Additive Manufacturing Strategies

LPW Technology Studies Micro-Cracking Issue in Metal 3D Printing

ST Medical Devices

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As 3D printing with metal continues to rise in popularity around the world and on all user levels, the conversation deepens as well. LPW Technology has been extremely involved in metal 3D printing—not only in creating metal powders but analyzing them as well.

They have helped other companies to use metal powders, as well as producing a study regarding porosity, creating a plasma spheroidisation process for advanced AM, and teaming up with numerous partners to advance metal printing technology. Their latest case study, ‘Maraging Steel: The effects of alloy chemistry on processability,’ focuses on LPW M300 maraging steel.

“A metal powder can not only move out of, but can also vary within, specification,” says Dr Rob Deffley, LPW’s Research & Development Manager. “In this particular case study, we follow the analysis of batches of M300 maraging steel as our highly skilled applications engineers identify the issues that led to a problem with the final mechanical properties of a built part. We also explore the solution they designed to resolve the problem.”

M300 offers characteristics such as:

  • Low carbon content
  • Exceptional mechanical properties
  • High tensile strength and hardness
  • Easy treatment with heat
  • Ease in use for tooling applications like injection molding and die casting

The LPW team began examining the material further though after two batches right in a row produced parts with micro-cracks—a condition that is not acceptable, and especially not for parts that require a smooth finish. Their suspicion was that the correct controls were not being used.

SEM images of micro-cracks observed at grain boundaries of M300 AM parts. Cracks are aligned in the vertical/build-direction.

The parts indicated vertical micro-cracking that occurred at grain boundaries, tipping off the researchers that they were dealing with a material issue. Because of that, they honed in on the alloy chemistry, discovering variances in elements such as silicon and manganese. Boron also showed itself to be an outlier:

“Whilst still within the levels of the specification, it was significantly higher than the previous, problem-free batches. The limits of the three residual elements were tightened sufficiently to ensure the powder delivered the required results,” states the team in their paper. “This has resulted in no recurrence of the micro-cracking in subsequent maraging steel powder batches.”

Relative concentrations of boron and silicon+manganese in the failing M300 batches (‘before’) and subsequent batches where no cracking was observed (‘after’).

The researchers examined a total of 23 batches of powder, searching for deviations. Boron was the ‘stand-out’ element in the two batches with cracks—and was four to five times higher than in the successful batches.

“These results prompted a revision of the alloy chemistry specification to impose tighter limits on these critical elements to ensure control is reestablished and future deviations are kept to a minimum,” stated the researchers.

As they continue to work with this metal powder and others, the goal is to make sure that it is not ‘overengineered.’ If that were to happen, it would make the specification even more challenging to produce. The researchers are busy conducting further studies centered around simulation and experimenting with alloys. They expect to learn more about the micro-cracking as well as gaining information for case studies in the future. Discuss in the LPW forum at 3DPB.com.

Relative levels of key residual elements across 23 powder batches.

[Source / Images: LPW Technology]

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