Crystal plasticity evaluation of laser peening effects on improving high-cycle fatigue life of Al-Li friction stir welded joints

Abstract

The strengthening mechanisms behind surface modification processes must be evaluated to optimize surface treatment methods. This study developed a physics-based crystal plasticity finite element framework to evaluate multiple-laser-peening impacts on enhancing the high-cycle fatigue properties of AA2195-T6 friction stir welded joints. For this purpose, different contributions of laser peening effects were quantified on the variations of fatigue indicator parameters in the joint region. Extreme-value fatigue indicator parameters evaluated the laser peening effects in decreasing the driving force for fatigue crack initiation at near-surface grain boundaries. Moreover, grain-average fatigue indicator parameters investigated the fatigue property modification trends with depth. Experimental high-cycle fatigue tests were applied to validate the related modeling outputs. According to the crystal plasticity model, the improvement in joint near-surface fatigue properties majorly stemmed from cyclic mean stress alleviation under laser-peening-induced compressive residual stresses. This effect was followed by the increased resistance against cyclic plastic deformation under the growth in dislocation density and crystallographic texture intensity and the reduction in texture heterogeneity under the crystal morphology homogenization effects after laser peening. Fatigue tests revealed the enhancement in the joint fatigue life under the relocation of fatigue crack initiation regions to depth after LP and validated the numerical approaches.