Influence of equal-channel angular pressing on the microstructure and corrosion behaviour of a 6xxx aluminium alloy for automotive conductors

Abstract

An Al-Mg-Si aluminium alloy was shaped by using a two-pass equal-channel angular pressing (ECAP) process. This led to fragmentation of the coarse Fe-rich intermetallics (IMCs), a decrease in grain size and an increase in the high angle grain boundary (HAGB) density, with overconcentration of HAGBs around the IMCs. Corrosion tests in NaCl solution showed that, before and after ECAP, only pitting corrosion occurred. However, for ECAP samples, pits were more numerous due to the fragmentation of the IMCs; they were also larger and less deep, their propagation being strongly influenced by the presence of very small grains around the IMCs.

Influence of equal-channel angular pressing on the corrosion fatigue behaviour of an Al-Mg-Si aluminium alloy for automotive conductors

“…The crack propagation zone was characterised by a granular aspect (Fig. 5d), which was consistent with the grain size of as-received, ECAP-RR and ECAP-RH samples measured in a previous study [30]. Therefore, it was considered that the crack propagated by intergranular fracture.…”

“…The IMCs fraction of surface area was found to decrease from 0.53% for the as-received samples to 0.32% and 0.34% for ECAP-RR and ECAP-RH samples, respectively. This was most likely related to fragmentation of the IMCs during ECAP, which resulted in a higher amount of small IMCs not detectable during image analysis [30]. This particular effect was also observed by other authors [14,31].…”

“…The microstructure of the as-received, ECAP-RR and ECAP-RH samples was described in detail in a previous paper [30]. Only major results are reproduced here.…”

“…4 µm, were lower than those of ECAP-RR sample, with a grain size of 16 µm. This was explained by the observation of an incomplete recrystallisation process for the ECAP-RR sample, whereas a complete recrystallization occurred for the ECAP-RH sample [30,33]. Indeed, the increase in processing temperature for ECAP-RH during the second pass led to a complete recrystallisation process, and then to a decrease in the deformation stored in grains, which induced a slight softening of the alloy.…”

“…In agreement with this last statement, the slight increase in both density of corrosion defects and corroded surface ratio for ECAP-RR and ECAP-RH samples could explain the decrease in fa tigue lifetime for those samples. This should be associated with the fragmentation of Fe-rich IMCs during ECAP processes [14,30], those particles acting as preferential corrosion sites [33]. The presence of a higher amount of cathodic sites led to the formation of a higher number of corrosion defects at the sample surface and to premature crack in itiation in the case of ECAP samples.…”