A 3D-printed object being removed from the printerUniversity of Cambridge
A team of researchers led by the University of Cambridge has developed a new technique that uses high-energy lasers to fine tune the properties of 3D-printed metal without compromising the complex shapes it forms.
Additive or 3D printing is proving an increasingly powerful tool for engineering and manufacturing, but it's far from a panacea. In fact, it often has some major drawbacks that require new approaches to overcome.
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3D printing metal usually involves a machine that lays down thin layers of metal alloy in the form of a fine powder. This layer is then melted or sintered using a laser or electron beam guided by a digital model, then another layer is added. When the printing is complete, the excess powder is swept away, revealing the final product.
By means of such printing, very complex shapes can be formed very quickly. The problem is that there's more to making something out of metal than its shape. There is also the complex interaction of the metal's physical, chemical, and mechanical properties. If these are not properly controlled, then the end product may be garbage.
A very simple example of this would be a 3D-printed knife. It's possible to make a remarkably fanciful blade that shows curves and details that would normally be all but impossible to achieve by conventional means, but if the properties of the metal itself aren't addressed, that blade might break like peanut brittle or be so soft that butter would hold a better cutting edge.
This is an obvious challenge when making complex shapes. However, metal workers have developed tried and true techniques to control the properties of metals thanks to thousands of years of practice aided by some Johnny-come-lately science in the past couple of centuries.
Essentially, this involves altering the crystalline structure of metals by different methods of heating and beating them. By controlled heating, cooling, and forging, a piece of metal can have its structure fine tuned until it is suitable for everything from a scalpel to an I-beam.
That's fine for simply shaped metal objects, but you can't stuff an intricate 3D-printed shape into a furnace or pound it with a hammer, which would defeat the whole purpose of using 3D-printing technology to make it in the first place. Instead, the Cambridge team, which included researchers from Singapore, Switzerland, Finland, and Australia, opted for using lasers to alter the metal in situ.
The idea was that the laser would selectively melt spots on the completed object made out of stainless steel, altering the crystalline structure. In this way, they could make the printed metal strong while removing the brittleness that such printed metal tends to exhibit. The selective reheating on a tiny scale turns the laser into a microscopic hammer.
The technique can't duplicate conventional metal working, so the team turned to an ancient technique to achieve a similar result. One method of making high-quality sword blades is to use two different metals like steel and iron, and weld and fold them together many times over. The result is a finely layered blade where the two metals stand out against one another and allow the sword smith to control not only the properties of the entire blade, but of particular sections, so the center of the blade is flexible, while the edges are hard enough to take a sharpening.
The Cambridge team came up with something similar by alternating the spots treated by the laser with ones left untreated. This allowed them a large degree of control over the object's final properties.
"We think this method could help reduce the costs of metal 3D printing, which could in turn improve the sustainability of the metal manufacturing industry," said Dr. Matteo Seita from Cambridge’s Department of Engineering and team leader. "In the near future, we also hope to be able to bypass the low temperature treatment in the furnace, further reducing the number of steps required before using 3D printed parts in engineering applications."
