Cyclic Fatigue Behavior of Friction Stir Welded AZ-Series Magnesium Alloys

Abstract

Fully reversed stress controlled cyclic fatigue testing in ambient air was conducted on friction stir welded magnesium AZ31 and AZ61 to magnesium AZ80 specimens to study the fatigue behavior and properties of the weld and to grasp an understanding of the rela- tionship between the material substructure and fatigue life. The resulting stress amplitude vs. cycles to failure (S-N) curves show an increase in fatigue life with a decrease in stress amplitude. For the AZ80/AZ31 weld, a sharp bend in the fatigue data was observed with an endurance limit of 70 MPa. A sharp bend was also observed in the AZ80/AZ61 weld which had an endurance limit of 90 MPa. Almost all the specimens fractured at the interface be- tween the heat-affected zone of the weld and the AZ80 base material for both dissimilar welds. SEM fracture surface observations from the AZ80/AZ31 weld show crack initiation occurring from the surface and crack propagation through the material for the high and moderate stress amplitudes. The fracture surfaces were neither true brittle nor true ductile, instead, they were a combination of the both with evidence of secondary angled cracking. The fracture surfaces also exhibited macroscopic and microscopic features unique to fa- tigue failure, which includes fatigue striations and beach marks. Analysis of the material dislocation substructure was conducted using TEM microscopy for the AZ80/AZ31 speci- mens tested at a high stress, moderate stress, and the endurance limit stress. A difference in the dislocation substructures and densities is observed from the different specimens which can be related to the stress amplitude applied. X-ray diffraction was also conducted on specimens tested at a high stress, moderate stress, and the endurance limit stress, as well as a specimen that was undeformed. The pole figures show a strong initial basal texture for the undeformed specimen that becomes rotated off the tensile axis as the specimens are subjected to increasing fatigue damage. The pole figures for the moderate stress and the endurance limit stress show a basal texture rotation of approximately 85 degrees.