Session: 16-01-04: Mechanical Performance III

Paper Number: 172708

Mechanical Test Artifacts for Evaluating Process and Material Variability in Laser Powder Bed Fusion 

Process anomalies and powder feedstock contamination in laser powder bed fusion (L-PBF) additive manufacturing are considered top priorities by the manufacturing community. Accordingly, the goal of this research is to develop mechanical test artifacts for detecting process anomaly (e.g. laser degradation) and powder contamination during L-PBF processing economically.

First, a test artifact design is designed to evaluate the ductility variation of L-PBF processed materials by measuring the length of cracking in the artifact, which occurs in the presence of residual stress. The test artifact consists of an inverted L-shaped cantilever structure whose fixed end is anchored to the build plate and the free overhanging beam end is attached to a support structure. By designing the beam/support interface to have a half-V notch, the residual stress generated in the L-PBF process can be concentrated to cause cracking to occur along the interface. It is found that the crack lengths measured in test artifacts printed in Inconel 718 using different process parameters have a decaying exponential relationship with elongations measured in tensile testing of notched tensile specimens, having an R-squared value of 0.9238 in the regression. This strong correlation indicates that an increase in crack length in a test artifact means smaller notch ductility in the printed material. To demonstrate its application, the test artifact is used to evaluate the impact of moisture in feedstock powders on the ductility of the printed materials. Our experimental result shows that the test artifacts printed using dry powders consistently have shorter cracks and hence higher ductility than those printed using powders with 70% humidity for a wide range of process parameters.

Next, we present another test artifact that breaks mid‑build at a layer count governed by the balance between layer‑wise residual stresses and the alloy’s ultimate tensile strength (UTS). The artifact comprises two arms joined by thin ligament. As successive layers shrink, tensile stress accumulates in the ligament until it snaps, ejecting a visible puff of powder that can be recorded with an ordinary webcam. Because only the break‑layer number is required, the test artifact integrates seamlessly into any production plate and adds less than four minutes of print time. By printing thirty specimens across the build plate, breakage happens within two layers except the ones placed near the argon gas inlet where location dependency is observed.  In the third experiment, laser degradation is simulated by stepping layer power from 285 W to 245 W and 205 W.  It is found that the breaking layer extends by an additional 2-4 layers at reduced laser power, which is attributed to reduced residual stress and strength of the printed material.

Presenting Author: Albert To Univ of Pittsburgh

Presenting Author Biography: Dr. Albert To is currently William Kepler Whiteford Professor in the Department of Mechanical Engineering and Materials Science at University of Pittsburgh, where he also serves as the Director of the Ansys Additive Manufacturing Research Laboratory. He is also the Founding Director of the MOST-AM Consortium, which is a public-private partnership of 30+ companies and national labs to address the most pressing needs in modeling and simulation for additive manufacturing. Dr. To received his B.S. degree from University of California, Berkeley and M.S. degree from Massachusetts Institute of Technology. He received his Ph.D. from U.C. Berkeley in 2005 and conducted postdoctoral research at Northwestern University from 2005-2008. His current research interests lie in metal additive manufacturing (AM), specifically in process development, in-situ monitoring, process simulation, and design optimization enhanced by artificial intelligence. Professor To has over 150 peer-reviewed journal publications in journals such as Additive Manufacturing and Computer Methods in Applied Mechanics and Engineering. He is currently an associate editor for Additive Manufacturing responsible for the modeling and simulation area. He has been a recipient of the NSF BRIGE Award in 2009, the Board of Visitors Faculty Award from the School of Engineering in 2016, and the Carnegie Science Award in the Advanced Manufacturing and Materials Category in 2018.

Authors:

Albert To Univ of Pittsburgh
Dinh Son Nguyen University of Pittsburgh
Sina Nejati Eghteda University of Pittsburgh