Ductilization by multi-pass Friction Stir Processing of thick 7475 aluminium alloyTuesday (05.11.2019) 17:15 - 17:35 Part of:
The exceptional tensile strength of 7XXX aluminium alloys is due to a fine hardening precipitation. In peak-aged T6 state, the yield strength reaches more than 500MPa. However, heterogeneous precipitation occurs along grain boundaries. This induces a softer precipitate free zone (PFZ) due to depletion of alloying elements. Industrial 7475 aluminium alloy plates are obtained by rolling. The elongated grain length may reach 1 mm.
Under loading, impurities like iron-rich particles (IM) break or detach with the matrix. These first micro-cracks will propagate along the grain boundaries following the heterogeneous precipitates inside the PFZ. Finally, trans-granular failure cuts the remaining ligaments of material by void coalescence and shearing. Both inter-granular and trans-granular modes play a role in the damage mechanism, as showed by Ludtka et al .
According to Rometsch et al , the presence of PFZ and heterogeneous precipitates cannot be removed by solution heat treatments. However, by applying Friction Stir Processing (FSP), a refined microstructure is obtained, leading to a new distribution of PFZ and heterogeneous precipitates in the material. Investigations on 10mm thick 7475 plate are performed from 1 to 4 passes. Further heat treatments can homogenize the precipitation and restore the yield strength of the initial rolled material. FSPed materials show a significant enhancement in ductility compared to the rolled one at the same yield strength. Up to 60% true fracture strain is obtained, improving by 150% with respect to the rolled material in the hardest state.
The number of passes plays a crucial role on the ductility improvement. FSP breaks the IM particles into fragments. Crack initiation can however be affected if the fragments form clusters due to insufficient FSP passes. Investigations are performed to converge to the optimal number of passes to obtain the best distribution of IM particles.
 G. Ludtka and D. Laughlin, Metall.Trans. A, 13A (1982) 411-425
 P. Rometsch, Y. Zhang, S. Knight, Trans. Nonferrous Metals Soc. China, 24(2014) 2003-2017