Resistance spot welding has established itself across a wide range of industries as a cost-effective method for joining steel sheets. In modern vehicle manufacturing in particular, high-strength steel sheets of varying strengths and quality need to be joined. This type of work is too demanding for conventional spot welding. There is also a growing need for more effective joining systems for aluminium sheet. For these types of difficult applications, manufacturers often have to turn to cost-intensive mechanical joining processes, such as punch rivets or screws. An optimised resistance spot welding process for complex, alternative applications is required. DeltaSpot® has successfully met this challenge.
The defining feature of DeltaSpot® is the robot welding gun with running process tape that runs between the electrodes and the sheets being joined. The continuous forwards movement of the process tape results in an uninterrupted process producing constant quality over a number of shifts. This results in precision in the welding process, accurate quality control and high electrode service life. Regular cap cutting of electrodes is no longer necessary. The process tape means that electrodes are effectively protected against wear and deposits from sheet coatings. This means that constant quality and reproducible welding points are assured over multiple production shifts.
Visualisation of the DeltaSpot® process: The process tape runs between the sheets and the electrodes ensuring that the electrodes are effectively protected.
The process tape transfers the welding current and, at the same time, protects the contact surfaces of the electrodes from contamination by zinc, aluminum or organic residues. This protection results in a significantly increased service life for electrodes. Tests with aluminum sheets (AlMg3 alloy) showed an extremely long service life of approx. 30,000 welding points. The process tape provides indirect sheet contact producing a largely spatter-free welding result. It eliminates the otherwise unavoidable rework necessary to meet new quality standards. The coating of the process tape has proved to be particularly advantageous for aluminum sheets. Optimised contact with the aluminum material prevents spattering and the associated damage to components. The process tape needs to be replaced infrequently and this takes little time and effort. In normal use, the process tape produces 7,000 welding points. If every segment of the welding tape is used two or three times, the service life can be extended accordingly.
The process offers additional features to the physical properties already described. These includes the servo-electric holder drive that saves on expensive compressed air supplies, and the integrated software that provides central configuration of the individual welding guns. The comprehensive monitoring and diagnostics system is also integral. When the welding process is complete, the tape contact surfaces used provide information on the work process. Image recording systems detect this unique fingerprint of the welding point. Suitable analysis systems can analyse this data. The software has a central role to play in this. The quality assurance system that can be integrated as an option enables complete recording of process data and thus reduces subsequent time-consuming quality control.
DeltaSpot® is primarily suitable for steel and aluminum sheets. It can also be used in combination with commonly used anti-corrosion coatings and for black/white joints. The latter are unalloyed or low-alloy steel joints with non-rusting, high-alloy steels. Different material combinations and thicknesses are also possible. Examples include deep-drawn steels in combination with chrome-nickel steels or Usibor, as well as material combinations with coated steel sheets and aluminum. Development focused mainly on the applicability of the process for high-alloy steel joints as well as magnesium joints. These materials provide the greatest potential in modern lightweight construction. Two-sheet or multi-sheet joining of practically all the above material combinations can also be achieved. The same welding gun can be used to weld different sheet thicknesses and material combinations simply by changing the process tape.
The high efficiency level is produced by additional process heat that arises as a result of the internal resistance and the contact resistances of the process tape. For traditional resistance spot welding of 2 x 1.0 mm AlMg3 sheets, a current of 35,000 A to 40,000 A is required. The process tape reduces the current to 16,000 A. The low welding current and the individual adaptation of the process tape to the welding process permit controlled heat application and allow the position and shape of the welding point to be controlled. This is described in further detail below.
Three-sheet joints (two thick sheets and one thin sheet) are problematic for traditional spot welding. The welding point forms in the area of the thicker sheet and does not cover the thin sheet sufficiently. To compensate for this effect, the additional heat applied by the process tape ensures targeted control of the welding point depth. This means it is possible to compensate for the reduced amount of heat in the area of the thin sheet by using a process tape with greater resistance. In this way, the welding point is displaced sufficiently towards the thin sheet. At the same time, the shape of the welding point is symmetrical and will now show increased volume in the area of the thinner sheet.
With aluminum, there is the problem of high electrical and thermal conductivity and a correspondingly low electrical resistance. The accompanying limited heat generation can also be compensated for by selecting the correct process tape. In contrast to traditional spot welding of aluminum that until now has been virtually impossible to achieve, the new resistance spot welding process can successfully weld aluminum for the first time.
To summarise, the spot welding with tape process is expected to find a wide range of applications in joining technology. Last but not least, the DeltaSpot® process promises to offer new perspectives for innovative working and opportunities for cost saving particularly in vehicle manufacturing.