| Hot-dip galvanizing is the most important way to prevent steel from corrosion. It is widely used in rail transport, communication, electric, construction and other industries. The comprehensive performance of hot-dip galvanized Zn-Al-Mg alloy coatings is extremely excellent. Among all kinds of Zn-Al-Mg alloy coatings, Super Dyma(Zn-11%Al-3%Mg-0.2%Si) alloy coating is particularly prominent. Therefore, the researches on microstructure of galvanizing Zn-Al and Zn-Al-Mg alloys as well as the coatings play a very important role in promoting the production of high quality hot-dip galvanized zinc alloy coatings.In the present experiments, pure iron plates were dipped in Zn-11%Al、Zn-11%Al-1.5%Mg, Zn-11%Al-3% Mg and Zn-11%Al-4.5% Mg baths at 510 oC with different immersion time, respectively. The effect of Mg on solidification structure of Zn-11%Al coating and growth of Fe-Al intermetallic layers in the coating was investigated by using X-ray diffraction(XRD), scanning electron microscope(SEM) and energy dispersive spectrometer(EDS). The result shows that the solidification microstructure of Zn-11%Al alloy coating is composed of Al rich phase and Zn/Al ternary eutectic. With the increase of Mg content in Zn-11%Al-x%Mg alloy, Zn/Al/MgZn2 ternary eutectic and blocky MgZn2 phase form gradually. The intermetallic layers in Zn-11%Al alloy coatings containing different amount of Mg consist of Fe2Al5 and FeAl3 phase, the thickness of intermetallic layers increases with the prolongation of immersion time. The increase of Mg content in the hot dip bath reduces the growth-rate index and the growth rate of intermetallic layer. At the beginning of hot dip process, the compact and stable Fe-Al alloy layer in Zn-11%Al alloy coatings forms on the iron surface. After immersing 120 s, the Fe-Al intermetallic layer is instability and broken because of the move of diffusion path. The Mg element in the Zn-11%Al-x%Mg alloy can significantly delay the time of the liquid zinc into the Fe-Al alloy layer, so that the Fe-Al alloy layer is more stable and compact.Pure iron specimens were immersed in Zn-11%Al-3%Mg-x%Si baths containing different amount of Si at 510 oC for different immersion time. The effect of Si in Zn-11%Al-3%Mg bath on phase constitution and the growth of Fe-Al intermetallic layer in Zn-11%Al-3%Mg-x%Si coatings were investigated. The result shows that the existence of Si in Zn-11%Al-3%Mg bath could affect the phase constitution of Fe-Al intermetallic layer and remarkably restrains the growth of the layer. The thickness of Fe-Al intermetallic layer is minimum when Si content in Zn-11%Al-3%Mg was 0.2 wt.%. The growth of Fe-Al intermetallic layer in Zn-11%Al-3%Mg-0.2%Si coatings is controlled by diffusion mechanism. 0.2 wt.% Si in Zn-11%Al-3%Mg bath obviously decreases the growth-rate index and the growth rate constant of Fe-Al intermetallic layer in coating.The Zn-11%Al-3%Mg-0.2%Si-x%Ti alloys containing different amount of Ti were prepared by adding 0.1, 0.3 and 0.5wt.%Ti into Super Dyma alloy, respectively. Scanning electron microscopy(SEM), energy spectrometer(EDS), X-ray diffraction(XRD) and differential thermal analysis(DTA) were used to study the effect of Ti addition on solidification structure of Super Dyma alloy and to determine the initial solidification temperature of Al-rich phase in the alloys. The result shows that the solidification structure of Super Dyma is composed of Al-rich phase, MgZn2 phase and Zn/Al/MgZn2 ternary eutectic structure. Because of the addition of Ti into the alloy, the initial solidification temperature of Al-rich phase in Super Dyma alloy decreases. Besides, the Al-rich dendrites ware obviously refined, the size of the MgZn2 phase is greatly reduced and the Zn/Al/MgZn2 ternary eutectic structure is significantly refined. |
PDF Full Text Request