Surface hardening of aluminum alloy by shot peening treatment with Zn based ball

A study was carried out to investigate the surface hardening of aluminum alloy 2024, when it was shot peened with soft Zn-based shot ball. Shot peening process was carried out at 0.3 MPa with the variable process times, using the compressed air. After shot peening treatment, the surface hardness of AA 2024 increased up to about 140 HV from 65 HV of the base material. Shot peening treatment caused the severe plastic deformation onto surface region and Zn based phase was alloyed during the surface treatment, which was originated from Zn based shot ball. A lamellar structure in the stacking order of Al and Zn based phase was formed ranging from the treated surface to the depth of several hundreds of nanometers. The Al and Zn based phase presented nanocrystallized microstructure, showing the average grain size of about 87 nm and 13 nm, respectively. The structural characteristic of the lamellar structure and nanocrystalline grain for two phases could be main factors for the surface hardening of AA 2024. Besides, small precipitations that remained undissolved and kept its original shape in spite of severe plastic deformation could be considered to be a secondary factor for the surface hardening.