Research On Inclusions In ZA27 High Aluminum Zn-based Alloy

High aluminum Zn-based alloys have excellent mechanical properties, damping properties, wear resistance and casting process performance, and low production cost, easy molding, broad application prospects. However, the presences of inclusions severely affect the mechanical properties of the material, and easily lead to premature failure of components. Taking ZA27 high aluminum zinc alloy as the research object, corresponding detection samples were produced by gravity casting and squeeze casting, and then inclusions and second phase in the alloy were for analysis and research.In the process of smelting, purification of ZA27 alloy, variation of the inclusions, and its effect on mechanical properties were studied. The results showed that: when the alloy melting, melt will produce a lot of gas and inclusions, when not refined, gas and inclusions are difficult to self-discharge; after solvent refining and blowing refining, holes in the alloy basically has been eliminated, the number and size of inclusions significantly reduced, the organization greatly improved. Shrinkage, porosity and inclusions in gravity casting organizations are the cause of the decline of mechanical properties, and melt purification can improve the mechanical properties of the alloy, especially the ductility. Inclusions generated during melting of the alloy mainly were Zn and Al oxides, and other non-metallic compounds and impurities which introduced from raw materials and unclean foundry returns.Al-Ti-B and RE were used in modification of the ZA27 alloy, and then the RE-rich phase and iron-rich phase in the ZA27 alloy were studied. The results showed that: rare earth elements and Fe elements are segregated at the grain boundaries, and form complex compounds with Zn, Al and Cu in ZA27 alloy. The compounds rich in rare earth are polygonal blocks, containing a large amount of rare earth elements, not containing Fe; the compounds rich in iron are mainly needles, and a few are small pieces, not containing rare earth elements. These hard and brittle intermetallic compounds have fragmentary effect on the body, easy to produce stress concentration, leading to formation of micro-cracks and extensions. With strict control of possible sources of the impurity Fe and adjust of the amount of modifier accordingly, the mechanical properties of squeeze casting ZA27 alloy worm gear were greatly improved.This paper also studied the effects of a small amount of Mn on the squeeze casting ZA27 alloy, as well as the formation characteristics of Mn-rich phase in the alloy. The results showed that, Mn can refine the grains and reduce the role of the secondary dendrite arm spacing, while a large amount of Mn-rich phase were produced. Most of Mn-rich phase are present in the grain boundary, and a few throughout grains. There are three common form of Mn-rich phase. Among them, the long needle and polygons massive manganese-rich phase have very high Fe content, much higher than the alloy impurity content of Fe and 3~4 times of the large block Mn-rich phases. When alloying the high aluminum Zn-based alloys with Mn, the content of impurity elements of Fe should be strictly controlled, and long needle Mn-rich phase must be minimized or avoided to form, and massive Mn-rich to grown up.