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A sound method for testing and increasing the strength of metal surfaces

Peening may be one of the most important engineering processes you’ve probably never heard of. It is used to strengthen metals by increasing the hardness of their surfaces, increase their compressive strength, increase their resistance to corrosion and, ultimately, increase their lifespan. Scientists working at STFC’s Central Laser Facility (CLF) have developed a new quality control method that can be used to check whether the laser peening process has been successful.

Peened metals are a crucial part of the aerospace industry – allowing the development of lighter and stronger metals that, in turn, enable engineers to produce lighter and more fuel-efficient aircraft. In jet engines, the peening process allows the metals used in compressor blades and fan blades to withstand being subjected to intense heat and pressure for hundreds of hours.

Outside of the aerospace industry, peened components can be found in automotive technology, power generation, drilling, medical implants, and nuclear power plants – where pressurised water reactors have to be protected against stress corrosion cracking to ensure safe operation over the increasingly long lifetimes of reactors. In fact, they can be found anywhere in which metals are subjected to extreme mechanical stress, degradation, and corrosion factors.

At its most basic, peening can involve striking a metal with a hammer, which induces a shock wave that travels through the metal and compresses the surface layer – increasing its hardness and resistance to cracking and fatigue. A more sophisticated method, called shot peening, involves blasting the surface with metallic or ceramic pellets, which allows more complex three-dimensional surfaces to be treated. The third, and most high-tech, method is called laser peening.

The laser peening, or laser shock treatment, process involves using a series of laser pulses to generate a hot plasma on the metal’s surface that rapidly expands – creating a shock wave that is driven into the material. Unlike shot peening, which leaves the surface rough and pitted, laser treatment leaves the metal’s surface largely unaltered. This advantage over shot peening has meant that laser treatment has become an indispensable tool for sectors such as aerospace and nuclear power generation.

However, the material processing industry still lacks an effective quality control technique for monitoring and evaluating the process in-situ. Current methods involve either complex X-ray diffraction techniques, or drilling a hole in the peened material and making an extensive series of measurements, which is destructive, expensive and time-consuming.

The new technique developed by scientists from CLF is based on the time-honoured technique of striking metal with a hammer and using the sound it generates to judge its quality and hardness. But, instead of using a hammer, the technique relies on listening to the sound generated by the laser pulses striking the metal during the peening process.

In the experiments, which were carried out using the DiPOLE laser system at CLF, the researchers recorded the acoustic signature of the sound waves generated by laser pulses striking a variety of metals and metal alloys. They then built up an acoustic profile that could be applied to the metals, which included aluminium, titanium and stainless steel alloys. They were able to demonstrate that each metal can have a unique acoustic signature assigned to it that gives information on the material’s strength, hardness and residual stresses. This unique signature can then be used to indicate the success and quality of the peening process without having to subject the metal to further testing.

 


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Last updated: 30 August 2019

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