Experiments　were　carried　out　in　this　paper　first　to　clarify　the　characteristics　of　a　digging　process　during　aluminum　alloy　of　type　5A06　variable　polarity　plasma　arc　welding　(VPPAW).　A　special　＂Blasting　type＂　penetrating　phenomenon　was　observed　through　measuring　the　penetrating　time　by　electrical　and　optical　signals.　A　3D　model　was　established　to　numerically　investigate　heat　transfer,　fluid　flow　and　surface　shape　during　the　digging　process.　An　adaptive　heat　source　was　adopted　to　describe　heat　transfer　process　involving　deformation　of　the　keyhole　tracked　by　volume-of-fluid　(VOF)　method.　The　plasma　arc　pressure　attenuated　in　a　parabolic　form　as　the　depth　of　the　keyhole　increases.　It　is　found　that　the　weld　pool　surface　becomes　concave　after　sufficient　melting　of　metal.　The　keyhole　depth　exhibits　responsive　changes,　which　corresponds　with　the　different　stages　of　welding　parameters.　In　the　late　stage,　the　depth　increase　rapidly　to　fully　penetrating　state,　which　shows　a　＂blasting　type＂　penetrating　process.　The　keyhole　formation　was　interpreted　by　considering　energy　accumulation　and　mass　conservation　of　fluid　flow.　The　maximum　velocity　during　digging　process　always　occurs　on　the　keyhole　surface,　where　a　vortex　in　the　opposite　direction　appears　inside　the　weld　pool.　The　calculated　penetrating　time,　keyhole　size　and　fusion　line　were　basically　in　agreement　with　the　experimental　results.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.