To　solve　the　problem　of　insufficient　strength　of　thick　aluminum　alloy　welded　joints,　this　study　introduces　a　pioneering　technique　known　as　variable-polarity　plasma　arc　keyhole-tungsten　inert　gas　cover　hybrid　welding　(VPPA　keyhole-TIG　covering　welding),　which　is　designed　for　welding　large-thickness　aluminum　alloys　without　any　weld　groove.　Using　13-mm-thick　2319　aluminum　alloy　plates　as　experimental　materials,　plasma　arc　butt　welding　experiments　were　conducted　by　varying　the　plasma　gas　flow　rate　and　application　of　cover　welding.　The　research　findings　underscored　that　the　use　of　cover　welding,　coupled　with　an　increase　in　the　plasma　gas　flow　rate,　markedly　improved　the　overall　performance　of　the　weld.　An　average　tensile　strength　of　299.0±4.0　MPa　and　an　elongation　rate　of　4.5±0.3　%　were　achieved　for　the　welded　joint.　A　microscopic　analysis　revealed　that　the　heightened　plasma　gas　flow　rate　did　not　precipitate　the　θ＇　phase　but　instead　resulted　in　undesired　grain　size　enlargement,　leading　to　reduced　joint　hardness.　The　secondary　thermal　cycle　effect　of　covering　welding　played　a　pivotal　role　in　facilitating　the　nucleation　of　needle-like　θ＇　strengthening　phases　within　the　matrix,　ranging　in　size　from　50　to　150　nm.　This　phenomenon　has　emerged　as　a　principal　factor　contributing　to　the　substantial　enhancement　of　the　mechanical　properties　of　welded　joints.　©　2024　Elsevier　B.V.