningful to him and the team.
“I worked on this research during my doctoral studies at Purdue and this is my first peer-reviewed publication as a lead author,” says Dr. Davis. “Tantalum research from all over the world was considered for the Ekeberg Prize. I believe this award will bring more attention to the Center for Materials Processing and Tribology at Purdue and validates the excellence of the research they perform.”
The Ekeberg Prize is an annual award recognizing excellence in published research about the element tantalum (Ta). The TIC describes the long term future of the tantalum market, “will depend on technology-driven innovations and a new prize dedicated to this rare and critical element will encourage research and development. The Ekeberg Prize increases awareness of the many unique properties of tantalum products and the applications in which they excel.”
Dr. Jonathan Dilger, the Director of Research for NSWC Crane, says these efforts continue to positively impact NSWC Crane’s mission to support the Fleet.
“I’m quite proud of the achievements of Dr. Davis that are recognized with the receipt of this prestigious international scientific medal,” says Dr. Dilger. “Dr. Davis’s research was co-sponsored by the NSWC Crane Ph.D. Fellowship Program and DoD SMART Scholarship Program, with investments that continue to pay dividends for our regional ecosystem and for the warfighter.”
The authors of the winning paper are Dr. Jason M. Davis, Dr. Mojib Saei, Debapriya Pinaki Mohanty, Dr. Anirudh Udupa, Dr. Tatsuya Sugihara, and Dr. Srinivasan Chandrasekar. The team mostly work at the Center for Materials Processing and Tribology at Purdue University, IN, USA, while Dr. Tatsuya Sugihara is based at the Department of Mechanical Engineering, Osaka University, Japan. Dr. Davis also works at the US Special Warfare and Expeditionary Systems Department, Naval Surface Warfare Center in Crane, IN, USA.
[Researching for a Solution]
Dr. Davis has worked in Small Arms at NSWC Crane for 15 years, and says he has seen the difficulties and limitations with how commonly-used metals are employed in weapons manufacturing.
“A lot of weapon performance is built on how weapons are manufactured. I’ve seen this challenge with metals that are difficult to use, and tantalum is one,” says Dr. Davis.
Dr. Davis says tantalum has many unique properties besides low thermal conductivity that make it worthwhile to research.
“Tantalum is highly resistant to heat and wear,” says Dr. Davis. “It is extremely corrosion resistant, like glass. In the healthcare industry, it is used for medical implants, like hip joints, as well as dental implants, like denture posts. The human body doesn’t react to it; it’s like it’s not there. It is also used in capacitors due to its very thin dielectric layer which results in a high capacitance level per unit volume.”
Tantalum is used across different industries due to its unique and high-quality properties, but Dr. Davis explains why it is not used as often as we might think.
“Tantalum is difficult to work with, so people choose to work with metals with inferior properties,” says Dr. Davis. “It’s a very difficult metal to cut with conventional methods, and therefore difficult to ‘machine’ and use. Our basic research shows a method that works.”
In the article, “Cutting of tantalum,” Dr. Davis and the team of researchers write, “Tantalum has long drawn the ire of machinist, being particularly difficult to cut.” The article continues, “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.” Dr. Davis writes, “These unfavorable attributes of the cutting” have usually been attributed to several things, such as its relative softness and low thermal conductivity.
Dr. Davis writes it is actually due to “the prevalence of a highly unsteady plastic flow – sinuous flow – characterized by large-amplitude folding and extensive redundant deformation.” This ‘sinuous flow’ and the ‘folding’ that occurs in materials are more significant in tantalum than metals such as copper and aluminum.”
In the case of copper and aluminum, there are options available for machinist to alleviate some sinuous flow through simply changing cutting parameters: increase cutting speed, use a tool with a more positive rake angle, work harden the metal prior to cutting, and/or take lighter cuts.
Dr. Davis describes the process for a machinist.
“When cutting copper and aluminum, the cutting forces are relatively low; so, the tool rake angle can be made more positive without too much concern of it breaking. Work hardening decreases the degree to which the metal plastically deforms, thereby decreasing the degree of folding. Smaller depths of cut also decrease the force. In addition, copper and aluminum are somewhat forgiving in that a single cutting pass will sufficiently strain harden the material below it and make the next few cutting passes easier until the strain affected material is removed. The high cutting forces and folding associated with tantalum put these options out of reach for the machinist.”
The research didn’t just study what made tantalum different than other metals to machine; the research conducted what could be done with the element in order to work with it.
Dr. Davis writes “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.” He continues in the article that this shows a “promising new opportunity” to improve its “gumminess.”
Dr. Davis describes how this research is different than other tantalum research.
“This research is unique in that it clearly establishes that this effect is not due to a chemical reaction since tantalum is highly corrosion-resistant. In the cases of copper and aluminum, this is not as easy to defend due to their reactivity with many chemicals. The research using tantalum leaves no doubt the effect of the surface-adsorbing media is altering the deformation through a mechanical means (i.e., a change in surface stress).”
Dr. Davis says this improvement provides exciting results for future capabilities.
“The research we conducted can lead to an increase in the use of tantalum, which could result in better performing technologies. Our research shows that if you can machine tantalum, it opens up a wide variety of uses most people thought wasn’t possible. Tantalum has really perplexed machinists to use…it’s pretty exciting to think we solved it. ”
[From Basic to Applied Research]
According to a description on the National Science Foundation’s website summary of the updated Frascati Manual, basic research is “experimental or theoretical work undertaken primarily to acquire new knowledge of the underlying foundations of phenomena and observable facts, without any particular application or use in view.” Applied research is “original investigation undertaken in order to acquire new knowledge,” and “directed primarily towards a specific, practical aim or objective.”
For gun barrel manufacturing, many refractory metals, or metals that are resistant to thermal wear, are difficult to machine. Metals used in the process would exhibit poor surface quality and shorten tool life, among other challenges.
The broaching process was particularly difficult for rifling a refractory-lined gun barrel – and so far it has been unsuccessful. Through a SMART Seed grant, Dr. Davis is now applying the new knowledge of cutting tantalum to rifling a gun barrel.
The basic research conducted by Dr. Jason Davis as the Principal Investigator (PI) and SMART Scholar not only reveals the fundamental reason behind the difficulty in cutting these soft and/or highly strain-hardening metals -- but also provides a means to suppress the behavior through the use of a mechanochemical surface effect.
“The basic research shows a method that works, that’s the important part,” says Dr. Davis.
The proposed effort will aim to demonstrate that conventional broaching can be used to rifle a refractory-lined barrel, expanding the use of refractory liners to small-caliber weapons as well as providing much needed guidance for traditional machining of refractory metals for other defense applications, such as hypervelocity projectiles.
“Other processes such as grinding, honing, and comminution stand to benefit as well. In principle, many of the same physical parameters are present as in a cutting process,” states Dr. Davis. “I am excited to see how others make use of this research.”
About NSWC Crane
NSWC Crane is a naval laboratory and a field activity of Naval Sea Systems Command (NAVSEA) with mission areas in Expeditionary Warfare, Strategic Missions and Electronic Warfare. The warfare center is responsible for multi-domain, multi- spectral, full life cycle support of technologies and systems enhancing capability to today's Warfighter.