The influence of mechanical surface treatments on fatigue behavior of extruded AZ61 magnesium alloy

Surface treatments such as shot peening which introduce compressive residual stresses into the near surface region can have a beneficial effect on fatigue behavior. In the present study, tension-compression fatigue tests were carried out to understand the influence of three different mechanical surface treatments on fatigue properties of extruded AZ61 magnesium alloy at a stress ratio of −1 and frequency of 20 Hz under ambient environment. The mechanical surface treatments used in the present study were: (a) shot peening, (b) micro-peening and (c) barrel processing. It was found that the fatigue strength of shot-peened specimens actually degraded by about 30% as compared to the specimens which had not been shot peened. In the shot peened specimens cracks were nucleated at shot-peened induced surface defects which then propagated to failure, a direct indication that the shot-peening induced compressive residual stresses were not able to offset the detrimental effect of process-induced high surface roughness and surface defects. On the other hand, both micro-peened and barrel processed specimens exhibited an improvement in fatigue strength of about 15%. In these latter two instances, even though cracks were nucleated at sites of specimen surface roughness, the process-induced compressive residual stress retarded fatigue crack propagation, thereby resulting in the observed improvement in fatigue strength.