Welding techniques boost aerospace

British-developed technologies allow the fabrication of more advanced rocket fuel tanks and nozzles as well as ground based products.

Large lithium aluminium sections for rocket fuel tanks being developed for NASA depend on friction stir welding – originally developed by TWI – while upper stage nozzles for Ariane 5, are being welded using a UK laser technique that allows a vision system to guide welding when no features are discernible.

These techniques not only advance the use of weight-saving living aluminium lithium alloys for general aerospace use, but also the improved manufacture of very high-precision metal fabrications generally.

Crucial to NASA's fuel tank project is the production of two single-piece, 5.5m diameter spun formed domes manufactured from flat plate blanks made from friction stir welded commercial 2195 aluminium copper lithium manganese plate. 2195 is only 5% lighter than the 2219 aluminium copper manganese alloy used presently, but is 30% stronger at cryogenic temperatures.

The current method to make a fuel tank dome is to weld eight pie shaped pieces of 2219 alloy together. This requires 10 welding steps and multiple operations and inspections. In addition, the conventional welding technique is plasma arc welding, which is problematic for aluminium lithium alloys because of the ready take of oxygen by the lithium. Friction stir welding gets over this, and is to be used in the manufacture of the complete tanks, which are expected to be 25% lighter than those made conventionally, and significantly less expensive.

The project was conceived by engineers at NASA's Marshall Space Flight Center in Huntsville, Alabama, and the Langley Research Center in Hampton, Virginia. The parts are being manufactured by MT Aerospace in Augsburg, Germany, which owns the patent on the concave spin forming process.

The manufacture of the Ariane rocket nozzles by Astrium in Germany posed different problems. Each one is made from 242 nickel alloy tubes, 4mm square and with a 0.32mm wall thickness, allowing cooling in flight by liquid hydrogen. A totally reliable method of joining them is essential if such a system is going to work. Before welding, the tubes are bundled together and spiral wound around a copper coated aluminium mandrel. The tubes are held together with binding wire which is unwound as the welding process (tungsten inert gas with no filler) proceeds.

The Kuka robot carrying the welding torch is guided by a 'Meta-Scout' vision system supplied by Oxford based Meta Vision Systems. During most of the process, the vision system is able to resolve between the adjacent tubes and follow them conventionally. However, near the engine end of the nozzle, the tubes are flattened to achieve the correct fit, and there is no visible feature to track. In this region, five laser lines are projected onto the workpiece to allow measurements to made in six degrees of freedom ( three orthogonal, and three rotational). A structured light technique together with grey scale vision analysis establishes seam position, height and orientation with respect to the tool. The only requirements are that the material on either side of the gap must have similar machined finishes and the sensor has to be oriented at close to 90° to the surface. Software allows the path to be predicted and followed to an accuracy of 0.1mm. Weld parameters are automatically selected to suit gap dimensions. A calibration system aligns the sensor with the welding torch and calculates any deviation of the tool centre point.
Similar systems are in widespread use in various industries where they facilitate extreme precision welding, including the manufacture of ducts for the Airbus A380 and the reconditioning of land-based turbine rings in the USA.

Tom Shelley

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