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ROBOTIC RESOURCES

Resistance spot-welding of aluminium moves from test phase to production-line implementation

Fronius

Georg Fischer Automotive uses innovative resistance spot-welding process for vehicle door-frame joins

The automobile industry supplier Georg Fischer Automotive has deployed an innovative version of the highly productive resistance spot-welding process to weld joins on the door-frames of the new Porsche Panamera. Delta Spot has allowed the car-making specialists at GFA’s Austrian plant in Altenmarkt to overcome the cost/efficiency barriers and technical quality limitations that had always held back the use of conventional spot-welding for joining aluminium. At the heart of the process is the Fronius solution based on spooling process tapes. The users at automotive vendor Georg Fischer report on the objectives, the history, the special features and the benefits of their project, and on the prospects it holds out.

Background to the decision
A 2 mm thick aluminium stiffening plate has to be joined onto the approx. 3 mm thick frames of the four die-cast aluminium doors of the Porsche Panamera. This stiffening plate is also made of an aluminium alloy. Alois Edtbauer, a toolmaker and foundryman who now works as a specialist buyer for foundry equipment and materials at Georg Fischer’s Austrian plant in Altenmarkt, explains the main outlines of the project: “In order to explore our production-engineering options, we looked at a number of joining processes to determine their suitability and cost efficiency”, he reports. Wolfgang Hintsteiner, the engineer in charge of coatings who is also responsible for the Porsche Panamera doors, adds: “The choice boiled down to conventional resistance spot welding, friction-stir welding, clinching, punch riveting with solid rivets and an adhesive bonding technique combined with a spot-welding process. Then we heard about DeltaSpot, a special resistance spot-welding process that was said to be particularly good at joining aluminium. We got more information about it from the developers at Fronius, and included DeltaSpot in our selection process as well.”

After the first test results, conventional resistance spot welding, riveting, bonding and clinching were all ruled out for production-economic or process-engineering reasons, leaving only two remaining alternatives – friction-stir welding, and resistance spot welding with DeltaSpot. The defining feature of this process is a spooling ‘process tape’ that prevents direct contact between the electrode and the workpiece, but mediates this contact indirectly instead (see also Box 2).

Boundary conditions
The four different doors of each set have to have inside stiffeners spot-welded to them. The doors are made of die-cast aluminium, each weigh 4.5 kg and are furnished with a titanium-zirconium (TiZrSiO4) anti-oxidation layer. “On the weldment, there is a main seal between the door and the frame at the point that has to be joined. This means that the welding carried out here must be as spatter-free as possible. The thermally induced distortion on the workpiece has to be kept within tight limits, and we have to be able to cancel it out with subsequent straightening”, explains Wolfgang Hintsteiner. “These are all reasons why conventional resistance spot welding is unsuitable”, he continues. “Firstly, because it causes too much spattering, and secondly, because its uncontrollable and very intense point-focused thermal input risks badly distorting the metal sheet around the weld-spots. Unlike large-area shape-distortion, this is a defect that cannot subsequently be corrected, which means that the whole door, with its unsightly undulating surface, is then only fit for the scrap bin.”

Because of the extremely high dimensional accuracy required on the outside flange, clinching and punch riveting were not viable options either. The main exclusion criterion here was the shape-distorting action of mechanical forces. Punching and riveting at such close-spaced intervals in the workpiece would cause kinks and undulations which would mean an unacceptable degree of surface unevenness. Adhesive bonding, and combinations of bonding with other joining processes, were also disqualified. Although adhesive-bonded joins are made without impairing the shape of the workpiece, they cannot be subjected to loading while they are curing, and the adhesive would cause unacceptable surface contamination.

Friction-stir welding also failed to make the grade, remembers Wolfgang Hintsteiner. “For our application, it was simply inferior to joining using DeltaSpot, from the production-engineering point of view. The reason is that in friction-stir welding, you have to allow for the overall wall thickness of the parts. Because of the casting tolerances, we would have had to ascertain and key in the exact thickness of the parts to be joined before every single welding operation, which would hold up the production process for too long.”

Benefits and usefulness
DeltaSpot has now proved its merits in everyday production-line practice since 2008. “The machine runs with complete process-reliability”, reports a very satisfied Alois Edtbauer.

Wolfgang Hintsteiner lists some features of the application that are particularly relevant in terms of user benefit: “The process tape lets us produce a uniform, exactly replicable spot that is exactly 5 mm in diameter, and with 16 spot-welded joins on every workpiece. We weld one of these doors in an approximately 100-second cycle, after which we don’t need to do any finishing-work on the surface. As far as the appearance is concerned, we’re getting a very clean weld-spot. It takes less than 15 minutes to change the process tape, and this only has to be done after about 5000 weld-spots, which means that we can weld around 300 Porsche Panamera doors without interruption. Every day, we produce all four doors for 185 Panameras.”

The electrodes only need changing once per tape. By way of comparison: in suitable conditions, users would only be able to weld 20 spots using conventional resistance spot-welding. In effect, this would mean cleaning the electrodes once for nearly every door!

Adds Alois Edtbauer: “We work round the clock, that’s 15 to 16 shifts per week. The DeltaSpot process creates a strong join between the two components without any difficulty – it only exerts minimal mechanical force and doesn’t need any extra mechanical materials. In our area of use, we’ve found it to be very cost-effective.”

Client requests, product prospects
Based on the goals that have been reached and on their positive experience of the system so far, Wolfgang Hintsteiner has come up with some more features he’d like to see implemented: “Being able to keep a sort of log of all the spots that are welded, identifying which door each spot was welded on, would be helpful for our quality documentation.”

Both experts are very positive about the prospects for DeltaSpot. “For applications like ours, with weldable castings, a defined surface, an anti-corrosive coating and adequate accessibility, DeltaSpot is the system of choice”, explains Wolfgang Hintsteiner. “Now we can offer our clients an alternative production step for applications like these, and this gives us a competitive edge.”

Georg Fischer Automotive
The history of Georg Fischer (GF) goes all the way back to 1802. Right from the firm’s earliest days, metal castings were one of its core competences. This global-playing enterprise has long been known as a pioneering user of innovative technologies.

With 12 production locations and a worldwide workforce of 5500, GF Automotive posted 2010 revenues of 1.12 bn euros. All the way from the first idea for a product to realising it in practice and then supporting its series production, GF Automotive partners the automobile industry and its suppliers.

The Altenmarkt plant in Austria has been part of Georg Fischer since 1999 and specialises in structural components such as strut brackets and doors for the ‘body-in-white’ stage. Around 600 people are employed in Altenmarkt, benefiting the company with their R&D expertise and widely acclaimed know-how in pressure die-casting, especially of aluminium and magnesium.

Resistance spot-welding with DeltaSpot – functional principle and practical utility
A process tape is spooled between the electrode and the workpiece, in the same rhythm as the spot-welding operations. Instead of alloying onto a fixed electrode, the aluminium now alloys onto this tape, which is spooled forward after every spot-weld so that the ‘used’ length of process tape is moved out of the contact zone each time. This means that for every single weld-spot, exactly the same defined conditions apply.

The process tapes prevent any direct contact between the electrode and the workpiece, protecting both of them from soiling, alloying or other workpiece-induced influences. This stabilises the weld process and greatly prolongs the electrode service life. They also improve the contact situation and avoid damaging the surface of the workpiece. The process tape helps to prevent surface spatter, and widens the process window.

A third and very significant advantage of DeltaSpot is the scope that it gives for systematically influencing the heat balance in the workpiece. The result is more heat in the workpiece at the same time as lower electrical input power.

The process tape can be used as a flexible tool for creating optimum conditions in each case, e.g. for joining materials of different compositions and thicknesses, or for multisheet joints.

Principal technical data:
Pincer welding gun
Design type X 350
Process tape, length max. 70 m
Weight 153 kg
Electrode diameter 16 mm
Welding amperage used in application  22.5 kA
‘Trafofluid’ transformer 250 kVA
Bosch PSI 6200 inverter 1200 A

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