New research challenges the one-size-fits-all approach to aluminum powder selection.
As Additive Manufacturing (AM) expands to a widening range of applications for aluminum parts, the need for more control over surface roughness is intensifying. While there’s a substantial body of research into strategies for reducing surface roughness, such as optimizing contour parameters and using finishing techniques, these methods add time and cost to the manufacturing process. Exploring the root of the problem has, until now, been a missing link in the data.
A new study from researchers at McGill University fills the gap by exploring the correlation between aluminum powder characteristics – specifically particle size distribution (PSD) – and sidewall surface roughness, a key contributor to fatigue failure. Their findings refute the traditional view that powder selection is a one-size-fits-all decision.
Part of the LPBF Process
Surface roughness is an unintended by-product of the laser powder bed fusion (LPBF) process. LPBF builds parts layer by layer, using a high-energy laser to selectively melt and fuse metal powder. While this technique enables the production of lightweight parts with complex geometries, it can also introduce surface asperities, particularly on vertical or angled sidewalls.
Asperities are the result of unmelted or partially melted particles that fuse to the melt pool edge. Unlike the top surface, which is directly exposed to the laser and typically smoother, sidewalls are more prone to roughness. Any surface irregularities resulting from the LPBF process can become stress concentrators, potentially degrading fatigue performance over time – a critical factor in the safety and reliability of a component part.
Exploring the Role of PSD
Is surface roughness the unavoidable outcome of the LPBF process, or is there a way to control it at the source? The McGill research team set out to answer that question by exploring correlations between surface roughness and different powder characteristics. For the experiments, they fabricated a series of thin-wall samples using a 400W laser system and four different AlSi10Mg powders from two manufacturers, documenting:
- particle size distribution and breadth
- apparent density: ratio of the total powder to container volume
- powder coordination number: contacts a single particle has with its neighbors
- sidewall coordination number: powders in contact with a sidewall
The predominant finding of the research was that coarse powder with a narrow PSD creates smoother sidewall surfaces. Why? Fewer contact points between particles and the sidewall reduced the number of partially melted particles adhering to the surface. Although the coarse powder introduced larger individual asperities, they were fewer in number, generating lower average surface roughness values.
Measuring the effects of PSD on surface roughness provides another data set for manufacturers to use when selecting powder for their applications.
“Understanding the role of PSD in surface roughness gives manufacturers another tool in the toolbox to determine the best powder for their application.” — Mathieu Brochu, associate professor at McGill University
Increasing Specialization
In the past, powder suppliers and manufacturers have relied on a wide PSD range to simplify inventory and ensure general usability across multiple part types. Those days are numbered. As new designs and applications for aluminum AM emerge, more specialization – not less – is needed, especially for aerospace and other industries with stringent mechanical and performance requirements.
Understanding the role of PSD gives engineers more precise control over laser processing and the intended surface finish. Study principal investigator Mathieu Brochu, associate professor at McGill University, believes that this kind of data is essential for manufacturers to make the best decisions for their designs.
“In powder metallurgy, fines are problems,” says Dr. Brochu. “If you widen the PSD on the low end, you’re introducing fines. What problems does a specific proportion of fines introduce into the fabrication of your part? Understanding the role of PSD in surface roughness gives manufacturers another tool in the toolbox to determine the best powder for their application.”
Data-Driven Powder Selection
Fines are just one part of the decision matrix. Taking a data-driven approach to powder selection involves reviewing the advantages and disadvantages of a given powder for the proposed application and production environment. Coarse powders offer faster printing, improved flowability, and lower humidity sensitivity but may not be suitable for thin structures. Fine powders enable better feature resolution but introduce challenges in handling, consistency and oxidation control.
Adds Dr. Brochu, “If you have a part without thin structures, choose coarse powder. If it’s thin printing, fine powder will provide the resolution that you need. The powder selection criteria have to come from your application requirements.”
As the AM industry matures, applications are becoming more complex and highly specialized. Getting better alignment of powder characteristics with surface finish, performance, mechanical requirements and other properties can unlock new possibilities for growth in metal AM.
Related reading:
Print and Repeat: Improving the reliability and economics of powder reuse in aluminum AM
