In　contrast　to　other　welding　processes　of　aluminum　alloys,　variable　polarity　plasma　arc　(VPPA)　nonvertical-up　welding　produces　pores　in　specific　positions　(on　the　upper　side　of　the　welded　joint)　rather　than　throughout　the　whole　welded　joint　due　to　the　effect　of　gravity.　In　horizontal　welding,　the　porosity　area　percentage　on　the　upper　side　of　the　welded　joint　reached　about　similar　to　20%.　Asymmetric　metal　flow　in　the　keyhole　molten　pool　caused　either　side　of　the　weld　to　have　different　cooling　rates.　This　difference　in　cooling　caused　the　metal　on　this　side　of　the　weld　to　rapid　crystallization,　leading　to　higher　concentrations　of　copper　and　manganese　around　the　weld　that　are　conducive　to　higher　porosity.　The　Cu　content　reached　similar　to　50%　around　the　pores　but　was　only　similar　to　7%　in　the　filler　wire　and　similar　to　5%　in　the　base　metal.　The　root　reason　for　porosity　is　the　asymmetric　metal　flow.　To　correct　this,　the　free　point　was　found　according　to　the　force　balance　on　the　leading　side　of　the　keyhole.　A　proposed　method　to　produce　symmetric　metal　flow　was　used,　and　the　resulting　welds　were　determined　to　be　of　high　quality　via　X-ray　phase-contrast　imaging.