Standards body ASTM International has just announced four $10,000 scholarships for graduate students. Selected from 50 applicants out of ASTM’s 5,000 student members, these ASTM International graduate scholarships have been awarded to Patricio Carrion, Laura Luna, Alex Thomas and Omid Vakili. First off, I’d like to congratulate everyone on having really lovely names.
Laura Luna’s work centers on rock standards, while Alex Thomas is active in the area of waste management and stainless steel. Omid Vakili’s work on sports equipment, particularly helmet safety, is 3D printing-adjacent, since a lot of new helmet designs are using 3D printing to create unique methods for managing impacts.
However, Patricio Carrion’s work is the most relevant to our publication. Carrion is actually on the F42 committee that is setting standards for 3D printing, or additive manufacturing, as we should be calling it. He works at the Auburn National Center for Additive Manufacturing Excellence. As well as his F42 committee work, he is doing research into the Multiaxial and Variable Amplitude Fatigue of AM Ti64.
Fatigue properties of 3D printed parts are notoriously difficult to discern. Due to run-to-run differences in machines, orientations, build paths and CAD, it is extremely difficult to take into account all the properties that can affect fatigue. The variables and variations make this much more complicated to do than with conventional manufacturing, where you’re usually starting off with a known billet of material whose properties are understood. Next, you usually cut a bunch of that block away. So, this is rather simple to predict and anticipate. In our case, we’re going to turn that all-understood block into powder of varying shape, roundness, and size, spread it out, and then melt it using different laser paths, differently every time. Depending on orientation or location on the build bed as well as the packing density of the powder, results can vary.
In other words, fatigue is extremely important and also notoriously difficult to predict and define. In an episode of the 3DPOD, you can actually hear Max and I burst out laughing when we hear Domin’s Marcus Pont explain that he was on a mission to find the fatigue strength of AM parts.
So, Patricio’s work is timely and much needed. It is wonderful to see ASTM supporting something this important. Previous work by Patricio concluded, that
“The microstructural characteristics, monotonic tensile deformation behavior, and fatigue behavior of as-built/annealed LB-PBF Ti-6Al-4V were investigated. The Morrow and Smith- Watson-Topper (SWT) mean stress correction models were employed to correlate R = 0 fatigue data. The linear damage rule (LDR) in conjunction with the Coffin-Manson (C-M) fit was used to capture the cumulative damage for high-low and low-high tests. Based on the experimental results and discussions, the following conclusions are drawn:
- The microstructure characteristics and mechanical properties of LB-PBF Ti-6Al-4V did not present any significant differences from results found in the literature for vertically fabricated LB-PBF parts. Moreover, the monotonic stress-strain response exhibited similar values for yield and ultimate tensile stress due to negligible work hardening, which was attributed to the martensitic α microstructure combined with the prior-β grain boundaries.
- LB-PBF Ti-6Al-4V is sensitive to mean stress across both low and high cycle fatigue regimes.
- The Smith-Watson-Topper mean stress correction model was able to provide appropriate fatigue life predictions while Morrow offered non-conservative predictions due to failure being driven mainly by the maximum tensile stress.
- Relatively small load sequence effects were observed for LB-PBF Ti-6Al-4V under high-low and low-high loading. These results were justified by the as-built surface.”
There’s a lot of optimism and future work there that could be super useful. Patricio has also looked at powder recycling and how this influences fatigue stress. Here he and his fellow researchers conclude that,
“Comparison of new and used Ti-6Al-4V Grade 23 powder characteristics, and their effects on the resultant microstructure, tensile deformation, and fatigue behavior of L-PBF Ti-6Al-4V specimens in the as-built and machine surface conditions were investigated. Based on the experimental results and observations, the following conclusions can be made:
As a result of a narrower PSD, as well as of lower cohesion and interparticle frictions, the used powder exhibited better flowability as compared to the new powder. The improved flowability and the less compressibility of the used powder may result in smaller internal pores.
Powder recycling had negligible effects on the microstructure as no differences in grain size and shape were observed between specimen sets fabricated from new and used powder.
Powder recycling did not have any significant effects on the tensile behavior and fatigue performance of the L-PBF Ti-6Al-4V specimens in the as-built surface condition.
The effect of powder recycling on the fatigue performance of L-PBF Ti-6Al-4V specimens fabricated from new and used powder in the machined surface condition was substantial in the high cycle regime, with specimens fabricated from used powder exhibiting significantly longer fatigue lives.
The average effective defect size, calculated based on the Murakami approach, was found to be smaller for specimens fabricated from used powder, which can explain their longer high cycle fatigue lives as compared to the specimens fabricated from new powder.”
Both papers are downloadable and well worth your time. Additionally, he has been involved in more papers on Ti fatigue, fatigue models, strain based fatigue and more. Standards are an oft overlooked part of our world but are of crucial importance to our future in critical applications where performance and safety are key components. Standards cause for increased interoperability and trust in our world. We’d like to congratulate Patricio and the others on their scholarships and also thank them for contributing to the development of standards.
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