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Automotive Industry

Alloy cat: Jaguar's XJ finds the right combination of aluminum construction techniques to produce a car that is spectacularly light and incredibly strong - Process

Automotive Industries, Dec, 2002 by Michael Scarlett

The new Jaguar XJ6 and XJ8 is the seventh generation of a car range with a 34-year heritage. Although some may disagree, many customers still like the distinct and svelte Jaguar sedan shape first evolved under Jaguar founder Sir William Lyons' stylist eye. And fittingly, the new XJ is, at first sight, hard to tell apart from its predecessor.

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But under those familiar sports-coupe lines, it is a radical departure from conventional practice with its monocoque aluminum alloy unibody construction. To make it, Jaguar spent a still undisclosed amount on a very up-to-date new press shop in a former warehouse at its assembly plant at Castle Bromwich, in the heart of the English Midlands, the U.K. The car is considerably bigger outside, inside and in trunk capacity. All models sit on air springs--another first for the Coventry-headquartered company. Reduced weight means the return of a six-cylinder XJ6, even if not a classic in-line six, but rather the high specific power output 3L V-6 first seen in the Jaguar S-type and Lincoln LS. But the most interesting aspect of it is its aluminum alloy construction, and how the light alloy is wrought and used.

Although mild and high tensile steel has long dominated automotive structures, aluminum alloy is, of course, not totally new to automaking. Nevertheless, Audi can accurately claim to have led the modern move towards aluminum for production car bodies, with their A8 luxury sedan introduced in 1994. The AS uses a skeleton frame of alloy extrusions between cast aluminum--alloy joints as its basic load-bearers, and partly thanks to its high structural strength, is not spectacularly light.

In the new XJ body, there are some magnesium--alloy castings, and a few aluminum extrusions, one of which is hydroformed to make the cross beam over the radiator assembly. Steel intrudes structurally only for the suspension and engine bearing sub-frames. But the rest of the body/chassis, about 85 percent is sheet aluminum--alloy, stamped to shape just like a conventional steel bodyshell. Because of those non-sheet parts, this is strictly speaking a hybrid monocoque, but a stressed skin monocoque none-the-less.

The sheet used ranges in gauge between 0.9 mm (0.035 in.) in skin panels (compared to between 0.028-0.030 in. for steel bodies) to a maximum of 3 mm (0.118 in.) in areas of concentrated load. Aluminum and magnesium alloy castings are brought in for places where more than 3 mm might have been needed, as in the tops of the housings for the air-spring/damper and top front suspension member mount. Reinforcement and any necessary complexity are easier to achieve in the mostly vacuum-die castings used, their wall thickness varying between 8 mm (0.315 in.) and 2.5 mm (0.098 in.) at places where the cast piece is joined to sheet.

Extrusions are used for their concentrated strength, in bumper beams, crash absorption cylinders and door impact beams. They lend themselves to roles where there are relatively simple shapes. They can also allow a reduction in the number of stampings required, as in the seat reinforcement.

The doors are good examples of all three forms of light alloy; the hinge pillar is a casting, the bottom and side panels are sheet stampings, with high-strength extrusions for the impact beam and door frame. Total body-in-white parts count adds up to 334--284 stampings, 35 extrusions and 15 castings.

Jaguar, which worked closely with aluminum supplier Alcan, ending up using two alloys thought to be unique to the European motor industry. In sheet, the XJ employs 5754 (an aluminum alloyed with 3 percent magnesium, around 0.5 per cent manganese and less that 0.1 per cent of iron and silicon) for non-cosmetic inside structures in either NG grade where higher yield strength was the priority, or AA grade where formability mattered most. Some inner closure panels are stamped from 5182 alloy (which has 4.5 percent magnesium content).

For skin panels, others use 6016 alloy; Jaguar chose 6111 alloy, which has increased copper (0.5 percent) for better aging characteristics and therefore superior dent resistance. Like all 6000 alloys, this one bake-hardens at 170 degrees Celsius, in the paint oven, its yield strength rising to between 15.5 and 16.8 tons/sq. in. (240 to 260 Mega Pascals). This compares to the 11.7 to 15.5 tons/sq. in. (180 to 240 Mega Pascals) of bake-hardened steel.

As well as providing much of that critical dent resistance necessary in exterior bodywork, 6111 alloy gives, in the pre-baked state, good enough formability, particularly important to allow reproduction of those famous Jaguar curves. The rest of the strength is generated by work-hardening.

As Mark White, Jaguar body structures manager says, "we had to make sure very early on that we could replicate those shapes in aluminum alloy. We did full simulations of each of the skin panels, using computer simulation tools, not only to ensure we could press them without splits or serious metal thinning, but also that we had good strain energy. We could actually simulate the amount of strain energy that we got in each of the parts, and we set a minimum strain energy of about 4 percent which meant that we could get good dent resistance once they'd been through the paint shop."

Automotive Industry

Alloy cat: Jaguar's XJ finds the right combination of aluminum construction techniques to produce a car that is spectacularly light and incredibly strong - Process

Automotive Industries, Dec, 2002 by Michael Scarlett

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