A　variable-polarity　plasma　arc　(VPPA)　is　an　effective　energy　source　for　the　welding　of　thick　aluminum　alloy　plates.　However,　the　mechanisms　influencing　the　fluid　flow　and　the　inhomogeneous　distribution　associated　with　thick-plate　VPPA　welding　remain　unclear,　restricting　the　application　of　this　technology　in　welding　of　thick　aluminum　alloys.　Here,　the　relationship　between　the　microstructure　of　the　weld　bead,　energy　transfer,　and　fluid　flow　is　clarified　by　combining　in　situ　three-dimensional　x-ray　imaging　and　multi-physics　modeling.　We　find　that　heat　conduction　at　the　keyhole　wall　is　the　main　factor　influencing　the　morphology　of　the　weld　pool.　The　plasma　arc　pressure　hinders　the　upward　flow　of　liquid　metal,　while　shear　forces　promote　this　flow.　This　causes　the　metal　close　to　the　weld　pool　surface　to　flow　slowly,　while　that　inside　the　weld　pool　has　much　higher　velocity.　It　is　also　concluded　that　the　large　crystal　size　observed　in　the　lower　layer　of　the　weld　is　partly　caused　by　heat　treatment　from　the　upper　layer　of　the　thick　plate.　An　eddy　with　a　high　flow　velocity　to　the　rear　of　the　weld　pool　destroys　the　crystal-growth　process,　and　this　is　considered　to　be　one　of　the　reasons　for　fine　crystals　appearing　in　the　upper　part　of　the　weld.　The　mechanisms　revealed　here　will　help　us　to　guide　the　use　of　VPPA　technology　in　the　production　of　stable,　high-quality　welding　of　thick　aluminum　alloys.　Published　under　an　exclusive　license　by　AIP　Publishing.