Get the Facts on… Plasma and Metal Additive PowdersAugust 24, 2021
Plasma Atomization is a very effective way of creating high-quality metal powders for additive manufacturing. While many people talk about the machines used to create the parts, knowing how to get the best quality raw material is also key.
We recently caught up with Jens Kroeger, lead engineer, and Matthieu Balmayer, technology & innovation leader at AP&C - a GE Additive company, to find out what plasma is and what is so special about using plasma atomization methods to create additive powders.
What is plasma, exactly?
Plasma is essentially an ionized gas. It’s been traditionally referred to as the fourth state of matter after solid, liquid and gas. When you raise the temperature of a gas, you start to ionize the molecules and separate the electrons from their nucleus. That creates a lot of light, and that’s an ionized gas. So, the atoms are going to become positively and negatively charged ions and when the charges combine, light is emitted.
But you don’t always need a very high temperature. For example, in fluorescent tubes, the temperature of the glass tube is not that high, but the density of atoms inside is low, such that the total amount of required energy is low, and it becomes much easier to strip the atoms of their electrons ─ to ionize them. Plasmas are found in many aspects of your life: the flame in a candle, the sparks when you plug in an electrical appliance, when you take your sweater off, and in lightning.
How do you make a plasma?
There are different ways to create plasma; ours is generated by injecting electricity into a gas to obtain a thermal plasma. To create a plasma, we take two electrodes — one positive and one negative — in DC mode and generate an arc between them. To create the arc, you need to also bridge those electrodes together to make a contact. When you generate the arc, electrons are pulled from one electrode to the other, exciting the gas. The excitement of the gas causes electrons to be pulled from it, and this produces light.
The temperature of an electrical arc can be very high and can easily reach 30,000°C. If you compare this to the surface of the sun, which is 6000 °C, you start to see how hot a plasma can get. When you use this method to create a plasma torch, you can pull on the two electrodes to create a length of plasma that sits between the electrodes. When you have a length of plasma, you can blow gas through it and pick up the heat from the arc, creating a plasma jet.
Basically, we use plasma to heat up the gas, and then we use this gas to atomize metals and create metal powders. The plasma torch is essentially an effective giant gas heater that can melt metals.
How does AP&C use plasma, and why?
Plasma is very hot, and this is good for melting different metals. The approach we use is called plasma atomization. In the plasma torch, we use a tube to blow the gas through, and because the gas is very high temperature, it has a very high gas velocity. This gives us the momentum required to liquify and pulverize the metal in front of the torch.
In this process, we deliver the greatest amount of energy and shear force possible to a feed of metal. Our process means that we can put in more energy per amount of gas delivered. The raw metal alloy is fed into the atomizer as a wire towards the apex of three plasma torches. We push a lot of energy onto a single point, which enables us to tear apart the metal wire and atomize it, creating a high-quality metal powder in the process,
Is using plasma advantageous for your products?
Plasma is the hottest gas that we can easily produce here on Earth. AP&C uses these plasmas to create premium metal powders, which have exceptional surfaces, morphology, and properties for 3D printing. The ability to consistently produce high-quality powders using plasma is a key driver towards the additive industry’s ability to produce metal 3D-printed parts on an industrial scale.
It is advantageous, because we can achieve much higher momentum than with other techniques, such as gas atomization (which uses high pressures but a cold gas). The higher the speed of the gas, the faster you will blow on the metal, and the thinner the powder you produce. Essentially, the faster the gas hits the metal, the better the powder.
There are also other features of the powder that are improved by our process. The first is the shape of the powder. By blowing hot gas on the molten powder droplets, you give them a few more milliseconds to cool down and adopt the morphology with the lowest level of surface tension. So, instead of settling in an elongated shape, perfect spheres can be created. This is not possible with gas atomizers as the cold gas freezes the elongated shape of the atoms in the powder. The sphericity of a powder is important for governing how a powder will behave in a 3D printer.
With spherical particles, the powder will flow better, it will spread more evenly on the powder bed and will prevent voids from being formed—meaning that you get a lower degree of porosity in your final part. Additionally, because we blow a hot gas on the metal, it has a higher enthalpy (energy per liter of gas), so fewer gas molecules get entrapped inside the metal. This also helps to reduce voids that could arise from inside the powder grains themselves.
Is plasma-atomized powder different from other metal powders?
Yes, with a plasma-atomized powder, you have very high sphericity, very good flowability, and a very good powder. The powders from other technologies (such as gas atomizers) may not be as high quality. However, for some powder metallurgy technologies, gas-atomized powders are good enough, but other applications, they are not suitable. Essentially, the higher the performance requirements will be for the powder, the more important it’s going to be to have a plasma-atomized powder. This is especially true for EBM and laser powder bed fusion processes.
How do you compare plasma atomization to the other technologies on the market?
AP&C typically works with reactive metals, and these metals would erode the crucibles used in gas atomizers when they are melted (due to the volume produced). The liquid metal effectively reacts with the crucible materials, dissolving the outer layer. This dissolved crucible material would then be present in the resulting powder material.
Because we have a wire and a plasma, the volume of molten metal produced is very small, so there are no issues with contamination from the wall of the crucible. Where gas atomizers focus on scale, the market for plasma atomized powders is currently more niche. However, what we might lack in production throughput, we make up for in the quality of powder.
They’re all related. The idea is always to apply energy on a feed of metal to rip it apart and atomize it. Gas atomizers are better for certain materials, such as cobalt and steel, as these materials are not very reactive, and you can produce several kilograms a second. So, there are some advantages in using gas atomization in certain scenarios, especially as the technology is widely available.
What is the future of plasma atomization?
In the past, we had the stone age, the bronze age, and the iron age. Now it is the plasma age.
The nature of plasma makes it extremely useful for processing refractory metals (metals with very high melting points). In the military, there are applications that require materials that can withstand the most extreme temperatures. The same is true for airplane engines and will still be true if airplanes adopt hydrogen energy technologies. Titanium alloys have demonstrated the need for plasma-atomized powder in recent years—by showcasing that if you integrate materials that can elevate the temperature of the engine, you can make it more efficient.
Looking towards future applications, there’s a need in the space industry for both rockets and in satellites. This space race is going to rely on 3D printers to build smaller and cheaper rocket launchers that can withstand high temperatures. Plasma-atomized powders are going to be ideal for these applications.
Going forward, we are also going to need evermore heat-resistant and corrosion-resistant materials, and plasma is an efficient system to produce and study these materials.
Are you ready for the dawn of the plasma age? If you are ready to join us, you can find out more by getting in contact with us to discuss.