Toward Understanding Pore Formation and Mobility During Controlled Directional Solidification in a Microgravity Environment (PFMI) - 01.09.14
Science Objectives for Everyone Using a transparent model material, this experiment studies the fundamental phenomena responsible for the formation of certain classes of defects in metal castings. Investigators examine the physical principles which control the occurrence of defects in manufacturing on Earth in order to develop methods to reduce flaws, defects or wasted material.
Science Results for Everyone
Bubbles can cause troubles, creating defects during solidification of metals and alloys. This experiment studied the physical principles controlling this phenomenon, using controlled solidification in microgravity. Observations include that thermocapillary forces provide a potential mechanism for avoiding bubble-caused defects in space processing; ground-based (two-dimensional) and flight (three-dimensional) samples showed significant differences in interface structure; and bulk samples from microgravity and the ground are needed for adequate comparison. Researchers are attempting to replicate results on the ground and expect they will provide insights into bubble behavior directly applicable to understanding solidification for materials processing.
Marshall Space Flight Center, Huntsville, AL, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration:
June 2002 - September 2006
Previous ISS Missions
ISS Flight UF2, STS-111 Space Shuttle Flight; samples will be returned on ISS Flight ULF-1, STS-114
- Porosity in the form of bubbles and pipes can occur during solidification processing of pure metals and alloys. It is detrimental to material properties and prevents obtaining meaningful scientific results.
- The Pore Formation and Mobility Investigation (PFMI) is a systematic effort directed towards understanding porosity formation and mobility during controlled directional solidification (DS) in a microgravity environment.
- The investigation will utilize a pure transparent material, succinonitrile (SCN) as well as SCN mixed with water, for the direct observation and recording of pore generation, mobility, and interactions can be made.
On Earth, bubbles that form in molten materials rise to the surface and release trapped gas prior to solidification. In microgravity, where there is no buoyancy or convection, bubbles can become trapped inside the material, leaving pores as the material solidifies. These pores can greatly reduce the finished material?s strength and structural integrity, making it a less desirable product. One of a couple of experiments investigating melting and solidification of materials, the Pore Formation in Microgravity (PFMI) experiment was designed to learn how bubbles form and move during phase change (from liquid to solid) inside molten material. The PFMI experiment used succinonitrile (SCN), a clear organic compound that is a transparent metal analog material, and SCN-water (1%) mixtures to observe bubble formation and bubble movement. The experiment was designed to methodically investigate pore formation and growth using SCN loaded with an excess amount of dissolved nitrogen gas, and examine the role of thermocapillary forces in transporting the bubbles away from the solidification interface.
Experiments were conducted inside the Microgravity Science Glovebox (MSG), a sealed and ventilated work volume in the U.S. Destiny laboratory. The samples were melted inside a thermal chamber with temperature-controlled hot zones and one thermoelectric cold zone. Flow visualization technology was used in support of the experiment to observe bubble movement.
PFMI provides insight on how materials solidify in the space environment. Once this process is understood and improvements are made, future manufacturing processes can take place in the microgravity environment providing robust products.
On Earth, materials that contain pores created and trapped during solidification degrade properties and cause a distinct weakening in the overall structure of the cast product. Examples of these materials include semiconductors and aircraft turbine blades.
PFMI is a partially hands-on experiment, but the crew will never come in direct contact with the molten samples. The succinonitrile samples will be loaded into the tubes preflight, and during the experiment, the crew will manipulate the samples using the sealed glove ports. The temperature inside the thermal chamber zones will be controlled automatically via the MSG's laptop. During Increments 5 and 7, sixteen experiments were performed. During Increment 8, six experiments were performed.
The crew will load the furnace/thermal chamber, sample tubes, and assembly, into the MSG work volume via a port underneath the work volume and make all necessary video and power connections. They will then activate the experiment using the MSG laptop. The zone will reach a maximum of 130 degrees C (266 degrees F), melting the succinonitrile sample. The hardware will then move the sample toward the cold zone, causing directional solidification as the sample passes through the temperature change. In some of the runs, the investigator will try to influence how the bubbles move through the sample. The video system will record the process and download, via the Station's Ethernet, real-time images to the Telescience Support Center (TSC) at the Marshall Space Flight Center (MSFC). The investigator will analyze the downlinked video to determine the parameters of the next run. To allow time for this analysis, no more than two runs per week will be scheduled. In addition to loading the samples and observing the progress of the experiment, they will conduct preventive maintenance on the MSG systems supporting PFMI.
The PFMI experiment used glass tubes (1 cm inner diameter and 30 cm in length) filled with SCN and water in concentrations ranging from pure SCN to 1% SCN water mixture. Of the 24 experiment runs, 21 were successful. The data from this experiment were provided by downlinked images during real-time operations on ISS. In addition, images were recorded using a videotape recorder (VTR) inside the MSG.
Grugel et al. (2004) observed bubble migration up the temperature gradient due to thermocapillary forces and reported that thermocapillary forces do play a role in bubble removal during solidification, thereby providing a potential mechanism for avoiding porosity in space processing.
Strutzenberg et al. examined the morphological evolution of the solidification of the PFMI samples (0.25% water to SCN mixture and 0.50% water to SCN mixture). Direct comparison between the ground-based thin (two-dimensional) samples and the flight bulk (three-dimensional) samples showed significant differences in the interface morphology. The flight samples achieved planar growth, an emergence of dendrites (crystallizes in the shape of a tree or branch), in less time than ground-based samples. When comparing the planar interface recoil, the flight sample was steeper than the ground-based sample. Additionally, the dendrite spacings in the flight bulk samples were closer together than the ground-based thin samples. This highlights the researchers’ premise that the use of two-dimensional (thin) samples in one-g to obtain quantitative data for comparison with theoretical models has significant shortcomings.
Strutzenberg et al. concluded that the thin samples are not adequate to provide data on the initial planar front dynamics, the dynamical condition for the planar interface instability, and the steady-state primary dendrite spacing. Solidification of bulk samples in a microgravity environment and in the lab setting is necessary for a suitable comparison. The flow visualization images obtained for the PFMI experiment allowed Grugel et al. (2005) to study bubble formation in SCN. The bulk solidification samples, which were filled with SCN, were melted and resolidified to observe the bubbles that formed. During controlled re-solidification, aligned tubes of gas were seen to be growing perpendicular to the planar solid/liquid interface, inferring that the nitrogen previously dissolved into the liquid SCN was now coming out at the solid/liquid interface and forming the little-studied liquid=solid+gas eutectictype reaction. The flight sample results could not be duplicated in the ground-based samples.
Cox et al. (2006) are now attempting to better replicate the space phenomena in a ground-based lab by using small-diameter channels to minimize bulk convection and buoyant bubble rise effects. The experiment team expects that the results will be directly applicable to understanding solidification for materials processing by providing insights into fundamental behavior of bubbles. (Evans et al. 2009)
Grugel RN, Luz PL, Smith GP, Spivey RA, Jeter LB, Gillies D, Hua F, Anilkumar AV.Experiments conducted aboard the International Space Station: The Pore Formation and Mobility Investigation (PFMI) and the In-Space Soldering Investigation (ISSI): A current summary of results. 57th International Astronautical Congress, Valencia, Spain; 2006 October 2-6 10 pp.
Strutzenberg LL, Grugel RN, Trivedi R. Morphological Evolution of Directional Solidification Interface in Microgravity: An Analysis of Model Experiments Performed on the International Space Station. 43rd Aerospace Sciences Meeting and Exhibit, Reno, NV; 2005
Grugel RN, Anilkumar AV. Bubble Formation and Transport during Microgravity Materials Processing: Model Experiments on the Space Station. 42nd Aerospace Sciences Meeting and Exhibit, Reno, NV; 2004
Grugel RN, Luz PL, Smith GP, Spivey RA, Jeter LB, Gillies D, Hua F, Anilkumar AV. Materials research conducted aboard the International Space Station: Facilities overview, operational procedures, and experimental outcomes. Acta Astronautica. 2008; 62: 491-498.
Grugel RN, Anilkumar AV, Cox MC. Observation of an Aligned Gas - Solid Eutectic during Controlled Directional Solidification aboard the International Space Station - Comparison with Ground-based Studies. 42nd Aerospace Sciences Meeting and Exhibit, Reno, NV; 2005
Grugel RN, Anilkumar AV, Lee CP. Direct Observation of Pore Formation and bubble mobility during controlled melting and re-solidification in microgravity, Solidification Processes and Microstructures. A Symposium in Honor of Wilfried Kurz, The Metallurgical Society, Warrendale, PA; 2004 111-116.
Grugel RN, Brush LN, Anilkumar AV. Disruption of an Aligned Dendritic Network by Bubbles During Re-melting in a Microgravity Environment. Microgravity Science and Technology. 2012 March; 24(2): 93-101. DOI: 10.1007/s12217-011-9297-y.
Ground Based Results Publications
Pettegrew RD, Struk PM, Watson JK, Haylett DR. Experimental Methods in Reduced-Gravity Soldering Research. NASA Technical Memorandum; 2002.
Struk PM, Pettigrew RD, Downs RS. The Effects of an Unsteady Reduced Gravity Environment on the Soldering Process. 42nd Aerospace Sciences Meeting and Exhibit, Reno, NV; 2004
Grugel RN, Anilkumar AV, Smith GP, Luz PL, Jeter LB, Volz M, Spivey RA.Toward Understanding Pore Formation and Mobility During Controlled Directional Solidification in a Microgravity Environment Investigation (PFMI). Conference and Exhibit on International Space Station Utilization, Cape Canaveral, FL; 2001 5119.
Cox MC, Anilkumar AV, Grugel RN, Hofmeister WH. Isolated Wormhole Growth and Evolution during Directional Solidification in Small Diameter Cylindrical Channels: Preliminary Experiments. 44th Aerospace Sciences Meeting and Exhibit. Reno, NV; 2006 1140.
Diagram and descriptions of the PFMI experiment hardware. Image courtesy of NASA, Marshall Flight Space Center.
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The Pore Formation and Mobility Investigation will melt samples of a transparent modeling material, succinonitrile and succinonitrile water mixtures, shown here in an ampoule being examined by Dr. Richard Grugel, the principal investigator for the experiment at NASA's Marshall Space Flight Center in Huntsville, Ala.
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Video Screen Shot of the PFMI experiment on ISS. This photograph shows a quiescent planar solid/liquid interface that is melting back at a rate of 10mm/s. (For scale, the thermocouple wires seen are 0.5mm in diameter).
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Video Screen Shot of the PFMI experiment on ISS. Shortly after, a bubble is seen moving up the temperature gradient in the liquid, after dislodging from the interface, and another, slightly smaller, is seen at the interface (time t = 0).
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NASA Image ISS013E14515 - Expedition 13 Science Officer and Flight Engineer Jeff Williams conducts the first run of PFMI in the Microgravity Science Glovebox (MSG) in the U.S. Destiny Laboratory.
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NASA Image: ISS013E14573 - Astronaut Jeffrey N. Williams, Expedition 13 NASA space station science officer and flight engineer, conducts the first run of the Pore Formation and Mobility Investigation (PFMI) in the Microgravity Science Glovebox (MSG) in the Destiny laboratory of the International Space Station.
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