Effect of different cooling conditions on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys repaired by additive friction stir deposition

Additive friction stir deposition (AFSD) provides innovative solutions to circumvent problems related to repairing defects of material by means of solid-state filling. In this work, the cooling conditions during the AFSD on microstructural evolution and mechanical response of repaired Al-Zn-Mg-Cu alloy were investigated for the first time. Repaired Al-Zn-Mg-Cu alloy under air-cooling and water-cooling conditions exhibited distinctive microstructure, i.e., the grain size, dislocation density, and precipitations, as well as resultant mechanical properties, owing to the different thermal features (peak temperature, thermal exposure duration, and cooling rate). Specifically, the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of air-cooling specimens were 262.3 MPa, 370.9 MPa, and 9.8 %, respectively. The YS, UTS, and EL of water-cooling specimens were 302.1 MPa, 421.1 MPa, and 11.3 %, respectively, which were 15.2 %, 13.5 %, and 14.3 % higher than that of air-cooling specimens. For the AFSD repaired Al-Zn-Mg-Cu alloy with water-cooling condition, the enhancement of mechanical properties mostly arises from increased cooling rate and decreased thermal duration compensating for the deterioration of the dislocation density and precipitated phases during the thermal process.