The　keyhole　welding　process　in　Variable　Polarity　Plasma　Arc　Welding　enhances　weld　quality　significantly,　yet　it　introduces　pool　instability.　Dynamic　stability　is　pursued　by　investigating　molten　metal　flowing　within　keyholes　and　their　characteristics.　Weld　pool　flow　patterns　are　obtained　via　particle　tracking,　while　instability　characteristics　are　captured　employing　a　high-speed　camera.　Analysing　directional　molten　metal　flow　unveils　Convergence　and　Separation　Points　in　weld　pool　flow.　The　latter’s　displacement　induces　weld　pool　instability,　resulting　in　an　increase　in　keyhole　area　and　a　reduction　in　the　pinch　angle.　The　keyhole　area　signifies　pool　instability,　while　the　rear　wall　angle　reflects　the　welding　state.　These　findings　provide　a　theoretical　basis　for　weld　control　and　advance　vision-based　intelligent　manufacturing　welding　techniques.　©　2023　Institute　of　Materials,　Minerals　and　Mining.　Published　by　Taylor　&　Francis　on　behalf　of　the　Institute.