The German Federal Institute for Materials Research and Testing (BAM) has gained new insights into the origin of liquid-metal embrittlement (LME) of steels. The research focused on high-strength galvanised steels, which are especially used in the automotive industry. The results make it possible to develop innovative alloying solutions that suppress LME, potentially paving the way for their widespread use in the industry.
Zinc coatings are essential to protect steels from corrosion. This involves immersing steel components in a bath of molten zinc at temperatures of around 450°C. The zinc then reacts with the surface of the steel to form a robust zinc coating that provides excellent corrosion protection and extends the steel’s service life.
Cracking poses challenges for automotive industry
When welding parts that need to be joined together, the zinc coating can cause micro-cracking in the steel components. ”LME is a decades-old problem that also occur in galvanised steels,” says Professor Robert Maaß from BAM. LME can be a particular challenge in the automotive industry, where vehicle frames contain up to 5,000 spot welds and where the integrity of the materials used is critical to minimise safety risks.
To make more accurate predictions on crack susceptibility and allow preventative measures to be taken, an in-depth understanding of the mechanisms governing LME is vital. New material characterisation methods and simulation techniques can help identify the origins of LME and develop mitigation strategies.
Innovative research method to make steels last longer
With this in mind, researchers at BAM have concentrated on investigating the early stages of LME, focusing on the structure, thermodynamics and atomistics at the steel’s interfaces and surfaces. They have developed an innovative approach that combines electron microscopy methods with computer-aided simulation models, including the “density-based phase-field technique” developed at BAM, to explain defects.
Using this approach, the research team discovered that intermetallic phases are formed at the interfaces between the grains of the steel before micro-cracks occur. These phases form when zinc accumulates at the grain edges. This considerably weakens the steel. With this finding, there is now a move towards approaches in which zinc accumulation and phase formation are controlled, preventing LME.
“Our findings make it possible to develop LME-resistant, advanced high-performance steels that are longer-lasting and more resource-efficient,” explains a summary from the team led by Robert Maaß, Reza Darvishi Kamachali and Tilmann Hickel. “As a result, our research makes an important contribution to sustainable and energy-efficient automotive manufacturing.”
The research was conducted in collaboration with partners ArcelorMittal Global Research, General Motors, the Max Planck Institute for Iron Research, and the Department of Materials Science and Engineering at the University of Illinois. It recently received recognition in the form of an award from the American Iron and Steel Institute (AISI).
