Scaling Laser Shock Peening Process Towards High Repetition Rates: a Demonstration on Industrial Grade Al2024-T351

Micro laser shock peening (mu LSP) with pulse energies well below 1 J proved to be a useful technique to obtain fatigue-life performances similar to those reported for traditional LSP processes on metallic bulk materials [1, 2]. However, it suffers from a lack of productivity as spot sizes are reduced and pulse overlaps are increased in order to obtain compressive residual stresses, deep below the surface of the bulk material. To overcome these limitations of mu LSP, we have investigated strategies to scale up the productivity by increasing laser repetition rate while keeping constant the total amount of energy deposited on the peened surface [3-5]. We have built a laser processing cell to meet industrial grade applications. Complex surfaces are mounted on a KUKA robot to control the laser orientation and pulse overlap on the 3D workpiece surface. The pulse energy is provided by an 8 ns, Nd:YAG Laser, operating at 1064 nm, with a variable repetition rate from 10 to 100 Hz and delivering a maximum energy of 450 mJ/pulse on Al 2024-T351 samples with a thickness of 10 mm. We present high speed video analysis as diagnostic tool illustrating limitations in upscaling of repetition rates. As a proof of the mu LSP effectiveness we present compressive residual stress profiles with up to -500 MPa peak and a return to zero down to 1.8 mm below the surface. This represents a 5-times improvement of the maximum stress depth, when compared to conventional peening processes widely used in the aeronautic industry.