Engineers conducting experiments in the unique environment of microgravity – known as Zero-G – have gained important new insights into the way metals solidify. Data from experiments installed on rockets that travelled some 260km above Earth helped develop a new method for assessing how solidification takes place – and that will allow for the production of stronger metal components. The global market for AM products and services in 2015 was £3.59bn and has grown at a rate of 31.5 per cent annually.

Additive manufacturing of metal components

Metal solidification is important in the casting, welding, and additive manufacturing of metal components. For example, the engine block of an automobile engine is made by pouring liquid metal into a mould and then allowing it to solidify to take up its final shape. Additive Manufacture (AM) or 3D printing of metal relies on the melting of metal powder by a laser and then allowing it to solidify layer by layer to build up a complete part. Metal solidification is one of the most fundamental processes in manufacturing. For example, in metal casting or welding, engineers must melt all or part of the metal and then allow it to freeze over to give a solid structure again.

Tiny crystal structures appear in the molten metal

Practically all metals in service would have existed as molten liquid at some point in their manufacturing cycle. As they begin to cool and solidify, tiny crystal structures appear in the molten metal and then grow to form the complete solid. A video can be viewed here. “These crystals, known as dendrites, grow to form a complete solid structure. In the end, the final crystal structure leads to something known as grain structure," said adjunct assistant professor at TCD and lecturer at Ulster University, Dr Shaun McFadden. "The grain structure ultimately determines the strength and toughness of every cast metal part because defects occur in the grain structure and small grains give the best performance.” “Because metals are used in countless ways, and the defects in them limit their suitability for certain tasks, we are really motivated to gain a deeper understanding so as to improve crystal nucleation and build tougher components.”

Fluid flow due to gravity in melted metals on Earth this does not occur in zero-G

The zero-G environment is evidenced by the weightlessness on board the International Space Station or on Sounding Rockets, which are launched into space and then go into freefall as they return to earth. Crucially, while you see fluid flow due to gravity in melted metals on Earth, this does not occur in zero-G. This means the crystals that would have floated to the top or sank to the bottom of the metal sample on Earth will appear to be stationary in zero-G, which means it is much easier to observe and accurately record their behaviour as they nucleate and grow in zero-G. In late 2015, the Swedish Space Corporation launched their MASER-13 (MAterials Science Experiment Rocket) sounding rocket campaign from the Esrange Space Center. The rocket reached a height of 260km above the earth, and it was aboard this rocket that the CETSOL research team had included a unique microgravity experiment known as the MEDI experiment.

Transparent material that forms crystals during solidification

“We used a transparent material that forms crystals during solidification in exactly the same way that a metal does, which allowed us to see the opaque crystals growing with the naked eye," said research fellow at TCD Dr Robin Mooney. "And by using low-level magnification we recorded video sequences of the crystal nucleation and growth. Along with insights provided by temperature readings, we then had a unique dataset for crystal growth.” These datasets led the team to develop a fundamentally new mathematical approach – the ‘Nucleation Progenitor Function approach’ - to modelling multiple crystals growing simultaneously in metal constructs. Dr McFadden added: “Our Nucleation Progenitor Function approach is the fruit of all this labour and we have used it to prove the functional progenitor-progeny relationships in the nucleation and growth of the crystals. It is our increased understanding of these relationships that will lead to better understanding of the grain refinement process.” The outcomes of this work have been published across three leading scientific journals: the 'Journal of Crystal Growth'; the 'International Journal of Thermal Sciences'; and the leading journal in metallurgy and metallurgical engineering, 'Acta Materialia'. The collaborative team included researchers from the School of Engineering at Trinity College Dublin, the German research institute Access e.V. (Technical University of Aachen, RWTH), and Ulster University. The research was supported by the Irish Space Delegation at Enterprise Ireland and ESA PRODEX, and was part of the ESA CETSOL programme. A video (credit: ESA, Swedish Space Corporation, CETSOL PROJECT partners, ESA PRODEX and Enterprise Ireland, DLR and Airbus) is available here