INFLUENCE OF PRECIPITATE MICROSTRUCTURE ON FLOW AND FORMING PROPERTIES OF AN ALUMINUM-ALLOY SHEET

A 7075 aluminium alloy sheet was cold rolled, solution heat treated and artificially aged to underaged (UA) and overaged (OA) conditions. A careful fabrication and heat treatment procedure was followed in order to obtain UA and OA sheets having the same isotropic crystallographic texture and uniaxial yield strength. Hydraulic bulge tests, plane strain, and uniaxial tension tests were performed to characterize the mechanical behavior of these materials. The forming limit diagram (FLD) was also obtained for these two conditions. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the precipitate microstructure, slip line patterns and fracture surfaces. Experimental observations showed that both macroscopic and microscopic responses of the materials to plastic deformation was strongly affected by the precipitate microstructure. FLDs were theoretically computed with the Marciniak and Kuczynski (MK) analysis, using the yield locus calculated with the Taylor/Bishop and Hill (TBH) theory for polycrystalline plasticity and the stress-strain curve corresponding to both UA and OA conditions. The predicted results were in good agreement with experiments for sheet in the OA condition, and they were not for the material in the UA condition. This was explained in terms of yield surface shape, consistent with the slip line characteristics, though differences in fracture behavior may contribute to these observations. The yield surface shape effect was emphasized since it provides, besides fracture, an alternate interpretation of failure mechanism near balanced biaxial tension, and since it can be described fairly well with a simple phenomenological description of plastic behavior.