Tech Front: Research Team Develops New Ultralight, Ultrastiff Additive Materials
A team of researchers from Lawrence Livermore National Laboratory (LLNL; Livermore, CA) and Massachusetts Institute of Technology (MIT; Cambridge, MA) has developed a new material for additive manufacturing processes that is as dense and light as an aerogel, but has 10,000 times more stiffness.
This material is described in the researchers’ paper published in a June 20 article in the journal Science. The paper, “Ultralight, Ultrastiff Mechanical Metamaterials,” outlines the development of micro-architected metamaterials that have properties not found in nature and that maintain a nearly constant stiffness per unit of mass density. The materials, the researchers say, hold promise for future use in developing components used in automobiles, aircraft or space vehicles.
“These lightweight materials can withstand a load of at least 160,000 times their own weight,” said Lawrence Livermore Labs Engineer Xiaoyu “Rayne” Zheng, the lead author of the Science article. “The key to this ultrahigh stiffness is that all the microstructural elements in this material are designed to be over-constrained and do not bend under applied load.”
While most lightweight cellular materials have mechanical properties that degrade substantially with reduced density because their structural elements are more likely to bend under applied load, the materials developed by the research team maintain ultrastiff properties across more than three orders of magnitude in density. The observed high stiffness is shown to be true with multiple constituent materials such as polymers, metals and ceramics, according to the research team’s findings.
“Our micro-architected materials have properties that are governed by their geometric layout at the microscale, as opposed to chemical composition,” said LLNL Engineer Chris Spadaccini, corresponding author of the article, who led the joint research team. “We fabricated these materials with projection micro-stereolithography.”
The team’s additive micromanufacturing process involved using a micro-mirror display chip to create high-fidelity 3D parts one layer at a time from photosensitive feedstock materials. This allowed the team to generate materials with complex 3D microscale geometries that are otherwise challenging or, in some cases, impossible to fabricate. “Now we can print a stiff and resilient material using a desktop machine,” said MIT professor and key collaborator Nicholas Fang. “This allows us to rapidly make many sample pieces and see how they behave mechanically.”
The research team was able to build microlattices out of polymers, metals and ceramics. They used polymer as a template to fabricate the microlattices, which were then coated with a thin-film of metal ranging from 200 to 500-nm thick. The polymer core was then thermally removed, leaving a hollow-tube metal strut, resulting in ultralightweight metal lattice materials.
Spadaccini, who also leads LLNL’s Center for Engineered Materials, Manufacturing and Optimization, said the team was able to fabricate an extreme, lightweight material by making thin-film hollow tubes. The team repeated the process with polymer microlattices, but instead of coating it with metal, ceramic was used to produce a thin-film coating about 50-nm thick. The density of this ceramic micro-architected material is similar to aerogel.
“It’s among the lightest materials in the world,” Spadaccini said. “However, because of its micro-architected layout, it performs with four orders of magnitude higher stiffness than aerogel at a comparable density.”
An ultrastiff micro-architected material was also created using a slightly different process, by loading a polymer with ceramic nanoparticles to build a polymer-ceramic hybrid microlattice. This polymer was removed thermally, allowing the ceramic particles to densify into a solid.
The new solid ceramic material also showed similar strength and stiffness properties. “We used our additive micromanufacturing techniques to fabricate mechanical metamaterials with unprecedented combinations of properties using multiple base material constituents—polymers, metals, and ceramics,” Spadaccini added. The new materials are 100 times stiffer than other ultralightweight lattice materials previously reported in academic journals.
In addition to Spadaccini, Fang and Zheng, the LLNL-MIT research team consisted of LLNL researchers (Todd Weisgraber; Maxim Shusteff; Joshua Deotte; Eric Duoss; Joshua Kuntz; Monika Biener; Julie Jackson; and Sergei Kucheyev); and MIT researchers (Howon Lee and Qi “Kevin” Ge).
The Department of Defense’s Defense Advanced Research Projects Agency (DARPA) and Lawrence Livermore’s Laboratory Directed Research and Development (LDRD) program funded the team’s research. ME
Tools and Machines Still Turning Research Heads
As component events of THE BIG M, held in Detroit in June, the co-located NAMRC, MSEC and ICM&P conferences highlighted the impressive scope of manufacturing innovation in progress worldwide. The University of Michigan (Ann Arbor) hosted the research forums organized by the North American Manufacturing Research Institution of SME, the ASME Manufacturing Engineering Div. and the Japan Society of Mechanical Engineers.
Machining, tools and materials are ever-evolving research areas that were well represented in papers at all three conferences. From hardening tool coatings by shot peening, to monitoring milling tool wear independently of cutting condition, to experimental validation of active fixture design, international authors discussed new techniques and materials designed to improve efficiency, machinability and accuracy.
Thejas Menon (Wichita State University; KS) and Vis Madhavan (Wichita State and NIST; Gaithersburg, MD) presented the use of infrared (IR) thermography to obtain accurate temperature measurements to validate machining simulations, a key to the proliferation of digital manufacturing. The optical access to the tool surface that IR requires was gained by making the tool out of a transparent material, YAG (yttrium aluminum garnet). This study produced the first successful machining of Ti-6Al-4V with transparent tools and measurement of the distribution of temperature at the chip-tool interface, providing data to verify and fine-tune material and friction models used in simulations of cutting (NAMRC paper #4523).
In their paper, Mehdi Nouri and Barry Fussell (University of New Hampshire; Durham) investigated the feasibility of a new method of tracking tool wear in milling using force model coefficients, which are shown to be independent from cutting conditions and correlated with tool flank wear. The authors are proceeding with further work with a variety of tools and materials to obtain quantitative data to estimate tool wear using this method. Implementation of the model will require an open architecture controller for online access to the cutting program line number and an inexpensive, nonintrusive, wireless force sensor to measure tangential and radial forces (NAMRC #4507).
The root cause for flank and nose wear when machining titanium alloys was explored by Trung Nguyen, Patrick Kwon, D. Kang and Tom Bieler of Michigan State University (East Lansing). The outstanding mechanical properties of Ti alloys and their inherent low thermal conductivity combine to present machinability challenges in a high-production environment. A Ti-6Al-4V bar turned under various conditions was examined for microfracture damage at the cutting edge and for scoring markings. The study results indicate that the root causes are the hard orientation of the alpha crystalline phases and secondarily by the adhesion layer detaching parts of the tool material from the nose and flank surfaces (MSEC #4116).
To clarify the mechanism of hardening DLC (diamond-like carbon) coatings by shot peening for tools and sliding parts such as piston rings, an analysis by XPS (X-ray photon spectroscopy) was conducted by Fumitake Nonoyama and colleagues at Nagoya University (Nagoya, Japan). It was discovered that DLC coating hardness increased most before and after shot peening was carried out at 0.3 MPa, without rapid change in surface roughness, distortion of the coating and generating of a crack (ICM&P #5010).
The selection of cutting conditions is crucial to achieve cost-effective machining of titanium alloys and has been the subject of much research. Continuously changing part shapes and variable cutting conditions make tool life prediction difficult in the roughing operations that are increasingly in demand in the aerospace industry. The paper by Shogo Nakashima, Zhigang Wang and Mark Larson (Makino Inc.; Mason, OH) focused on the milling operation of heat-resistant titanium alloys on several state-of-the-art machining centers, with the production cost variation in several industrial scenarios detailed as case studies (NAMRC #4449).
Optimization-driven machine tool design was addressed by Kiran G. Bhat et al. from the Indian Institute of Technology (Delhi, India) and Dinesh Sharma of Micromatic Grinding Technologies Ltd. (Ghaziabad, India). Finite element analysis-based static and dynamic computational tools, previously used for post-design verification only, help manufacturers deliver machine tools with high precision and light weight. Two improved design-analyze-modify iterative methodologies have been implemented for design of lean machine tool structures: parametric optimization with design sensitivity analysis, and feature sensitivity analysis with topology optimization (NAMRC #4414).
The outcome of final part quality from a manufacturing process depends significantly on fixtures. Experimental validation of an active fixture design methodology was presented by Thomas N. Papastathis and colleagues from the University of Nottingham (UK). Active fixtures constitute a promising approach with the ability to adapt fixturing parameters such as clamping forces and fixture layout to overcome problems of static deformation, displacement, and liftoff and slippage. Careful placement of a clamping element can noticeably improve part surface quality characteristics, among other conclusions noted by the authors (MSEC #4166).
For more information on these or other research papers from NAMRC, MSEC and ICM&P, contact email@example.com. ME
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This article was first published in the August 2014 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 8/1/2014