A keyhole is a subcategory of void, or pore, that can sometimes be found in AM components.
Anthony Rollett, a co-author of the paper and Professor of Materials Science and Engineering at Carnegie Mellon University, said, “The research in this paper will translate into better quality control and better control of working with the machines.”
“For additive manufacturing to really take off for the majority of companies, we need to improve the consistency of the finished products. This research is a major step in that direction.”
Metal powder bed fusion
Powder bed fusion (PBF) technologies use a high-power laser to melt polymer or metal powder layer by layer till a full 3D object is formed. PBF is one of the most widely used methods for metal 3D printing. But one issue with PBF metal printing is the formation of pores in the metal part known ‘keyholes.’
In PBF, when a laser hits the metal powder it forms a melt pool. This melt pool expands due to heat and then cools down to form a solid layer. If the laser power is too high, the evaporation rate and melt pool temperature also increase. Due to this, the melt pool penetrates into the layer forming a ‘keyhole’ pore. The keyhole can lead to cracking and the general weakness of the part.
The understanding of what happens to the metal powder when it is hit by a laser is still limited. The recent paper, which used Ti-6Al-4V powder as a sample, hopes to advance our understanding of keyholes pores in metal 3D printing.
“We are really studying a very basic science problem, which is what happens to metal when you heat it up with a high-power laser,” commented Cang Zhao, a post-doc at Argonne National Laboratory and co-author of the research.
“Because of our unique experimental capability, we are able to work with our collaborators on experiments that are really valuable to manufacturers.”
Synchrotron imaging is a non-destructive analysis used to visualize the interior structure of an object (in particular microscale components like particles).
The Argonne and Carnegie study utilized the ultrahigh-speed x-ray synchrotron imaging developed at the U.S. Department of Energy’s (DOE) Advanced Photon Source (APS) to observe what happens to the metal during laser printing.
Tao Sun, a physicist at the Argonne National Laboratory, “The keyhole phenomenon was able to be viewed for the first time with such details because of the scale and specialized capability developed at Argonne”
“The intense high-energy X-ray beam at the APS is key to discoveries like this.”
Finding key relations
With the APS technology, it is now possible to observe clearly what actually happens to the metal powder on contact with the laser. This lets researchers predict certain properties of the keyhole pore such as its depth and adjust the parameters of the machine.
As Ross Cunningham, a co-author of the study, explained, “Based on this research, we now know that the keyhole phenomenon is more important, in many ways than the powder being used in additive manufacturing”
“Our research shows that you can predict the factors that lead to a keyhole – which means you can also isolate those factors for better results.”
Rollet added further, “Most people think you shine a laser light on the surface of a metal powder, the light is absorbed by the material, and it melts the metal into a melt pool. In actuality, you’re really drilling a hole into the metal.”
“We’re drawing back the veil and revealing what’s really going on.”
Quality control in 3D printing
Quality control in 3D printing, in particular metal, is one of the most vital aspects of the technology, because, as the number of print runs necessary to create a final part increase the cost of the 3D printed part also rises.
Institutes and companies have developed in-situ process control and quality assurance (QA) methods. Among these include, Arcam’s LayerQam, photodiode-based monitoring by EOS, software and hardware package by Sigma Labs.
Effective QA methods are key to unlocking the full protentional of 3D printing. “It’s [QA] important because 3D printing, in general, is rather slow […] It takes hours to print a part that is a few inches high. That’s OK if you can afford to pay for the technique, but we need to do better,” adds Rollett.
The research discussed in this article is titled Keyhole threshold and morphology in laser melting revealed by ultrahigh-speed x-ray imaging. It was published in Science and was jointly written by Ross Cunningham, Cang Zhao, Niranjan Parab, Christopher Kantzos, Joseph Pauza, Kamel Fezzaa, Tao Sun, Anthony D. Rollett.
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Featured image shows x-ray images of vapor depression forming a keyhole pore. Image via Science