Influence of pulse energy on surface integrity of AZ31 magnesium alloy processed by femtosecond laser shock peening

Abstract

Compared with traditional laser shock peening by nanosecond pulse laser, femtosecond laser shock peening (FLSP) exhibits superior flexibility and efficiency since it can be carried out directly in air without any ablative or confinement layers. In this work, FLSP was successfully performed on AZ31 magnesium alloy sheet. The influence of pulse energy on surface morphology and roughness, microstructure, surface residual stress and hardness was investigated in details. The results show that varying the laser pulse energy could effectively improve the surface integrity of the material. Regular dents are produced at the material surface with relatively low pulse energies, and the regular dent morphology disappears gradually with increasing pulse energy, suggesting an increased surface roughness. Surface microhardness and residual compressive stress are increased significantly after FLSP. The surface microhardness increases from the untreated 58.13 HV to the maximum value of about 69.58 HV at 37 μJ, by an increase of 19.7%. At 73 μJ, the residual compressive stress rises to a maximum value of approximately 59.6 MPa from an untreated 16.3 MPa. Moreover, the nano-hardness near the surface at the cross section is 1.68 GPa at 37 μJ, with an increase of 46% compared to the untreated 1.14 GPa. Particularly, twins and grain refinement layers are observed near the surface after FLSP, also indicated by the XRD analysis. The plastic deformation and grain refinement with low pulse energies contribute to the surface strengthening while the thermal effect and surface oxidation with increasing pulse energy may inhibit the enhancement of surface integrity.

Keywords

• Laser shock peening
• Femtosecond laser
• Pulse energy
• Surface integrity
• AZ31 magnesium alloy