Metallurgical quality is gaining importance for metalcasters, and for producers of aluminum castings that issue is even more critical because of the material cost of aluminum alloys: primary aluminum costs can erode the revenues of high-volume orders, and poor quality secondary aluminum can reduce productivity for a foundry or diecaster. In particular, trace elements of ferrous material in secondary aluminum can make the final material brittle, and limit its use for high-value automotive or aerospace applications.
A researcher at England's University of Birmingham, School of Metallurgy and Materials, Dr. Biao Cai, has developed a method for identifying and removing iron from secondary aluminum using magnets and a temperature gradient. The invention has been patented by University of Birmingham Enterprise, and supported by the Midlands Innovation Commercialisation of Research Accelerator, which awarded Biao a grant to build a large-scale prototype.
Recently, Dr. Cai published a report on his findings concerning microscopic changes that take place when molten alloys cool and solidify under a magnetic field. For FM&T, he described his research and how it is relevant to both the manufacturing and recycling of metals and alloys.
"Magnets have long been used in metal manufacturing, including casting, welding and recently, additive manufacturing. What we are interested in is the underlying physics, how the solidifying liquid metal interacts with imposed magnetic fields. To do this, we used two advanced methods.
"One is high-speed synchrotron X-ray tomography, which can capture 3D images in seconds. This technique is known as 4D X-ray imaging, available at synchrotron sources such as the European Synchrotron Radiation Facility and Diamond Light Source. Using this, we directly visualize what happens at the interface of solid and molten alloys as they cool and solidify with and without magnetic fields.
"The second is a computational model that learned all the physics and runs on high-performance computer clusters, which allows us to look into the same phenomenon in the virtual world.
"What we have discovered is that magnetic fields can control the flow of the liquid metals, leading to serious segregation, screw-like structure, and fine crystals.
"Further, what we have seen is a phenomenon known as magneto-hydrodynamics. The interaction of liquid metals with the imposed magnetic field, drives or disrupts liquid metal flow, and subsequently alters microstructural evolution.
"This is significant because the microstructure of metals and alloys ultimately determines their physical properties on a macro scale. The application of magnetic fields to control element distribution has already been used in semiconductor crystal growth. If controlled properly, we should be able to produce favorable microstructure and hence lighter or stronger superalloys. I am now interested in using this concept in laser-based additive manufacturing to control microstructure formation.
"Also, this technique can be used to remove impurity elements in metals and alloys. Here my work has concentrated on removing iron from aluminum alloys.
"Iron is normally detrimental to aluminum alloys, leading to the formation of intermetallic. Iron is concentrated with successive recyclings, which makes recycled aluminum brittle and limits its use in premium applications such as aircraft.
"Existing methods for removing iron from aluminum during recycling are either expensive or inefficient.
"I applied a magnetic field to control melt flow of an iron-containing aluminum alloy during solidification and found out that iron distribution can be controlled on both macro- and micro-scale. The governing force is the induced Lorentz force, which drives the flow of iron impurity in molten aluminum alloys. This inspired me to develop a technique to remove iron contamination from recycled aluminum alloys.
"The University of Birmingham has patented this technique. We’ve already approached small and large-scale companies and industry groups to seek advice on how the technology can be developed, and we are interested to hear from potential commercial partners in the recycling sector.
"We’ve been awarded funding to build a lab-scale prototype, and we hope to have this ready to show to industry by early 2021."
Dr. Cai’s research was published recently by Acta Materialia, and is available at: https://bit.ly/3ml2Luw