Jian Lu in his laboratory at CityU. Photo: City University of Hong Kong.
Jian Lu in his laboratory at CityU. Photo: City University of Hong Kong.

It has been estimated that over 80% of engineering failures are due to material fatigue, which is thus a key parameter for lightweight structures in mechanical systems such as aircraft, automobiles and energy-production systems.

Now, researchers at the City University of Hong Kong (CityU) and Shanghai Jiao Tong University in China have achieved a breakthrough by creating an aluminum (Al) alloy with unprecedented fatigue resistance using advanced 3D-printing techniques. Their new fatigue-resistance strategy could also be applied to other 3D-printed alloys to help develop lightweight components with increased load efficiency for various industries. The researchers report their work in a paper in Nature Materials.

“The fatigue phenomenon in metals was discovered about two centuries ago,” said Jian Lu, dean of the College of Engineering and director of the Hong Kong Branch of the National Precious Metals Material Engineering Research Center (NPMM) in CityU, who co-led the research. “Since then, fatigue failure has become one of the most important issues in the lifespan and reliability of all dynamic mechanical systems, such as those in aircraft, automobiles and nuclear power plants.”

Conventional metals exhibit fatigue strength that is generally lower than half their tensile strength. “Low fatigue strength is caused mainly by multi-scale defects in the materials, which continue to grow and evolve with cyclic loading, forming macroscopic cracks and expanding eventually into larger cracks that destroy the entire material structure,” Lu explained. “This challenging phenomenon also happens in alloys produced by additive manufacturing, also known as 3D-printing, limiting further applications of 3D-printed materials.”

To overcome the issue of low fatigue resistance in 3D-printed alloys and other metal materials, a joint-research team from CityU and Shanghai Jiao Tong University used laser powder bed fusion (LPBF) – one of the most widely used metal additive manufacturing techniques – to successfully fabricate a novel aluminum alloy made from AlSi10Mg powders decorated with TiB2 nanoparticles. They found that the fatigue resistance of this 3D-printed nano-TiB2-decorated AlSi10Mg (NTD-Al) alloy is more than double that of other 3D-printed Al alloys and surpassed those of high-strength wrought Al alloys.

The team also used micro-computed tomography to investigate the 3D-printed NTD-Al alloy and found, throughout the sample, a continuous 3D-dual-phase cellular nanostructure that acts as a strong volumetric nanocage to prevent localized damaged accumulation, inhibiting fatigue crack initiation.

“The three-dimensional network of nano eutectic silicon (Si) generated by additive manufacturing inside the alloy due to rapid solidification could block the movement of dislocations, thus suppressing fatigue crack initiation,” said Lu. “With controlled defects through process optimization, the fatigue limit of the bulk NTD-Al alloy is superior to that of all existing Al alloys.”

In a series of fatigue tests, the research team showed that the printed bulk NTD-Al alloy could achieve fatigue resistance of 260MPa, more than double that of other additive manufacturing Al alloys. The high-fatigue-strength limit of the bulk NTD-Al alloy surpassed that of all other Al alloys, including conventional high-strength wrought Al alloys with limited metallurgical defects.

The researchers have already used the NTD-Al alloy to fabricate prototypes of large thin-walled structures. These include the fan blades of aircraft engines designed for high fatigue strength, which successfully passed a qualifying fatigue test.

“These findings indicate the potential applicability of our alloy for the lightweight structures necessary in industries where fatigue properties are the key design criterion,” said Lu. “Our alloy can help reduce weight by increasing the load efficiency of moving components.

“Combined with the advantages of 3D printing, the latest discovery will boost lightweight design and reduce carbon emissions in modern industries. And the same strategy can be also used for other materials to help solve the fatigue failure challenge in metal additive manufacturing.”

This story is adapted from material from the City University of Hong Kong, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.