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ROBOTIC RESOURCES

Innovative 'DeltaSpot' Spot Welding Process

Fronius

Innovative process for resistance spot-welding of aluminum sheets proves its worth among users
 
Although resistance spot-welding has a decade-long track record in the joining of steel sheet, when used on aluminum it always met with tight constraints in terms of productivity and feasibility. For in the conventional resistance spot-welding process, the specific properties of this lightweight construction material either detract from process capability, or cause unjustifiably high costs due to the extreme wear-and-tear on the electrodes and the great amount of electrode-dressing needed. With the innovation of a continuous process tape, development engineers at Fronius found a solution that does away with these obstacles. Named ‘DeltaSpot’, this special spot-welding process also boasts a number of other advantages: Because the heat balance in the workpiece can be selectively influenced by contact with the process tape, it is possible to achieve top-quality joins between sheets made of different materials, even when these are of different (and indeed only minimal) thicknesses. First trialed in the trendsetting automotive manufacturing sector, this innovative process from Fronius has since been put to the test in a range of different practical applications, with positive results. This article explains the technological background and presents several exemplary applications.
 
Aluminium spot-welding: preconditions and proposed solution
Used instead of steel in e.g. autobody construction, aluminum is a lightweight material that is central to today’s strategy of significantly decreasing vehicle weight. This is a response both to competitive pressures and to the environmental imperative to conserve resources and cut emissions of climate-harming carbon dioxide.
For steel sheets, the high reliability and productivity of resistance spot-welding make it one of the most commonly used thermal joining processes. However, although it is such a well-proven process for joining steel sheets, anyone wishing to use resistance spot welding on aluminum used to have to tackle two main areas of difficulty: First, the relatively higher electrical conductivity of aluminum than steel, requiring a significantly higher amperage to obtain a comparable result. Secondly, another major difference from steel has to do with aluminum’s oxidation behavior: at welding temperatures, aluminum forms a high-melting oxide skin under the influence of oxygen, and it is this oxide skin which constitutes the electrically and thermally insulating material as such. The force acting via the electrodes breaks open the brittle aluminum oxide and enables the current to flow. The current flows through the first cracks in a highly concentrated manner and so is exceedingly high in these places, where it briefly results in extremely high material resistance with correspondingly high temperatures. The varying thickness of the naturally occurring oxide layer, with the differing contact resistances that result, together contribute to the process instability. A further factor is the tendency of aluminum to be ‘picked up’ by (i.e. alloy to) the base metal of the electrode, causing yet more changes in the current-transfer points.

Frequent dressing and changing of the electrodes interrupts production and pushes up the operating costs. Not even continuous readjustment of the control system can compensate for the reduction in the quality of the products. Welding spatter soils both the weldments and the welding cells, necessitating costly post-weld machining and clean-up work. In practice, all these peculiarities of aluminum have given resistance spot welding of this material a bad name among welding professionals, with the result that other materials-joining methods have tended to be preferred.
 
The process tape: principle of operation, and details
With its process tape, the innovative DeltaSpot process opens up some new and interesting perspectives. The principle is based on a process tape running between the electrode and the base metal, in the same rhythm as the spot-welding operations. Unlike fixed electrodes which always present the same contact surfaces, in this case the aluminum alloys to the process tape, which is then wound on a few mm after each spot-weld. In other words, the ‘used’ length of process tape is moved out of the contact zone every time. This means that for every weld-spot, exactly the same conditions obtain as for the previous one and the next one.
 
The process tapes perform several very different functions. First, they prevent any direct contact between the electrode and the workpiece, protecting the electrodes from soiling or other influences emanating from the surface of the workpiece. This stabilizes the weld process and greatly prolongs the electrode service life. Secondly, they improve the contact situation: for every single spot, the process tape creates a new electrode contact surface to the workpiece. This prevents surface spatter and widens the process window.
Compared to a conventional sequence of 20 to 30 spot-welded joints (of not very satisfactory quality), DeltaSpot achieves around 10,000 top-quality spots with just one process tape. This corresponds to a 70 m long tape and a 7 mm interval between weld-spots. When the tape is used up, it is simply replaced by a new one for the next series.
 
Influence on the heat balance in the join
A third and very significant advantage of the process tape is the scope that it gives for directly and selectively influencing the heat balance in the workpiece. This is because the existing contact resistances and material resistances are now augmented by the material- and contact resistance of the process tape. When the current is switched on, this resistance generates additional heat which ‘shields’ the join against electrode cooling, thereby increasing heat-generation in the join. This acts as additional heat input into the base metal from the outside.
The result is more heat in the workpiece at the same time as lower electrical input power. By using process tapes made of different materials and with different coatings, the user can modify and ‘fine tune’ the overall balance and distribution of heat in the workpiece.
 
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 multisheet joints. The extra resistances bring with them a further advantage: by focusing the added heat input onto the point being joined, they reduce the shunt effect. Because of this, unwanted current transfer at other positions on the workpieces hardly ever occurs. This is especially relevant for light-gauge sheets.

The standard electrode has a convex shape. Thanks to the slightly elastic process tapes, this results in an optimally shaped circular contact area. However, a concave electrode with a ring-shaped contact area can also bring a number of alternative advantages, leading to higher current density on the ‘ring’ compared to a flat electrode and resulting in higher process reliability. Another benefit of this ‘Expo electrode’ is that the surfaces of aluminum sheets are barely marked by any ‘stamping’ indentations at the weld-spots.

Unlike in the conventional process, in which the weld-pool almost only ever takes a nugget shape, DeltaSpot will also create a cylindrical weld shape if the boundary conditions allow. This is mainly due to the insulation against electrode cooling, and to the extra thermal input.
 
Examples from practice
Some noted European and Asian automobile and engineering firms have tested DeltaSpot welding systems and decided to introduce them for series production. One example is the Georg Fischer company from Austria, which is spot-welding the aluminum die-cast doors for the new Porsche Panamera. Here, a 1.5 mm thick stiffening plate has to be spot-welded onto the 2 mm thick aluminum die-cast door-frame.
 
On Hyundai’s new luxury models Equus and Genesis, subcontractors Sungwoo Hightech are using DeltaSpot to manufacture the engine bonnet (hood) from aluminum. The critical criteria for Hyundai were the high quality, process reliability and enhanced corrosion resistance which it made possible, and its cost advantage over punch riveting.
 
A pilot application in the field of railed-vehicle construction is underway with the Oslo Metro, which now has a 5-year ‘track’ record with DeltaSpot-welded electronics control cubicles on its trains. DeltaSpot welds around 120 spots, of diam. 7 mm, on each set of four control-cubicle doors. These spot-welds join two 2 mm thick AlMg3 sheets. The contract from the manufacturer of the trains, Siemens, is being fulfilled by the Austrian system partner Gebr. Bach with great success in terms of both quality, cost-effectiveness and technical proficiency. The flawless technical quality of this work is confirmed by the testers from TÜV Bavaria SZA, who have inspected the intersheet joints in accordance with the relevant railway standards.
 
DeltaSpot has also established itself in the general mechanical engineering and apparatus construction sectors. The Irish company C&F Tooling manufactures aluminum and steel housings for truckborne refrigerating sets. Steel and aluminum housings have to be welded alternately, with identical electrode and process-tape configurations. One steel and one aluminum housing are then installed for each refrigerating set.
 
Rittal, a leading German manufacturer of control-cubicle systems, insists on uncompromisingly cost-efficient high-tech production. Following intensive testing and trials, the company integrated a DeltaSpot installation into its series production operations early in 2011. The machine welds hinges and guidepieces to the doors of aluminum control cubicles.
 
Important technical details and data on these applications will be found in the table below!

Conclusion and outlook
DeltaSpot means that a technically and economically attractive spot-welding process is now available for aluminum. Compared to conventional resistance spot welding, the differentiated process parameters open up new possibilities with regard to different materials, workpiece thicknesses and multisheet joints. From the point of view of lightweight construction, the system’s ability to reliably join aluminum sheets, and aluminum to steel sheets, makes it highly relevant. Its greatest advantage is that it minimizes the costs for consumables.
 








Technical details

Georg Fischer Automotive Products
Aluminium die-cast doors for Porsche Panamera


Sungwoo Hightech for Hyundai
‘Equus’ engine bonnet

Gebr. Bach GmbH
Doors (4 per cubicle) of electronics cubicles for metro trains in Oslo


C&F Tooling
Aluminium and steel housings for truckborne refrigerating sets

Rittal GmbH
System control cubicles

Material combinations
 

Die-cast Al
with Al stiffening plate

Al 5000
with Al 6000

AlMg3
 

Mild steel,
hot-dip galvanised;
AlMg3,



AlMg3

Thickness of material in mm

2 + 1.5

0.85 + 8.85

2

St: 1+1; 1+2.25
Al: 1+1

1.5+1;5;
2+2; 1.5+2; 2+2.5

Number of weld-spots per workpiece

16

19

120

Steel component: 35
Al component: 114

 ≤ 30

Number of weld-spots with one electrode and one process tape, approx.:

5500

10,000

7000

10,000

7000

 

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