Cause of cracks in high-performance galvanised steels discovered

One way to keep steels from rusting is galvanisation. This involves dipping the steel parts in molten zinc at around 450°C. The zinc then bonds to the steel surface, creating a protective zinc layer that shields the steel from corrosion and increases its durability. When welding the individual components, however, micro-cracks can form in the steel parts, which are caused by the zinc coating.

This presents a particular challenge in the automotive industry, where a vehicle chassis can 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, a deeper understanding of the mechanisms governing liquid-metal embrittlement (LME) is vital.

In this context, researchers at the German Federal Institute for Materials Research and Testing (BAM) have devoted their attention to studying the early stages of LME, focusing on the structure, thermodynamics and atomistics at the steel’s interfaces and surfaces. They have devised an innovative approach that combines electron microscopy methods with computer-aided simulation models, including the density-based phase-field technology developed at BAM, to explain defects.

Using this approach, the research team has discovered that intermetallic phases are formed at the interfaces between the grains of the steel even before micro-cracks occur. These phases form when zinc accumulates at the grain edges, considerably weakening the steel. Building on this finding, the researchers are now pursuing approaches that involve controlling zinc accumulation and phase formation, preventing LME. Their goal is to develop LME-resistant, high-performance steels that are longer-lasting and more resource-efficient, in turn making an important contribution to sustainable and energy-efficient automotive manufacturing.

The research was conducted in collaboration with BAM partners such as 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. Their partnership recently received recognition in the form of an award from the American Iron and Steel Institute (AISI).