Formability and Fracture of Some Higher Strength Aluminum Alloys

This work presents the comparative behavior of some formable grades of aluminum alloy sheet and an aluminum killed drawing quality (AKDQ) steel sheet in a number of formability tests. Tensile data were assembled from specimens cut at different orientations in the plane of the as-received sheet. An attempt was made to fit the stress-strain data with an empirical equation of the form σ¯ = k (ϵ0 + ϵ¯)n. The following quantities were also obtained from the tension tests: uniform strain, the plastic strain ratio (r value) and fracture strain. The aluminum alloys showed inferior r values and lower values of fracture strain when compared with the steel sheet; the n value did not always correlate well with the measured uniform strain in the alloys. The aluminum alloys were deformed under different conditions of biaxial straining by stretching over a rigid punch, and forming limit diagrams were constructed. From the same test it was also possible to assess the fracture strains. The fracture thickness strain does not appear to be a single valued material property but can be dependent upon the deformation mode rather like the limit strain. Additional tests performed on the materials included bulging in a circular die under hydraulic pressure, an in-plane torsion test, and an in-plane stretch test designed to promote a plane-strain deformation mode. The results from these tests supplemented the limit and fracture strain data gathered from the punch-stretch tests. The aluminum alloys could be formed but showed inferior formability to the AKDQ steel. The lower fracture strains exhibited by the alloys may influence the behavior in certain plastic forming processes in a manner that would not be anticipated on the basis of n value predictions.