Characterization and corrosion behavior of groove pressed and shot peened AZ31 Mg alloy sheets targeted for biomedical applications

Shot peening (SP) process was applied to grain refined AZ31 Mg alloy sheets processed by groove pressing (GP) with an objective to investigate the role of surface grain refinement on enhancing the surface properties of AZ31 Mg alloy targeted for degradable implant applications. Microstructural studies and phase analysis by x-ray diffraction were carried out. Microhardness measurements by Vicker’s indentation, surface wettability by contact angle measurements and corrosion behavior by electrochemical and immersion studies were conducted. From the results, it can be observed that GP resulted a grain refinement from 41.6 ± 3.3 µm to 6.2 ± 2.4µm after one cycle. After shot peening for 60 s on the groove pressed sheets by using 316L stainless steel balls of 2 mm, grain refinement at the surface was further refined to 1.7 ± 1.1 µm. Due to grain refinement at the surface, higher hardness (98.2 ± 2.6 HV0.1) was measured for GP+SP sample compared with the GP AZ31 (81.5 ± 5.5 HV0.1) and the base material (54.8 ± 7.4 HV0.1). Increased wettability was observed for GP+SP sample due to the superior level of grain refinement as reflected from lower contact angles (56.1 ± 1.8°). Corrosion experiments carried out by using simulated physiological solution (SPS) clearly indicated lower corrosion current density (3.49 ± 0.95 × 10-4 A/cm2) for GP+SP sample due to the stabilized passive film attained by grain refinement. Immersion studies of the samples in SPS for 3 days also confirmed the decreased degradation rate for GP+SP sample. The results suggest that combining GP and SP brings the combined benefits of bulk grain refinement and surface nano-structuring to enhance the performance of AZ31 Mg alloy with improved corrosion resistance targeted for degradable bone implant applications.