How Laser Peening Enhances Fatigue Life of Aviation Additively Manufactured Components

Over the past decade, additive manufacturing (AM) has been attracting the attention of various industries. This has driven the maturation of the technology and implementation of strategies to adopt and integrate AM.

One of the first industrial applications of AM is GE Aviation’s LEAP engine fuel nozzle that started production in 2015.

Why Do Additively Manufactured Parts have Lower Fatigue Life?  

Powder bed fusion AM processes generate surface defects such as unfused powder layer and a high porosity layer before reaching the fully solidified layer. Voids in the metal part (microstructural porosity) act as a crack generation site and have a negative impact on fatigue life. In addition, the AM process generates high tensile residual stresses due to the melting and solidification that takes place during the layer build process.

Currently, several post-processes are being utilized to enhance the material properties of the AM components. Hot Isostatic Pressing (HIP) is a typical post-process used on AM parts to reduce porosity and relax built-in tensile residual stresses. Research is being conducted to assess other post-processes to enhance the performance of AM components.

Why Do Additively Manufactured Parts have Lower Fatigue Life?

Laser peening has demonstrated its ability to enhance the fatigue life of AM parts by 10-15 times. The laser peening process is localized to the critical failure location of the component, mitigating the need for full component post-processing.

“Laser peening proves again its capability to significantly enhance the fatigue life of AM parts,” says Micheal Kattoura, Materials Research Engineer at LSP Technologies, Inc. (LSPT).

You can read more about this here: “Laser Peening to Enhance Addictive Manufacturing”

National Institute for Aviation Research on Enhancing Fatigue Life of Additive Manufacturing through Post-Processes 

The National Institute for Aviation Research (NIAR) at Wichita State University conducts research to support maturation and transition of technologies into aviation industrial applications. This collaboration was part of a large investigation that NIAR performed to assess various AM post-processes to enhance the fatigue life of Ti-6Al-4V, a material with vast array of applications in aviation. The parts were produced using Direct Metal Laser Sintering (DMLS) – Powder Bed Fusion (PBF), then underwent a Hot Isostatic Pressing (HIP). Laser peening was performed on these parts using two laser peening conditions which are discussed below. Bending fatigue tests were then performed by NIAR on all parts to provide a comparison of the benefits between laser peening and the as-machining surface. Five samples for each laser peening condition were tested in this initial investigation.

A set of laser peening conditions typically used for processing titanium alloy, denoted LSP-tape, shows that this process method was capable of consistently enhancing the fatigue life by 4x compared to the machined parts. In addition, an experimental laser peening condition, denoted LSP-bare (no opaque overlay was used), was able to enhance the fatigue life up to 15x compared to machining. Fractured surface analysis showed that the laser peening process pushed the failures to subsurface failures. Thus, the variability of the fatigue life for this condition is a resultant of the variability of the subsurface defects among the different samples.

As for the underlying reasons behind this enhanced performance, additional testing of the residual stresses and surface roughness of the parts was conducted. Laser peening generated high compressive residual stresses that counteracted the applied stresses resulting in enhanced fatigue life. Worth noting, laser peening did not significantly alter the surface roughness of the part when providing the fatigue enhancement compared to machining that significantly enhanced the surface roughness of the parts.

Benefits of Laser Peening for the Aerospace and Aviation Industries

Some benefits of laser peening are localized treatment in which the aerospace and aviation industries can add laser peening to the production process thus saving on time, while adding increased performance and reliability. The ability to control the whole process in-house is another benefit for production managers.

How the Aviation Industry is Using Additive Manufacturing

Additive manufacturing allows users to produce new prototype products quickly and cost-effectively, oftentimes on-demand or at the point of need. This can be beneficial in numerous industries such as aerospace, medical, transportation, and energy. To move from prototype, through qualification and into production, AM materials and processes must be honed and controlled with rigorous specifications and characterized via validated material allowable databases. In addition, AM-produced parts require a variety of “post-print” production processes to ensure the required physical and mechanical properties are achieved. Laser Peening has demonstrated its ability to significantly enhance the properties needed to enable AM-produced parts. 

The need to explore technologies such as laser peening to enable more AM-produced is demonstrated by the Department of Defense (DoD) releasing its first-ever comprehensive Additive Manufacturing (AM) Strategy in January of 2021. This strategy will improve AM implementation efforts.

“The Air Force eagerly partnered with OSD and our other Services to develop a much-needed DOD AM Strategy that addresses our collective needs. This Strategy is vital as it provides a roadmap to fully adopt and integrate AM in our acquisition and sustainment communities,” says Angela Tymofichuk, Deputy Assistant Secretary for Logistics and Product Support, Office of the Assistant Secretary of the Air Force for Acquisition, Technology and Logistics. “We envision AM as an effective tool that can mitigate diminished manufacturing sources of supply and long lead time supply chain shortfalls, while empowering Airmen to create innovative solutions for the myriad of sustainment challenges they encounter.” https://www.cto.mil/dod-additive-manufacturing-strategy/

Sources

National Institute for Aviation Research, Wichita State University (NIAR):

“NIAR’s mission is to strengthen university research capabilities; provide applied learning opportunities for students; and support the aviation and manufacturing industries – while driving innovation and prosperity for the community, region and state.”

LSP Technologies:

“LSP Technologies is the trusted leader in laser peening for metal forming and fatigue mitigation for customers around the world. In addition providing laser peening as a services business, LSPT also provides laser peening equipment and custom-engineered laser peening solutions.”