Development of a portable laser peening device and its effect on the fatigue properties of HT780 butt-welded joints

Laser peening (LP) is a well-established technique for introducing compressive residual stress (RS) near the surface of metal components, to improve their high-cycle fatigue properties. The authors have developed a compact LP device with a thumb-sized Nd:YAG microchip laser mounted on a collaborative robot arm. The device was applied to 9-mm-thick HT780 high-strength steel plate samples with irradiated pulse energies of 7.5−8.0 mJ, spot sizes of 0.42−0.58 mm and pulse densities of 100−1,600 pulses/mm2. X-ray diffraction showed that the maximum compressive RS was over 500 MPa near the surface, and the LP effect reached a depth of approximately 0.1 mm from the surface. Butt-welded HT780 samples were laser-peened with a pulse energy of 7.7 mJ, spot size of 0.49 mm and pulse density of 800 pulses/mm2. Then, the samples were subjected to a uniaxial fatigue test with a stress ratio of 0.1. The results showed that the fatigue strength at 107 cycles was improved by at least 50 MPa, comparable to the improvement attained by LP in a previous study with a pulse energy of 200 mJ from a conventional Nd:YAG laser.