Improvement in fatigue performance of friction stir welded A6061-T6 aluminum alloy by laser peening without coating

The authors have applied laser peening without coating (LPwC) to fatigue specimens cut out from friction stir welded (FSWed) A6061-T6 aluminum alloy plates with a thickness of 3 mm. Both crown and root sides of the specimens were peened by laser pulses with an energy of 60 mJ and a peak power density of 2 GW/cm2 from a frequency-doubled Nd:YAG laser. The effects on the fatigue properties were studied through plane bending fatigue tests with a stress ratio of R = −1. The results showed that the fatigue strength of unwelded specimens (base material; BM) was 110 MPa at 107 cycles and LPwC enhanced the strength by 60 MPa in spite of increase in surface roughness due to the direct irradiation of the laser pulses to the bare surface of the specimens. Meanwhile the fatigue strength of FSWed specimens was 90 MPa and LPwC enhanced it by 30 MPa to 120 MPa. This increment is a half compared to that in the BM, however the fatigue strength of the FSWed specimens after LPwC was higher than that of the BM. The surface roughness, hardness and residual stress were assessed and characterized as well.