Research shows how microscopic crystals grow in molten metals, and paves the way for improving the tensile strength of alloys used in casting and welding
From: University of Birmingham
June 1, 2022 -- Scientists
from the University of Birmingham have described how microscopic crystals grow
and change shape in molten metals as they cool, in research that is breaking
new ground in alloy research and paves the way for improving the tensile
strength of alloys used in casting and welding.
Their research,
published today in Acta Materialia, used high-speed synchrotron X-ray
tomography to ‘photograph’ the changing crystal structures in molten alloys as
they cool.
The study shows that as
aluminium-copper alloy cools the solidification process starts with the
formation of faceted dendrites, which are formed by a layer-by-layer stacking
of basic units that are just micrometres in size. These units start out as L
shaped and stack on top of each other like building blocks, but as they cool
they change shape and transform into a U shape and finally a hollowed out cube,
while some of them stacked together to form beautiful dendrites.
The study was led by Dr
Biao Cai, from the University of Birmingham’s School of Metallurgy and
Materials, whose research has already demonstrated how magnetic fields
influence crystal growth1.
Dr Cai commented:
"The findings from this new study provide a real insight into what happens
at a micro level when an alloy cools, and show the shape of the basic building
blocks of crystals in molten alloys. Crystal shape determines the strength
of the final alloy, and if we can make alloys with finer crystals, we can make
stronger alloys."
He added: “The results
are in direct contrast with the classical view of dendrite formation in cooling
alloys, and open the door to developing new approaches that can predict and
control the formation intermetallic crystals.”
Dr Cai’s previous
research has resulted in a novel technology to improve the quality of recycled
aluminium by removing iron from molten alloy in a simple, inexpensive process
that uses magnets and a temperature gradient.
The technology is the
subject of a patent application filed by University of Birmingham Enterprise.
It has also attracted funding from the Midlands Innovation Commercialisation of
Research Accelerator and the EPSRC-Impact Acceleration Account, which has
enabled Biao to build a large-scale prototype that runs to 1000oC,
and uses a 1 Tesla magnet.
The prototype is
currently being tested using ingots provided by the Tandom Metallurgical
Group, which operates an international trading operation from its base in
Congleton, Cheshire, where they produce aluminium alloys, master alloys and
recycle aluminium products, scraps and drosses.
Dr Cai expects to
publish the results of the testing and showcase the demonstrator to industry
before the end of the year, with the aim of finding industrial collaborators
willing to run tests in foundry settings in combination with existing
production lines.
https://www.birmingham.ac.uk/news/2022/research-paves-the-way-for-stronger-alloys
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