Nearly all additive manufacturing processes result in components with inherent weaknesses, but laser peening AM parts used in demanding applications can extend fatigue life 10-15x, experiments show.
“If you think of a part lasting hundreds of thousands of cycles, just think of pushing that up to a million cycles or more.”
Micheal Kattoura, Ph.D.
“Every additively manufactured part, especially in the as-built condition, inherently has two issues going on: microstructural porosity, consisting of microscopic voids in the metal, as well as tensile residual stresses from the layers building in the AM process,” said LSP Technologies Materials Research Engineer Micheal Kattoura, Ph.D.
“Near surface, laser peening can reduce the porosity and create high magnitude and high depth compressive residual stresses . Those compressive stresses not only address the near-surface weaknesses of AM parts, but they can also mitigate the stress concentration effects of the porosity.”
Kattoura explored laser peening additively manufactured (AM) CoCrMo alloy parts in a variety of strategies to optimize the process for the material, one of the most commonly used alloys for AM manufacturing.
At the recent 2020 TMS (Minerals, Metals & Materials Society) Annual Meeting & Exhibition in San Diego, Kattoura discussed “Effect of Laser Shock Peening Processing Parameters on the Microstructure, Residual Stress, and Fatigue Behavior of Additive Manufactured CoCrMo Alloy.”
The research was the product of a team that also included:
- Boetang Twum Donkor, University of Cincinnati
- Jie Song, University of Cincinnati
- Jan Kaufman, Czech Technical University, Prague
- Seetha Ramaiah Mannava, University of Cincinnati
- Vijay Vasudevan, University of Cincinnati
“We found that laser peening improves the fatigue life of additively manufactured parts by 10 to 15 times,” he added. “If you think of a part lasting hundreds of thousands of cycles, just think of pushing that up to a million cycles or more.”
Kattoura had also presented an overview of metal surface issues at the 2019 TMS conference, examining a number of strategies for treatment of known AM issues.
CoCrMo for Additive Manufacturing
Dr. Kattoura’s work on AM laser peening used CoCrMo — cobalt chrome molybdenum, an alloy already being used in AM for industries such as aerospace and biomedical implants. CoCrMo was most famously used by GE Aviation and GE Additive for an additively manufactured leap fuel nozzle. GE’s Arcam EBM unit used CoCrMo for femoral knee components and tibia trays as well.
Across a wide variety of conditions – including the use of vinyl or aluminum tape to cover the part and different types of laser wavelength – laser peening consistently improved yield strength by 15% and fatigue life by 15-20x untreated fatigue life, the team’s experiment proved.
“This work has continued to encourage materials scientists about the benefits of laser peening parts in a wide and flexible array of additive manufacturing applications,” Kattoura said. “We hope to track the various methods of additive manufacturing and optimize the impact of laser peening on those parts.”
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