Ruari McCallion reports that new technologies and techniques in vehicle production are being blended with established methods in the modern assembly line, but new vehicle platforms may require a change in the balance

The methods, techniques and technologies for joining parts that come together to create finished vehicles have, for a century or more, relied on mechanical techniques: welding, screwing, nuts and bolts. Joining techniques such as glueing and stitching were restricted to such areas as interior upholstery, but modern vehicles use a broader mix of materials, including thermoplastics, and the methods used to join them to the metal chassis have changed as well.

Viewed as a single area of expertise, fixing could be regarded as being in a state of flux, as the drive to reduce vehicle weight - particularly to reduce emissions and also improve the range of electric vehicles - becomes even more pressing. But before a given company decides to rip out their lines of robotic welders for replacement with a series of adhesive dispensers, it is worth considering both the value of established technologies and the limitations of new methods – as well as the actual and expected market penetration of plastics and other new materials.

Reduced plastic usage - for now

The Polyamide Report 2011, published by German company Heidelberg Business Media, is about to be released and it is a timely addition to the information pool. Based on data collected by automotive benchmarking company A2Mac1, which examined 129 cars constructed between 2004 and 2010, it reveals that usage of the two main polyamides used in automotive production, PA6 and PA66, is actually in decline. According to the report, total average weight (in usage) per car fell from around 20kg per car in 2004 to 14kg in 2010. Polypropylene usage also declined over the same period, from 62kg/car to 60kg, while polyurethane usage fell quite dramatically, from 18kg/car to just 10kg, a 44% reduction.

These things did not happen in isolation. Overall vehicle weights also declined as there has been an on-going shift from larger to smaller platforms in Europe and North America. Additionally, the BRIC markets - the countries enjoying the strongest growth in the automotive sector - are dominated by smaller platforms, which naturally have a lower material consumption.

But even individual applications that rely on thermoplastics, such as HVAC systems, have demonstrated material usage reductions, as OEMs maintain their drive to lightweight vehicles. However, plastics remain an important material with regards to auto production, and usage is sure to rebound as more electric vehicles come on-stream, and in greater volumes. More than other powertrains, they accentuate the need for lighter weight and models already on the market, as well as concepts revealed to date, show an increased use of plastics as body panels and in other applications.

Mixing in adhesives

Despite a drop in overall usage, which could be temporary, incorporating plastics into the production mix has had a profound effect on the way vehicles are assembled, as for a number of reasons, traditional mechanical methods are not appropriate. Chief among these are that conventional methods, such as spot welding, riveting, and clinching and hemming, may not work and plastics in some applications will, at some point, be required to form a solid bond with metal. Joining two types of material – even steel and aluminium – has always required special consideration; the thinking simply has to be deeper with metal-to-plastic joints.

The solution has often been to use adhesives, and yet while much has been made of their value and effectiveness, they are not without challenges and limitations.

“Adhesives have been used in varying degrees for decades, but we have seen massive advances over the past 50 years and a lot of adhesive technology is relatively young; the Plexus technology has been available for only about 15-20 years,” says Chris Coll, Territory Manager, UK and Scandinavia, for ITW Plexus. “What we are seeing now is a growing acceptance. More and more applications and parts are being evaluated as adhesives projects than was the case in the past. Standard plastics have been used for years, in bumpers and spoilers, for example. Now we are seeing more composites too, such as carbon fibre. The ability to join carbon fibre to metal is an aspect in expansion.”

However, while windscreen fixing is now an established application and certain adhesives are used in chassis, tub and underbody bonding, usage is more widespread at lowervolume manufacturers. Even beyond panel and chassis joining, it is not usual to find adhesives used to create suspension sub-assemblies or in load-bearing structures.

“In those areas, even in aerospace, you will tend to find mechanical methods alongside adhesives,” says Coll. Another area where adhesives are widely used is in the construction of power boats and yachts. While not designed to travel at the same speed as a car, they are subject to considerable impact stress. “Bolts used in marine applications may be under more stress and strain than in a car, but the critical factor is the adhesives themselves,” he adds. So much so that the adhesive may be stronger than the materials it is used to join - meaning a composite hull could fail before the adhesive does. “Adhesives spread the load of an impact all around the joint. In a chassis tub, adhesive allows the stress and strain to be spread around; a nut and bolt localises it,” Coll explains.

The right specification

But is excess strength a positive or a negative? Automotive OEMs generally do not want excess, especially if it means higher costs. “In certain applications, such as the front end for example, you need the maximum strength possible,” Coll replies. “Adhesives are also lighter than welds or mechanical fixings.”

While adhesives might be an over specification in some situations, certain aspects, such as the fact that glues can provide an invisible or barely-visible join, can be an advantage over screws, rivets or other fixings - and products will continue to change as new challenges arise: “Material advances, such as carbon fibre and new and complex blends of plastics, are dictating more and more adhesive advances.

Another angle is electric vehicles, which present certain technical challenges with regard to conductivity.” Another advantage to selecting adhesive over another solution is that they are very good at dealing with heat, although while temperature issues are not a problem, the thermal cycle can have a detrimental effect. Yet while such temperature cycles could limit adhesive usage, a new Plexus technology enables it to cope in an effective manner, Coll maintains.

“Our ‘core shell’ technology makes the material behave like microscopic squash balls,” he says. “Expansion is a shock, as is cooling. As the temperature drops, the ‘squash balls’ will return to their original shape and do so through millions of cycles.” Applications develop as the material becomes more trusted. Adhesives are already being asked to do things they never had to do in the past and Plexus believes they will become more and more prevalent as materials evolve, driven by the quest for lightness, as the electric vehicle experience illustrates. That is likely to continue and even accelerate – but that does not signal the end for conventional mechanical fixings. Far from it, the domination of adhesives in auto assembly remains a long way in the future.

The old methods still have their redeeming qualities, including ease of use. An adhesive bond can be absolutely outstanding, it can even be the strongest part of the car, but the only way to achieve it is by allowing time for the bond to cure. In low-volumes this may not be too much of a problem, but in higher numbers it certainly can be.

Even while savings have been made in terms of paintshop processing, allowing time for even quick-curing adhesives to bond could negate any of those gains. There is also the question of investment. OEMs have spent a lot of money on robotic welding systems, and replacing them with adhesive stations would involve further capital commitment. The indications are that carmakers are not yet prepared to make a fundamental switch any time in the near future.

The old reliables still do the job

“Initially, new and alternate joining technologies tend to be niche applications. I don’t see them taking the place of spot welding very soon, for example, as car designs are already in place for the next 10 years, through 2020,” says Dr. Srinivas Nidamarthi, Technology Manager with ABB. Based in Auburn Hills, Michigan, Nidamarthi is responsible for ABB Robotics Systems – Automotive Systems, which makes turnkey assembly lines for aerospace and automotive companies, including areas for riveting, welding, clinching and hemming and also adhesive applications.

He points out that in modern cars, body-in-white features between 15 and 20 different joining technologies, with the top three automated processes including: spot welding (from a minimum of 30%, up to 60% of all joins); arc welding; and then glueing. These are augmented with a series of lesser-used techniques, including riveting (particularly with aluminium bodies), and clinching and hemming, which while one of the older joining technologies, is still a staple of today’s automotive joining hierarchy, Nidamarthi believes.

Cold comfort

“Hemming, as found at the edge of doors, offers a good aesthetic feel and addresses safety considerations in these exposed areas,” he says. “Tailgates and boot doors need to be hemmed, as do wheel arches.” Hemming, like clinching, are cold-form processes; hemming requires a lot of force to bend metal edges or flanges and three or four passes to complete the process. Clinching uses plastic deformation of metal sheets with a punch or die to make the join. As both processes produce a mechanical lock, it is possible to join several layers with either technique as long as the materials are not brittle. It is a relatively simple process, with low noise and low energy consumption. It doesn’t need high voltage, like welding.

Hemming limitations include the fact that it works best with relatively flat surfaces, and that it needs a flange – which translates to added weigh - but the benefits outweigh the disadvantages. “Hems are difficult to spot weld, but other techniques are emerging for these areas, including laser welding and brazing, which can be almost flangeless.” Naturally, ABB is involved with delivering those technologies.

“We group clinching, riveting and hemming together, as cold-force application technology,” continues Nidamarthi.

“Different technologies have their pros and cons and particular technologies are chosen depending on the material mix and physical constraints of the car. Coldforce technologies need a heavy press tool, while ductile materials are easier to clinch and hem. If the material is brittle – like the magnesium alloy used on the Chevrolet Corvette – then cold clinching and hemming is not a good idea. In such cases the optimum joining method is selected.” In fact, Nidamarthi has a preference for adhesives, as they are cleaner, they don’t require heavy press forces and can be delivered in one shot. But clinching and hemming still has its advantages.

“If you put adhesive on one surface, you don’t automatically get the other surface at the same time and the same place,” he explains. “Clinching and hemming automatically brings them together, you know they are in the right place. You want to ensure the alignment is correct and clinch/hem automatically makes the decision.” As with most automated processes, there is a chance of misalignment. “[Misalignment] can be overcome with vision systems and accurate computing that includes robots controlling the adhesive gun, and Dow Chemical has undertaken research that shows a half-kilogram dispensing can is able to save up to 23kg of bodyweight.”

Capital investment and operational costs can still dissuade potential users from choosing advanced technologies such as lasers, but flexibility is a main driver behind these systems. While adhesive glue guns are emerging technologies, robotic welders and clinch/hem equipment can be moved around the bodyshop in order to achieve a cost-effective joining process.

Developing effective welding

Welding and mechanical fixing must evolve to adapt to new demands. More than half of the industrial robots throughout the world are used in the automobile industry and their most important application is spot welding in BIW construction. From the 1950s to the 1990s, virtually all welds in automotive body parts in volume production used resistance spot welding, but the advent of laser welding has delivered a number of advantages, including single-side access, smaller flange widths, improved torsional stiffness, smaller heat-affected areas, automated processing, and greater design flexibility.

A paper published by Hong-Seok Park (and others), of the University of Ulsan, South Korea, addressed the advantages of laser welding over resistance spot welding, and presented experimental simulation and real-world testing outlining the steps needed to get the most out of the process.

Despite the high performance of laser welding equipment, the authors found that Korean companies had used the joining technique in only a few areas due to the complex, time-consuming and cost-intensive process of planning the laser welding cell. The paper went on to describe the process of conceiving, designing and implementing such a cell, using digital manufacturing, examining process flow, material combinations, quality and other variables, while describing how simulation using MES (Manufacturing Execution System) and CAPE (Computer Aided Production Engineering) was used to validate the cell’s operation.

A further study, conducted by Jialiang Zhang, Jianguo Yang and Beizhi Li, of the College of Mechanical Engineering at Donghua University, Shanghai, China, puts forward that the special fixtures used by many automotive manufacturers are only suitable for specific welding processes and that reconfigurable welding fixtures are an important development. However, the paper also presents details outlining a cost-effective prototype of a reconfigurable weld assembly fixturing system for parts with similar characteristics. As stated, reconfiguration can further extend the life expectancy – and applicability – of automated welding systems. As such, hemming may still give way to laser welding, but, as ABB’s Nidamarthi suggests, this would depend on the total number of laser-welding operations that could be performed, and the investment required to remodel the shopfloor, including any safety enclosures.

Piercing and punching

One method that can join components made of the same or different types of material, without welding or adhesives, is pierce nuts and studs, a technique in which German company Profil specialises. The company describes itself as ‘a pioneer in mechanically joined fastener technology’ and claims a number of advantages for its system, including the ability to join different types of metals, composites, and surface-coated and pre-painted panels.

“An interesting application for Audi is the front carrier, which houses the radiator and headlights and is bolted to the front wings and cross-member,” says Adrian Ellis, Sales Manager with Profil UK.

“It is made of steel, faced with plastic. We provide the bonnet latch fixing, too. What is driving the method forward is the combination of materials and we have undertaken testing in GRP and carbon fibre, although 99.9% of what we do is metal to metal.”

Profil’s fastening methods include self-piercing and riveting; self-riveting and pressed-in parts. Its fixings are largely made of heat-treatable and corrosion-resistant carbon-boron steel with a zinc-nickel surface lining. The company claims that the fixings can also help reduce overall vehicle weight.

“We started off making special parts for customers in applications such as a box section with a spacer,” Ellis says. “A spacer with a thread inside saves a weld and enables the use of shorter bolts.” Profil developed a range of aluminium fasteners some years ago, but Ellis points out that the cost of such parts could only be amortized in higher-cost vehicles.

On the other hand, the company’s RND flange nut is described as a ‘premium’ product. “We have taken the flange as the reinforcement, which does away with the need for additional reinforcement. The next step is to reduce the weight of the fastener itself, and we think the market might go for high-strength and ultra-high strength steels.” The fixing system is widely used in chassis components and in applications such as seat attachments. The screws have antiloosen and anti-rotation webs, which are clinched from the bottom side.

“We’ve had no issue with loosening over several decades,” explains Ellis. “When the fasteners are attached to the bodyin- white before painting, there are no corrosion problems at all. The current Range Rover uses Profil for its bonnet hinge fixing and smaller fixings in the front.” Although the basic idea of Profil – screwing and riveting together – is longestablished, the company’s emphasis on development of new materials and lightweighting, as well as the relative simplicity of the method, has helped it to retain and even grow its market share.

Room for all

The decision that must be made by OEMs is to either adapt existing joining techniques to work with new materials, or to switch to new methodologies designed to accommodate these advances. New welding techniques, the resilience of clinching and hemming and the inherent ability of fixings to join metals of two different types will keep all these alternatives in the joining mix, but demand for adhesives is still likely to grow, due to the increased interest in vehicle lightweighting and the join strength that can be achieved.

This, though, will be balanced in the medium term by extended curing times and the capital expenditure required to adopt the joining system.