Welding process delivers Bentley quality

The cold metal transfer welding process delivers excellent results on the bodywork for the Phaeton and Bentley Continental GT limousines.

The bodywork for the Phaeton and Bentley Continental GT limousines is built at Volkswagen Sachsen's Mosel facility in the German state of Saxony.

The purchasers of models such as these are discerning clients who expect nothing less than perfection in terms of travel comfort and safety, married to classic handcrafted quality: In other words, the bodywork finish has to make a visual impression that underlines the car's sheer exclusivity.

This explains why this craft manufacturing operation is characterised by fairly small bodywork production runs, with high machine availability and a relatively low degree of mechanisation.

The vehiclebuilder craftsmen at Volkswagen in Saxony have to juggle a number of tasks relating to quality, deadlines and costs.

A key factor helping them to do this is the Fronius-developed CMT (cold metal transfer) welding process.

CMT's reduced heat input and absolute freedom from spatter deliver excellent results, as well as cutting down on the amount of work needed for clamping devices and post-weld machining.

The extremely narrow time window between the investment decision and the planned start of series production presented the VW and Fronius professionals with a challenge to which they rose with bravura.

The "leitmotif" underlying the making of exclusive automobiles is the discerning client's insistence on prestige.

This rules out a high degree of mechanisation.

The bodies of the Bentley and the Phaeton are joined together from high-strength galvanised steel sheet, with several mounted parts being made of plastic and aluminium.

A mere 25% of the manufacturing processes can be automated (by way of comparison: the Lupo and Polo production lines are 90% automated).

A key area of work is the seams and joins.

These tie up a high proportion of manpower and logistical resources.

Engineer Andreas Kruger, 38, was Head of Autobody Construction in Mosel for the top-of-the range D-Class during the relevant period from 2004 to 2005.

He compares the job with controlling a "magic triangle" of quality, cost and time management.

Where the forces acting on the vehicle superstructure intersect - and thus at the focal point of the problem solving effort - come the load-bearing constructional components: the undercarriage, the rear-wall cross member, the drain-channel in the engine-bay bulkhead and the A-pillar.

One of the most important of these points of intersection, or focal points, is the C-pillar, for here the quality of the joins is of twofold significance.

It has to be just right in terms of both stability and appearance.

To begin with, in an accident situation it is this strut which is decisive for the safety, or otherwise, of the vehicle.

Secondly, the weld-zone is exposed to view here, with a lustrous glow shining in the car's reflective paintwork.

Whatever welding system is used here simply has to be one that can fulfil these two sets of requirements.

All three of the parts to be joined in the C-pillar are of differing sheet thicknesses.

The deformation and sinkage parallel to the seam which often resulted from the high thermal input taking place in MIG/MAG welding had to be given suitable post-weld machining.

This job, and the job of completely removing all the welding spatter, are an area of competence for skilled metalworkers.

The production engineers realised that these laborious operations on the C-pillar were accounting for an undue share of the time being consumed per car-body and employee - and were eating up the time of experts whose high-value manpower was urgently needed in other departments.

This was the field, then, in which Kruger's team defined its goal: Realising potential savings.

There are 4.3m of laser-welded seams and 18m of fusion-welded ones inside the finished bodywork of a Bentley, and not significantly fewer in the Phaeton.

Welding thus accounts for a correspondingly high proportion of the value added.

All the more reason for the technicians at VW's plant in Saxony to be exceedingly critical in their choice of welding-system partner.

They already had good client-vendor relationships with Fronius, the European market number one and worldwide technological leader.

Jens Rickmann is the Fronius point-man responsible for taking care of clients Volkswagen at their Mosel facility.

At the beginning of the second half of 2004, Rickmann approached Kruger to alert him to a possible solution to the labour-intensity problem on the C-pillar.

The approach he had in mind was based on the CMT process and would open up scope for working with lower heat input, and without spattering.

These factors would make it possible to achieve results that were qualitatively superior to those obtained with conventional MIG/MAG welding - quite apart from all the cost-cutting potential.

Kruger got to know this innovative process for himself at the Fronius expo stand at the EuroBlech fair in Hanover, Germany.

One thing was clear to him there and then: "It's not a matter of whether we want CMT or not, but of when and how soon we can introduce it".

Before he even left the Hanover expo, Kruger gave his welding-system partner Fronius a clear outline of what he needed.

Fronius R and D Head Heinz Hackl and Jurgen Bruckner of Fronius got to work straight away on a concept for implementation, while local customer adviser Jens Rickmann kept in constant direct touch with Kruger as the link-man at the client.

In the course of the first meetings, Kruger specified his exact objectives and gave Rickmann key phase-by-phase time-goal data.

The number of employees per day was to be reduced - corresponding to a 2-hour reduction in working time per car-body.

The several-day-long test phase before the start of series production could only take place during a holiday period, so the decision was made to use Weeks 52 and 53 of 2004.

By waiting until the next time-window at Easter 2005, he and his colleagues would have minimised the risk, but lost an opportunity to jump a step ahead of the competition.

In this situation, the technical, leadership and teamworking qualities of all those involved came very much to the fore.

It was not long until the definitive plan had been agreed: A test phase at Fronius using Fronius demo systems - Testing with demo systems in VW's central R and D Depatment in Wolfsburg - Installation in Mosel - Adjusting and optimising the CMT seam - Series deployment for the first and second stages of the workflow sequence.

CMT has now been in service for autobody construction at Mosel since 3rd January 2005.

"The bottom line is that it's been a great success", says Kruger.

Despite the thorough conceptual design, the client still felt bound to allow for a certain residual risk that the new installation might have to be dismantled and removed after all.

"Of course we all heaved a sigh of relief when the report came in about the satisfactory test results", recalls Kruger.

"Experience curve" is the graphic term he uses to describe what they went through in these suspense-filled weeks.

As he points out, all the hard work paid off in the end, in that "We won the race - hands-down".

The objective has been more than met: Skilled employees are now available for redeployment to other challenging tasks, sooner than expected.

Before moving on to talk about the technical details of the CMT process, the Head of D-Class Autobody Construction at the time describes the practical-benefit side of CMT once again.

The decision makers reviewed several different offers, of course, but at the end of the day, says Kruger, the sophisticated and transparently presented technology from Fronius was the most convincing solution.

Regularly collected test data give further proof of how right the choice was: according to Kruger, the strength of the CMT-brazed seam is excellent - even higher than that of the base metal.

One car-body per month, he says, is subjected to destructive testing, with random samples being made by way of micrographs, and other locations being checked by tensile testing and visual and ultrasound examination.

"The main advantage is the 20 to 30% reduction in the heat input".

"Given the relatively thin sheets we're dealing with here, the principal effect of this is to reduce sheet sinkage by half", explains Kruger.

Being able to go gently on the metal sheet in this way is well worthwhile, as it means that significantly less time has to be spent on the labour-intensive sequence of raising and smoothing-operations on the C-pillar.

Now the metalworkers only need to raise the sheets a maximum of 0.1mm.

The reduction in heat input is due to the repeated interruptions in the arc caused by controlled short circuiting.

"CMT is more than just low-spatter, it really is zero-spatter", says Kruger, moving to the next item on his list of plus-points.

Because the wire electrode is retracted by digital control at a frequency of up to 70Hz, what happens is that 70 times per second, exactly one droplet, and one droplet only, of the melting electrode ends up in the seam being welded.

Cutting to zero the amount of reworking needed for spatter-removal is highly relevant when it comes to luxury class car-bodies: the dust that this reworking would cause would settle inside the complicatedly shaped bodywork, acting like poison in the cleanroom conditions of the paintshop.

"Thirdly, CMT is more accurate, and fourthly, it is faster than the commonly used MIG/MAG process".

Kruger sums it up like this: "I don't see normal GMA welding as having any technical advantage that the CMT process can't offer as well".

"Also, CMT does away with a lot of clamping and logistics work because there is so much less heat distortion".

Kruger reckons that the investment in this new equipment has definitely paid off, and that it will not be long before they introduce a second, complete CMT seam.

"We're the first automobile manufacturer to use the process as standard", he explains, "and that gives us a prototype of how the process works that we can apply to other models as well".

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