High strength 7XXX aluminium alloys reach unmatched tensile strength compared to all other industrial aluminium series. However, it remains an important technological challenge to improve the ductility of 7XXX alloys especially in high strength condition resulting from peak aging heat treatments (HT). Large micron-sized iron-rich intermetallic (IM) particles promotes void nucleation by particle cracking during the fracture process, which is one of the reasons for low ductility of 7XXX aluminium alloys. Beside IM influence, grains morphology and size affect the 7XXX alloys ductility. Indeed, softer precipitates-free zones along the grain boundary act as “pre-cut path”, facilitating crack propagation [1].

Friction Stir Processing (FSP) can break the detrimental IM particles and modify the grains shape and size. Significant ductility improvement was observed for 6XXX alloys mainly because of the IM size reduction and redistribution [2]. In this work, 7075 rolled Al plates are processed by FSP under various processing parameters. After FSP, IM particles are broken into smaller fragments and the large-size rolled grains become significantly smaller and equiaxed. Post-FSP T6 HT is applied for restoring the peak-aged precipitation and rising the strength up to T6 level [3]. The ductility of this FSP + T6 HT material is greatly improved while maintaining high strength, opening a way to better ductility and formability of 7XXX aluminium alloy series.

[1] Ludtka G M, Laughlin D E. (1982). The influence of microstructure and strength on the fracture mode and toughness of 7XXX series aluminum alloys. Metallurgical Transactions A, 13(3): 411-425.

[2] Hannard F, Castin S, Maire E, et al. (2017). Ductilization of aluminium alloy 6056 by friction stir processing. Acta Materialia, 130: 121-136.

[3] Lezaack M, Simar A. (2021). Avoiding abnormal grain growth in thick 7XXX aluminium alloy friction stir welds during T6 post heat treatments. Materials Science and Engineering: A, 807: 140901