Asymmetry of strain rate sensitivity between up- and down-changes in 6000 series aluminium alloys of varying Si content

Abstract

Increasing demand for a reduction in fuel emissions in passenger vehicles has generated the need for lighter weight materials to be used in automobile manufacture for body-in-white applications. Aluminium alloys in the 6000-series, containing Mg and Si are ideal candidates for these applications but lack the formability found in commonly used steels, providing a need to more fully understand the factors influencing the formability of these alloys at high strains. Conventionally, a high strain rate sensitivity (SRS) is tied to increased formability because it retards the increase in the local strain rate found in the diffuse neck interior. However, most experimental work neglects that the regions exterior to the neck will undergo a local decrease in the strain rate which causes a corresponding material softening. Observations of an asymmetry between up-change and down-change SRS of these alloys in the natural aged condition show that different mechanisms are controlling the SRS depending on the direction of rate change. Following a characterization of the state of clustering by differential scanning calorimetry, continuous tensile and precision strain rate sensitivity testing results are presented, elucidating the differences between the up-change and down-change SRS tests. It is shown that these differences are due to the activation of different thermal obstacles during the two directions of rate changes. The role of a change in Si content on the mechanical properties is explored and its suspected role on the asymmetric SRS is discussed.