‘Military-grade’ Aluminum

  • 27-Jul-2016 12:20 EDT
USS_Belknap_collision_damage.jpg

USS Belknap post-fire under tow.

When Teddy Roosevelt led his Rough Riders in their famous charge up San Juan Hill during the Spanish-American War, he was carrying a piece of leading-edge military technology: an aluminum canteen. Since that epic day 118 years ago, aluminum alloys have helped lighten all manner of land-combat equipment, including howitzers, 5-ton trucks, Humvees and even assault bridges.

And the stuff is tough. Armor-tough. In fact, 5000-series (or 5xxx) aluminum alloys widely used in the auto industry are also used extensively in the world’s army vehicles. The stronger heat-treatable 7xxx alloys—not yet in volume automotive use because they’re difficult to form—provide superior ballistic protection in specialized aluminum armor, versus rolled homogenous-steel armor. Both 5xxx and 7xxx alloys protect the crews of U.S. Bradley Fighting Vehicles. Military duty cycles also demand supreme corrosion and shock resistance. Aluminum delivers.

So it was no stretch for Ford Truck marketers to create the tag-line “military-grade” for the 5xxx and heat-treated 6xxx series aluminum bodies of its new 2015 F-150. They knew they had to convince skeptics that light-metal bodies would equal or even outperform steel in overall strength, dent resistance and crash protection.

Aligning the new pickup with the “bulletproof” image of combat tanks in the public’s mind would be a clever play. Never mind the TV ad from Chevrolet that appeared soon after the F-150’s launch. It showed a nuclear submarine—steel hull, of course—surfacing through thick Arctic ice with a Silverado in the foreground.

A downright sobering naval image came to mind when I first heard the Ford guys trumpeting “military grade aluminum.” My thoughts were of the USS Belknap, a Navy cruiser that collided with the aircraft carrier USS John F. Kennedy off the coast of Sicily in 1975. A fire then broke out on Belknap that burned and melted the ship’s aluminum superstructure right down to the main deck. Seven sailors on Belknap were killed and one died on the carrier. Newspaper photos of the burned out and gutted cruiser, its steel hull under tow to the U.S., remained etched in my memory.

While aluminum does not burn, it starts to lose its structural integrity at 500°F (260°C) and severely deforms—in effect, melts—at 1500°F (81°F). By comparison, steel doesn’t begin to soften until it reaches 1800 to 2000°F (982 to 1093°C). The Belknap investigation taught the U.S. Navy a lot about aluminum’s survivability. But it wasn’t until 1987 that the Naval Sea Systems Command, in charge of vessel design and engineering, announced it was reverting to steel superstructures for new shipbuilding programs.

The reason: Aluminum superstructures were cracking in 263 ships, half the U.S. fleet at the time. Repairing the stress-induced cracks cost billions, averaging $445,000 per ship, said Alcoa’s marketing manager who had to face the media. Navy admirals also admitted that the aluminum sections required far more maintenance than those of steel ships.

While the cracking debacle didn’t cause the Navy to halt aluminum use in all ship construction, it did prompt a re-think of future architectures and materials. The subsequent 62-ship Arleigh Burke class destroyers are built of steel.

But none of them come in a Crew Cab 4x4 King Ranch model.

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