Effect of specimen orientation and extrusion welds on the fatigue life of an AA6063 alloy

The effect of specimen orientation and extrusion seam welds on the fatigue life of specimens milled from a hollow AA6063 aluminum alloy extrusion profile have been characterized in the form of S–N curves. The fatigue behavior of the fully recrystallized AA6063 alloy reported here is compared to previously reported results on a partially recrystallized AA6082 extruded aluminum alloy [Nanninga N, White C, Furu T, Anderson O, Dickson R. Effect of orientation and extrusion welds on the fatigue life of an Al–Mg–Si–Mn alloy. Int J Fatigue 2008; 30(9): 1569–78]. Specimen orientation with respect to the extrusion direction has a large effect on fatigue life, and is attributed to the orientation of extrusion die lines. Transverse specimens exhibit only a fraction of the high cycle fatigue life exhibited by longitudinal specimens. The presence of seam and/or charge welds, oriented both longitudinal and transverse to the loading direction, may also reduce the fatigue life. However, the effects of these welds are not as significant as the effects of orientation alone, and appear to be related to increased surface roughness at the welds. Variations in fatigue life are explained by surface roughness and variations in microstructure. Grain boundary separation was the primary mechanism of fatigue crack initiation in this study. Results from Auger energy spectroscopy and transmission electron microscope analyses suggest that localized strain in precipitate free zones may assist in this behavior.