May 26, 2016 | By Alec

Titanium alloys can be found everywhere nowadays. Being extremely strong, lightweight and resistant to corrosion and high temperatures, its used in everything from military equipment to commercial jets and tennis rackets. What’s more, thanks to 3D printing the costly material can be used more effectively and on a smaller scale. But a new study by researchers from the Carnegie Mellon University (CMU) suggests that 3D printing inserts several flaws into the material, which can decrease the material's resistance to fatigue and lead to breakage. The solution might be in the powder’s production process.

This has become apparent from a study led by Anthony Rollett, Professor of Materials Science and Engineering at CMU. The study also included graduate students Ross Cunningham and Tugce Ozturk, and Professor of Mechanical Engineering Jack Beuth. They have already published their findings in The Journal of The Minerals, Metals & Materials Society.

The problem is, they say, that powder-based 3D printing techniques increase the material’s porosity. The 3D printed parts feature miniscule pores, which can affect the overall quality of the component, making it less strong than titanium alloy parts made with other production techniques. “Having a strong understanding of the fundamental science of additive manufacturing materials is necessary in order to use them in aerospace and other demanding applications,” says Beuth.

It was already known that metal 3D printed parts suffer from porosity, but the CMU researchers teamed up with the Argonne National Laboratory in Chicago to find out exactly how that porosity is created, and what can be done to counter it. “In a conventional material like steel, there aren’t any of these pores. Besides that, you work so hard to avoid any sort of nonmetallic inclusion,” says Rollett. “In additive manufacturing materials, there they are. You have to figure out how to understand them and deal with them. It is a new challenge for the field of materials science.”

The team travelled all the way from Pittsburgh to use the Argonne’s Advanced Photon Source (APS), a unique installation that uses microtomography to rapidly image anything. The APS is powerful enough to see through metal particles down to miniscule detail— to roughly one millionth of a meter. The unique equipment is in such high demand among researchers, that the CMU team could only secure access to the APS device for 48 hours.

The material that was put to the test was Ti-6Al-4V, a common titanium alloy that includes 6% aluminum and 4% vanadium. It is widely used in aerospace and biomedical industries for its excellent material properties. But as Rollett, a material microstructure expert, explained, they are the first to study the material at such a small scale for 3D printing. “Carnegie Mellon is bringing really unique research to the table because we are one of the only universities chasing after advanced characterization techniques in the metals 3D printing space,” he says.

The material itself is commonly 3D printed using either selective laser melting or electron-beam melting (EBM) technologies, and the latter technology (which uses electrons to melt and compress powder particles) was the focus of this study. Using the APS tool, the titanium alloy samples were inspected at a micron scale. "We can observe hundreds or even thousands of pores at a high resolution of about two microns," Rollett said. The samples themselves were 3D printed with a commercial EBM machine with different electron beam power levels. They also imaged a sample of preprinted powder.

The APS reconstructed the samples at numerous angles, almost like a CT scanning device. 1,500 images were made in just two minutes for each of the samples. “The APS microtomography system is capable of scanning objects in minutes, compared to laboratory systems that can take several hours,” said Xianghui Xiao, physicist in the Imaging Group of Argonne's XSD. The results provide a lot of info on the density and chemistry of any scanned part.

The CMU researchers theorized that the samples, each realized through different 3D printing settings, would highlight a ‘sweet spot’ of parameters that would significantly reduce or eliminate porosity. To find out, the number, volume and distribution of pores were all recorded using a range of parameters. Typically, the pores can be anywhere from a few microns to a few hundred microns in size, and are unevenly distributed throughout parts.

In particular, the melt pool (the size of the material melted at once) was altered, but the results were disappointing. “Relative to printing speed and spacing, if you decrease the power level and the melt pool becomes too small, you may leave behind unmelted powder, which is a source of porosity,” Rollett said. “However, if you increase the power level too much, you risk creating deep holes, called keyholes, with the electron beam that also leave behind voids.”

In fact, none of the different settings proved to be a ‘sweet spot’. “Porosity was present in every piece,” Rollett said of the 3D printed samples. “To us it was a surprise that it was always there.” While some samples exhibited fewer or smaller pores that others, these were then clustered most strongly at the part’s surfaces, which is still detrimental to fatigue resistance.

These results are problematic, as it means that part qualification – one of the biggest obstacles for commercial 3D printing – won’t become any easier. While it won’t be such a problem for medical implants as titanium is stronger than bone anyway, aerospace parts – which are exposed to a lot more stress – could suffer. It could become an obstacle that the aerospace industry would rather avoid altogether.

The CMU team is therefore looking at other options. Rollett revealed that their results further suggest that porosity can be traced back to powder processing, as the images of pre-printed powder revealed smaller pores that could expand during 3D printing. “Our next step might be to ask if there is a better way to process the powder before printing,” Rollett said. Changing the production methods, or even the compound, could provide the solution to making 3D printed titanium every bit as reliable as it should be.

 

 

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Fabien Degré wrote at 6/2/2016 1:54:55 PM:

Did you study the effect of Hot Isostatic Pressing on theses samples ?

Scott Simmons wrote at 5/31/2016 2:07:23 PM:

Hello, my name is Scott Simmons. I work for Henkel Corporation. We are currently working withy several 3D printing companies in filling the pores with thermoset anaerobic resin. I realize this does not address the root cause- nor will it help in every application, however, we are successfully filling the porosity found in 3D printing, allowing for not only leak free parts, but also parts that are stronger and in some cases, more easily machined. Please contact if I can be of any further technical support- Thanks! (scott.simmons@henkel.com)

Funk_Soul wrote at 5/27/2016 12:32:15 PM:

That's why it's great for some medical devices



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