Microstructure development, phase reaction characteristics and properties of quaternary Zn-0.5Al-0.5Mg-xBi hot dipped coating alloy under slow and fast cooling rates

In this study, the effect of Bi addition on the microstructure, phase evolution, solidification characteristics and hardness property of hot dipped Zn-0.5%Al-0.5%Mg coating alloy have been investigated. An excellent agreement was obtained among the thermal analysis parameters, solidified structure and energy dispersive spectroscopy results. The results show that Zn dendrite, Zn2Mg, Zn6Al3Mg precipitated at 407 °C, 352 °C, and 333 °C, respectively, and the ZnMg4Bi5 and/or Zn2Mg2Bi3 compounds formed simultaneously at the last stage of solidification at 329 °C after the addition of Bi. With determining the coherency point, time interval coherency (tN-tDCP) increased gradually when Bi addition increased up to 0.5%; however, temperature interval of coherency (TN-TDCP) remained constant. The Zn dendrites became coherent at a much longer time period which corresponds to a decrease of secondary dendrite arm spacing (SDAS) by 8.3%. In addition, increasing the cooling rate from 0.1 °C/s to 3.4 °C/s decreased SDAS significantly from 220.8 μm to 81.1 μm. Solid fraction at coherency point increased from 22% to 31% and decreased from 18% to 12% for Zn2Mg after increasing the Bi concentration; however, the solid fraction of Zn6Al3Mg remained constant. In addition, the hardness of Zn-0.5Al-0.5Mg alloy increased by 15% with the addition of Bi.