Abstract: In a system for welding thin section aluminum castings to elongated sections of thin walled hollow aluminum extrusion, an expandable mandrel specific to the weldment to be assembled serves also as a holder and positioner while welding is in progress. A powered carriage rotatably supports the normally cantilevered mandrel and advances it into position in the bore of the extrusion. Sets of hydraulically powered expansion shoes of the mandrel are spaced along its length so as to register internally with the location of external weld joints between casting and extrusion. Selective independent actuation of shoes or sets of shoes applies outward pressure locally to remove distortions in the "as received" extrusion, so as to maintain consistent fit up between extrusion and casting, facilitating consistently good welded joints. This mechanical positioning and form control system lends itself readily to integration with robot welders for fully automated operation.