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Lockheed Martin Michoud Space Systems

In New Orleans, Lockheed Martin finished fabricating the first Type 2195 aluminum-lithium super-lightweight external fuel tank for the Space Shuttle in January, 1998. The firm saved 7,500 pounds from the previous design by replacing 2219 aluminum alloy (6 percent copper) with 2195 aluminum-lithium alloy (1 percent lithium, 4 percent copper).

Lockheed Martin uses soft-plasma-arc welding to fabricate the 154-foot-long 28-foot-diameter super-lightweight external Space Shuttle fuel tank, of Type 2195 aluminum-lithium alloy.

Assembly of tank parts and pressure vessel components requires 15 specialized weld tools. Lockheed Martin engineers modified each tool for backside weld shielding with a gas mixture of helium and argon.

The $7-million weld-equipment upgrade, made from 1994 through 1998, to take on welding of alloy 2195 included installing Hobart Advanced Weld Control Systems (HAWCS) II on each weld tool. Some HAWCS were outfitted with automatic penetration control using a laser beam-based displacement sensor for arc-height control on flat-position welds and programmable mechanical oscillation.

One process implemented for this new alloy involved using an oversized plasma orifice along with a protruding electrode to produce two concentric rings of gas shielding. The process, referred to at Lockheed as soft-plasma-arc welding (SPAW), is used on flat-position welds or welds that transition to the flat position on a curvature.

The HAWCS controller allows multi-process welding with the same torch - a non-keyhole variable-polarity plasma-arc weld (VPPA) as a seal pass, a DC gas-tungsten-arc-weld (GTAW) penetration pass, then a non-keyhole VPPA cover pass. The equipment also makes automatic tack welds prior to the seal pass on circumferential welds.

"Even with the equipment and process enhancements, at elevated temperature the lithium in the alloy can react readily with occasional contaminants to form porosity. And, it was prone to hot cracking," explains Barry Keegan, Lockheed Martin senior facilities engineer in New Orleans.

Another recent implementation at the site: seven DC GTAW orbital pipe welding systems for automated repair welding. Running on straight or slightly curved track, the orbital weld heads, from Arc Machines, Inc., ride along the skin of the external tank to deposit repair welds. "Many automated repairs were successful where previous manual weld repairs were having difficulty," states Keegan


Reprinted from Welding Design & Fabrication, November 1998