New generation application: laser shock peening ensures deeper RCS, using the new THEIA laser
Harnessing the energy from pulsed lasers provides a new tool to improve the reliability and lifetime of metal parts. Laser shock peening offers significantly deeper residual compressive stresses (RCS) than traditional shot peening methods. With the introduction of THEIA, a diode-pumped Nd:YAG laser, Thales enters a new sector, that of laser shock peening, shifting metal fatigue prevention to a higher plane.
History indicates ‘shot peening’ (SP) has been used over the past five decades in surface modification to metals and alloys, for the purpose of changing the surface integrity, obtaining superior mechanical properties, preventing metal fatigue, or surface cracking and in so doing improve the service life of parts. The result is a dimpling effect. Following the impact, the recovery process induces residual compressive stresses (RCS) on the surface of the substrate making it stronger and resistant to surface cracking. Shot peening is widely used on various metals and alloys across industrial, aerospace, medical, and automobile sector.
Recently laser shock peening (LSP) has been adopted as a more suitable process in critical industries such as nuclear, biomedical and aerospace. In the aerospace industry, LSP is used in passenger and military aircraft to enhance the fatigue performance of the compressor blade, landing gears, shafts, valves, discs, and complex forming of wing surfaces. In the United States, laser shock peening is now being used on F-35B’s short take-off aircraft used by the U.S. Marine Corps. Following months of testing and accreditation, laser shock peening is being applied to strengthen the materials in the aircraft’s bulkhead and airframe subjected to excessive performance stresses of landing in small spaces or aircraft carrier decks.
The aim is to enable the aircraft to reach its full structural life limit. Aircraft components are subject to critical life limits and require scheduled maintenance procedures, laser shock peening provides a new improved solution. LSP has been shown to enhance cracking resistance up to ten times deeper than shot peening for fatigue prevention and part performance. What is important for the F-35Bs, is that this maintenance process doesn’t add any additional weight nor does it reduce the fuel or weapons carrying capacity.
LSP techniques can be applied to strengthen turbine fan blades with testing of fatigue strength demonstrating it is two times more effective than with traditional shot peening processes. Recent research has shown that LSP can be successfully applied to thin wall welds of Ti6A14V, used to strengthen components of aircraft engines. While residual stress profiles vary based on part thickness and material properties, LSP consistently produces residual compressive stresses many times deeper than those produced with SP. As a result, laser shock peening is being heralded as the ultimate fatigue enhancement solution.
LSP is widely used in post-processing welds, for example in the nuclear sector where stress corrosion cracking (SCC) is a major threat to nuclear reactors and canisters, laser shock peening is proving an effective solution. Other industry usage examples are found in connecting rods in heavy equipment (bulldozers, etc), metal forming dyes, aluminium ship decks, power plant components even racing car parts.
To meet the growing demand, Thales Group has introduced THEIA, a high-power and high-frequency laser system delivering nanosecond pulses at different wavelengths in the near infrared (1064nm), in the visible (532nm), and in the near ultraviolet (355nm). The new configuration can be used for LSP without any thermal protective coating on the targeted part to receive the peening treatment. THEIA boasts a very high laser frequency at 200Hz with a small laser spot size of 0.8-1.5mm and high overlap ratios (>1,000%). This capability opens up new opportunities for enhanced peening.
The principle behind the process of laser peening is to create compressive stresses, deformations or plasticisation on the surface of a fabricated component causing the material to compress inwards leading to denser sections. Laser peening creates high-pressure plasma, and on expansion, strong shockwaves penetrate through the treated metal, creating a residual compressive stress field that fights crack propagation and fatigue issues. The process works by using a high-energy pulsed laser to generate high-amplitude stress waves on the surface of the target part. The laser is not used for heat effects, but for the dynamic mechanical effects of the shockwave generated by the laser beam. Where the laser beam strikes the metal surface a plasma is created.
To confine the energy from the laser beam along with confining the plasma to the metal surface, and gain maximum mechanical force, a thin layer of water overlays the part. The water is transparent to the laser beam. It is only recently that water has been applied, previously parts had to be coated in aluminium, copper, zinc and black paint. These were considered effective coatings to help prevent thermal effects like laser ablation, melting, and generation of tensile stress, and, enhance the peak pressure of the shock wave induced during direct laser interaction. Providing a protective coating is generally a costly and time-consuming requirement, but with recent developments in Thales’ new THEIA, a high-power, high-frequency laser system, the ‘tamping material’ now required is a thin film of water. By laser shock peening the treated metal’s properties are improved, resulting in higher strength and greater lifetime.