Fast laser shock peening shows promise for industry
Laser Shock Peening (LSP) is a mechanical surface treatment process that has been used mainly in the aerospace and nuclear industries for more than 50 years.
Through a collaboration between Thales LAS and the PIMM laboratory, a fast and user-friendly configuration of laser peening has been developed.
This has been achieved using a new generation of high-power and high-frequency laser systems.
As shown in figure 1, LSP consists of focusing a high-power laser pulse (1-10J, 5-20ns) on a metal target to reach a great intensity, typically around 2-8GW/cm² (depending on the targeted application).
As a result, a high-pressure plasma (>GPa) is created, and its expansion generates strong shock waves that propagate through the treated metal. These shock waves create a compressive residual stress field (due to yielding of the metal) that fights crack propagation and thus fatigue issues. Therefore, the treated metal’s properties are improved, resulting in higher strength and greater lifetime.
However, in modern applications a thermal protective coating has to be applied (see figure 1), as the plasma reaches high temperature (>10,000K). Altogether, the cost and time required to treat a given surface are increasing because of this thermal coating.
Figure 1: Main mechanisms of the laser shock peening process
Why should fast laser shock peening bait the industry?
Fast laser shock peening (FLSP), also called laser shock repeated dense peening (LSRDP), is an enhanced version of the current LSP process. This new configuration is used without any thermal protective coating and at a higher laser frequency: 200Hz has been demonstrated, compared to the 20Hz used in current industrial applications.
Furthermore, small laser spots are used (0.8-1.5mm) with high overlap ratios (>1,000 per cent). More compact laser systems are also associated with this configuration, as no more than 1J of energy is required per laser pulse to make the peening process effective (compared to more than 5J for current applications).
In addition to no longer requiring the treated metal to be covered with a thermal coating before any processing, this configuration offers reduced surface treatment times, enabling it to become a great asset for industry as it could be applied to a wider range of applications and at a lower cost.
Eventually, this configuration may even be delivered through optical fibres, as typical laser pulses carried this way are in the nanosecond range and carry around 0.5J of energy. This would enable a safer and more manoeuvrable FLSP process.
Overview of the solution developed by Thales
As illustrated in figure 2, FLSP was developed and implemented under an all-in-one platform that enables the user to perform either metrology (to increase process accuracy or to check the applied parameters: mainly laser pulse energy and duration, and spatial distribution of energy) or FLSP treatments on metals. Al 2024 and Ti6Al4V were subjected to FLSP treatments; a deep and high level of compressive residual stress was obtained in each case.
Figure 2: Rogue Laser facility dedicated to the FLSP process
This platform relies on Thales’ new Theia generation of diode-pumped solid state (DPSS) lasers. It delivers laser pulses of up to 1J of energy, with a pulse duration of 10ns and at either 1,064, 532 or 355nm (in the presented configuration, treatments were performed at 1,064nm). Most importantly, this laser was operated at a very high frequency of 200Hz. Theia will be upgraded to even higher frequencies by Thales in the near future: up to 500Hz.
While developing the FLSP solution, the following issues were identified and addressed:
- An air-blowing system has to be implemented to clean the laser beam path and avoid detrimental parasitic plasmas (see figure 3). These parasitic plasmas are mainly induced by metal particles and water droplets ejected out after laser ablation of the metal target; this results in a loss of energy transmission. This also helps to protect upstream optical elements from high-speed ejected particles.
- A water jet with a suitable flow rate, depending on the process parameters (energy, frequency and laser spot size), must be integrated to ensure the water confinement (used to increase the plasma pressure by slowing down its expansion, compared to a free expansion in the air) is constantly renewed shot by shot.
Figure 3: Air-blowing system to clean the laser beam path
Conclusion and prospects
The FLSP solution has been studied experimentally through many research works at the PIMM laboratory. It is, as of now, fully operational to reinforce metals in lab conditions. However, the critical point remains that an air-blowing system must be used to thwart ejected particles and ensure a repeatable process; this may be a tough challenge for some industrial environments.
It is highly expectable that this new configuration will pave the way for a larger spread of laser peening in industry. As this solution is fast and solves one of the current pains inherent in LSP (the use of a thermal protective coating), it may help to address new use cases or make current ones more cost-effective.
Dr Alexandre Rondepierre is an optical engineer at Thales LAS France SAS
Dr Olivier Casagrande is a laser architect at Thales LAS France SAS
This article was co-authored by Phil Goldberg, Avinash Hariharan and Annett Gebert, of the Leibniz Institute for Solid State and Materials Research Dresden