Unraveling the impact of the substitution of Si by Al on liquid metal embrittlement behavior of 3^rd generation AHSS

This study addresses one of the most crucial ongoing discussions concerning the mitigation of Zinc-induced Liquid Metal Embrittlement (LME) in third-generation Advanced High-Strength Steels (AHSS). The role of Silicon (Si) and Aluminum (Al), both common alloying elements in AHSS, in triggering LME-induced cracking is thoroughly investigated. Two chemical compositions, 0.2C–3Mn-0.8Si and 0.2C–3Mn–1Al, featuring martensitic microstructures, were selected. Hot tensile tests were conducted in the temperature range of 600–900 °C to simulate the thermomechanical cycle during spot welding. This resulted in a significant reduction in ductility for both Si and Al-alloyed steels in the coated samples compared to the uncoated ones. The loss of ductility was notably more pronounced in Si-alloyed steel than in Al-alloyed steel. These findings were also evident in a specialized LME-triggering spot-welding test, suggesting that Si, as an alloy element in steel, leads to higher LME susceptibility compared to Al. This can be attributed, as indicated by thermodynamic equilibrium calculations and investigations of the Zn–Fe layer, to an increased destabilization and retardation of the protective intermetallic Zn–Fe phases by Si. The insolubility of Si in intermetallic phases and liquid Zn is considered a thermodynamic constraint. In contrast, the theoretically possible full solubility of 1% Al in intermetallic phases as well as in liquid Zn was experimentally demonstrated and supported by thermodynamic simulation. Furthermore, Al, by increasing the Ac3 temperature, results in lower amounts of austenite in the microstructure. It is known that austenite plays a detrimental role in triggering LME. Based on this comprehensive study, it is reasonable to conclude that the formation of intermetallic phases plays a critical role in controlling the contact time between steel and liquid Zn as an embrittling agent, and this is strongly influenced by the Si and Al content of the steel.