Liquid metal embrittlement during FAST joining of magnesium and galvanized steel

Joining magnesium (Mg) alloys to steel is difficult due to metallurgical incompatibility. Applying a zinc (Zn) coating to steel enables formation of a thin Mg-Zn eutectic phase layer during welding, which promotes strong bonding. However, in joints created with Friction-stir assisted scribe technology (FAST), this Mg-Zn eutectic phase layer occasionally extends from the interface to the surface of the Mg sheet. This phenomenon is attributed to the formation of a liquid-state Mg-Zn eutectic phase, coupled with the distinctive material flow induced by the FAST tool. Microstructural analysis confirmed that the Mg-Zn eutectic phase comprises α-Mg and the Mg₂₁Zn₂₅ intermetallic compound. Lap shear tensile tests revealed that when the Mg-Zn eutectic phase migration pathway aligned with the stir zone boundary, it led to reduced joint strength and premature fracture along the eutectic phase pathway. This indicates that liquid metal embrittlement (LME) occurred during FAST joining of Mg alloy and galvanized steel. These findings highlight the critical importance of controlling tool features and process parameters in FAST welding to prevent LME- related failures in dissimilar Mg/steel assemblies.