Influence of natural aging and ramping before artificial aging on the microstructure of two different 6xxx alloys

In this study the effect of a ramp up to artificial aging (AA) temperature in combination with natural aging (NA) was analyzed for two different industrial aluminium alloys. The alloys differ in their Mg/Si ratio as well as their copper (Cu) content. Both alloys have, however, a comparable amount of solute and dispersoid forming elements. The peak aged conditions of the alloys were investigated. NA prior to AA reduces the strength as compared with directly aged regardless of ramping. The hardness up to peak is very similar for conditions with and without the ramp if NA is performed, but for the Cu rich alloy a clear increase in peak hardness could be measured when ramped from the supersaturated solid solution (SSSS). The peak strength plateau is shorter for the ramped samples regardless of the NA time. Investigations by (scanning) transmission electron microscopy found longer but fewer precipitates when ramped from the SSSS and an increased precipitate number density when ramped after NA. Fewer hybrid-type precipitates and less overaged precipitates, e.g. B’, and more L/C could be found for all ramped conditions. Atom probe tomography measurements showed that more solute is left in the matrix between the precipitates when ramped directly from the SSSS. This was related to a lower precipitate number density