This　study　investigated　heat　and　mass　transfer　as　well　as　the　effect　of　gravity　on　keyhole　formation　in　the　molten　pool　of　keyhole　plasma　arc　welding.　The　flow　trajectory　of　the　molten　pool　was　obtained　via　high-speed　camera　images　illuminated　by　a　laser　light　system,　and　the　temperature　distribution　was　obtained　using　an　infrared　thermal　camera.　The　experimental　results　showed　that　the　molten　metal　flowed　rearward　and　downward　at　the　top　surface　of　the　weld　pool　and　thereafter　to　the　rear　side　of　the　keyhole.　There　are　two　types　of　reverse　flow　of　molten　metal　on　the　rear　side　of　the　keyhole.　The　different　welding　positions　had　minimal　effect　on　the　flow　pattern;　however,　the　flow　velocities　were　different.　Further,　for　the　temperature　distribution,　although　the　keyhole　front　side　yielded　a　similar　high-temperature　area　under　different　welding　positions,　the　area　of　the　high　temperature　on　the　keyhole　rear　side　in　the　vertical　position　welding　was　much　larger　than　that　in　the　flat　position.　Consequently,　the　influence　of　the　force　on　the　keyhole　molten　pool　was　evaluated,　and　the　flow　pattern　of　the　molten　pool　was　found　to　be　dominated　by　the　sheer　force.　Moreover,　the　flow　velocity　and　solidification　morphology　of　the　molten　pool　were　affected　by　the　gravity　at　different　welding　positions.　In　addition,　the　correlation　mechanism　of　the　temperature　distribution,　flow,　and　solidification　morphology　of　the　keyhole　molten　pool　was　analyzed　using　the　combined　action　of　shear　force　and　gravity,　which　further　revealed　the　mechanism　of　the　influence　of　gravity　on　the　keyhole　welding　process.(c)　2022　Elsevier　Ltd.　All　rights　reserved.