Astronauts, get your welding goggles on – the space station is going into the foundry business. The International Space Station (ISS) is set to do a spot of industrial research this June, when ESA’s Materials Science Laboratory-Electromagnetic Levitator (MSL-EML) heads for the station aboard Europe's’ Automated Transfer Vehicle 5 (ATV-5) Georges Lemaître unmanned space freighter as part of a program to study the casting of alloys in a weightless environment.
Most metals are crystalline and their properties depend on this microstructure, which develops as they cool. An everyday version of this is tempering, where a steel knife blade is heated to red hot and then plunged into cold water. The sudden cooling alters the crystalline microstructure of the steel, making it hard and able to hold a sharp edge.
The example is a simple one, but the process is actually extremely complex. It’s even more so when molten metal is cooled inside a casting. The temperature and density differences, convection forces as the cooling molten metal rises and falls in the mold, and any number of other factors are among the many reasons why casting metals, especially exotic alloys, is often as much art as science.
Microgravity is one way of reducing this complexity, so scientists are better able to understand it. In the absence of gravity, there aren't any convection forces, so metal castings have an even temperature. Furthermore, in a gravity-free environment metal samples can be suspended in a magnetic field and heated using conduction coils. This means there are no complicating factors, such as the molten sample sticking to a crucible wall or being contaminated by it.
By means of microgravity, scientists hope to gain a better understanding of an alloy’s surface tension, viscosity, melting range, fraction solid, specific heat, heat of fusion, mass density, and thermal expansion among other things. This would be of tremendous importance for everything from casting turbine blades to developing lighter weight alloys.
The problem is, there isn't a lot of of microgravity on Earth and most of that involves falling. You can get 20 seconds in an airplane during a parabolic trajectory and six minutes in a sounding rocket, but neither of those are very practical for carrying out metallurgical research. To get serious, you need a space station. And on the ISS, there’s all the microgravity you want.
Weighing about 360 kg (795 lb), the MSL-EML was built by Airbus Defence and Space in collaboration with ESA and the DLR Space Administration. It consists of an automated chamber that keeps samples in a vacuum or a controlled gas mixture. In addition to electromagnetic levitation and induction heating coils, there is a digital video observation camera, a high-speed data camera capable of capturing up to 30,000 images per second, and a pyrometer.
When activated, the MSL-EML automatically feeds one of 18 spherical samples, 5 to 8 mm in diameter, consisting of various aluminum, copper, and nickel alloys into the process chamber using a rotating magazine. The machine uses electromagnetic fields to levitate samples in a the container, keeping them out of contact with the walls or any other materials. Then the inductive heating pushes the sample temperatures up to 2,000⁰ C (3,600 ⁰ F), reducing them to a liquid state.
In such a controlled environment, scientists will be able to dial-in various factors and study how such samples change as they cool and solidify. There’s no need for crucibles, which could contaminate the samples, and the samples aren't under the influence of gravity, which would deform the developing crystals or set up convection currents, resulting in uneven cooling. Meanwhile, the sensors record every detail of the process.
According to ESA, the microgravity containerless system produces a purer sample with fewer variables to take account of. The findings from the MSL-ELM can be compared to computer models and findings from experiments conducted on similar samples on Earth on parabolic flights.
The EML will travel to the space station this June aboard the Georges Lemaître, along with the first batch of samples for experimentation. It will be installed in the MSL, which sits in the European Drawer Rack in the Columbus Laboratory, and it will be controlled from the ground at the German Aerospace Centre’s User Control Centre (MUSC) in Cologne. After each batch of experiments, some samples will return to Earth for further analysis.
ESA says the information generated by the MSL-ELM may one day be used to scale up manufacturing processes that can produce the same properties on Earth on an industrial scale.
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