Ductilization by multi-pass Friction Stir Processing of thick 7475 aluminium alloy

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 [1]. According to Rometsch et al [2], 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. References [1] G. Ludtka and D. Laughlin, Metall.Trans. A, 13A (1982) 411-425 [2] P. Rometsch, Y. Zhang, S. Knight, Trans. Nonferrous Metals Soc. China, 24(2014) 2003-201

Friction Stir Processed Al alloys for damage mitigation and healing

During the service life of an aluminum part, damage initiates on large iron-rich intermetallic particles and leads to final failure by growth of these cavities and their coalescence. This damage leads to significant economic loss for their replacement. A first step towards an increase in fracture strain is to decrease the size of these intermetallic particles. This has been shown to be highly efficient when 6xxx series Al alloys are processed by Friction Stir Processing (FSP) [1]. A second step, which is the aim of the present work and related to the ERC-ALUFIX project, is to integrate particles in the aluminum alloy by the very same process. These newly integrated particles can either favor crack deviation in order to increase toughness or be used for damage healing. Crack deviation can be favored by the integration of NiTi particles surrounded by controlled internal stresses. The internal stresses are introduced via pre-straining and shape recovery of the embedded NiTi particles. In addition, the NiTi particles enhance the mechanical strength of the composite. A homogenous NiTi distribution and a good bonding at the Al/NiTi interface is the key to a successful crack deviation. This is a challenge in a 7075 Al matrix while easily achievable in a 1050 Al matrix. An alternative is to introduce healing particles in the Al matrix and perform a healing treatment to heal micro-damage. Here the damage healing is assessed trough in-situ heating in the TEM and X-Ray nano holotomograph. It was found that small damage (up to 1 µm size) can be healed at 400°C within 10 min of heat treatment