Scientists have captured X-ray images of metal dendrites forming in microgravity for the first time, in research that could help improve manufacturing processes including casting, welding and metal 3D printing.
An international research team, including scientists from the University of Manchester, conducted the experiment aboard the MASER-13 sounding rocket, which launched from the Esrange Space Centre in Sweden.
During around six minutes of microgravity, a sample of aluminium-copper alloy was melted and cooled inside a specially designed furnace. An onboard X-ray imaging system allowed researchers to watch branching crystal structures known as dendrites form and grow inside the metal in real time.
Dendrites develop as molten metal solidifies and have an important influence on the properties of the finished material. Their size, shape and orientation can ultimately affect the strength, reliability and overall quality of a manufactured component.
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Understanding exactly how they develop is therefore important for processes ranging from conventional casting and welding to additive manufacturing.
Studying dendrite growth on Earth, however, presents a problem. Gravity drives convection within liquid metal as differences in temperature and composition cause the material to move. These flows affect heat and solute transport around a growing dendrite, making it difficult to separate the fundamental mechanisms of crystal growth from the effects of convection.
Taking the experiment into microgravity allowed the researchers to largely remove that influence.
The resulting X-ray sequences produced a large volume of data, which the team analysed using a machine-learning system trained to identify and track individual dendrites. It measured their direction, growth rate and interactions with neighbouring structures.
The researchers found that dendrites grew more stably and underwent considerably less rotation without gravity-driven convection. While most followed expected growth patterns, some crystals still developed in unexpected directions, suggesting that mechanisms other than convection can influence their behaviour.
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‘There’s something really striking about seeing how the same metal can grow differently depending on whether gravity is present,’ said Dr Fan Wu, research associate in Manchester’s Department of Materials.
‘The rocket gave us six minutes to observe this process, with X-ray imaging capturing what was happening inside the metal in real time.’
Following the flight, researchers used facilities at Diamond Light Source to produce 3D images of the solidified sample and map its internal crystal structure.
The findings could now provide a useful reference for validating computer models used to predict metal solidification.
Better models could ultimately give engineers greater control over microstructure during manufacturing, improving the consistency and performance of components produced through casting, welding and metal additive manufacturing.
Dr Wajira Mirihanage, senior lecturer in materials science at Manchester, said the microgravity dataset provides researchers with a valuable way of testing whether existing manufacturing simulations are accurately capturing the underlying physics.
The research is published in the journal Acta Materialia.