Aluminium is making greater and greater inroads into car body construction. In the first part of this article we take a look at the aluminium/steel construction of the BMW 5 series and then in the second part, look in more detail at the self-piercing rivets and bonding techniques used to join the material.
Aluminium/Steel Construction of the BMW 5 Series
The BMW 5 series uses a unique construction. The front-end structure from the cowl forward is all-aluminium, while the rest of the vehicle structure is steel. The front portion of the lower rails is aluminium; the portion of the lower rails under the floor pan is steel. A rear portion of the upper rails is steel, however the rest of the upper rails are made of aluminium. The floor pan and the lower portion of the A-pillars are steel, both joined to an aluminium cowl panel.
The first questions that come to mind are why and how. Why make only the front-end structure out of aluminium? How is aluminium and steel joined while avoiding galvanic corrosion?
The "why" can be answered by considering that an aluminium structure can be just as strong as a comparable steel structure with about one-third less weight. The aluminium front assembly allows the new 5 Series to be slightly longer, wider, and taller than previous model years but still weigh about 20 kg (44 lb) less. With the front structure made of aluminium, there is almost a near equal weight distribution between the front and rear of the vehicle. Also, with the rear half made out of conventional steel, there’s better repairability, necessary for a high volume production vehicle.
When bare aluminium joins with bare steel, contact or galvanic corrosion can result. Where aluminium meets steel in the 5 Series main structure, the joints are made using adhesives and rivets, a process known as rivet-bonding. (More on the rivets in a moment.) Everywhere aluminium and steel come together, the adhesive protrudes at least one millimetre outside the joints. This helps ensure that direct contact between bare steel and aluminium is completely avoided. The steel panels are also galvanized and the aluminium panels are treated with a titanium/zircon coating designed to both hinder oxidation and enhance the adhesive bonding surface. After construction, the entire vehicle is also subjected to a phosphate bath and E-coat.
Aluminium part construction used for vehicles is either stamped, extruded, or cast, and the 5 Series uses each of these in the front-end structure. The strut tower/apron assemblies are cast construction. The inner part of the lower rails (engine carriers) are extruded construction, while the outer portion is stamped. The upper cowl panel is extruded and hydroformed. The upper rails and cowl panels are stamped. The alloys are 5000 and 6000 series.
Besides the front structure, the driveshaft, suspension system, engine mounting brackets, transmission crossmember, rear subframe, and several exterior panels are also aluminium.
The aluminium front section, which is available as a complete assembly, is originally assembled with coated steel self-piercing rivets and adhesive. Repairs are made using coated steel blind rivets and adhesive. This is because the installation of self-piercing rivets requires a dedicated tool that isn’t readily available. Also required is two-sided flange access that is not always available during repairs. The blind rivets can only be ordered from BMW, and there is a specific adhesive.
The recommended process for removing the self-piercing rivets during repairs is a unique process in itself, involving a special stud welding tool and blind rivet gun to pull the rivets out. Where access prohibits this process, the rivets can be drilled out from either side, or ground down on the backside.
The stud weld pulling method begins by grinding the rivet heads to bare steel. A special stud welder is then used to weld on stainless steel studs to each rivet head. A special blind rivet gun is used to pull out the rivet.
Think of the process of installing a blind rivet, how the gun pulls on the mandrel of the rivet to compress the head before cutting the mandrel off flush with the head. This is how the blind rivet gun is used with the stainless steel stud to pull out the self-piercing rivet. The high pulling force that is required to do this is why most blind rivet guns will not work. The blind rivets are installed in the resulting holes, using the same blind rivet gun. BMW holds the patent on the stud welder and rivet extractor and they are only available through BMW dealers.
Preparation of the flanges for adhesive is done with a Pyrosil® (flame) kit. This is the same process used on the 2004+ Jaguar XJ. The flame is applied to the flange. While the flange is still warm, adhesive primer is applied. After allowing the primer to dry, adhesive is applied.
Extruded sections and cast parts, which show any sign of visible or measurable deformation, are replaced, and not straightened. This includes the front lower rails, because the inner part of the lower rails are extrusions.
There is a lower front rail service part available for partial replacement of the rail, forward of the front axle. The procedure for attaching this part to the existing rail is unique. The aluminium lower rail section is cut and fitted to the existing rail, just like any sectioning procedure. But instead of welding the section in place, it is bonded, using two insert-like "repair elements." Screws, installed in the repair elements, are turned to form a tight fit. The screws are removed after the adhesive has cured and the screw holes sealed with a PVC sealing compound.
Rivets, most commonly self-piercing rivets (SPRs), have replaced resistance spot welds for joining structural and non-structural parts on some vehicle models. These coated steel rivets are found primarily on aluminium-intensive vehicles.
What is a Self-Piercing Rivet?
A self-piercing rivet is a tubular rivet by design, with a partially hollow shaft. This allows the rivet to pierce through the material it is going to join without pre-drilling any holes. After piercing the top panel, the rivet radially expands into the bottom panel.
When you look at the backside of an installed SPR, you don’t see the rivet, but the impression of the rivet flared out in the bottom panel. This joining process provides very strong and reliable joints, without changing the property of the material around it or using as much energy as resistance spot welding. SPRs may be used to join coated and dissimilar materials of varying thickness, as well as being used for multiple layer riveting, as long as the material hardness and rivet length allow.
Use and Removal of Self-Piercing Rivets
SPRs are currently found on the 1997+ Audi A8, 2004+ BMW 5 Series, and the 2004+ Jaguar XJ. These three vehicle makers all have different procedures for removing SPRs.
Audi recommends pressing out the rivets with a pneumatic multi-purpose riveting tool.
BMW recommends using a weld-on bolt extractor for SPR removal. This involves cleaning any coatings off of the rivet head and welding on a bolt that is used to extract the rivet. If the bolt extractor repeatedly fails, or the rivet is inaccessible, drilling the rivet and punching out the remnants is an option. Grinding SPRs is only recommended by BMW if the panel being ground is also being replaced.
Jaguar recommends using an electronic dual-purpose riveting tool for removing SPRs. If they are not accessible, drilling or grinding with a belt sander is also an option. If an SPR falls into the body cavity during removal on any of these models, attempt to remove it with a magnet. If it cannot be removed, use anti-corrosion compound in the area to coat the lost rivet. This will help prevent galvanic corrosion.
Just as there are different recommendations for removal, there are different recommendations for replacement of SPRs.
Currently, Jaguar is the only vehicle maker recommending installation of SPRs when making certain repairs to the XJ. SPRs are installed using the same electronic dual-purpose riveting tool that is used for removal simply by changing the dies on the tool. Jaguar-specific, coated steel blind rivets are used where SPRs are not.
Audi uses specific aluminium solid rivets and aluminium MIG plug welds to replace SPRs.
BMW uses specific coated steel blind rivets to replace SPRs. All of these vehicle makers recommend using adhesive in conjunction with rivets, also called rivet-bonding, in their respective repairs.