Every increasing complexity of each new vehicle unveiled around the world shows a strong trend which is not going to go away any time soon. From the humble tyre through to the paint system technology is being developed at a pace far faster than ever before.
There are some awkward contrasts however. Just as not only are manufacturers creating more product lines than ever before to reach those all important market niches, the remains of the Financial Industry gift that just keeps giving (the 2007 / 2008 financial collapse which affected big parts of Europe and the USA) caused an important shift in policy right across the automotive manufacturing sector.
Spare parts are frequently limited to known ‘fast moving’ components typically used in routine servicing, in order to reduce the number of parts held by dealership as well as the national hub(s). Quite simply vehicle manufacturers were forced to do the unthinkable and reduce stock.
The result? Waiting a few more days for parts to arrive from a national warehouse operation through to waiting for a few weeks for the next batch of imported spare parts to arrive at the local dealership. Even if the vehicle is made / built inside the same country, the spare parts are treated exactly the same way to reduce the amount of stock held locally or nationally for the after-market. Not one automotive consumer market around the world is exempt from this policy.
The one automotive sector most affected by this policy shift is the vehicle collision repair business.
Let’s look into a new trend which is gradually gaining traction in mass market vehicles – putting together panels not with MIG / MAG welding, spot welds, or indeed any type of weld. The rapidly improving technology of structural adhesives along with an entire sub-culture of rivet technology its advancing. Why?
Quite simply the modern mass market steel alloy based body shell can have an array of steels and material thicknesses which make successful welding inside the automotive manufacturing plant a bit challenging. That is a challenge in spite of much higher tip pressures and energy levels that are available from typical aftermarket spot welders, for example.
Step one for the automotive manufacturers is to reduce the environmental impact of the manufacturing plant – which means trying to use fewer spot welds, for example. To recover the joint strength but reduce the number of spot welds a bonding agent is used – applied before the panels are assembled together and then the joint is spot welded. Locally around the spot weld the bonding agent will fail, but the area between the spot welds – which would otherwise not be attached between adjacent panels – is now a fully functional joint. The spot welds hold the panels in position whilst the bonding agent cures.
Step two is the use of bonded joints held together by rivets during the curing process. One of the first production vehicles to feature this technology way back in 1997 was the Lotus Elise, but more famous (and slightly more common) the technique has been used on three generations of Audi A8, the Audi A2, half of the soon to be replaced Audi TT, Mercedes–Benz AMG SLS, two generations of Chevrolet Corvette chassis, two generations of Jaguar XJ, the latest Porsche 911 (type 991), the latest Boxster/Cayman (type 981), the latest Range Rover (L405) as well as Range Rover Sport (L494).
Indeed there are many more vehicles which have used a hybrid of steel alloys and aluminium alloys (such as the Mercedes–Benz C class W205. From 2015 the highest volume production vehicle line, the Ford F-150, retains a steel alloy chassis but otherwise all the body panels are made from aluminium (or was that aluminum?). Whilst this is big news for the automotive industry, the aerospace sector has had fully bonded structures flying round the world for decades.
Bonded versus welded joints
The main advantages of ‘cold’ joining processes (‘bonding’) over localised fused joints (‘welding’) are:
• More efficient load distribution in the joint compared with welding.
• No change in the panel properties around the joint zone due to heat during the joint formation.
• Ability to join materials which do not ordinarily weld.
• Reduced strip of vulnerable components in the area due to the lack of sparks/grinds.
So, why use rivets? The bonding agents needs time and stability between adjacent panels to build strength. The rivets pin the shell together to allow the bonding agent to fully cure while the assembly is sealed, painted and dressed into a complete vehicle during the manufacturing process.
The body assembly using bonded joints cannot be completed at anything like the same pace as for a welded structure without the use of rivets. Just as with welding processes, access to both sides of the panel joint is not always possible and has produced an ever-increasing array of rivets – some of which do not even use rivets but mechanically lock panels together.
The primary task of the rivet is to clamp panels together along a joint flange, and as such perform a relatively minor role in the final joint strength. Vehicle manufacturers who blazed the trail for aluminium intensive mass production vehicle structures have and continue to share technology information with each other along with suppliers of sheet metal/bonding agents/rivets.
Some pretty impressive things have been developed to improve repairability as a result, but the bottom line is there are too many rivet variations. Given the present and rapidly increasing variety of rivets, this product complexity is hard to understand apart from a relatively narrow manufacturing perspective.
Rivets, rivets everywhere
Vehicle manufacturer support of rivet identification via dealer outlets in the aftermarket has remained poor to average. Add to that the number of different types of rivet forming tools required, and then add to that the limited number of product approvals by each vehicle manufacturer together with a relatively small (but increasing) number of applications.
Check this out. The Range Rover twins used the platform from the Jaguar XJ (X351), so the joining technology is a mix of solutions used for the past 11 years along with some new features. Compared with the previous Range Rover’s spot welded steel alloy body, the number of joints has gone down from more than 6000 spot welds to 3886 rivets, with a total energy saving in the weld shop of 50%. Each body shell has 161 m of structural adhesive applied to it without a single weld.
The JLR body structure uses 10 different diameter/length/hardness combinations of self piercing rivet, and a further seven different types of rivet with disposable stem. The production plant uses more than 250 robots, each of which can handle more than one type of rivet to build shells. One can see from these details the process is fully optimised for manufacture.
Repair methods and training offered by vehicle manufacturers for bonded structural repair have been detailed as well as thorough. However, the picture is not complete since the methods refer to a rivet type – but does the local dealer know what that rivet part number is?
The main issue is a proliferation of dedicated tools that exist mainly because of the proliferation of rivet types and limited vehicle manufacturer approvals for each rivet tool machine. For most common repairs (body side, front and rear rails) there should only be two to three different types of rivet. Indeed this aftermarket issue will become the main focus as future vehicle generations adopt bonded structural joints.
The trend is firmly underway with the increased steel content in the 2015 TT (all new body structure that happens to use aluminium and steel alloys), rapid expansion of the JLR aluminium intensive vehicle lines and the hybrid structures on offer in the Mercedes–Benz W205 C class as well as the W222 S class. Indeed the biggest volume player so far – the new 2015 Ford F-150, is set to bring many more repairers face to face with structural bonding.
Pure bonded structures. But whilst the bonding agent technology already exists and is even used in things like buses let alone aircraft, it would require a complete re-think about how vehicle bodies are manufactured. For the repairer it would be almost the same as welding in new panels – except paying close attention to degreasing/surface matching/temperature control/humidity control to ensure maximum bond strength once cured.
So, what we have here is a evolving mass production technology which will purge less successful joint types in coming product generations. Until then, an element of chaos reigns in the aftermarket.
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Auto Industry Consulting is an independent provider of technical information to the global collision repair industry. Products include EziMethods, our online collision repair methods system and Auto Industry Insider, our collision repair industry technical information website.
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