Microstructure Evolution and Mechanical Properties of 7A52 Aluminum Alloy Thin Sheet Repaired with Friction Stir Surfacing.

A solid-state repair technique based on surface friction welding is investigated in depth to achieve excellent mechanical properties of damaged 7A52 aluminum alloy. The results show that the yield strength and tensile strength along the repair direction are 436 MPa and 502 MPa, respectively, at a rotational speed of 1400 rpm and a travel speed of 300 mm/min, which are about 157.9% and 129.7% of those before the defects were repaired, respectively, while the elongation is 17.2% compared to the base material. Perpendicular to the repair direction, the yield strength and tensile strength are 254 MPa and 432 MPa, which are 111.4% and 129.7% of those before the defects were repaired, respectively, while the elongation is 11.8% compared to the base material. The mechanical properties of the repaired areas are still improved compared to those of the defect-free sheets. On the one hand, this is attributed to the dynamic recrystallization of the nugget zone due to the thermo-mechanical coupling, resulting in the formation of a fine, equiaxed grain structure; on the other hand, the precipitated MgSi phase, which is incoherent within the base material, transforms into the Al(Fe, Mn)Si phase, as well as the precipitation of the AlMn phase and η' phase, resulting in the enhancement of the properties. The material fracture at the junction of the nugget zone and the heat-affected zone occurs after repair, which is attributed to the significant difference in the texture of the nugget zone and the heat-affected zone, as well as to the stress concentration at the junction.