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

• E-mail
• Print
• Link

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.

A 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 onepiece 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 Jaguar's 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 pretences 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 won't 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.