Stretch formability of AI–Mn alloys

Abstract

The stretchability of strip-cast and DC-cast Al–Mn alloys as measured by their forming limit curves has been studied. Optimization seems to be achieved with as high a work-hardening exponent as possible in uniaxial tension when instability is approached. This work hardening rate is, in the present case, influenced more by the slope of the stress-strain curve than by the flow-stress level which also has an effect. At these high strains the increase inflow stress is, in general, proportional to the square root of the dislocation density. Consequentially, both the rate of production and the annihilation and polygonization of dislocations have an important effect on the work-hardening rate. The strip-cast and non-homogenized alloys have the lowest work-hardening rates of the alloys investigated. They contain a large number of finely dispersed secondary particles which give rise to an irregular and tangled dislocation structure after straining to maximum load in uniaxial tension. It is suggested that this kind of structure facilitates a more pronounced dislocation annihilation than one consisting predominantly of cell walls. In general, the observed da/dE v. strain curves at large strains increase with decreasing dispersion ratio (volume fraction/average radius) of the particles.