Vorpal Weapon Fusion
(from Starfinder Core Rulebook)
With the vorpal fusion, a weapon can tear between the molecules of a creature. The weapon gains the severe wound critical hit effect. If the weapon already has a critical hit effect, when you score a critical hit, you can apply either the weapon’s normal critical hit effect or the severe wound effect. Only a weapon that deals bludgeoning, piercing, or slashing damage can benefit from this fusion.