The　consideration　of　environment-friendly　and　low　energy　consumption　tends　to　control　and　guide　the　progress　of　metal　joint　manufacturing　processes.　Variable　polarity　plasma　arc　(VPPA)　welding　is　an　environment-friendly,　high　quality　and　efficiency　joint　method　for　aluminum　alloys　due　to　its　characteristics　of　such　as　little　welding　fume,　and　high　energy　density　etc.　However,　the　three-dimensional　morphology　and　physical　phenomena　of　penetration　weld　pool　in　VPPA　welding　are　not　clearly　understood,　which　slows　down　its　process　development　and　optimization.　This　work　aims　to　clarify　the　three-dimensional　keyhole　detouring　flow　of　VPPA　welding　for　aluminum　alloy.　The　liquid　metal　distribution　of　penetration　weld　pool　was　generally　obtained　by　metallographic　observation.　The　measurement　results　show　that　the　liquid　metal　layer　attached　to　the　three-dimensional　keyhole　boundary　is　thin.　And　the　liquid　metal　mainly　distributes　in　the　bottom　side　of　the　keyhole.　In　order　to　understand　the　weld　pool　dynamics,　the　stereo　synchronous　imaging　of　tungsten　tracer　particles　using　two　sets　of　X-ray　transmission　system　was　adopted,　for　the　first　time,　to　measure　the　3D　velocity　field　of　convection　flow　inside　weld　pool　of　VPPA　welding　of　aluminum　alloy　in　real　time.　The　flow　on　the　weld　pool　surface　at　different　locations　of　keyhole　boundary　was　also　measured　by　tracing　zirconia　and　slag　particles.　The　liquid　metal　flows　simultaneously　to　the　top　and　around　the　keyhole.　The　metal　at　the　bottom　of　keyhole＇s　front　wall,　flows　around　the　keyhole　towards　the　backside,　then　separately　flows　to　top　side　and　bottom　side,　forming　a　vertical　stationary　point.　The　key　to　weld　and　avoid　defect　proofing　are　to　ensure　stable　detouring　flow　on　the　keyhole　bottom　and　proper　stationary　point　position.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.