Fatigue strength improvement of Mg alloy AZ61 by double ultrasonic shot peening

Abstract

Rotary bending fatigue tests using magnesium alloy AZ61 treated by double ultrasonic shot peening (USP) and peening under other conditions were performed to investigate the effect of shot peening (SP) on the fatigue behavior and to obtain high and stable improvement in fatigue strength. The single USP treatment was conducted under the condition of the shot diameter of 0.6 mm, 1.0 mm and 2.0 mm and the coverage of 80 %, 200 % and 300 %. The air blast SP was conducted using Zirblast B120 shot and under the coverage of 300 %. The first treatment of the double USP was conducted under the condition of the shot diameter of 1.0 mm and the coverage of 300 %, and then the second treatment was conducted under the condition of the shot diameter of 0.6 mm and the coverage of 300 %. Microstructures, hardness, residual stress and surface roughness of each specimen were investigated and compared. The fatigue strength of USP specimens which were treated under the low coverage and the high coverage with the large shot diameter was decreased, while that of the other USP and the air SP specimens was improved. High fatigue strength improvement was achieved by the double USP compared with the base metal and the other single USP and air SP specimens. It is considered that the improvement of fatigue strength of double USP specimen is attributed to the enough increase in hardness and compressive residual stress in the first treatment and the decrease of surface roughness and the removal or the modification of the thin work-softened layer in the second treatment.