Residual Stresses in Dengeling-Treated Aluminum Alloy AA 7050

Abstract

Abstract. Dengeling is a new method for mechanical surface treatment of metallic parts to enhance fatigue properties. The treatment produces consecutive lines of indents by a spherical indenter, resulting in compressive residual stresses in a surface layer. This paper investigated residual stresses, surface roughness and near surface deformation, in specimens of AA 7050 T7451 after different dengeling treatments. The effect of indent overlap (0%, 25% and 50%) and indenter size (∅3 and ∅8 mm) were studied. X-Ray diffraction measurements revealed that plastic deformation and compressive residual stresses were generated in a surface layer of about 1 mm by treatments using the ∅3 mm indenter and about 1.2 mm by treatment using the ∅8 mm indenter. Anisotropic residual stress fields were observed with higher compressive residual stresses (-360 MPa for 50% indent overlap, independent of indenter size) perpendicular to indent lines and lower (max -270 MPa for all the treatments) parallel to indent lines. Increasing the overlap between indents gave higher subsurface compressive residual stresses only in the transverse direction of indent lines. It also reduced the surface roughness. Best surface finish (Ra below 1 μm) was obtained when using the ∅8 mm indenter and 50% indent overlap. IntroductionDengeling is a new method for mechanical surface treatment of metallic parts to enhance fatigue properties. The treatment is carried out on a numerically controlled machine using a special indenter which moves and strikes the surface of the metal part in a given pattern and at a high frequency, up to 600 Hz. The transfer of kinetic energy from the indenter upon impact causes in-plane plastic deformation, producing consecutive lines of indents in the treated surface. Similar to the widely used shot peening process in which the surface is bombarded by hard shots, the dengeling treatment induces compressive residual stresses in a surface layer. As a considerable number of publications on shot peening have demonstrated [1][2][3], compressive stresses in near surface regions can greatly improve the fatigue resistance of a part. Dengeling has certain benefits over shot peening. The process is NC controlled, thus the operator can control exactly the location and magnitude of the generated residual stresses. Furthermore, the dengeling treatment is performed on the same machine that is used for machining the part; it can therefore be integrated into the machining process of metallic parts. That machining and surface mechanical treatment are carried out on the same machine means a great reduction of production cycle time and cost.