Free Form Fabrication in Space Ken Cooper, NASA Marshall Space Flight Center Robert Crocket, California Polytechnic State University, San Luis Obispo Floyd Roberts, Mabel’s Prototypes and Coffee Shop “…5…4…3…2…1…Liftoff! Of NASA’s first manned mission to Mars! For a total 900 day mission, including 500 days on the Red Planet’s surface1…” It has been a dream of mankind for ages…to set foot on that elusive Red Dot in the sky. To see what is really there, to see more than robotically broadcast images, to reach down and grasp a handful of that rustic soil and let it sift slowly through gloved fingers. But what will it take to get there? It is not a trivial issue to maintain a crew of humans for three solid years on artificial life support systems, nor is it easy to transport the required mass of structures needed to sustain that life over the millions of miles to Mars. No coming up for air, no two-week re-supply from Mother Earth. Even the small robotic missions have had a hit-and-miss history regardless of the height of technology stacked into them. A critical part of long term manned space presence will be the capability to build and repair a large array of components on the go, from circuit boards, to brackets to complete living structures. How will we be able to do it? This presentation will focus on one aspect of manufacturing in space. A recent NASA-sponsored conference on In Space Fabrication and Repair specifically asked these difficult questions to a group of experienced manufacturing professionals from NASA, industry and academia. A significant amount of the discussion focused around a specific set of new manufacturing technologies, collectively deemed as “rapid prototyping” (RP) or “solid free form fabrication” (SFF). These fabrication techniques are unique in that they build objects additively by selectively depositing material to “grow” a part from the bottom-up, as opposed to traditional subtractive or forming manufacturing techniques. While these technologies may or may not apply to large scale structure fabrication, there is a high likelihood that they can be used on parts built in the “shirtsleeve” environment of a spacecraft. Solid Free Form Fabrication Technologies Fused Deposition Modeling (FDM) is an extrusion-based process for depositing thin layers of polymer to form a solid model. The build stock comes in a filament (wire) form, which is pushed through a heated orifice to extrude a semi-molten bead of material in a directed manner (See Figure 1). Current build materials encompass plastics like acrylonitrile-butadiene-styrene (ABS), Polycarbonate and Polyphenylsulfone (PPSF), but successful experiments have deposited higher strength polymers and green ceramic materials. The FDM process has been previously tested in short-term microgravity conditions, which will be discussed later. 1 “Application of Solid Freeform Fabrication Technology to NASA Exploration Missions”, Watson, Petersen, Crockett.

42nd AIAA Aerospace Sciences Meeting and Exhibit5 - 8 January 2004, Reno, Nevada

AIAA 2004-1307

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Selective Laser Sintering (SLS) forms parts in layer-wise fashion in a powder bed of polymer, sand, or polymer-matrix metal. A powder bed is heated up to within a few degrees of the glass transition temperature, then thin layers of the powder are spread across the build area with a roller, and finally a laser beam is used to selectively melt the desired part area at that particular “slice” (See Figure 2). Current materials used included nylon, silica and zircon sand, as well as stainless steel and A6 tool steel alloys. A similar process utilizing an electron beam for a power source has recently been introduced as well. Since the build stock comes in powder form that must rest in a contained bed, these processes will be more suited to planetary surface operations where gravity is significant.

Laser Engineered Net Shaping (LENS) and Precision Optical Manufacturing (POM) deposit metal powder into a high-powered laser beam on a moving substrate to form solid metal parts. These processes can be imagined as an artistic welding technique, where a 1-millimeter diameter laser weld pool is injected with the stock material from an inert gas jet as the process “draws” the part in free space (See Figure 3). Since these processes build fully dense structural materials, they are highly being considered for in space fabrication and repair applications. The issues lie in the control and containment of the powder build medium.

Electron Beam Welding (EBW) is a high intensity welding process used in conventional joining applications. The NASA Langley Research Center is conducting research in applying the process to forming net-shape parts, as well as joint work with the NASA Johnson Space Center developing a low-voltage e-beam system for potential hand-held repair operations in space2 (See Figure 4). This technique provides advantage over powder-feed processes in that the stock material is in wire form, reducing the complexities faced with microgravity powder handling.

2 “Development of a Prototype Low Voltage Electron Beam Free Form Fabrication System”, Watson, Taminger, Hafley, Petersen.

Ultrasonic Object Consolidation (UOC) utilizes solid-state joining techniques to deposit layers of tape to form solid aluminum parts (See Figure 5). This unique SFF technology lays up complex three dimensional structures employing the simple low temperature, low heat solid state welding process often used in electronics, fabrics and foil industry. The UOC process may also have application in bonding dissimilar materials as well as on-demand repairs, as it gets around many of the safety issues involved with handling lasers or electron beams in space.

Laser Precision Metal Deposition (PMD) deposits spooled flat metal wire into a laser beam to form up solid metal parts (See Figure 6). The part/repair area height is determined by the number of layers and thickness of the wire. The bonded area doesn’t actually reach a melting state, but is more akin to a tack-welding process. The head swivels to turn corners as the build wire is continuously fed to the contact area. This technology may also be attractive for in space fabrication due to the fact that it requires less laser power than powder-based processes, as well as it eliminates the microgravity powder handling issues as stated before. Ground and Reduced-Gravity Flight Experiments In a joint project between NASA/MSFC and NASA/JSC, a commercial Stratasys Fused Deposition Modeling System (FDM 1600) has been used in ground experiments to build parts perpendicular and opposing to the gravitational field. These experiments provided a preliminary understanding of the deposition phenomena without gravity assist, and served to define the experiments that were conducted in the NASA KC-135 reduced-gravity aircraft. Flight tests in the KC-135 occurred in June, 1999. These tests involved a total of 160 parabolas. Experiments conducted aboard the reduced gravity aircraft were designed to develop a better understanding of the controlling factors and various parameters for FDM in reduced gravity. The key to FDM in reduced gravity was found to be ensuring continuous bead contact with the substrate (the previous layer). In 1g, bead tears due to unbalanced forces produced by the liquid surface tension can create flaws or gaps in the final part -- the unidirectional force of gravity, however, acts to stabilize the bead and overcome minor fluctuations during deposition (e.g. nozzle speed, tip height, flow rate). In reduced gravity, the consequences of deposition fluctuations are more severe; as droplets will likely form and bead continuity will be lost.

It was verified that continuous contact between the deposited melt and a solid surface was critical in the absence of the stabilizing gravitational force. Figure 7(a) illustrates this result; while the gaps were actually created by the 2-g pullout of the reduced gravity aircraft, the propagation and magnification of the error is of interest. It should also be noted that the ABS plastic used in these experiments is a “self-healing” system due to its high viscosity and solidification rate; extending layer wise deposition to a wider range of materials requires a better understanding of the impact of material rheology and processing parameters on deposition, which requires an extension of the analytical model described previously. Conversely, with such knowledge of the impact of material rheology on deposition, it is also possible to take advantage of reduced gravity. Under certain conditions, material can be extruded into free space (without a support), as shown in Figure 7(b). These experiments were primarily of a technology demonstration nature, serving as “proof of principle” of layer wise fabrication in microgravity. What is required now is to create a solid foundation of applied materials research for the use of deposition-type fabrication in a microgravity environment. One variable to be studied in microgravity is an important issue that occurs during the first phase of the deposition process. The phenomenon of die swell is the dramatic observable increase in diameter of a viscoelastic stream leaving a die or extrusion orifice. Die swell is related through state equations to heat transfer, surface tension, gravity, and pressure differential, flow rate and various material-specific parameters. Current viscoelastic work is empirically based.3 Tanner4 developed a theory of jet-swell which allows the prediction of the swelling ratio D/d (where D is the jet diameter and d is the pipe diameter) from the dimensionless Weissenberg number Wi, the product of the shear rate at the pipe wall

and the relaxation time of the fluid λ. The resulting relation is:

where (D/d)i = 1.13 is the inelastic swelling ratio. Gravity does have an effect on the jet: the jet becomes increasingly thin as the viscoelastic fluid is drawn down by its weight. This effect is less important in the immediate vicinity of the outlet, but the result is an underestimate of the swelling and hence the relaxation time and the elasticity. Thus, effects of gravity (and surface tension which also acts to reduce the jet diameter5), as well as nonlinearities in the constitutive

3 VISCOELASTIC CHARACTERIZATION OF LOW-GRADE MOLASSES BY JET-SWELL MEASUREMENTS By G. D. McBain*, J. A. Harris*, Y. K. Leong* and S. Vigh** *School of Engineering, James Cook University **CSR Ltd.. 4 Tanner, R. I. (1970). A theory of die-swell. J. Poly. Sci. Pt A-2 Poly. Phys. 8:2067-2078. 5 Fredrickson, A. G. (1964) Principles and Applications of Rheology. Prentice-Hall, Englewood Cliffs, New Jersey

equation, have meant that an adequate jet-swell model remains elusive. Further ground based research, along with flight experiments, are required to verify the viscoelastic theoretical foundation necessary for control of extrusion processes such as deposition in both 1g industrial settings and in a reduced gravity environment. This is just one of the many variables to be considered for the flight of a solid freeform fabrication apparatus, with others including initial extrusion, solidification rates, bead spreading, or in the case of metallic welding-type processes, surface transport phenomena and weld-pool formation. Where To Go From Here In addition to internal NASA studies, there are several directives from NASA to private industry and academia to pursue these interesting technologies for fabrication in space. Under the Small Business Innovative Research program, the 2003 call for proposal cites… “Of particular interest are free form fabrication or forming technologies that can utilize direct or minimally processed local materials and are easily transportable.”6 Also, from a 2003 NASA Research Announcement call for proposals: …“Free space” fabrication processes will be involved in the manufacture of a variety of objects of different geometries: surfaces (e.g., solar sails and panels, solar collectors, radiators, shields), structural elements (e.g., trusses, beams, shells), ribbons and fibers (e.g., electrical and optical circuitry, antennas) and small complex parts (e.g., fasteners, mechanical devices). To achieve these goals, research is needed on the behavior of materials throughout the entire process, from handling raw materials to incorporating them into finished parts, under the unique conditions defined by the space environment…7 Regardless of the path in which we get to Mars and beyond, there will almost certainly be a need to fabricate or regenerate components along the way. Solid free form fabrication technologies, whether as we know them today or in some adaptation thereof, may provide at least one key to enabling long-term human mission success.

6 F2.01 In Situ Resources Utilization of Planetary Materials for Human Space Missions, Lead Center: JSC 7 http://research.hq.nasa.gov/code_u/nra/current/NRA-02-OBPR-03/index.html