Lightweight　manufacturing　reduces　energy　consumption　and　protects　the　environment;　therefore,　it　offers　an　important　direction　of　development　in　engineering.　Variable　polarity　plasma　arc　(VPPA)　welding　is　a　light　gauge　aluminum　and　magnesium　alloy　joining　technology　capable　of　the　highly　efficient　welding　of　medium-thickness　metals　via　the　keyhole　effect,　and　without　the　complex　pre-processing　required　by　traditional　methods.　However,　the　stability　of　the　keyhole　weld　pool　is　easily　lost,　resulting　in　a　much　narrower　parameter　window　than　that　of　conventional　methods.　Here,　through　keyhole　morphology　analysis　and　x-ray　in　situ　imaging　experiments,　we　reveal　the　material　flow　behavior,　the　mechanism　behind　the　stability　of　the　keyhole　weld　pool,　and　the　reason　for　the　very　narrow　process　parameter　window　in　VPPA　welding　processes.　We　discovered　that　the　polarity　pressure　difference　of　the　plasma　arc,　which　is　induced　by　the　keyhole　boundary　shape　and　plasma　arc　intensity,　drives　the　flow　pattern　to　the　top　side　of　the　keyhole,　which　is　beneficial　for　keyhole　filling　and　the　formation　of　a　stable　weld　bead.　The　influence　of　the　difference　in　plasma　arc　pressure　and　the　keyhole　boundary　on　the　flow　field　revealed　in　this　study　may　guide　the　optimization　process　of　light　metal　joining　to　achieve　the　highly　efficient　and　defect-free　manufacturing　of　large　and　complex　structures.