Effect of friction stir processing and heat treatment sequences on the microstructural evolution and mechanical properties of wire arc additively manufactured aluminum alloys

Friction stir processing (FSP) is an effective method to improve the porosity, microstructure, and properties of aluminum alloys produced by wire arc additive manufacturing (WAAM). Solution treatment followed by aging is essential to achieve peak strength in heat-treatable aluminum alloys. The sequence of FSP and heat treatment significantly affects the microstructure and properties of additively manufactured aluminum alloys. This study employed three different sequences of FSP and heat treatments—FSP followed by solution and aging (FSA), solution and aging followed by FSP (SAF), and solution followed by FSP and then aging (SFA)—to regulate the microstructure and properties of ZL205A aluminum alloy produced via WAAM. Microstructural analysis reveals that the as-deposited (AD) sample features a coarse-grained microstructure with an average grain size of 42.87 μm, along with a network-like θ phase predominantly precipitated at the grain boundaries. The tensile strength in the vertical direction is relatively low at 249.7 MPa, with an elongation of only 9.6 %. After FSA processing, the network-like θ phase at the grain boundaries completely dissolves and reprecipitates as finely dispersed needle-like θ'' phase, resulting in the highest tensile strength (425.4 MPa) among the three processing conditions. However, the fine-grained structure formed by dynamic recrystallization during FSP underwent abnormal grain growth (AGG) during the solution treatment owing to insufficient thermal stability, reducing ductility and limited an elongation to merely 3.3 %. By contrast, the SAF and SFA processes altered the formation of metastable phases during FSP, yielding substantially lower tensile strengths compared to FSA. During the SAF process, some metastable phases were not fully re-dissolve, retaining more metastable phases which contributed to higher tensile strength than the SFA sample, with tensile strengths of 328.5 MPa and 289.9 MPa, respectively. Both SAF and SFA samples maintained the fine-grained structure produced by dynamic recrystallization during FSP, with average grain sizes refined to 3.10 μm and 2.88 μm, respectively. This significant grain refinement greatly enhanced the elongations of the SAF and SFA samples, reaching 24.4 % and 20.7 %, respectively.