Finding Keyholes in Metal Additive Manufacturing
A relatively recent phenomenon in the field of additive manufacturing (AM) has been the discovery of “keyholes” (i.e., flaws) that form during the metal AM process. AM’s promise to revolutionize industry is currently constrained by a widespread problem: tiny gas pockets in the final product, which can lead to cracks and other failures.
Until now, manufacturers and researchers using AM processes such as laser powder-bed fusion (LPBF) haven’t known much about how the laser itself drills into the metal. We know that the laser produces cavities called “vapor depressions,” but largely the field assumed that the laser strength or the type of metal powder used were to blame for these defects. As a result, manufacturers have been using a trial-and-error approach with different types of metals and lasers to seek to reduce the defects.
Our lab, in partnership with Argonne National Laboratory, recently published new research in Science that identifies how and when these gas pockets form, as well as a methodology to predict their formation.
Our team used the extremely bright high-energy X-rays at Argonne’s Advanced Photon Source (APS) – a Department of Energy Office of Science User Facility and one of the most powerful synchrotron facilities in the world – to take super-fast video and images of LPBF, in which lasers are used to melt and fuse material powder together.
Under perfect conditions, the melt-pool shape is shallow and semicircular, called the “conduction mode.” But during the actual printing process, the high-power laser, often moving at a low speed, can change the melt-pool shape to something like a keyhole in a warded lock – round and large on top, with a narrow spike at the bottom. Such “keyhole-mode” melting can potentially lead to defects in the final product.
Our research shows that keyholes form when a certain laser power density is reached that is sufficient to boil the metal. This, in turn, reveals the critical importance of the laser focus in the AM process.
We believe this research could motivate makers of AM machines to offer more flexibility when controlling the machines, which could lead to significant improvements in the final products. In addition, if these insights are acted on, the process for 3D printing could get faster.
The research in this paper will translate into better quality control and better control of working with AM machines. For AM 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.
So, essentially, we’ve drawn back the veil and revealed what is really going on. Most people think that in AM 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 are really drilling a hole into the metal. We believe this discovery could dramatically improve the metal 3D-printing process.