Effects Of Alloying Elements And Densifiers On Properties And Microstructure Of ZnAl4 Alloy

Low-aluminum zinc alloys are widely used in the machining industry due to their excellent properties such as low melting point,low energy consumption,good fluidity in liquid state,and stable mechanical properties.A major problem that has been plaguing domestic enterprises is that most of the zinc-aluminum alloys produced by domestic enterprises suffer from low plasticity and toughness,high impurity content,poor dimensional stability and corrosion resistance,which limit their development.In order to allow the domestic zinc-aluminum alloys to better meet the industrial needs,it is of practical significance to improve the quality of domestically-made low-aluminum zinc alloys.This thesis mainly focuses on the basic research of gravity casting zinc-aluminum alloys and hot dipped zinc-aluminum alloys,and provides technical support for enterprises to produce high-quality low-aluminum zinc alloys that meet the market demand.Using Vickers hardness test,tensile test,electrochemical test and neutral salt spray corrosion test,combined with scanning electron microscope（SEM）,energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）and high-angle annular dark field scanning transmission electron microscopy（HAADF-STEM）,the effects of the addition of three typical modifiers and Mg content on the mechanical properties and microstructure of as-cast ZnAl4 alloy,and the effects of Mg content on the corrosion resistance and microstructure of hot-dip ZnAl4 alloy have been systematically studied.The main conclusions are as follows:Al-5Ti-B,Sr and rare earth（RE）elements can significantly improve the elongation of ZnAl4 alloy,where the RE modifier can also improve the strength of the alloy.The additions of the modifiers do not change the types of phases in the as-cast ZnAl4 alloy,but changes the morphology and size of the phases.The morphology of the primaryη-Zn phase changes from a dendritic structure to one exhibiting nearly equiaxed graininess,and the morphology of the（α+η）eutectoid structure changes from a lamellar structure to an ellipsoidal structure and the size is refined.Part of the modifier elements added to the ZnAl4 alloy will be precipitated from the alloy liquid as intermetallic compounds,which increases the resistance to grain boundary migration and inhibits the growth of the primaryη-Zn dendrites,resulting in the improvement of theη-Zn phase morphology.Part of the modifier elements will segregate at the front of the solid-liquid interface to form a supercooled region,where the equilibrium crystallization temperature is lower,inhibiting the growth of the eutectoid structure and thus refining its size.The addition of Mg element to the as-cast ZnAl4 alloy can enhance the strength and hardness of the alloy,and the level of enhancement increases with increasing the Mg content.After adding Mg element,a Zn/Al/Mg ternary eutectic structure,which is more effective than the binary eutectic structure in strengthening the ZnAl4 alloy,is formed in the alloy,and the proportion of the ternary eutectic structure increases and its size decreases with the Mg content,which is beneficial to the improvement of alloy strength and hardness.The Mg element exists in the as-cast ZnAl4 alloy in two forms:partly by forming intermetallic compounds,and partly by being uniformly distributed in theη-Zn phase of the ternary eutectic structure.The corrosion rate of hot dipped ZnAl4 alloy coatings on steel appears to decrease,then increase,and then decrease with increasing the Mg content.The low corrosion rate of the coatings with Mg contents in the range of 1 to 1.5 wt.%implies that the hot dipped alloy coatings with these Mg contents possess good corrosion resistance.The Mg addition improves the corrosion resistance of the hot dipped ZnAl4 coatings by forming a dense Fe₂Al₅Zn_0.4reaction layer,refining the solidification structure and forming more protective corrosion products Zn₅（OH）₈Cl₂·H₂O.