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Welding

When worlds collide

By Gretchen Salois

Above: Engineers plan to test the parachute systems for NASA's Orion spacecraft in August by dropping a representative Orion capsule from an aircraft's cargo bay at an altitude of 35,000 ft. Photo: Radislav Sinyak / NASA

NASA uses friction stir welding to withstand the rigors of space

March 2016 - In February, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that it could see vibrations of space—gravitational waves—spelling out Einstein’s view that space and time are stretchable. Scientists have spent years painstakingly maneuvering lasers and tubes to detect these oscillations. It took two black holes 1.3 billion light-years from earth to smash into one another, resulting in a ripple-like effect of detectable gravitational waves. 

Why is this important? It opens a new portal to studying the universe and emphasizes the innovation that hitches a ride along with space exploration. Often the breakthrough technology we experience in other areas like the cars we drive or the power behind our computers, started in some form while exploring the unknown beyond our planet. 

NASA’s Orion EM-1 pressure vessel is the latest in welding technology feats, using friction stir welding to ensure the needed “strength and ductility is on par with the parent material,” says Ronald Baccus, Orion structures and thermal protection system area manager at NASA.

FFJ 0316 welding image1

Every parameter of the weld process is monitored including cone welds. Friction stir welding ensures strength and ductility without sacrificing weld integrity. Photo: NASA

The crew cabin structure pressure vessel shell welded elements were all manufactured from a tempered aluminum 2219 alloy. “This is a heritage alloy that has been used in welded crew cabins for several aircraft, including the space shuttle orbiter and International Space Station modules,” Baccus says. 

A team of about 625 NASA employees and 2,650 contractor and support employees in 49 states are working on Orion. Several production houses are contracted to manufacture various piece parts based on forging, forming and machining capabilities, Baccus says. Parts are delivered to the Michoud Assembly Facility in New Orleans and then staged for sequential welding into the friction stir weld fixture. 

How it’s assembled

Orion’s welded pressure shell is made from seven primary sections: a tunnel, forward bulkhead, three cone panels, a barrel and aft bulkhead. “The tunnel and barrel components are single-piece machined forgings, the cone panels are machined from formed plate, and the aft bulkhead (an airtight bulkhead located between the cabin and tail of an aircraft) is a single piece spun-formed part that is then machined,” says Baccus. 

The aft bulkhead and barrel are a little over 12 ft. in diameter and each 120-degree angle cone panel has a cone height of about 3 ft. The forward bulkhead and tunnel have a diameter of approximately 8.5 ft. and 4 ft., respectively, says Baccus. “Each part is either forged or formed, then machined into an orthogrid configuration for optimum strength and stiffness.”

Orion first used the friction stir welding process on an Exploration Flight Test-1 vehicle which flew successfully in December 2014. “We are leveraging that experience and applying lessons learned in the production of the current Exploration Mission-1 vehicle and beyond,” Baccus says, adding that the team at NASA is also implementing a closeout plug weld operation for all circumferential welds as an improvement to mechanical closeouts (joining the cone to the barrel) used on EFT-1.

Instead of using MIG or TIG welding, which require filler wire that is melted into the joint, friction stir welding is a uniform weld. “Temperature and weld wire properties cause a loss of material properties in the surrounding materials,” explains Jim Bray, Lockheed Martin’s Orion crew module director. “There can be losses of 50 to 60 percent of the parent material. The friction stir welding blends two plates together at a very low temperature, caused by friction alone, and there is no (filler) weld wire required.”

Strain gauges were used to monitor stress put into the material during the welding process. “All stresses need to be accounted for when determining if the materials are sufficient to meet the mission loads,” Bray says. “The strain gauges are attached to wires to transmit the data to our Data Logger.”

Every parameter of the weld process is monitored, including the rotational speed of the pin and the travel speed through the two plates, says Bray. “We use a Phased Array Ultrasonic Inspection after the weld is completed to verify the weld integrity.”

FFJ 0316 welding image2

Lockheed Martin's team completed the final weld of the cone section of the Exploration Mission-1 crew module pressure vessel. Photo: NASA

No surprises

NASA makes it its mission to avoid surprises. Engineers faced welding challenges for the pressure vessel long before any welding took place, says Lockheed’s Bray.

Most problems were worked out during the friction stir weld process on the EFT-1 when the manufacturing team and structural engineers, along with materials and process engineers, developed weld parameters to produce quality welds, including spindle speed and feed rates, self-reacting pin clamping force, and fixture design/stiffness. 

From the first weld on the flight article (tunnel to forward bulkhead weld) and the final closeout weld (cone to barrel), took approximately four months, says Baccus.

According to Bray, the biggest challenge was verifying the final closeout weld tool since it’s a new and complex tool. “We completed a pathfinder for all the welds to make sure the tooling, procedures and processes work before subjecting the flight article to the welding,” Bray says. 

One major change from EFT-1 to the EM-1 is going from six cone panels at a 60-degree arc each to only three cone panels at 120-degree arcs, each with integrally machined longerons (thin strip of material to which the skin of an aircraft or propellant tank is fastened). “This reduced the number of piece parts required, eliminated nine longitudinal welds from the assembly, and reduced the mass of the crew module by several hundred pounds,” says Baccus. The team achieved this by a series of trials to find the best sequence for forming, incremental machining and final heat/temper that would keep the surface profile dimensions within tolerance for subsequent welding.

FFJ 0316 welding image3

Orion is moved to the work stand at the Operations and Checkout Facility at NASA's Kennedy Space Center. Photo: Radislav Sinyak / NASA

Space-grade strength

In addition to the load the pressure vessel must experience while in orbit, engineers had to factor in how the structure would react to the launch and entry loads and environments, “with the most critical loading conditions determined by off-nominal scenarios such as launch abort or splashdown with only two of three chutes deployed,” Baccus says, adding such factors are vital to crew survival.

The delicate balance between determining the mass allowed—how many crew members, fuel, equipment—means that engineers spend a lot of time on structural analysis. “It also drives the need to implement material systems with higher strength-to-weight ratios where they are most effective,” he says. An example would be the graphite composite systems used in large fairings, the heat shield and other structural panels on the Orion spacecraft. “Earth-bound systems” aren’t going to be restrained by these factors like those headed to outer space.

Inspecting a space-bound system entails a different kind of weld inspection. “After the welds are completed, each one is subjected to a nondestructive inspection technique called phased-array ultrasound,” explains Baccus. “This allows for the engineers to inspect the quality of the weld and determine if any voids or other indications exist that may require a repair.” 

According to Bray, “We used adjustment capabilities built into the tool to achieve a perfectly round condition for the upper half and the lower half of the crew module, and we committed to welding only after everything was perfectly aligned. Our people are the best in the world,” he claims, “at designing tooling to enable exact positioning and then at aligning parts to achieve tight tolerances. They got it right and produced a crew module that met engineering requirements with defective-free welds.” 

Every flight structure also undergoes a proof pressure test to a factor of 1.5 times the design pressure. “This exercises the weld and ensures that it has the capability to withstand the applied stresses prior to flight,” says Bray.

The crew cabin shell is the primary structural component that will be welded into the spacecraft, as well as subsystem components such as propulsion tubing, which is welded based on the need to contain pressure or hazardous elements.

In February, the pressure vessel was loaded into a tooling structure where engineers will perform a series of proof pressure and structural load tests to verify its strength, says Baccus. After which, an assembly team will begin integrating the Orion systems and subsystems to go from a structure to a functioning spacecraft. 

Extensive research and rigorous testing is what will send Orion into orbit, propelling it 40,000 miles beyond the moon during its next mission, Exploration Mission-1. “It will [travel] farther than any spacecraft built for humans,” says Baccus. FFJ

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