Additive manufacturing or solid freeform fabrication (SFF) is a general term used to describe techniques of build­ing three-dimensional parts, layer-by-layer, based on computer mod­els. Interest in this technology as a research tool started in the mid-1990s.  This is a very important materials processing technology and has spread widely since. Additive manufacturing, as the name suggests, is a method of building components by addition of material as opposed to removing material from a larger piece. Traditionally, a component with a complex geometry was manufactured by the removal of material; by drilling, grinding, machining, etc. Additive manufacturing creates a part, layer-by-layer, from “nothing” into a finished component. Ideally, there is no need for a machining process. Some additive manufacturing or SFF techniques are:

• Laser sintering/curing
• Three dimensional printing
• Stereolithography
• Fused deposition

Stochastic and nonstochastic voids

A very important feature of SFF is the ability to create both stochastic and nonstochastic foams. A stochastic foam structure is a cell structure having a random variation in cell size, shape, and type (open or closed porosity). The term stochastic usually refers to the cell characteristic obtained by traditional foam blowing techniques such as chemical and physical foaming agents, i.e. the type found in seat cushions and Styrofoam insulated coolers. One could make a stochastic foam using SFF. A blown foam cellular structure can be imaged using, for example, X-ray computer tomography. The output file from the X-ray computer tomography can then be used to reproduce the foam using one of the SFF techniques. In contrast, a three-dimensional nonstochastic foam structure consists of repeating units of open cells, see Fig. 1 (Ortona, D’Angelo et al. 2012). These structures and void geometry can be generated based on analysis for individual applications thus minimizing weight while maximizing performance and functionality. Because of the ordered nature of nonstochastic structures, one can optimize it via computer modeling with far better accuracy that can be done with stochastic foams. So once the structure is designed, one of the SFF methods can be used to make the optimized nonstochastic structure.

Creating functional voids

The advancements in SFF techniques represent a significant and promising shift in developing unique structures ideally suited for individual applications. Significant advances have been made using this technology in the area of biomaterials and biostructures, e.g., creating bone and tissue scaf­folds for implanting into the body. We discussed the need for designed porosity in biomaterials in a previous The Space Between blog, Biomaterials and Much Ado About Nothing(ness). Another promising application is that of structural sandwich composites.  When thin sheets of, for example, metal or polymer are placed at top and bottom of the structure in Fig.1, one gets a sandwich composite.  A sandwich composite is simply face sheets or skins with a lightweight core sandwiched in between.  Figure 2 shows a schematic of the most common sandwich structure that uses a honeycomb core.  Honeycomb is a two-dimensional nonstochastic foam. This type of structures dates back to 1915 when Hugo Junkers patented sandwich composite incorporating a honeycomb core with metal face sheets.  This structure went on to revolutionize aircraft and aerospace construction – but that is a story for another blog!

When used in sandwich composites, the three-dimensional, nonstochastic struc­tures outperform the random cell structure of a stochastic foam.  With proper design, the high strengths of 3-dimensional nonstochastic foams cores can be superior to even honeycomb at a lower relative density.  This higher strength of the three-dimensional cell structure is due, in part, to their superior buckling resistance(Wadley, Fleck et al. 2003).

SFF and hierarchical porosity

Hierarchical porosity or the design of porosity on several length scales is important for many applications ranging from biomaterials, structural materials to anti-wetting (self cleaning) surfaces. Each scale of porosity is designed to perform an essential function. SFF allows one to design macroscale, nonstochastic porosity. However most SFF techniques are not able to create smaller features, such as porosity in micrometer range. Therefore, to overcome this and create hierarchatical porosity via SFF, a traditional foam forming methods are typically employed. A sacrificial template method has been used to produce ceramic (Ortona, D’Angelo et al. 2012) and metallic (Ryan, Pandit et al. 2008) struts making up the nonstochastic structures. Chemically blown polymeric foam struts of a stochastic nature can be made by simultaneous UV curing/porogen decom­position, releasing CO₂gas (Schlögl, Reischl et al. 2012).   In addition, hollow glass microspheres can be incorporated into the strut formulations to create syntactic foam scaffolding.

The important point to realize is that the pore structure can be con­trolled on multiple length scales; stochastic on the micrometer scale and nonstochastic on the macroscale.  As the dimensional control of SFF processes improves, researchers will gain the ability manipulate smaller and smaller features.  This means that SFF will not only be a tool for creating complex parts economically, it will become a tool for imparting interesting functionality; functionalities that come with the control of voids and porosity.

In our book, Voids in materials: from unavoidable defects to designed cellular materials we provide details on the design, processing, characterization, and functionality derived from atomic scale defects up through macroscale porosity.  The basic premise is that,at some level, all solids contain voids, whether intrinsic or intentional.  Sometimes the voids are ignored, at other times they are taken into account, and other times they are the focal points of research.  In this book, we take due notice of all of these occurrences of voids, whether designed or unavoidable defects. We define these voids (or empty spaces in materials), categorize them, characterize them, and describe the effect they have on material properties. Read more about ‘The Space Between’ and materials voids by Gary:

• Biomaterials and Much Ado About Nothing(ness)
• To the Bottom of the Sea Using Buoyancy Voids
• A Cause and Solution to Global Warming

Gladysz and Chawla’s upcoming book Voids in Materials: From Unavoidable Defects to Designed Cellular Materials is now available for pre-0rder on the Elsevier Store. Use discount code “STC3014” and save up to 30% on your very own copy!

About the Authors

Dr. Gary Gladysz (Twitter: @GMGladysz) is an Associate at Empyreus Solutions, LLC, Seattle WA, USA, where he consults and leads university and government technical interactions. He received his PhD from the New Mexico Institute of Mining & Technology where he participated in the NATO Collaborative Program with the German Aerospace Institute (DLR).   Since receiving his PhD, he has led research efforts in university, government and industrial settings. He has extensive research experience designing and characterizing fibrous composite materials, ceramic composites, polymers, composite foams, and thin films.

As a Technical Staff Member at Los Alamos National Laboratory (LANL), he was technical lead for Rigid Composites and Thermoset Materials. In 2005 he was awarded the LANL Distinguished Performance Group Award for his work leading materials development on the Reliable Replacement Warhead Feasibility Project. He has served on funding review boards for LANL, National Science Foundation, ACS, and the Lindbergh Foundation. He has been guest editor on four issues of leading materials science journals, including Journal of Materials Science and Materials Science & Engineering: A. Dr. Gladysz has organized five international conferences/symposia on syntactic foams and composite materials. He started and currently chairs the ECI international conference series on Syntactic & Composites Foams. Dr. Gladysz currently lives near Boston, Massachusetts, USA.

Krishan Chawla is Professor Emeritus of Materials Science and Engineering at the University of Alabama at Birmingham, USA. He received his Ph.D. from the University of Illinois at Urbana-Champaign. His research interests encompass processing, microstructure, and mechanical behavior of materials. He has taught and/or done research at several universities around the world. Professor Chawla has served as a Program Director for metals and ceramics in the Division of Materials Research, National Science Foundation.

He is a fellow of ASM International. Among his other awards are: Distinguished Researcher Award at New Mexico Tech, Distinguished Alumnus Award from Banaras Hindu University, Eshbach Society Distinguished Visiting Scholar award at Northwestern University, Faculty Fellow award at Oak Ridge National Laboratory. Professor Chawla is the author or coauthor of various textbooks in the area of materials and serves on the editorial boards of a number of journals. He is editor of the journal International Materials Reviews.

References

ORTONA, A., D’ANGELO, C., GIANELLA, S. and GAIA, D., 2012. Cellular ceramics produced by rapid prototyping and replication. Materials Letters, 80(0), pp. 95-98.

RYAN, G.E., PANDIT, A.S. and APATSIDIS, D.P., 2008. Porous titanium scaffolds fabricated using a rapid prototyping and powder metallurgy technique. Biomaterials, 29(27), pp. 3625-3635.

SCHLÖGL, S., REISCHL, M., RIBITSCH, V. and KERN, W., 2012. UV induced microcellular foaming—A new approach towards the production of 3D structures in offset printing techniques. Progress in Organic Coatings, 73(1), pp. 54-61.

WADLEY, H.N.G., FLECK, N.A. and EVANS, A.G., 2003. Fabrication and structural performance of periodic cellular metal sandwich structures. Composites Science and Technology, 63(16), pp. 2331-2343.