The relationship between extrusion die line roughness and high cycle fatigue life of an AA6082 alloy

The effect of surface finish on the fatigue life of hollow extruded AA6082 was studied by comparing results from specimens with as-extruded surfaces to results from specimens with polished surfaces. Extrusion die lines are the main contributor to surface roughening, and since die lines are parallel to the extrusion direction, distinct variations exist between fatigue lives of as-extruded specimens taken longitudinal and transverse to the extrusion direction [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]. Polishing specimen surfaces eliminated much of the variation between specimen orientations. Fatigue lives of polished specimens containing extrusion seam welds transverse to the loading direction were also studied. The seam weld did not appear to significantly affect the fatigue life. Die lines were modeled as notches and finite element analysis (FEA) was used to estimate a linear-elastic stress concentration factor for approximating fatigue run-out values for specimens with as-extruded surfaces loaded transverse to the die lines. The predicted run-out stress values based on the FEA match well with those obtained experimentally.