Requirements of aluminium spot welding

Reliable industrial spot welding of aluminium is an ambitious venture

The reproducible, i.e. process-safe resistance spot welding of different aluminium alloys is a technologically very demanding task, especially under the condition, that such welding processes are carried out with the lowest possible energy input - i.e. currents.

In contrast to spot welding of steels, for example, far more physical variables and influencing variables have to be considered when welding aluminium. In this case, the material composition of the electrodes is of great importance, as are the physical properties of the aluminium material to be welded itself.

Aluminium materials occur in numerous alloy forms and the respective electrophysical properties of these alloys increase accordingly. Even different storage times of aluminium sheets influence the electrophysical properties of individual batches.

In contrast to steel construction, e.g. during spot welding of car body components made of high-alloy steels in the automotive industry, spot welding of aluminium with the usual copper electrode caps poses the fundamental problem of alloying the aluminium to the copper caps - aluminium oxides are formed, which immediately influence the electrical properties of the contact surfaces on the electrode and aluminium sheet.

In addition, higher current strengths must generally be used in the aluminium spot welding process than in the spot welding of high-alloy steel grades, for example. This in turn requires larger power sections in the current sources and more powerful transformers in the spot welding guns.

In the picture: Standard electrode copper caps alloyed under heavy load. Clear layers of aluminium oxide have formed. Welding spots set with such caps do not provide process-safe quality.

The introduction of aluminium spot welding to the automotive repair market 2014

A practicable solution for vehicle repairs

In the years 2003/2004 ELMA-Tech develops in cooperation with Opel the first spot welding machine for process-safe resistance welding of high-strength steels.

With the introduction of the „Virtual Machine“ welding process control, ELMA-Tech will be able to offer the first fully automatic spot welding machine on the market from 2008: A system, that uses virtual measuring technology based on a calorimetric model to calculate the lens size suitable for successful welding from the total sheet thickness determined when the welding gun is closed, and that manages the spot welding process fully automatically accordingly.

An aluminium spot welding process for these machines has been offered for the automotive repair market since 2014. This also takes place fully automatically. However, expensive electrodes with special alloy caps are used for this process in order to solve the aforementioned problem of alloying. This is a feasible solution for the motor vehicle repair market, as comparatively few points are welded at low intensity. This solution cannot be used for industrial applications: On the one hand for cost reasons, on the other hand the spot welding machines used in the automotive repair market do not quite achieve the required performance.

About the current state of technology

Previously complex solutions necessary

The factors described above make aluminium spot welding in the industrial sector complex and thus also cost-intensive. Welding technology manufacturers have so far taken various measures to address the problem. Although special alloy admixtures in the electrode caps reduce the alloying of the caps, they are uneconomical in industrial applications and also exhibit poor thermal properties.

The main focus here is on preventing the electrode caps from alloying and on designing a reliable welding process.

For example, additional automated cap cleaning systems are used which clean the electrode cap surfaces after relatively few spot welds by brushing or grinding. These solutions therefore interrupt the welding process and ensure slower cycle times overall.

Other systems - see graphic below - operate with permanently circulating process belts above the electrode caps to avoid direct contact between the cap surface and aluminium sheets. This solution also works with complex additional equipment and causes the user additional costs for the material of the process belts.

The effective solution from ELMA-Tech: Keep it small!

Effective and slim: The aluminium spot welding solution from ELMA-Tech

As early as 2014, ELMA-Tech was the first company to introduce spot welding of aluminium thin sheets to the automotive repair market, at that time with tungsten electrode caps and process reliable for the typical (less intensive) applications in body repair.

One of the main premises for the further development of this process for use in the industrial sector in the following years was to develop such a welding process on the basis of existing ELMA-Tech system components:

• Process control „Virtual Machine“
• Industrial welding guns with associated control cabinet (power source)
• Virtual measurement technology
• 16 mm standard copper electrode caps

The process to be optimised should therefore do without the additional systems described above, as used by the competition. The aim was thus a maximum lean machine concept. This goal was achieved.

In the years of research & development since 2014, ELMA-Tech has also been confronted with the problem of alloying the caps, which was largely overcome with the help of process control in combination with virtual measurement technology.

System components used in the ELMA-Tech aluminium spot welding process for industrial applications

Picture above: ELMA VISION CM Alu. Industrial manual welding gun from ELMA-Tech in the aluminium version with higher transformer and cooling capacity. C gun with standard electrode shafts and 16 mm electrode caps (copper chrome zircon).

Picture below: ELMA VMC VISION Alu. Control cabinet or power source with process control „Virtual Machine“ (VM3) and special power section for aluminium applications. Process control VM3: the welding process control for reliable spot welding of aluminium. The process control „virtual machine“ is the heart of ELMA-Tech‘s welding technology. All process parameters are recorded 20,000 times per second. From this, the control reliably calculates the energy control required for the aluminium spot welding process.

Right next to the control cabinet a SMC HRS 50 Water recooling unit for optimum cooling of the spot welding gun and the control cabinet.

ELMA-Tech aluminium spot welding technology for industrial use from the perspective of experts

Significant increase in efficiency

After extensive test series and constant further development of the welding process control at ELMA-Tech, valid results crystallized in mid-2017.

In spring 2018, MAGNA Steyr (Graz, Austria) was able to reproduce the positive process results in large-scale test series with an ELMA-Tech prototype system and confirm them in laboratory analyses, e.g. in dynamic tensile tests, grinding and peeling samples.

In the tests, at least 150 weld spots could be set without changing, cleaning or milling the electrode caps. The virtual measurement technology developed by ELMA-Tech directly on the electrode caps proved to be essential for the technological breakthrough.

Werner Karner, head of the MAGNA Steyr Joining Centre for Experimental Construction, speaks in this context of a „very significant optimisation and increase in efficiency“ in the industrial welding of aluminium joints.

It is particularly important here that „the process works with standard equipment“ and has now been reliably adapted „from individual vehicle repair applications to industrial applications“

Excellent quality of the welding spots

First presentation of the aluminium welding process at EuroBLECH in Hanover, October 2018

The interested professional audience is enthusiastic

The aluminium welding process was presented for the first time at the ELMA-Tech stand on the occasion of EuroBlech 2018 in Hanover, designed as a pilot series and on the basis of a lightweight industrial manual welding gun type CM.

During a trial initiated at the trade fair, ELMA-Tech succeeded in setting a number of 650 welding spots on a defined aluminium alloy with only one standard copper electrode cap.

Minimal alloying without loss of function for the process

The picture shows slightly alloyed 16 mm standard electrode caps after 150 welding spots have been set: The caps have an alloy which forms during the first 12 welding spots and does not increase further during the welding process.

This alloy does not cause any loss of function of the further welding process, as the welding process control Virtual Machine (VM3) can react to these surface changes on the electrode caps with a highly flexible process management.

In the area of the electrode contact surfaces on the cathode side, the surfaces of the sheets show process-induced radial notched or also roughened impressions.

The current ongoing process optimization at ELMA-Tech confirms impressively, that the alloying and electrode imprint or surface deformations can be reduced to almost zero in the near future.

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Conclusion

The advantages of the aluminium spot welding process from ELMA-Tech

The special advantages of ELMA-Tech‘s industrial aluminium welding process compared to the competition are therefore clear to see:

• The industrial aluminium spot welding process developed by ELMA-Tech works validly with the ELMA-Tech (Virtual Machine) control system used for steel sheet connections - i.e. without additional equipment such as circulating process belts or cap cleaning systems.
  
  
• In addition, 16 mm standard electrode caps can be used. ELMA-Tech control cabinets in the VMC VISION Alu version will be used as welding power sources for the finished application.
  
  
• Depending on the sheet combination, 600 welding spots can be reliably spotted.

Outlook

Further development and some hints for interested parties

Further development

In addition to a constant development cooperation with our partner MAGNA STEYR in Graz, we will further expand our „product range aluminium spot welding process“ according to the requirements and demand of the market.

Basically, we will set parameters for the most common aluminium alloys in a permanent process in order to be able to spot weld as wide a bandwidth of aluminium grades as possible. Please note the notes below.

In the future we also plan to set up our stationary spot welding machine ELMAconcept VISION as well as our robot welding guns for this process.

Hints for interested parties

In view of the countless aluminium alloys on the market and the complexity of the aluminium spot welding process, at present ELMA-Tech cannot guarantee, that the aluminium welding process presented in this information will allow any aluminium alloy to be welded with the same maximum number of points in a reliable manner.

If you are interested in an application-specific process solution, please talk to our „Sales Resistance Spot Welding“ and agree on the appropriate procedure during the discussion.

As a rule, we will ask you to describe your requirements and send us materials so that our application technology can carry out test welds. These test welds are free of charge for you, as long as they are within the usual range.

Only after these welding tests have been carried out, you will receive a reliable statement from us as to whether we can support you with our applications.

Positive welding results have so far been achieved with the aluminium material classes 5000, 6000
and 7000.

Aluminium spot welding system

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