Authors: Dr Andrew G Murphy and Dr David J Browne, School of Mechanical and Materials Engineering, University College Dublin Sounding rockets offer short-duration, high-quality microgravity (weightlessness) providing optimal experimental conditions for cutting-edge scientific research into solidification of aluminium-copper crystals. In the absence of gravity effects, nucleation, growth, and interaction of equiaxed dendritic crystals can all be observed without the added complexity of fluid flow or grain motion, dramatically simplifying subsequent analysis and theoretical validation. Researchers from UCD’s Phase Transformation Research Group have recently performed a unique and significant solidification experiment on board the MASER 13 Sounding Rocket, which was launched on December 1, 2015, from Esrange, Kiruna, northern Sweden. [caption id="attachment_26042" align="alignright" width="300"]aaarocket1 Fig. 1 Click to enlarge[/caption] Figure 1 – Schematic illustration of the MASER 13 Sounding Rocket configuration, along with the actual MASER 13 installed in the Skylark launch tower in Esrange, Sweden. Dr Andrew Murphy (left) and Dr David Browne (right) standing in front of the Skylark launch tower days prior to the MASER 13 launch. Inset: MASER 13 payload experimental module configuration.

Solidification science, in situ X-radiography and microgravity

Modern manufacturing demands such as cost reduction, waste elimination, environmental impact, and increase in specific strengths, have led to a fervent increase into casting research in recent years. Although a process that people have been using for thousands of years, casting (pouring molten liquid metals into moulds) is still one of the most cost-effective and efficient methods of manufacturing complex metal shapes. For example, millions of tonnes of castings are produced worldwide every year, manufacturing everything from engine blocks and jet turbine blades, to hip implants and children’s toys. In the last 60 years, through advancement of experimental techniques and increased computational processing power, casting research has moved from a ‘black art’ to an area of dedicated scientific research, i.e. solidification science. While it is possible to cast pure metals such as copper, aluminium and gold, people have known for a long time that objects made from pure metals are typically weak, deforming and/or breaking easily, providing little more than decorative use. However, the addition of small quantities of ‘alloying elements,’ for example, copper added to aluminium or carbon added to iron (steel), results in a material with vastly improved mechanical properties and durability. When considering alloy solidification, the transition from liquid to solid is typically accompanied by both solid and liquid phases coexisting within the same volume. The coexistence of both solid and liquid phases typically leads to gravity-induced fluid flow, solid transport, and segregation, as a result of density differences between the solid and liquid. The consequence of these phenomena is castings with anisotropic mechanical properties and/or casting defects that may lead to catastrophic service-life failure of the cast component. Characterising these gravity-induced effects is extremely complex due to the chaotic nature fluid motion, requiring advanced experimental tools to adequately measure solidification phenomena of interest. Of the last 20 years, one of the most sophisticated methods of investigating alloy solidification has been real-time in situ X-ray radiography, wherein a small volume of an alloy system is melted and solidified at a controlled rate while an X-ray beam is projected through the sample onto an X-ray camera. Density differences between the solid and liquid phases show up as light and dark contrast on the detector, allowing measurements to be made of growth velocities, solid fraction evolution, grain motion, etc. Dedicated solidification furnaces have been developed to melt and solidify a range of materials, for example, aluminium-copper, aluminium-zinc, iron-carbon, and lead-tin. Furthermore, in recent years, advances in X-ray source and detector technology have allowed for the development of high-resolution, compact X-ray imaging diagnostics, providing the opportunity to perform solidification experiments on board microgravity platforms, such as parabolic flights and sounding rockets. The microgravity environment provides optimal experimental conditions when gravity-induced fluid flow, and its effects on solidification, need to be eliminated. The quality and duration of microgravity available varies from platform to platform. For example, Drop Towers provide relatively low-quality microgravity for less than 10 seconds. Sounding rockets provide extremely high-quality microgravity periods for up to 12 minutes, depending on the size of the rocket motor. Virtually unlimited microgravity is available on board the International Space Station (ISS); however, the quality of the microgravity may be somewhat less than that available on board sounding rockets due to the ‘manned’ nature of the space station. Typically, the needs of the experiment will dictate the level and duration of microgravity required. For instance, in preparation for the development of a specially designed isothermal solidification furnace for operation in both terrestrial and microgravity environments, the authors performed a near-isothermal solidification experiment, using a calibrated gradient furnace, on board the 58th ESA Parabolic Flight Campaign, operated by Novespace from Bordeaux, France. The quality of the microgravity on board parabolic flights varies within the range ± 0.05 g – approximately four orders of magnitude poorer than that available on board a sounding rocket. During the microgravity period on board the parabolic flight, free-growing aluminium-copper crystals were shown to move relative to the varying positive/negative g-level, demonstrating much higher quality microgravity was required to adequately eliminate gravity effects during solidification (https://goo.gl/k3pVUL). As a consequence of our parabolic flight trials, as well as extensive terrestrial-based solidification experiments, a new dedicated isothermal solidification furnace was commissioned by ESA, based on design inputs from UCD, and scheduled flight on board the MASER 13 sounding rocket to perform, for the first time, isothermal equiaxed solidification in microgravity using real-time in situ X-ray diagnostic imaging.

XRMON-SOL on board MASER 13

The MASER (Materials Science Experiment Rocket) 13 sounding rocket is a microgravity experiment platform capable of providing extremely low levels of gravity (0.000001 g) for six to seven minutes. Note, under normal conditions, terrestrial gravity is 1.0 g. MASER 13 was launched from Esrange in Kiruna, northern Sweden, on December 1, 2015, with four experimental payloads on board: XRMON, BIM-3, MEDI, and CDIC-3, each of which was investigating the effect of the absence of gravity on various scientific phenomena, for example, solidification, fluid flow, and biological growth. Figure 1 shows an illustration of the fully assembled MASER 13 Sounding Rocket, comprising two booster rockets, the payload, and the nose cone recovery system, overlaid on top of an image of the actual MASER 13 sounding rocket assembled in the Skylark launch tower at Esrange, Sweden. Standing in front of the launch tower are Drs Murphy and Browne (left and right, respectively) during a rocket inspection several days prior to the launch. Also shown (inset) is the stacked payload, i.e. the arrangement of the individual experiments, with XRMON-SOL located towards the top behind the nose cone and recovery system (parachute). Fully assembled MASER 13 stood approximately 13 metres tall. At ignition, the first stage solid-fuelled rocket booster fires providing a thrust of 102 kN for 20 seconds propelling the rocket out of the launch tower with a peak acceleration of 10g. With the fuel expended, the first stage drops off and the second stage ignites for 11 seconds, producing a thrust of 240kN with a peak acceleration of 7.5g, propelling the rocket up to an apogee of ~260 km. The second stage drops off, along with the nose cone, leaving the payload and recovery systems to complete the sub-orbital parabolic trajectory, encompassing the microgravity period, unaided. [caption id="attachment_26045" align="alignright" width="300"]aaarocket2 Fig. 2 Click to enlarge[/caption] During the launch phase, the MASER sounding rocket can achieve velocities of between 1 and 2 km/s, ultimately traversing the distance from ground to apogee in ~260 seconds. After completion of the microgravity period, the payload free-falls back to earth at a maximum velocity of 2 km/s, with the parachute deploying at an altitude of ~30 km ensuring the payload lands safety in the impact zone ready for recovery and subsequent disassembly. The entire flight is complete 11 to 15 minutes after launch. The payload is then recovered from the impact zone by helicopter and airlifted back to the launch site for inspection and breakdown. Figure 2 – XRMON Module breakdown. (a) Fully assembled module prior to payload integration. (b) XRMON assemblage of X-ray diagnostics, furnace, and control system, with outer shell removed. (c) Fully assembled XRMON-SOL Isothermal Solidification Furnace prior to module installation. (d) XRMON-SOL with furnace housing lid removed to allow sample installation. (e) XRMON-SOL sample pocket with aluminium-copper sample disc installed. The completed XRMON module, flown on board the MASER 13 sounding rocket, had a mass of 99kg, a height of 1,140mm and a diameter of 438mm, making it the largest of the four experiment modules in the payload. Figure 2 shows the breakdown of the XRMON module showing, (a) the outer shell with the access cover removed to allow furnace installation and removal from the module, (b) the inner assemblage of X-ray diagnostics, furnace, and control system, (c) the fully assembled XRMON-SOL Isothermal Solidification Furnace, (d) XRMON-SOL with furnace housing lid removed showing inner furnace body, (c) XRMON-SOL sample compartment with aluminium-copper sample installed. Figure 3 shows a schematic illustration of the inner working of the XRMON-SOL furnace, showing (a) the heater arrangement with respect to the X-ray aperture positioned in the centre of the furnace body, (b) an exploded view of the furnace components, and (c) the relative positioning of the XRMON-SOL heater with respect to the in situ X-ray diagnostics. The heater body is a machined boron-nitride monoblock with eight individually wound, controlled, and monitored heater coils, arranged into concentric inner and outer rings, Figure 3 (a). The heater design and configuration allows for precise control and manipulation of the temperature field across the field-of-view (FOV). [caption id="attachment_26046" align="alignright" width="300"]aaarocket3 Fig. 3 Click to enlarge[/caption] Figure 3 – Schematic illustration of the XRMON-SOL furnace assembly and heater arrangement. (a) Heater arrangement with respect to the X-ray aperture (hole in the centre). (b) Exploded view of heater assembly components. (c) Furnace arrangement within in situ X-ray diagnostics. The sample, shown in Figure 3(b), was a alloy of 80:20 (by weight) aluminium to copper in the form of a circular disc measuring 21 mm in diameter by 0.2 mm thick, giving a mass of ~0.2 grams. Included in the alloy were titanium-diboride inoculant particles, which in industry are added to aluminium alloys to promote a fine grain structure resulting in a high strength material. For our purposes, a sufficiently small volume of particles were added to promote nucleation of a few dozen grains within the FOV thereby providing useful information on various solidification phenomena of interest. For integration into the furnace, the sample was sandwiched between two discs of glassy carbon, which is X-ray transparent with a thermal conductivity one to two orders of magnitude lower than aluminium-copper, Figure 3 (b). With the entire assembly together, the furnace is positioned in the path of an X-ray beam, in this case a compact high energy polychromatic microfocus transmission-type, and an X-ray camera, in this case a structured X-ray scintillator mounted to a high resolution visible light CCD, as illustrated in Figure 3 (c). The X-ray scintillator converts X-ray photons into visible light photons, which can then be captured by the CCD creating an image on the sensor. With the X-ray imaging setup described, images were recorded on board the MASER 13 sounding rocket at a rate of three frames per second, with an observable FOV of approximately 4.1 × 2.7mm, and a spatial resolution of 6.2μm.

Microgravity solidification results

Figure 4 shows a sequence of X-ray images recorded in situ and in real-time on board the MASER 13 sounding rocket flight during the six-minute microgravity window. For the first time, we see solid α-aluminium equiaxed crystals (light grey), growing in an enriched aluminium-copper liquid (dark grey) under microgravity conditions, during isothermal solidification. Image contrast in situ is a result of density differences between the solid crystals and liquid. In this case, the low-density crystals absorb relatively little of the incident X-ray beam, thereby showing up as bright regions in the FOV. Conversely, the much denser liquid absorbs significantly more of the X-ray beam resulting in the dark regions surrounding the crystals. Also shown in Figure 4 is the predefined furnace temperature profile set for the experiment. Note, both the image and temperature timescale have been zeroed to coincide with the time of launch. Melting and stabilisation of the sample took place during the first half the microgravity window. The sample was then slowly cooled during the second half of the microgravity period, resulting in equiaxed nucleation and growth from approximately t = 241 s to t = 359 s. The cooling profile was then increased and the sample cooled relatively quickly to ensure complete solidification before the end of microgravity; the sample was fully solid at t = 425 s. Preliminary inspection of the microgravity results shows that isothermal equiaxed solidification was successfully performed during the microgravity period. Equiaxed crystals appeared, uniformly distributed throughout the FOV, at different times depending on the size of the inoculant particle upon which each individual grain grew. Particle sizes generally vary from a few hundred nanometres to a few microns, with larger particles generally nucleating grains earlier and smaller particles nucleating grains later during isothermal solidification.aaarocket4 Figure 4 – X-ray image sequence recorded in real-time on board the MASER 13 Sounding Rocket, and associated temperature profile showing image capture times (+) relative to the microgravity window (shaded). First three images show solid (light grey crystals) and liquid (dark grey background) coexisting during solidification. Last image shows fully solid sample. For comparison, a ground-based experiment was performed using the same sample and the same experiment timeline to characterise the effect gravity plays during solidification. While this work is still underway, some early observations have shown that crystals appear in the FOV earlier than expected in microgravity than on ground, suggesting that gravity acts to retard grain nucleation possibly due to sedimentation of the inoculant particles after the sample has been fully liquefied. During ground-based solidification, grain motion and rotation are clearly evident during solidification due to the gravity-induced buoyancy of the lighter solid crystals in the denser liquid. In microgravity, these effects are eliminated and grains remain stationary during the early stages of solidification. However, grain motion in microgravity was observed during the later stages of solidification due to what is known as ‘liquid feeding’ wherein the liquid surrounding the grains undergoes a volumetric contraction when it changes from liquid to solid. To accommodate this feeding effect, liquid must move towards the growing grains resulting in the observed grain motion. In industrial casting, this phenomenon is called solidification shrinkage and can result in casting defects such as porosity and cracks in cast components, leading to possibly catastrophic failure of the cast component, if not detected early, or more likely rejection and scrapping of the cast part.

Conclusions and future work

For the first time, isothermal equiaxed solidification of an aluminium alloy has been performed in microgravity on board the MASER 13 sounding rocket, monitored using real-time in situ X-ray imaging. This microgravity solidification experiment represents an ideal case study for theoretical and computational model validation. Further and more detailed analysis is planned for the future. Some of the questions regarding closeness of terrestrial experiments to microgravity have been answered, and, more importantly, additional questions based on the microgravity experiment have been raised regarding nucleation dynamics, growth kinetics, grain interactions, and liquid feeding, which may require further microgravity experiments in the future. Finally, this research could ultimately help in the development of improved manufacturing processes for casting of metal products such as medical devices, and energy-saving components for cars and planes. The UCD team is co-ordinating an ESA-funded European consortium of academic and industrial partners on this topic.

Acknowledgements

The authors wish to acknowledge the financial support of the European Space Agency (ESA) under the PRODEX program, (Contract No. 90392 and 4000110414). This work is part of the ESA-MAP (Microgravity Applications Promotion) project XRMON. Thanks are due to Dr Mark Gibson and Mr Daniel East of CSIRO, Australia, for supply of materials. The authors also wish to thank the Swedish Space Corporation (Ylva Houltz, Christian Lockowandt, Jianning Li, and Kenneth Henriksson), as well as ESA representatives Neil Melville and Antonio Verga, without whom this project would have not been possible. Thanks are also due to Dr Bryan Rodgers, of Enterprise Ireland, for support in securing the ESA PRODEX funding for the project.