Effect of warm peening on residual stress and microstructure of CNT/Al–Mg–Si alloy composite

An aluminum matrix composite reinforced with carbon nanotubes was fabricated and subjected to peening treatment at RT, 150, and 250 °C. The introduction of elevated temperature significantly enhanced the near-surface strengthening effect of the peening treatment. The surface residual compressive stress showed only a slight change with increasing treatment temperature, whereas the maximum compressive stress in the subsurface region increased markedly and the depth of the compressive residual stress layer was extended from about 400 μm at RT to 450 μm at 150 °C and 500 μm at 250 °C. In addition, warm peening promoted stronger microstructural refinement and defect accumulation in the affected layer, with reduced domain size and increased microstrain and dislocation density. Correspondingly, the depth of the microhardness-strengthened layer increased from about 300 μm at RT to 350 μm at 150 °C and 400 μm at 250 °C, while the maximum microhardness reached 197 HV at 250 °C, which was about 17% higher than that at RT. These results demonstrate that warm peening is an effective approach for improving the strengthening depth and near-surface mechanical performance of carbon nanotube (CNT)-reinforced aluminum matrix composites.