In the present study, the hole drilling and cold expansion hole are investigated. Three radius value and five expansion degrees have been used. The hole radius successively were 1.0; 2.5 and 3.0 mm. In this case the time life is controlled by the local radius and the local stress concentration factor. The cold expansion has been performed in order to get a final radius of 3 mm, the initial radius were slightly smaller so the expansion degrees varies from 0% to 9.0%. As we’ll see later there is a limit on top a certain level the cold expansion is damaging. EXPERIMENTAL PROCEDURE We used two aluminium alloys 60XX. that means its contents Magnesium and Silicium. These alloys are used for drawing at cold and mid temperature - working. The composition and the main mechanical properties are respectively given in the table 1 . The true stress versus true strain shows a low stress hardening. Alloy Mg Si Fe Cu Mn Cr Zn Ti Re Rm E A% Hv50 60XX1 0.45 0.5 0.26 0.2 0.23 0.16 0.09 0.04 245 273 68 12 90 10 60XX2 0.6 0.7 0.24 0.06 0.9 0.02 0.06 0.02 280 327 68 12 95 10 TABLE 1 CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES Photo 1 Microstructure Alloy 60XX1 Photo 2 Microstructure Alloy 60XX2 The first alloy presents a constituted microstructure of coarse grains without any precipitates (photo 1) the microstructure of the second alloys presents fine grains where we can detect very small precipitates (photo 2). The specimens were cut from aluminium alloys plates of 5mm thick, with the axe parallel to the rolling direction of the plates. A slit of 1mm in width and 15 mm in length was machined at one lateral side of specimen. A pre crack from the slit tip was introduced by cyclic loading and the total length of the slit and the pre-crack was (a0 + ) = 27.5 mm. A hole was machined at the pre-crack tip. The hole was first drilled conventionally and then carefully enlarged by a boring bar to a desired radius [2]. The cold working expansion process was realised by forcing a hard steel ball of 6mm in a diameter slightly smaller pre-drilled hole. A burnished surface of the expanded hole was obtained and the surface structure was refined by plastic deformation leaving at the hole surface.
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