Why is Aluminum more difficult to sinter than the other materials we work with at The Virtual Foundry? It’s actually a pretty interesting story, let’s talk through it.
In a word, Oxides. One of the things that makes Aluminum such a useful material is that it doesn’t corrode. But, here’s the catch, it does corrode. It just does it in a way that looks pleasant and it does it fast. So fast that it’s likely that you rarely see aluminum itself, you’re actually looking at an oxide of aluminum. This passivating layer happens at an atomic scale. All of this starts within 50 seconds and is many atoms deep by the time 80 seconds have passed. At 200 seconds the oxide layer is at complete saturation and nearly about 2 to 3 nanometers thick.
What’s more, this layer of oxide is self-healing. Meaning, if you scratch through this layer of oxide, it will simply regenerate. This property is a major part of what makes Aluminum so useful and resilient. It’s also exactly what makes Aluminum so different from the other materials in our list of products.
On top of all of this, the oxide of aluminum itself has some pretty amazing properties. The melting point of aluminum is 1,221F, but the melting point of aluminum Oxide is 3,762f. On mohs scale, which goes from 0 to 10, Aluminum is a 2.8 but it’s oxides are a 9 (diamond is a 10). You’ve likely used aluminum oxide in things like sandpaper.
Just how active are these materials? Well, I think the easiest way to look at this is to look at how much electricity it takes to reduce (remove the oxygen) from the material in it’s oxide state to its metallic state. Some googling shows that to convert Bauxite (aluminum ore) to 1 pound of pure aluminum requires 63,000 to 95,000 watt-hours of electricity for Alumnium and 250,000 to 261,000 watt-hours for a kilo of Titanium. This is a truly astounding amount of energy. For this to make economic sense, Alcoa, the worlds largest AL refiner, builds it reducing plants near massive sources of large-scale energy production. Picture the Three Gorges dam on the Yangtze in China, or near massive hydroelectric plants in the united states, Brazil, etc.
To further the anology, adding the Oxygen back into the pure aluminum takes a similar amount of energy. Here’s a video where I throw 1/2 teaspoon of Aluminum Powder into a Bunsen Burner. The effect is very dramatic.
So what does all of this mean for the future of 3D printing Aluminum?
It’s just a matter of time before we come up with a solution that will simplify this process to the point where we’ll be able to Additively Manufacture Aluminum and Titanium with relatively low-tech equipment. The Virtual Foundry is currently working with our key Aluminum powder vendor to create a blend that will simplify this process. In addition, at least 3 other labs around the world are working with us directly on the same goal with Titanium.
Please feel free to contact us if you have anything to add, or would like to participate in this project.
Bradley Woods, Founder/CEO
The Virtual Foundry, inc.