The manufacture of Fe-Co-V powder
The Fe-Co-V alloy metal powder is produced by gas atomization. This means that fine droplets of the molten alloy are sprayed by means of a pressurized gas jet. The droplets of different shapes and sizes will then solidify into a powder.
This Fe-Co-V alloy powder can then be used in an L-PBF (Laser powder bed fusion) machine.
The characteristics of Fe-Co-V alloy
As a rule, binary Fe-Co alloys have interesting mechanical and magnetic properties due to their disordered structures. Indeed, these alloys exhibit:
- a high saturation magnetization,
- a high permeability,
- low magnetostriction (little deformation under the effect of a magnetic field),
- a high Curie temperature (Temperature of transition between the ferromagnetic state to the paramagnetic state).
However, they also have a significant drawback of an inherent brittleness at ordinary temperature. As the material is highly conductive and brittle, a small amount of Vanadium (less than 3%) has been added to the alloy. This reduces the brittleness of the material and increases the ductility while retaining the magnetic properties of the Fe-Co alloys, but also decreasing the electrical conductivity. The addition of Vanadium makes it suitable for cold forming.
These magnetic materials such as Fe-49%Co-2%V alloy combined with additive manufacturing methods such as L-PBF offer enormous potential.
To further improve the properties of the alloy, additional heat treatments can be performed.
In tests, different heat treatments such as pre-annealing, normalization and/or annealing were performed on 3 Fe-Co-V alloy samples.
This led to the conclusion that the heat treatments modify the microstructure. The grain size and consequently improve the magnetic properties (saturation magnetization, high magnetic permeability, low coercivity). The ideal is to obtain a uniform structure with large grains in order to have optimal properties. Other factors modify the magnetic properties such as the size of the precipitates, the internal stresses, the state of order, the external stresses, etc.
Concerning the mechanical properties, tests have shown that homogeneous fine-grained structures prevent the mobility of dislocations. This leads to a very high strength. However, after heat treatment, the material loses its strength and the elongation at break also decreases. Nevertheless, the addition of vanadium improves the ductility.
The most effective heat treatment for this alloy consists of a pre-annealing phase at 700°C for 2 hours. This is followed by a primary anneal at 820°C for 10 hours.
In order to be used in 3D printing and then marketed, the electrical resistivity of the material must be increased to avoid eddy current losses. This can be done by changing the composition of the alloy, but this has a negative impact on the magnetic properties, or the microstructure. The resistivity of the Fe-Co-V alloy is much higher than that of the binary Fe-Co alloy.
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Application areas of a soft magnetic material
Fe-Co-V is a soft ferromagnetic material. This means that under the effect of a magnetic field, the material can be magnetized as many times as possible easily (the magnetic permeability is high). However, as soon as the magnetic field disappears, the material no longer emits any magnetism (the remanence is very low).
The development of a soft ferromagnetic material with good magnetic properties such as Fe-Co-V alloy allows to improve the aeronautical industry. Indeed, these materials allow amplifying the operating capacity of electrical equipment on aircrafts.
One application where 3d printed Fe-Co-V offers great potential for improvements is in electrical generators. An electrical generator is a device designed to produce electrical energy from mechanical energy. It consists of a magnet (the rotor) which is the moving part and a stator (coil) which is the fixed part. The rotor rotates in the center of the stator. Then, the stator has the role of receiving the induction produced by the electromagnetic effect generated by the rotation of the rotor, to transform it into electric current.
The Fe-Co-V alloy is a very good material to improve the electrical power of aircrafts in the coming years. The aim is to increase the speed of rotation of the machines in order to increase their power density. The use of this alloy could increase the permissible mechanical stress by 20%. This would improve the power of the machine. In addition, the alloy would reduce magnetic losses for a similar frequency.
A second example where additive manufacturing of a soft ferromagnetic material can achieve great improvements is in the transformer. The purpose of the transformer is to transform a current and voltage system into another similar system with different values. It works by magnetic induction. That is to say that an alternating voltage is applied to a primary coil and then passes to the secondary coil by magnetic induction. The voltage will be different at the input of the primary coil and at the output of the secondary coil. This depends on the number of windings in each coil. In this example, it is interesting to use a soft ferromagnetic material such as Fe-Co-V because in addition to having important magnetic properties, it avoids losses by eddy currents and energy dissipation by hysteresis cycle .
As electrical loads increase, the power and number of transformers installed in an aircraft increase as well. Transformers need to be lighter and more optimised in order to stay within safety standards.
Today, additive manufacturing offers exceptional flexibility, particularly in the choice of materials. Some materials added to 3D printing allow the creation of lightweight parts while keeping the mechanical properties essential for proper functioning.