Improvement of residual stress and fatigue properties by low-energy laser peening

Laser peening was applied to A7075-T73 aluminum alloy and HT780 high-strength steel with low energy pulses of 2.5 mJ to 20 mJ which is attainable by ultra-compact handheld lasers. Then, we measured surface residual stresses of both materials by x-ray diffraction using a cos α method and confirmed that compressive residual stresses were built on the surface. Depth distribution of residual stress was evaluated by alternately repeating the x-ray diffraction and electrolytic polishing. Effect of laser peening on the residual stress penetrated into a depth of about 0.3 mm to 0.4 mm for the HT780. Welded joints of HT780 were laser-peened with low energy pulses of 10 mJ and 20 mJ and followed by uni-axial fatigue loading with a stress ratio of 0.1. Resulting fatigue strengths and lives were comparable to those of the HT780 welded joint peened with pulse energy of 200 mJ. It was clear through these experiments that laser peening could introduce compressive residual stresses on the surface of the tested materials and improve the fatigue properties of HT780 even if low energy pulses were applied. Such results would lead to downsizing the device, reducing the cost and expanding the application of laser peening.