Roles for shot dimension, air pressure and duration in the fabrication of nanocrystalline surface layer in TC17 alloy via high energy shot peening

Abstract

In order to synthesize and assess the nanocrystalline surface layers induced by severe plastic deformation, TC17 alloy was peened by cast steel shots with dimensions of Φ0.3 mm, Φ0.6 mm and Φ1.2 mm at the air pressures ranging from 0.35 MPa to 0.60 MPa and processing durations ranging from 15 min to 60 min. The effect of peening parameters including the shot dimension, air pressure and processing duration on the surface structure characteristics of TC17 alloy was systematically analyzed based on electron microscopes and X-ray diffraction. The results showed that TC17 alloy underwent the dynamic processes among metal flow, severe work hardening and desquamation of surface layer during peening. The nanocrystalline surface layers have been fabricated in TC17 alloy at the shot dimension above Φ0.6 mm. Increasing shot dimension and air pressure or prolonging processing duration resulted in a decrease in the nanocrystalline size at topmost surface and an increase in the thickness of peening affected layer. For TC17 alloy, shot dimension of Φ0.6 mm should be chosen as too large shots led to the plastic instability during metal flow and too small shots resulted in the insufficient grain refinement degree and damage of surface finish. The largest thickness of ultrafine grains layer was up to 43 μm in TC17 alloy with a shot dimension of Φ0.6 mm at an air pressure of 0.35 MPa and processing duration of 60 min.