Optimized MAG Welding
Silicate deposits on weld seams can be significantly reduced in the future thanks to an improved welding process for metal active gas welding (MAG) and an optimized pickling process.
thyssenkrupp Steel has been a trusted partner of the automotive industry for decades, meaning it is always eager to further the development of steel as a material, and to keep the industry attractive and competitive. This is especially true when it comes to the further processing of the material, such as joining uncoated hot strips for components and assemblies made of steel.
Automotive industry suppliers, including KIRCHHOFF Automotive, predominantly use gas-shielded metal arc welding here. Under standard conditions, this process commonly results in deposits on and around the weld seams, which can reduce the durability of the components.
Fewer deposits, greater corrosion protection
After welding, the components are cleaned and then coated with a thick EPD layer to ensure they are protected against corrosion. However, the brittle deposits on the weld seams cannot always be removed completely. This can lead to the deposits flaking off together with the electrophoretic deposition coating (EPD) later as a result of thermal and mechanical stress on the components, which in turn makes them vulnerable to corrosion damage at the points where the coating has flaked off.
Development project with KIRCHHOFF Automotive
Thanks to a joint development project by KIRCHHOFF Automotive and thyssenkrupp Steel, the number of these deposits on and around the weld seams can now be reduced by optimizing the metal active gas welding process (MAG welding for short).
As a result, the corrosion resistance of the chassis is improved and the requirements of the automotive industry are fulfilled.
In our interview, the project participants tell us all about the MAG welding process, the issue with weld seams, and the jointly developed solution.
From KIRCHHOFF Automotive, we spoke to Dr. Jan Stuhrmann, R&D Manager, and Christian Dahmen, Technology Development Specialist. Melanie Dinter from the responsible Sales Customer Service and Marco Queller from Technology & Innovation supported the project for thyssenkrupp Steel.
Which structural components in the body is the reduced-silicate MAG welding process suitable for?
Dr. Jan Stuhrmann: In principle, the process can be used for many different steel components and assemblies that are subject to increased corrosion resistance requirements. The area of the chassis is worth mentioning in particular here since ungalvanized steel grades are often used to increase the quality of the weld seams due to the operating loads required. But other structural components made of steel, such as crash management systems, frontends, or battery casings are suitable as well.
Which processes are generally used to join structural components made of steel?
Dr. Jan Stuhrmann: There are a number of different joining processes that are used as well as MAG welding depending on the requirements set in terms of efficiency, joining properties, and the manufacturing conditions. These processes include resistance spot welding, laser welding, as well as mechanical joining and gluing, to name a few examples.
Why is gas-shielded metal arc welding mainly used in steel car body manufacturing?
Marco Queller: The various different degrees of mechanization offered by MAG welding is the reason why the process is so popular across every industry, from handicraft businesses to major companies. Its very nature means that the joining technology is partly mechanized when it is provided and can be fully automated without a great deal of effort. Its worldwide availability and high training standards in comparison to other processes are also major advantages.
What are the technical advantages of the welding process?
Marco Queller: Components can be accessed from one side and relatively large gap dimensions can be bridged and joined easily during the manufacturing process, meaning they require significantly lower cost tolerances than laser welding, for example. The linear weld seams mean the joined assemblies are extremely rigid and have an extremely high degree of joint strength.
Are there also disadvantages to MAG welding?
Marco Queller: Absolutely – when it comes to manufacturing steel structures, for example. Extremely tough, brittle silicate deposits form during the manufacturing process, which can have a negative impact on the durability of the assemblies.
Are silicate deposits on and around the weld seams in these steel structures a common problem?
Dr. Jan Stuhrmann: The conventional MAG welding process always causes silicate deposits to form – both on the weld seam and on the fusion line next to it. Since the deposits have a vitreous structure and don’t conduct electricity, they are often insufficiently coated during the electrophoretic deposition coating process (EPD). These can then fall out of the weld seam later, especially while the vehicle is being operated, leaving now unprotected weld seam areas behind.
What does this mean for the weld seams?
Christian Dahmen: Uncoated areas left behind by silicates that have fallen constitute points where corrosion may occur later down the line. But the oxide layers formed during the welding process, which are then not sufficiently removed during conventional EPD preprocessing, mean the coating often does not sufficiently adhere to the area next to the weld seam, the so-called heat affected zone (HAZ), either and can also reduce the component’s resistance to corrosion.
What sort of problems can this lead to?
Christian Dahmen: The corrosion processes mean that the required joining properties and thus the functionality of the component can no longer be guaranteed during operation.
What is the current solution to this problem?
Christian Dahmen: There are a number of different procedures that can be followed to remove both the silicate deposits on the weld seam and the oxide layers in the heat affected zone (HAZ). Mechanical procedures such as blasting with a different blasting material, optical procedures such as laser ablation, and chemical procedures such as pickling are prime examples of these.
That sounds like a lot of effort...
Christian Dahmen: The thing all these processes have in common here is the extra process step and the high costs associated with it. For example, pickling processes can be integrated into an EPD plant as an additional preprocessing step. However, very long process times need to be set in addition to the use of special pickling processes to completely remove the silicate deposits, which are heavily mechanically embedded into the seam.
What needed to be done to overcome the problem together?
Melanie Dinter: Our shared task was to work together to minimize the silicates that form during the welding process. The outcomes resulted from experiments, among other things, which means a lengthy, cautious approach to individual process parameters that had to be carefully balanced. As the famous physicist Albert Einstein once said: “If I had an hour to solve a problem and my life depended on the solution, I would spend the first 55 minutes determining the proper question to ask, for once I know the proper question, I could solve the problem in less than five minutes.”
And what was the result?
Dr. Jan Stuhrmann: The conventional MAG welding process has been optimized to the extent that the silicate deposits on the weld seam were reduced to a minimum. As a result, less aggressive pickling processes with drastically shorter pickling times during the preprocessing step of the EPD coating procedure are sufficient, since only the oxide layers on and around the weld seam in the HAZ need to be removed first and foremost.
What are the benefits of the solution for tkSE, customers, and end consumers?
Marco Queller: The results of the project meant the quality of the weld seam were improved in general, which contributed to the component being more resistant to corrosion at the highly stressed weld seams. This allows thyssenkrupp Steel to analyze the processing qualities of hot strips using current and future customer requirements and it can integrate these into future material developments.
Dr. Jan Stuhrmann: Reducing the silicate deposits on weld seams can increase the corrosion resistance of steel structural components after they have been EPD coated. No additional process step is required, unlike conventional silicate removal procedures.
To which OEMS can the process be expanded in the future?
Dr. Jan Stuhrmann: The process is suitable for use with all OEMs that place higher requirements on the corrosion properties of MAG-welded steel components.
How long did it take to research and develop this solution together?
Melanie Dinter: Over the course of the two years in which the project ran, we systematically investigated the different impacts of metal active gas welding on the formation of silicate. The findings from these experiments were considered and jointly evaluated by KIRCHHOFF Automotive and tkSE together with various welding plants in a series of joint experiments. Both companies shared equipment, enabling the project to achieve the maximum amount of success.
How was the collaboration shaped during the project?
Melanie Dinter: We have had a trusting relationship with the company for years now. We work as equals. Our collaboration was the synergy of good communication, direct communication channels, and everyone working towards a common goal.
Dr. Jan Stuhrmann: We always had open discussions during the customer project with regards to information and the jointly conducted experiments, which took place either at tkSE or KIRCHHOFF Automotive.
What was needed to be successful?
Melanie Dinter: In customer service and in our company, appreciation, cooperation, and working as equals are not empty clichés in a company’s mission statement; they are reality. Communication, relationships, structures, and processes need to be understood within the context of the company. The culture in the team is always a part of the culture in the organization as a whole.