Friction　stir　processing　(FSP)　technology　can　significantly　improve　the　performance　of　components　prepared　using　wire-arc　additive　manufacturing　(WAAM).　In　this　study,　a　hybrid　method　combining　WAAM　and　interlayer　FSP　(WAAM–IFSP)　was　used　to　prepare　multilayer　2319　aluminum　alloy　samples.　The　resulting　periodic　microstructures　with　alternating　bimodal　and　columnar　grain　regions　were　investigated　in　detail.　The　formation　mechanism　and　influence　of　the　microstructure　on　the　alloy　mechanical　properties　were　revealed　by　analyzing　the　texture,　grain　morphology　and　distribution,　grain　boundary　orientation　angle,　and　dislocation　density.　The　use　of　WAAM–IFSP　refines　the　grains　in　the　deposited　alloy　layers,　reduces　the　defect　density,　increases　the　internal　dislocation　density,　and　results　in　a　random　distribution　of　precipitates　in　the　matrix　via　the　crushing　and　mixing　action　of　the　stirring　tool.　Compared　with　samples　prepared　by　WAAM　alone,　the　top　layer　of　the　WAAM–IFSP　sample　showed　the　best　performance,　with　the　ultimate　tensile　strength　increasing　from　297　to　378　MPa　and　the　elongation　increasing　from　16.2%　to　30.6%.　Furthermore,　in　the　vertical　direction,　the　elongation　increased　nearly　threefold,　from　7.9%　to　23.2%,　which　is　attributed　to　a　higher　interfacial　bonding　strength　and　the　bimodal　grain　structure　formed　by　a　heat-source　coupling　effect　during　WAAM–IFSP.　These　findings　are　expected　to　provide　new　insights　into　the　further　development　of　methods　for　enhancing　the　performance　of　WAAM　components　by　introducing　IFSP,　thereby　extending　the　engineering　applications　of　additive　manufacturing　methods.　©　2023　Elsevier　B.V.