Design of low aluminum ZAM coating and research on inhibiting mechanism of liquid metal induced embrittlement

摘要

The liquid metal embrittlement (LME) cracks that form during the hot stamping process of zinc -based coatings significantly reduce the toughness of the coated sheets, representing one of the bottlenecks for the application of zinc -based coatings in automotive panels [1, 2]. In this work, based on the Zn -Al-Mg database, the li quid phase projection diagram enriched with zinc was calculated, and the coating alloys were designed to be located at the Zn/Al eutectic line, the Al/MgZn 2 eutectic line, and the Zn/Al/Mg 2Zn11 eutectic point. Among these, the Zn -4.5Al-3.0Mg coating posses ses the highest content of ternary Zn/Al/Mg2Zn11 eutectics, exhibits the finest microstructure, and shows the most uniform distribution of elements [3]. In the eutectic region of the alloy, both Mg 2Zn11 and MgZn 2 phases precipitate simultaneously, with the Mg2Zn11 phase growing and precipitating on the matrix of the MgZn 2 phase [4]. The corrosion resistance of the coating improves with the addition of Mg, Al, or Sn elements. Then, during the austenitization process under no -strain conditions, for the same coated sheet, as the austenitization temperature increases, the microstructure of the Zn-Al-Mg coated sheet transforms from a coarse layered structure to a fine, dense monolithic structure, with a more uniform distribution of Zn and Al elements. Under the same austenitization temperature, as the content of Al and Mg elements in the coating alloy increases, the coating microstructure becomes progressively finer and denser. Finally, the LME sensitivity of the coated sheets was evaluated through the relative loss of fracture energy [5, 6]. The relative loss of fracture energy increases and then decreases with the rise in testing temperature and decreases with the increased uniformity of element distribution after austenitization. At a testing temperature of 920 ° C, the Zn-xAl-yMg coated sheet exhibits the lowest LME sensitivity, with a relative loss of fracture energy below 10%.