The pilots reported that during the approach to the destination airport, the left inboard roll spoiler jammed in the partially deployed position. The airplane landed without further incident. Postincident examination revealed that the fusion welded plug in the piston of the roll spoiler servoactuator had separated. The manufacturer reported that there was one previous failure of a welded plug in a servoactuator piston. They have since changed the design of the piston to eliminate the welded plug.