EIt could be so simple: It is not only common in vehicle construction to connect parts made of sheet steel to one another in no time by pressing them together and using pliers to weld them together electrically with welding points. The current heats the material until it melts. The process is called resistance spot welding. It’s quick, holds up well and is the most cost-effective option in series production. Steel is a grateful material in this regard. But aluminum is increasingly being used in cars because safety requirements, comfort requirements and the spread of battery-electric drives are increasing the weight and therefore lighter materials should be used where possible.
The catch is that aluminum has so far resisted efforts to simply join it with spot welds like steel. Manufacturers and suppliers are therefore looking for ways in which it could still work. Matuschek, a medium-sized company from the Aachen area and specialist in resistance spot welding, has developed a process with which the problems can apparently be solved.
For a long time unsuitable for series production
They are in the properties of the metal. Compared to steel, aluminum has a much lower melting point, but higher electrical and thermal conductivity. In addition, an oxide layer quickly forms on the surface, which in turn changes the electrical resistance. The consequences are high power consumption, unclean welded joints and spatter that have to be reworked. In addition, the copper caps of the welding guns that are pressed onto the workpiece wear out much too quickly. That is why other methods of joining materials are preferred in series production. Gluing is an expensive method that is particularly suitable for joining different materials. The shape must of course be suitable for this, and a subsequent check whether it also holds as intended is difficult. For aluminum, gluing is used in addition to other joining methods. In vehicle construction, these are hollow punch rivets, but they pierce the material and riveting is also expensive.
Test series: spot welds on an aluminum sheet that were destroyed again for testing.
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Image: F. Pfaffendorf, Magna International Stanztechnik GmbH
There have been repeated attempts to use the spot welding process, explains Alexander Dumpies, head of the pressure welding department at the SLV industry research institute in Halle. It is the leading process in vehicle construction for joining the same materials. The safety-relevant parts for the crash test are all welded. A few years ago, a provider developed a type of tape that runs with the welding process, but it has proven to be susceptible. Others have experimented with different alloys for the welding caps.
The investment costs are high
Matuschek chooses a different route. The core element of the Alu-Spatz + system is an oscillating grinding unit. It keeps the adjustable radius and the defined surface roughness of the spherical copper electrode caps on the welding gun constant, it cleans and shapes them as required during ongoing production. Matuschek does not reveal the details of the patented process. The rough surface penetrates the oxide layer, says Gregor Hau, specialist in resistance spot welding in the company. Depending on the alloy, 100 to 150 spot welds could be set in production before the caps are ground. Little material is removed in the process. The caps could be reworked about 180 times before they had to be replaced. Around 20,000 welding points are possible per cap – with steel it is only around 4,000 because the sheet metal is usually galvanized. There are also disadvantages to the process. Hau says the investment costs are higher; however, the overall costs of the procedure are lower.
Welding robots at work: aluminum spot welding of an Audi underbody at the supplier Snop in Ghent
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Image: G. Hau, Matuschek Messtechnik GmbH
Nevertheless, the process itself is not very well known in the industry, although the beginnings date back to 2015. It is used by Jaguar Land Rover, the company has experience in the use of aluminum parts for vehicle construction since the Second World War. There are also a number of suppliers for German car manufacturers who use the Alu-Spatz +.
A monitoring tool is built in
The electronic welding control recognizes the most important parameters based on feedback from sensors. Then the current intensity, the welding time and the contact pressure are set for one point. The adaptive control recognizes malfunctions such as sheet cracks or electrode misalignments and compensates for fluctuating process conditions, explains Hau. Quality control is also important. The monitoring of whether a point meets the requirements or not is built in. Destruction or other test procedures are drastically reduced.
Matuschek’s method is conclusive, judges Dumpies. The use of an oscillating grinding head is technologically obvious, but has not yet been implemented in series production. And, as far as he knows, the combination of adaptive process control and integrated monitoring tool has not yet existed in resistance spot welding of aluminum.
Nevertheless, before the start of production of a new part, the welding gun had to be fundamentally adjusted and the production line set up for it. This costs time and initially leads to more errors and thus rejects. Matuschek has therefore recently developed software that determines the optimal setting before starting and prepares the system for possible deviations. This has been around for a long time for steel, but according to Hau not yet at all for aluminum. Reference curves have been determined on the basis of experience. Based on this, every customer can create a program library for their applications. This lowers costs and could ensure that aluminum welding is more widespread.
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