Introduction

Laser shock peening (LSP) is a mechanical surface enhancement process using high-pulsed energy laser beam to generate and propagate shock waves inside metallic materials, inducing deeper (≥1mm) and higher (≥700MPa) compressive residual stresses. LSP, compared to the conventional shot peening, is also a more targeted and controlled process, typically applied to high performance, critical components, such as fan blades, to improve their metal fatigue life and damage tolerance

Three factors which affect LSP:

  • Confining layer (i.e. water) – direct and amplify pressure wave into target material
  • Ablative layer (i.e. black tape) – absorbs laser energy and protects surface from thermal effects
  • Target materials: metals such as titanium, nickel-based superalloys (e.g. IN718), steel, aluminium

Benefits of LSP

PROCESS:

  • Deep, high compressive residual stresses with minimal cold work (fatigue enhancement)
  • Non-contact impact loading with very high strain rate (>107 s-1)
  • Targeted and controlled process

EQUIPMENT:

  • High speeds – shorter processing time
  • Feedback system to adjust performance for desirable output response
  • Local system to monitor and store real-time outputs of the laser
  • 6-axis industrial robot with 200kg payload to cater to large, heavy components of different geometries

How LSP Works:

High-energy short pulsed laser strikes the surface of the specimen. The energy of the laser beam is quickly absorbed by the protective overlay.

The quick absorption of energy results in plasma formation. The plasma expands and generates recoil pressure in GPa. As a result, shockwaves are created.

The expanding plasma is confined by the dielectric medium (water). This amplifies pressure to a much higher value than its free expansion (~1 order of magnitude higher). Powerful compressive shockwaves propagate into the material.

If the amplitude of the shockwave is above the dynamic yield strength, of the target (Hugoniot Elastic Limit (HEL)), it will form plastic deformation along the passage of the shockwave. After the shockwave passes, the residual plastic strain creates a comprehensive residual stress gradient below the target surface, which decreases as the depth increases.

LSP Vs Shot Peening

Commercial Applications of LSP