Issue 48

M. Laredj et alii, Frattura ed Integrità Strutturale, 48 (2019) 193-207; DOI: 10.3221/IGF-ESIS.48.21 195 P ROCESS CHOICE any cold expansion approaches have been developed and investigated according to the diverse expansion tools. The most widely used methods in practice are: ball expansion, direct mandrel expansion, hole edge expansion and split sleeve expansion processes [14, 15, 05]. In the present work the expansion by a mandrel with sleeve has been chosen in order to avoid direct contact between the workpiece and the mandrel to prevent both mandrel wear and the damage of hole surface. The cold expansion hardening is carried out by inserting a mandrel with a sleeve through the hole of a tool (a conical mandrel with sleeve) of greater diameter “D” than the initial diameter of the hole “d” as shown in figure (1). Split sleeve cold expansion is accomplished by pulling a tapered mandrel pre-fitted with a lubricated split sleeve through a hole. The function of the disposable split sleeve is to reduce mandrel pull force, ensure correct radial expansion of the hole, preclude damage to the hole and allow one-sided processing [16]. The expansion is obtained by interference of the initial hole diameter (d) and the maximum mandrel diameter (D) taking into account the sleeve thickness (t). The degree of expansion is defined as a percentage by the following expression: ( ) 2 % 100 D T d DE d + − =  (1) D : Maximum mandrel diameter T : Thickness of the sleeve d : hole diameter before hardening Figure 1 : Drilling of cold expansion process overview [17]. M ODEL DESCRIPTION AND MECHANICAL PROPERTIES everal studies have been conducted to compare the results of 2D and 3D hole hardening finite element models [15, 05, 18]. All these previous studies consider the process as a generated axisymmetric stress state. A conclusion consensus of the different researchers’ works that two-dimensional finite element models predict quietly different tangential residual stresses when compared to the surface results of 3D models. A 3-D model was preferred to the axi- symetric 2-D one because of rectangular plate shape. This observation justifies the 3D models to simulate this phenomenon. The models consisted of square plates with 70 mm length, three thickness 3, 5 and 7mm with 6 mm diameter hole centrally located. Models had undergone a cold expansion hardening by mandrel with sleeve elastically deformable. The sleeve wall thickness is 0.25 mm and the mandrel diameter has a variable diameter depending on the degree of expansion (2, 4 and 6%). The expansion was simulated using the commercial code ABAQUS 11 [19]. The materials used in our 3D simulation work are Aluminum alloys 6005-T6, 6082-T6 and AU 4G. The mandrel is supposed to be perfectly rigid. The sleeve has been modeled as an elastic body with the steel material properties (Young's modulus of 220 GPa and a Poisson's ratio of 0.3). The friction coefficient between the contact surfaces is taken equal to 0.1. The friction is generally present between the handle and the plate but often neglected between the mandrel and the handle since it is lubricated. M S