Study On Microstructure And Influencing Factors Of Hot-dip Aluminum Coating Of High Si-Mo Ductile Cast Iron

Hot-dip aluminizing is to immerse the metal in the molten aluminum liquid,and the liquid aluminum and the metal diffuse to each other to obtain an intermetallic layer composed of intermetallic compounds which have good corrosion resistance and high temperature oxidation resistance.High Si-Mo ductile cast iron is a popular material for exhaust pipes.It has high high-temperature strength,thermal fatigue,excellent oxidation resistance,growth and good high-temperature creep resistance.With the development of engine technology,the exhaust pipe material needs higher corrosion resistance and high temperature resistance,and the use of hot-dip aluminizied to improve the performance of high Si-Mo ductile cast iron is an effective way.In this paper,high Si-Mo ductile cast iron is used as the substrate,and the coating is obtained by changing different hot-dip aluminizing process conditions.The metallographic structure and morphology of the coating were observed with ZEISS metallographic microscope and JEOL-6360 LV scanning electron microscope,and the phase structure of the coating was analyzed with X-ray diffractometer.The results showed that:(1)The high Si-Mo ductile cast iron hot-dip aluminized coating formed an aluminum-rich layer and an intermetallic layer,and the intermetallic layer consists of an inner intermetallic layer close to the substrate and an outer intermetallic layer close to the aluminum-rich layer.This is the phenomenon of double-intermetallic layer found in the coating of high Si-Mo ductile cast iron substrate.Among them,the inner intermetallic layer is formed by the iron-rich Fe-Si phase and a small amount of iron-rich Fe-Al-Si ternary phase;the outer intermetallic layer is mainly composed of the aluminum-rich Fe-Al phase and a small amount of aluminum-rich Fe-Al phase Al-Si ternary phase composition;(2)With the increase of silicon contents in the plating solution,the Fe-Al-Si ternary phase formed in the intermetallic layer increases and the Fe-Al phase in the outer intermetallic layer decreases,and the wavy boundary between the inner and outer intermetallic layers tends to be gentle,and the thickness of the outer intermetallic layer gradually becomes thinner;(3)Increasing the immersion plating time and increasing the immersion plating temperature will intensify the diffusion process of the each atom,make the undulation of the wavy boundary between the inner and outer intermetallic layers more obvious,and make the thickness of the inner intermetallic layer thinning;(4)The growth direction of the intermetallic layer is from the initial interface of solid iron and liquid aluminum to the direction of the substrate,and graphite is included in the plating structure including the aluminum-rich layer,the outer intermetallic layer and the inner intermetallic layer;combined with the three stages of infiltration,dissolution and diffusion in the process of hot-dip aluminizing,thus,the formation process of high Si-Mo hot-dip pure aluminum coating is obtained;(5)At the hot-dip plating temperature,the silicon element and the iron element in the substrate form an Fe-Si phase,which prevents and slows the diffusion of the aluminum element,and which is the reason for the double-intermetallic layer phenomenon in the intermetallic layer.