High entropy alloy interlayers for mitigation of liquid metal embrittlement in galvannealed advanced high strength steel welds

Abstract

Liquid metal embrittlement (LME) is a long-standing problem for resistance spot welding (RSW) of Zn-coated steels for automobile applications, especially for advanced high strength steels (AHSSs). The problem originates form the fact that melting point of zinc is significantly lower than that of the steel sheet (difference in melting points is approximately 1000 °C), causing Zn to melt and vaporize during welding. The expulsed liquid zinc can penetrate into the steel grain boundaries, causing LME. This work aims to design and implement high entropy alloy (HEA) interlayers that can absorb zinc during welding and mitigate LME. A code for down selection of candidate alloys was developed to rank alloy systems based on their stability to maintain a single-solid solution phase over a wide composition range of Zn and Fe. The code included thermodynamic calculations (including configurational entropy and mixing enthalpy), atomic size difference, and valance electron concentration. Equilibrium phase and Scheil solidification diagrams calculated from ThermoCalc software predicted dominance of fcc phase with no Zn separation especially in the top two HEA candidates. DICTRA simulation results demonstrated limited zinc diffusion into steel when HEA interlayer was used and no indications of Fe-Zn intermetallic compounds were observed at the weld/HEA interface. In agreement with simulation results, EDS analysis at the weld/HEA interface showed the preferential diffusion of Zn into the HEA rather than steel. Optical microscopy examination demonstrated elimination of LME cracks and no obvious existence of harmful phases in weld samples with HEA foils. Furthermore, tensile shear test showed that joints with HEA interlayer exhibited a higher fracture load than conventional welds. Implementation of the HEA interlayer did not appear to affect the microstructure evolution and hardness profile of the nugget and heat-affected zones.