To explore the effect of Sn-Zn alloy coating on the wetting behavior of Al-Si melt and Q235 steel, in this paper, an improved sessile drop method was used to study the wetting of Al-Si melt on uncoated Q235 steel substrate and coated Q235 steel substrates with different coating thickness at 700 ℃. Various thicknesses (190-5 500 nm) of Sn-Zn alloy coatings were deposited on the surface of Q235 steel substrate by magnetron sputtering and hot-dip plating. Scanning electron microscope (SEM) and energy dispersive X-ray (EDS) were used to characterize and analyze the composition and microstructures at the cross-section and surface of the solidified wetting sample. Experimental results showed that the Al-Si melt and the uncoated substrate were partially wetted at 700 ℃, and the initial and final contact angles were 88° and 66°, respectively. By applying 190 nm Sn-Zn alloy coating on the steel substrate, the initial spreading rate increased significantly and the final contact angle was about 13°, and a precursor film was formed at the spreading front of molten Al-Si alloy on the coated Q235 steel substrate. With the increase of the coating thickness, in the wetting of Al-Si alloy melt on coated Q235 steel substrate, the final contact angle decreased, the melt spreading rate and the width of the precursor film increased, and it tended to be completely wetted at 1 680 nm coated substrate(the final contact angle was about 2°). The interfacial reaction products between Al-Si melt and uncoated steel substrate were Fe2Al5 phase adhering to the substrate side and Al8Fe2Si phase close to the melt side. The reactive layer grew rapidly towards the melt side perpendicular to the interface, which isolated the contact of the melt spreading front with the substrate and inhibited the melt spreading. The application of the Sn-Zn alloy coating promoted the rapid growth of the reaction layers (Fe2Al5 phase and Al8Fe2Si phase) along the spreading direction on the coated substrate surface. The surface layer of the precursor film was mainly composed of Al8Fe2Si phase. Some isolated low-melting tin phases exist near the interface of the solidified Al-Si/coated Q235 wetting specimen with an excessively thick coating (5 500 nm). The effect mechanism of Sn-Zn alloy coating on the wetting behavior of Al-Si melt and Q235 steel was analyzed based on interfacial reaction, wetting triple line configuration and wetting driving force. The Sn-Zn coating affects the wetting behavior of Al-Si melt and Q235 steel mainly by changing the growth of the interface reaction layer and the configuration of the triple wetting line. The tin-zinc alloy coating can effectively improve the wettability of Al-Si melt and Q235 steel, and contribute to the formation of the precursor film at the spreading front. As the coating thickness increases, the melt spreading rate increases and the final contact angle decreases. Continuing to increase the coating thickness after tending to complete wetting (Al-Si/1 680 nm coated steel substrate) has no significant effect on the wetting behavior, but an excessively thick coating will promote the formation of a low melting point tin phase near the solidification interface, which will weaken the interface bonding. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.