A new study by engineers at Monash University (Melbourne, Australia) has demonstrated improvements in the fatigue life of high-strength aluminum alloys by 25 times, which they believe could be a significant outcome for the transport manufacturing industry.
In their work,1 the researchers demonstrated that the poor fatigue performance of high-strength aluminum alloys was because of weak links called “precipitate free zones” (PFZs).
Led by Christopher Hutchinson, a professor of materials science and engineering, the research team made aluminum alloy microstructures that were able to heal the weak links while in operation. They describe this process as a form of self-healing.
Aluminum Alloy Facts
Aluminum alloys are the second-most popular engineering alloy in use today, according to the researchers. Compared to steel, they are light, with approximately one-third of the total density. In addition, they are non-magnetic and have excellent corrosion resistance.
Aluminum alloys are important for transport applications because they are light, which improves fuel efficiency. However, their fatigue properties are notoriously poor compared to steel of similar strength, the researchers explain.
When using aluminum alloys for transport, the design must compensate for the fatigue limitations, Hutchinson says. This means more material is used than manufacturers would like, and the structures are heavier than desired.
“Eighty percent of all engineering alloy failures are due to fatigue,” Hutchinson says. “Fatigue is failure due to an alternating stress and is a big deal in the manufacturing and engineering industry. Think of taking a metal paperclip in your hands and trying to break the metal. One cannot. However, if you bend it one way, then the other, and back and forth a number of times, the metal will break.”
Failure by Fatigue
“This is ‘failure by fatigue,’ and it’s an important consideration for all materials used in transport applications, such as trains, cars, trucks, and planes,” Hutchinson says.
Failure by fatigue occurs in stages. The alternative stress leads to microplasticity, in which the object undergoes permanent change due to stress. From there, the accumulation of damage results in localized plasticity at weak links in the material. The plastic localization then catalyzes a fatigue crack. This crack grows and leads to final fracture.
Using commercially available AA2024, AA6061, and AA7050 aluminum alloys, the Monash researchers used mechanical energy imparted into the materials during the early cycles of fatigue to heal the weak points in the microstructure (the PFZs). This strongly delayed the localization of plasticity and the initiation of fatigue cracks, and saw enhanced fatigue lives and strengths.
Implications of Research
Hutchinson believes these findings could be significant for the transport manufacturing industry as demand continues to grow for fuel efficient, lightweight, and durable vehicles.
“Our research has demonstrated a conceptual change in the microstructural design of aluminum alloys for dynamic-loading applications,” he says. “Instead of designing a strong microstructure and hoping it remains stable for as long as possible during fatigue loading, we recognized that the microstructure will be changed by the dynamic loading and, hence, designed a starting microstructure [that may have lower static strength] that will change in such a way that its fatigue performance is significantly improved.”
“In this respect, the structure is trained, and the training schedule is used to heal the PFZs that would otherwise represent the weak points,” Hutchinson concludes. “The approach is general and could be applied to other precipitate hardened alloys containing PFZs, for which fatigue performance is an important consideration.”
Further research findings were recently published in the Nature Communications journal.
Source: Monash University, www.monash.edu.
1 “Monash Engineers Improve Fatigue Life of High-Strength Aluminum Alloys by 25 Times,” Monash University Latest News, Oct. 15, 2020, https://www.monash.edu/news/articles/monash-engineers-improve-fatigue-life-of-high-strength-aluminium-alloys-by-25-times (Nov. 16, 2020).