The benefits of replacing traditional body welds with adhesives, fixings and fasteners

Not so long ago, the idea of using adhesive for body-in-white assembly operations would have been deemed unrealistic. However, slowly but surely, the numerous advantages of adhesives have ensured this innovative joining technique has made significant inroads into conventional methods of BIW construction.

So what are the principal benefits? According to 3M, a Tier One expert in BIW joining techniques, these high modulus adhesive bonding systems exhibit high fatigue strength and demonstrate excellent stress distribution, a factor that increases the torsional stiffness of the car body, leading in turn to an improvement in the load/driving behaviour. Additionally, such products offer easy solutions to joining different materials, improved corrosion resistance, NVH benefits, reduced energy consumption and significant weight savings.

Modern adhesives also offer longer shelf life, no odour, squeeze out control and slippage resistance, the elimination of welds, improved joint-fill, no panel distortion, excellent washout resistance and improved appearance and aerodynamics.

Above all, 3M suggests that the deciding factor should be the improved performance of adhesives in crash test conditions, particularly in dynamic wedge impact tests and drop tower tests. Here, the presence of microscopic phase transitions in the fabric of the adhesive provides a convenient damping effect, effectively absorbing impact energy.

Estimates suggest that many production cars already contain 5-6kg of adhesive (around 170m of body seams), and 3M predict that it won’t be long before OEMs will be witnessing per-vehicle weight savings in the region of 20% when compared to traditional welds and joins.

The most notable 3M development in this area is the introduction of two-part adhesives. For the first time, these materials provide ‘lock-up’ strength and stability at room temperature – a significant advance when compared with conventional, one-part adhesives that require temperatures greater than 165°C to achieve chemical reaction.

According to 3M, adoption rates for adhesives are growing rapidly, particularly among European OEMs such as BMW, Mercedes-Benz and Audi. For instance, the previous Mercedes S-Class featured around 77m of adhesives per body (primary structure and closures), whereas the new model has 194m – a 152% increase. Similarly, the previous generation Audi A6 incorporated 36m of adhesive. The latest model now includes 122m, an increase of 239%. Other models also showing notable gains in adhesive application include the Volvo V50, Mercedes SLK, BMW Mini and Skoda Octavia.

Such vigorous growth rates are predictable, states 3M, as the latest two-part adhesives meet a long list of customer requirements including: weight reduction; the ability to accommodate increased substrate hybridisation and diversity (high-strength steel, aluminium, SMC, magnesium); and improved crash performance and longterm durability. There is also a trend towards ‘cold’ BIW concepts, whereby pre-gel ovens are being removed and short- and low-temperature induction curing cycles are being introduced. The company is keen, however, to outline that structural adhesives are more than just a single product; different solutions are required to match specific customer needs.

Recent successful BIW applications for 3M include hem flange bonding on all closures on the Ford Transit, hood hem flange bonding on the Ford Mondeo, hem flange bonding on all closures on the Ford C195 Fiesta, aluminium hem flange bonding on all closures on the Jaguar XJ350, bonding of hem flanges, reinforcement brackets and door waist rails on the Aston Martin DB9 and V8 Vantage, and aluminium door cross beam bonding on the Smart ForFour. Additionally, two-part adhesives for complete body and crash structure solutions have been provided by 3M to manufacturers of high-performance composite and mixed substrate vehicles.

Most of 3M’s solutions have been fulfilled using two-part adhesives, SA 5027 for aluminium and steel structures, and SA 7036 for galvanised steel. Other products include: 3M Scotchweld DP480 for composite crash structures; 3M Scotchweld DP490 for composite vehicle construction; and 3M Scotchweld 7222 for aftermarket repair of aluminium and composite structures. Typically these products are supplied in cartridges for manual application, or in drums to suit automatic dispensing by robots.

Looking to the future, 3M says it has developed one-part and two-part adhesives that can achieve full structural performance to suit the requirements of low-energy production of low-energy consumption vehicles.

Counting the cost

According to specialist chemical company Degussa AG, which supplies materials used in bonding technology, a medium-class vehicle requires welding at approximately 5,000 points, with each weld costing around €0.05. Degussa says that approximately half of these can be replaced with adhesive joins, saving around €70 per vehicle. In addition, using 1kg of adhesive reduces the vehicle weight by 25kg.

With this in mind, rapid advances in adhesives technology are allowing the design of significantly lighter cars and enabling the use of thinner steels. Last year’s Ford Focus was significantly lighter due to the use of crash-durable structural adhesives in the upper body, while new GM vehicles developed on the Epsilon and Delta platforms will also utilise newly-developed crash-resistant adhesives.

It’s fair to say that the use of structural adhesives for car bodies is most advanced in Europe, where manufacturers are committed to reducing carbon dioxide emissions to a maximum of 120g/km for all new passenger cars between 2012 and 2015 and to 95g/km by 2020.

Dow Automotive for example is enjoying considerable success in Europe, supplying structural adhesives from its Betamate brand to various OEMs. Betamate 1496, for instance, is being used on Audi A5 structural elements such as pillars, bottom construction and front walls. Dow says the adhered joints return up to a 25% improvement in body stiffness and up to 15% more energy uptake in a crash, while their durability improves the strength of bonded joints up to 1,000 times the break time in comparison with spot welds.

Dow has also developed an adhesive grade for BMW X5 front rails, front of dash, shock towers and underbody. A plant in Michigan was modified to produce the material, which is formulated to stay secured to the body structure during body wash, phosphate and e-coat baths.

Another supplier working hard to further the advancement of structural adhesives in automotive environments is Momentive Performance Materials, which has just introduced two new high modulus SPUR+ pre- polymers known as 3100HM and 3200HM. “The new SPUR+ pre-polymers will allow adhesive formulators to develop structural adhesives demonstrating high tensile strength, one or two-part moisture cure, adhesion to many surfaces without the use of primers, and water, chemical and temperature resistance,” says Dominique Chizat, Momentive’s European Market Development Manager.

“They are presently under evaluation for a number of automotive and transportation applications.” The new pre-polymers are silylated polyurethane resins that moisture cure under ambient conditions. Because of their ability to build high tensile strength, they are good candidates to consider as binders for one- or two-part structural adhesive formulations. The low viscosity prepolymers are formulated without plasticisers and contain no free isocyanates.

Early test results show that SPUR+ 3100HM pre-polymer, when cured alone, provides a tensile strength of 450 psi and registers 40 on the Shore A hardness scale, while SPUR+ 3200HM pre-polymer is for even more demanding applications, potentially offering 1,000 psi tensile strength – approximately 10 times greater than conventional prepolymers, with a Shore A hardness of 60.

Sticking points

As with all emerging technologies, many challenges lay ahead for crash-resistant structural adhesives. One hurdle that remains is the basic conservatism of design engineers, who are familiar with, and trust, conventional joining methods, particularly spot welding. Another is the lack of software that can accurately simulate the mechanical properties of the latest adhesives on a large scale, although the Fraunhofer Institute for Manufacturing and Applied Materials Research in Bremen, Germany is working to solve this problem. There is also a need for adhesives with higher heat resistance that will allow use of the latest materials close to the engine block, adhesives that cure at lower temperatures (allowing OEMs to save on e-coat oven operations), and grades with shorter cure times.

Some of these caveats are the reasons why the joining techniques advancing most rapidly today employ hybrid bonding, where spot welding or another conventional fastening system is deployed to provide initial strength.

Aluminium body structures, for example, can benefit extensively from a combination of structural adhesives and mechanical fasteners, such as self-piercing rivets, which provide a joint with far greater peel resistance than bonding alone and at least three times the strength of riveting alone.

In addition, rivets allow de-jigging of the primary structure before the adhesive is cured, holding the geometry and dimensions without specialist fixturing through e-coat.

This joining strategy is being deployed on the Lotus VVA (Versatile Vehicle Architecture) to be found on the nextgeneration Lotus Esprit, as well as in the latest all-aluminium Jaguar XJ, built at Castle Bromwich, UK. This particular plant uses more than 3,200 self-piercing rivets and epoxy adhesives to hold each body-in-white’s 339 pieces together.

Once the pieces are stamped, epoxy adhesive is applied both by workers and robots, while fasteners are applied using 55 manual hydraulic Henrob rivet guns and 88 robotic servocontrolled Kawasaki rivet guns.

Choices, choices, choices

According to Adrian Ellis, Sales Manager at fastener supplier Profil UK, mechanically-mounted threaded fasteners in automotive sheet metal applications typically fall into three categories – self-pierce and rivet, rivet forming, and pressed insert.

“Self-piercing fasteners incorporate a cutting edge, which, together with the insertion tool’s bottom die, pierces a hole for the fastener to be located and riveted in position,” he says. “Rivet type nuts and studs are simply inserted into pre-formed holes and have their shanks formed into a rivet head by the lower tool, while insert-type fasteners are not themselves deformed during installation, but rely on the base material flowing into undercuts and recesses in the fastener to achieve the required location and retention.”

In the majority of sheet metal applications, the properties of each of the aforementioned will exhibit similar benefits over traditional weld nut or weld stud fixings. The high-volume production runs associated with vehicle manufacture lend themselves to the installation of mechanical fasteners as part of the pressed component’s own production cycle, using a dedicated station within its multistage progression tooling. However, mechanical fasteners are just as suitable for applications where assembly takes place ‘off-press’ – using pre-pierced components.

“Numerous variations on the theme exist – representing a range of forming configurations for the sheet metal and the respective fastener,” says Ellis.

Profil fasteners are offered in a vast variety of styles for use with materials from 0.6 to 6.0mm thick. They can be employed with coated or uncoated materials, high-strength, stainless and sandwich steels.

“When compared to other assembly methods – notably welding – the attractiveness of pierce, riveting or insert fastening technology can be encapsulated within the term ‘cost-effectiveness’,” states Ellis. “Unsurprisingly, load carrying performance and total installed cost are principal considerations.”

Mechanical fastener assembly offers a fast and reliable method of achieving vibration-resistant fixings that will accommodate static, dynamic and impact loads. Most products of this type incorporate a series of radial ribs that bite into the base material on installation to improve torque resistance. Recommended torque loadings will vary from 10Nm for M6 variants through to 80Nm for M12 fasteners, even in panel thickness of less than 1mm.

“By contrast, such light gauge material will normally be regarded as beyond the production limits of most welded fasteners,” says Ellis.

Equivalent fastener push-out resistance typically ranges from 4-5kN for M6 nuts and 5-13kN for M12 variants, depending on material thickness. Clearly, pull-through performance will also vary with panel thickness, as well as fastener configuration – such as the inclusion of a large flange to act as reinforcement for the base panel.

“Other important performance criteria include vibration and fatigue resistance,” explains Ellis. “Here, welded fasteners can suffer significant failures due to thermal disruption of the sheet metal structure. Further problems can arise in terms of initiating corrosion centres, as well as potential heat damage to coated or plated surfaces – if they can be welded successfully at all. By contrast, mechanically-attached nuts and studs are ideal for use with corrosion-resistant finishes and materials, such as the galvanised steel and zintec increasingly employed within today’s automotive sector.

“Mechanical fastening technology is often at its best in high-speed production processes, with the nut or stud installed as part of the component’s multi-stage forming operation,” he continues. “Under these conditions, well in excess of 100 fasteners can be installed per minute, at least an order of magnitude greater than welding. Even with manually-fed applications, cost savings of up to 30% can be achieved.”

According to Profil, through their installation as part of the presswork process, mechanically-applied fasteners offer significantly better positional accuracy and repeatability than welded equivalents. In most applications, tolerances of ±0.20mm can be readily maintained, while a figure of three times this value might be expected for corresponding welded parts.

Engineering strength and safety

In the world of automotive components, it will come as no surprise that fasteners aren’t the first thing engineers think about. They’re not exciting and they’re commonly treated as routine. That said, they have a pivotal role to play regarding vehicle safety and performance. Using the wrong type, size or strength of fastener can lead to premature failure, unwanted rattles and dissatisfied customers, particularly when it comes to body panels.

“The challenge is to keep critical joints fail-safe, not only under expected use but also under extreme use,” says Frank Metelues, a design engineer at Dana Corporation, a Tier One supplier of (among other things) frame and chassis technologies. “You’ve got to do this while hitting all the required price points for performance, manufacturing and warranty, and some drivers are always going to push the limits. Off-roading, for example, compounds extreme loading, vibration and thermal forces.”

Because traditional fasteners are susceptible to self-loosening rotational movement, stripping and shearing, their use in critical joints without additional locking methods isn’t always appropriate. Testing has found that the first two threads of traditional fasteners can carry as much as 80% of the load, promoting stripping or shearing, while subsequent male threads ‘float’ within female threads.

The standard methods used to ‘lock’ traditional fasteners, however, have their own limitations. Locking adhesives, for instance, progressively lose effectiveness as temperatures rise. In high volume, their use typically requires a large capital expense to purchase and program robot applicators. And when re-application is necessary, cleaning the threads of affected components takes added time and labour before re-application is possible.

Fortunately, vehicle engineers are successfully attacking these problems with a variety of new technologies that help optimise design reliability and profit, even under extreme conditions. One of the most interesting solutions is also the simplest – an innovative self-locking fastener called Spiralock. By design, Spiralock is capable of resisting loosening even under loads and vibrations strong enough to break the fastener. It can also be reused many times and is highly resistant to heat.

What differentiates Spiralock is a 30º ‘wedge’ ramp cut at the root of the female thread (while traditional fasteners use a 60º thread). The wedge ramp allows the bolt to spin freely relative to female threads until clamp load is applied. The crests of the standard male thread are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement. This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial-torsional loading, joint fatigue and temperature extremes.

Material world

There is increasing use of high-strength materials – such as dual-phase and residual austenitic steels – within the automotive sector, and this trend is driven by the need for greater passenger protection and increased fuel economy, combined with reduced carbon dioxide emissions. The result is stronger but lighter components in safety-critical areas such as seat mounts and structural members of the passenger compartment – notably side impact beams and in the A- and B-pillar areas.

The adoption of high-strength materials is accelerating, with vehicles such as the Honda Civic seeing more than a five-fold increase in the use of 600MPa steel in its construction within a single development generation. Elsewhere, 800 and 1200MPa materials are already in production, with trials of materials such as Usibor 1500 – an ultra-high strength boron steel that can be hardened, also being actively pursued.

Ultimately, the formability of the base metal will determine how suitable mechanical fasteners are for use with such materials, because the fastener requires sufficient base metal to engage with its anti-rotational features to prevent spinning of the fixing under assembly conditions. However, in principle, all high, very high and ultra high strength steels are suitable for use with mechanically-applied fasteners.

Profil fasteners are currently being deployed in a variety of high-strength steel applications – typically with the fastener inserted ‘in press’ during the component’s manufacturing cycle. For instance, Profil Class 10 M8 RND nuts are inserted in 1200MPa steel side impact beams used by Mercedes. RSN and RND high-strength rivet nuts have also been specified by Audi at the heart of a ‘safety-critical’ bonnet latch mounting application on its latest-generation A4 and A5 vehicles. The fasteners are installed in 0.8mm thick high tensile strength steel beams that form an integral part of each vehicle’s impact crumple zone. Profil fasteners are employed in similar applications at other motor manufacturers – including Land-Rover, Honda and Mercedes – where they have demonstrated their suitability for production use with up to 1200MPa steels.

Arnold & Shinjo offer a similar story, stating that both VW and Audi are gaining up to 30% reductions in body shell and assembly processing times by using its piercing nuts and fastening systems in the production of a variety of parts. And the lower-cost piercing nuts are also financially attractive, since bottom line costs fall below those of the traditional weld nuts used previously.

Crucially, the piercing nuts also offer companies a level of process security that weld nuts cannot match; weld process monitoring cannot reveal whether the weld is uniform, while punching in a press allows applied forces to be documented via end pressure gauging.

The fasteners – from the Arnold & Shinjo PIAS PN, HN, KP and RXM series – are used in the manufacture of, for example, side panel frames, wheel housings and roof rails. Applied in conjunction with existing production sequences on punch and press tools, or via tailor-made insertion systems (also from Arnold & Shinjo), the company says VW and Audi are meeting demands for consistent product quality, volumes and, crucially, cost savings.

The fasteners are utilised in a range of existing process lines embracing DQ800 presses, five-station transfer presses, press lines and vacuum transfer presses. The piercing nuts are fed at the rate of up to 400 per minute – large tools in transfer presses or press lines can apply 30 nuts per stroke – and six punch heads, for instance, can operate at 65 strokes per minute.

This includes body shell door panels for the VW Golf, where 24 nuts are applied to each door from outside, inside and at different angles to the panel. In other instances, the piercing nuts are applied via Arnold & Shinjo tailormade solutions. For example, in the case of a body shell component of high strength sheet metal, Arnold & Shinjo developed a fastener insertion system with five fixed C-frames to enable five nuts to be inserted in a single operation.

In Audi models, 44 nuts are used in the construction of the A3, 54 for the A4, 72 for the A6 and 24 for the TT. For VW cars, 30 piercing nuts are used on the Golf and 40 on the Passat.

Occasionally, special panel applications demand special fasteners, which is why the new Atlas SpinTite stainless steel half-hex shank-threaded inserts from Penn Engineering (PEM) offer high corrosion resistance and fastening solutions for ‘blind’ attachment applications where only one side of a panel is accessible for assembly. Their hex body design, for installation in hex holes, promotes high resistance to rotation, while a low-profile head minimises protrusions.

They can be used in panels of any hardness, as thin as 0.51mm.

Further, the overall cost of fasteners must be examined when considering installation methods for body assembly. For example, pierce nuts are significantly less expensive than weld nut alternatives, making them a bottom-line friendly choice. If a company’s annual usage can be estimated at three million nuts, opting for the pierce nuts could generate a savings of around £20,000.

Adhesive tapes offer flexibility in vehicle production

Of course, not all body adhesive applications are structural. Adhesive cloth tapes are commonly found in body applications such as gap filing and hole sealing.

The Scapa range of cloth tapes can consist of a carrier material constructed from a polyethylene film, with coated cotton cloth on one surface with a rubber-based, pressure-sensitive adhesive. The adhesive formulation ensures high initial tack, while the total tape construction gives product robustness for resistance against abrasion, tears and puncture.

On automotive trim assembly lines the need to permanently seal drain holes, gaps and apertures has always presented manufacturers with potential problems, such as the correct placement of sealing tape, the operator forgetting to apply tape and excessive use of tape when hand torn from a roll.

The solution offered by Scapa and now widely used by manufacturers such as Nissan is die cut cloth tape assembly sheets. To suit the specific vehicle and assembly sequence, Scapa cloth tape is supplied on sheets, where a set number of die cut cloth patches, in shapes to suit the holes being covered, are attached.

All waste material between each shape is removed, thus reducing waste disposal on the actual product line. The use of assembly sheets ensures that all intended holes are covered, material wastage is all but eliminated, while providing the correct size of cloth for the specific aperture.