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Process Tape for Better Spot Welding by Heinz Hackl, Fronius Fronius is an active player in solar electronics, battery charging systems and welding technology. Its Welding Technology Division recently unveiled a new resistance spot welding process that offers high stability and reproducibility of results. Its special feature is a “process tape” that is spooled around the electrodes.
Limitations of Conventional Spot Welding
Resistance spot welding has found broad acceptance in industry as a low-cost method for joining steel sheets. In this process, a pincer welding gun holds two or more sheets together and a brief high-voltage current pulse is applied via the electrodes, resulting in fusion of the sheets at a single point. Spot welding is in particularly widespread use in the field of vehicle body construction. The process has long been regarded as a “traditional” process with little
or no further potential for improvement. However, its limitations are being made increasingly apparent by the types of joints required in modern vehicle manufacturing – between steel sheets of different thicknesses and quality grades that often include highstrength steel. There is also a significant trend towards joining aluminium. Owing to the high conductivity of aluminium, it requires a current that is around three times as high as that required for steel. For this reason and because of the copper/aluminium contact, electrode life is currently very short, which is the reason why more demanding applications often have to resort to cost-intensive mechanical joining processes such as punch riveting or screwing. This explains the interest shown by industry professionals in an optimised resistance spot welding process for alternative applications.
The New Spot Welding Process
Figure 1: The process tape runs between the sheets to be joined and the electrodes, ensuring highly effective protection for the electrodes.
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One such spot welding process that has been optimised with regard to the above requirements is Delta Spot. While still exploiting the basic principle of resistance spot welding, this process aims to be an economically viable alternative to the – otherwise unavoidable – mechanical joining
processes. Its characteristic feature is a robot pincer gun with a spooling process tape that runs between the electrodes and the sheets to be joined, Figure 1 and Figure 2. The continuous onward motion of the process tape leads to an ongoing, constant-quality process throughout several shifts. The result is welding process precision and long electrode life, with no need for the frequent remilling of the electrode caps that is usual in today’s systems. The process tape protects the electrodes against wear and against deposits from the coatings on metal sheets, thus ensuring constant quality and reproducible spot welds for several production shifts in succession. As well as providing highly effective protection to the electrodes, the process tape also helps to achieve higher efficiency. The reason for this reduced power requirement is the additional heat input generated by the contact resistance of the process tapes. This contact resistance can be adjusted by choosing suitable process tapes. Depending on the welding task to be performed, there is thus scope for considerable variation with regard to the size and depth of the spot welds. The process is mainly suitable for steel and aluminium sheets, including those with common anti-corrosive coatings and for dissimilar (“black & white”) joints or bimetal joints AutoTechnology 6/ 2006
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of coated steel sheets to aluminium. In the development of the process, emphasis was placed on ensuring that it could also be used for higher-alloy and extra-highalloy steel joints as well as for magnesium joints, as these materials are among those with great potential in the field of advanced lightweight construction. The physical properties stated above are complemented by a number of potentially beneficial additional features. Among these are the servo-motorised pincer gun drive, which dispenses with the need for extensive compressed-air supply arrangements, and the integrated software for central parameterisation of each of the pincer welding guns as well as for comprehensive monitoring and diagnosis of the entire installation. As an optional extra, the quality assurance system uses spot weld image processing to ensure 100 % final inspection and automatically documents and archives the process data, Figure 3 and Figure 4.
Long Replacement Intervals for Wearing Parts The process tape has a dual function. It not only conducts the welding current but also protects the electrode contact surfaces against contamination by zinc, aluminium or organic residues. The protection afforded to the electrodes gives them a significantly longer service life. Tests with aluminium sheets (AlMg3 alloy) have demonstrated a very long service life of approximately 30,000 spot welds. The process tape needs changing only infrequently and is easy to replace. When used once, the process tape is sufficient for approximately 7,000 spot welds. If each section of the welding tape is used two or three times over, the service life is prolonged by the same factor.
Figure 2: The new process for resistance welding greatly enhances quality, electrode service life and productivity in the joining of sheets.
tive production shifts, the process also provides other efficiency gains. These result primarily from the elimination of labour-intensive post-weld machining and from the cleaner working environment made possible by the spatter-free joints. The indirect contacting of the sheets through the process tape prevents welding spatter, thus dispensing with the post-weld machining that would
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Difficulty in aluminium welding
Reduced Power Requirement
Spot Welding without Spatter Besides eliminating the need to change the electrodes, thus resulting in practically uninterrupted availability for several consecu-
otherwise be inevitable and putting new and higher quality standards within reach. The elimination of welding spatter has proven to be especially advantageous in the welding of aluminium sheets. A general difficulty in aluminium welding is contact problems caused by the oxide layer on the surface of the aluminium. This frequently leads to spattering, with the result that both the electrodes and the welding results are impaired in equal measure. This is counteracted by the coating on the process tape, which optimises the electrical contact to the aluminium material and thus prevents spatter formation and the attendant damage to the welds.
Figure 3: Image capture systems that focus on the used tape contact area provide insights into the work process.
The improved efficiency is a result of the extra process heat that arises from the internal resistance and the contact resistance of the process tape. In conventional resistance spot welding of 2 x 1.0 mm AlMg3 sheets, a power requirement of between 35,000 A and 40,000 A may be expected.
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Figure 4: As an integrated optional extra, the quality assurance system permits 100 % data capture. This means that fewer quality checks are needed at a later stage, which would involve greater effort and expense.
joints between all common coated and uncoated steels • welding different combinations and thicknesses of materials (deep-drawn steels in conjunction with CrNi or Usibor) • two-sheet and multi-sheet aluminium joints between all common aluminium alloys
The process tape reduces the electric power needed to 16,000 A. As a large proportion of the energy is input directly from the process tape, it cannot be lost by way of the familiar shunt effect. This makes a major contribution towards optimising process stability. The lower welding current and the scope for individually adapting the process tape to the welding process make it possible to control both the heat input and the position and shape of the spot weld.
Joining Different Types of Sheet Three-sheet joints with two thick sheets and one thin one are somewhat problematic for conventional spot welding, Figure 5. The spot weld that forms in the region of the thicker sheets does not fuse sufficiently with the thin sheet. In
order to compensate for this effect, the process tape enables the spot weld depth to be controlled by the additional thermal input from the tape. This makes it possible to compensate for the low heat input rate to the thinner metal sheet by using a process tape with a higher resistance. In this way, the spot weld can be shifted sufficiently towards the thin sheet. At the same time, the spot weld also becomes more symmetrical, with greater volume in the thinner metal sheet. Welding aluminium presents the problem of its high electrical and thermal conductivity and correspondingly low electrical resistance. The low heat generation that this entails can also be offset by the right choice of process tape. Compared to resistance spot welding which in any case is barely feasible for aluminium, the new process offers a means of spot welding aluminium with complete process reliability. Initial success was first demonstrated on aluminium sheets made of AlMg3 and AlMgSi alloys. Several series of tests using this technology have also confirmed its prospects for improving the cost-effectiveness of joining aluminium.
Adaptability is in Demand Figure 5: The resistance welding process can be used to weld any type of metal joint. Shown here are three aluminium sheets (0.3 mm, 2.0 mm and 1.0 mm AlMg3) with a coated process tape and parameters of 17 kA, 700 ms and 4 kN.
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In short, the new resistance welding system offers potential solutions for the following materials and applications: • two-sheet and multi-sheet steel
An example of how materials can be combined is the welding of electrolytically galvanised steel sheet, TRIP 800, 2.0 mm (Transformation Induced Plasticity), on the underside and hot-dip galvanised steel sheet, DP 600, 2.0 mm (dual phase steel), on the upper side, Figure 6. A pincer gun force of 5 kN was applied, with a welding current of 9 kA. Similar results were also obtained with bimetal joints, e.g. with a steel sheet, DC01+ZE25/25, 1 mm (low carbon steel) on the underside and an aluminium sheet (AlMg3, 1 mm) on the upper side, Figure 7. The reason for the flexibility of this welding procedure is its wide process window. This adaptability makes the new process just as suitable for welding galvanised and organically coated steel sheets as for spot welding untreated aluminium sheets. One of the reasons for the versatility of the new process is its indirect heat input. This makes it possible to use the same inverter and transformer for aluminium applications as for steel applications. Simply replacing the process tape and the electrodes is all it takes to change over from steel to aluminium applications using the same pincer welding gun.
Optimised Process for Quality Assurance When the process tape is used only once, a defined current flow takes place on a new contact area of the tape for each spot weld. An essential feature of this is the consistent optimisation of the current and the pincer gun force curves during the spot welding operation. This “welding current and force profile” must be separately tailored to each welding process in order to take account of the individual properties of the workpiece and materials. The key role AutoTechnology 6/ 2006
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in this is played by universal software for central parameterisation, monitoring and process data acquisition for every single pincer gun in the entire system. As a helpful tool that makes it easy to create a graphic “welding current and force profile”, this software also comes with a program editor that allows users to carry out swift step-by-step optimisation of the welding current and force profile in a rapid sequence. The software also contributes to consistent quality assurance with its remote maintenance capabilities, which include central monitoring of all the pincer welding guns connected. No separate network infrastructure is needed apart from the existing in-company network. If the usual “ready” signal is not detected, automatic error diagnosis is performed, with an exact description of the error. In addition, this networking permits easy transfer of parameters from the PC to any of the pincer welding guns, or from one gun to other guns.
After the welding operation is finished, the used contact area on the tape provides useful insights into the work process. Image capture systems detect this unique “fingerprint” of the spot weld, which is then evaluated by suitable analysis systems. The optionally integrated quality assurance system permits 100 % data capture, thus reducing the number of quality checks needed at a later stage, which would involve greater effort and expense. As well as facilitating quality assurance, this also ensures informative, meaningful documentation.
Figure 6: An electrolytically galvanised steel sheet on the underside (TRIP 800, 2.0 mm) and a hot-dip galvanised sheet on the upper side (DP 600, 2.0 mm). The pincer gun force was 5 kN and the welding current 9 kA.
Conclusion To sum up, a broad spectrum of applications can be foreseen in the materials joining sector for this spot welding process using a process tape. The process offers the prospect of even more production operations, particularly in vehicle manufacturing.
Figure 7 : A steel sheet (DC01+ZE25/25, 1 mm) can be seen on the underside and an aluminium sheet (AlMg3, 1 mm) on the upper side.
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