Issue: Dec 2002


Alloy Cat



Jaguars XJ finds the right combination of aluminum construction techniques to produce a car that is spectacularly light and incredibly strong.

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.

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 A8 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. 


Under the skin of Jaquar\'s new XJ is a monocoque aluminum alloy unibody

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.

Jags single hydroformed aluminum alloy extrusion includes the front cross beam over the radiator

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 theyd been through the paint shop.

Extrusions are drawn from three alloys AA6063-T6, AA6060-T6 and AA7108-T6. Castings are in three alloys    C446-F, whose alloying ingredients are 3.5 percent magnesium, plus silicon and manganese, C448-T6, an aluminum silicon (10 percent) plus magnesium alloy, and A356-T6, a seven percent silicon alloy with some magnesium. The materials are ductile enough to permit attachment to sheet using self-piercing rivets. This is vitally important because Henrob self-piercing rivets are half of the joining process used almost entirely throughout the new XJ body-in-white; the other component is the Betamate bonding adhesive.  This is damp enough to be nozzle-applied by robot or manually, yet stiff enough to stay in place prior to uniting the adjoining part, even when the panel is vertical. It is also entirely compatible with the PT2 oil used to pre-treat panels during stamping and transport, so no work has to be wasted degreasing them, and cures at the same temperature as the bake-hardening alloy.

Some blind riveting is done manually bonded joint has very good shear strength, but not so good resistance to being pulled or torn apart; riveting is the familiar answer. The combination is very strong and durable; add to that the fact that in contrast to spot welding, it provides the structural equivalent of seam welding. This explains the great increase in stiffness torsionally and in bending claimed for the new XJ body versus its steel forerunner; simply because it has 100 percent solid joins between all its panels and parts. The technique is of course very familiar to the aircraft industry, where it was first used. 

To avoid electro-chemical corrosion between the boron steel rivets and aluminum, the rivets are tin-zinc plated, which after sealing with the final paint coat is found to make a reliable corrosion protection. 3,200-odd rivets are set over 341.2 ft of bonding adhesive, which compares well with the 4,500 to 5,000 spot welds of a conventional steel body.

Welding is only found in four places, at the joints of roof to A- and C-posts, which are united using dual pulse MIG welding and subsequently hand-finished. This is driven by styling and production feasibility. Stamping constraints imposed by the forming limits of aluminum precluded a one-piece body side ring and Jaguar style could not permit the common fix of a badge or other piece of trim to mask the joint hence the hand finishing.

The new press shop is Jaguars first in the Midlands they inherited the Ford press shop at Halewood in the English north east where the compact X-type is built and it was installed and equipped by the German company, Schuler AG.  Jaguar makes no pre-tences about their experience of stamping; they use an Anglo-Dutch consortium of stamping specialists, Polynorm Stadco, to run this part of the historic Castle Bromwich factory.

You wont find Jaguar spokesman admitting it, but insiders tell how the new XJ started conception as another steel car.  Ford, which has done well known work on monocoque aluminum?alloy stressed skin cars was debating which of their two luxury car marques, Lincoln or Jaguar, would kick-off the new technology.  The design of the Lincoln was allegedly not ready, so Jaguar got the job which meant a virtually completely new engineering re-design, with four years to go to the same Paris Show 2002 reveal deadline. 

That may explain why Schuler AG got the job of supplying and fitting out the new press plant.  As Jens Anspacher, product manager of Schuler SMG says, we have enough capacity to provide a huge number of presses in a very short time, spreading the production amongst all our facilities. The purchase order and start of the project was in November 1999, and the arrival of the first press was on the 6th of November 2000.  We delivered 13 presses, of which seven were from the Schuler Hydrap facility, three presses came from Schuler SMG, and three presses we built in a facility in  France.  The first trial part came from the smaller press line in March 2001; the final handover of the two lines to Jaguar was in October 2001.  Since then, there has been a long trial period to get all dies ready and running and ready for production

At Jaguars framing station, the vehicles body sides, underpan and roof are riveted and bonded together by robots

The main hall contains all 13 Schuler presses, divided between two lines, A and B. Line A has five presses in line, the lead-off one being 2,000 metric tons; the other four are 800 ton capacity. Transport of the part between presses is by ABB robot. A sixth press, another 2,000 ton version working at a slower speed and having five die sets rather than the standard three, is both for try-out work and for production of three parts, the front and rear fenders and the lower rail, all of which require an extra operation. It is placed at right angles to the line.

Line B has seven presses, all 800 tons. It can be used as a single press line with five or six operations, or as two lines, with two de-stackers on each side of the line. In the middle of the line, there are flexible conveyors, which can be moved from cell to cell depending on where the output is. Again as with line A, one of the presses can be used for tryout. 

A key feature of the presses is that they are not the traditional wheel and crank type, but hydraulic, which allows a far greater versatility of press control, very important when stamping aluminum alloy.  Draw speed can be critical for aluminum, but with the hydraulic press it can be programmed to run at constant, increasing or decreasing speed, which is very helpful for making aluminum parts.  First contact with the material can be far smoother, because the slide can be braked before accelerating for forming.  The slow closing speed allows the aluminum time to flow into the die, reducing the risk of cracks. 

The spare tire well stamping provides a striking example of how successfully the Schuler hydraulic presses operate. Manipulation of the alloy selected and the use of part lubrication during stamping combines to achieve remarkably deep drawing in aluminum alloy, with a near vertically sided well that is 300 mm (11.8 in.) deep-formed in one piece.

The programmable retraction speed enables smooth retraction too when using slide cushions or spring dies; Jaguar is using spring dies. The lead-off press has a 600 ton hydraulic four point controlled cushion which adds to the flexibility of the press, especially for parts which are complex to draw. Producing parts for one car on just two press lines demands more die changes, and therefore flexibility to make all the different parts that in a higher volume plant would normally be produced on special lines.

Blanks are cut in Germany by a Schuler subsidiary, and protected during transport with a lubricant (MD 404 AL 3070), or a form of wax on structural non-cosmetic panels. Transport from Germany to Castle Bromwich is lengthy, and demands protection of the blanks.  But oil in particular acts like glue on a pack of blanks.  Gravity does not help, as the blanks are so light.   Schuler uses air knives to part the blanks, backed up by a double blank sensor.   

De-stacking a pack of steel blanks robotically is comparatively simple, because magnetic grippers can be used to pick up each sheet separately. These blanks are both floppier and non-magnetic, so the Schuler de-stackers here use appropriately closer-spaced vacuum grippers. The sensitivity of aluminum alloy pressings is obviously a problem.  Schuler meets this with a wash stage prior to loading into the first press, using a wash fluid composed of 10 percent oil (V 1303) and 90 percent water, plus a clean press shop atmosphere, made so by pressurizing it slightly (by 1 bar ? 14.5 psi).

The press frames are four-pillared rather than two-pillared, so that die changes can be made transversely; the new die, prepared on its movable bolster on one side during the operation and batch involving the previous die, moves in as the bolster carrying the previous die is withdrawn on the opposite side. This is a major feature behind the 15-20 minute die change time achievable.

And the result of all this light alloy effort Jaguar says that the body is 60 percent stiffer in twist and bend than a steel equivalent, whilst the overall weight of the car is lower by 440 lb (200 kg) than the previous XJ, in spite of sitting on a 119 in. wheelbase (a 5.7 per cent increase), being 1.3 percent longer, 17 percent wider, and 6.5 percent higher. Put another way, comparing very roughly the bulk of the two cars via the product of overall length, width and height, it is 26 percent bigger.  No wonder the 400 hp XJR claims a 5.0 sec and 0-60 mph time, or that the European combined fuel consumption of the 3L is equivalent to 22.5 U.S. mpg.

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