Study On Feathery Ciystal Formation And Morphology Of Zn-Al Alloys

Zn-Al alloys have good casting properties, excellent wear and friction properties, anideal alternative to expensive copper alloy materials. However, the very developed columnardendrites, especially the feathery crystals formed under certain conditions, can seriouslyweaken the mechanical properties, its organization. In this paper, the feathery crystalformation conditions such as temperature gradient, alloying elements have been studied,which will not only enrich metals’ solidification theory and crystal growth theory, but alsoprovide the foundation to eliminate the feathery crystals and improve mechanical propertiesby forming fine equiaxed grains.This thesis has studied the formation mechanisms of the feathery crystals. The effects ofalloying elements of ZA27alloy on this kind of crystals have discussed. The results indicate:For the Zn-Al binary alloys, the grains change from equiaxed grains in turn to columnardendrites, feathery grains, columnar grains and equiaxed dendrites with decreasing the Alcontent. Namely, the grains change from randomly distributed equiaxed dendrites todirectional lined columnar dendrites, and again to randomly distributed equiaxed dendrite.The growth direction of secondary dendritic arm is affected by the alloying elements, havingthree regimes, two, four and six directions.For the Zn-27Al alloy, its microstructure changes from developed equiaxed grains in turnof dendrites with developed secondary dendritic arms, feathery grains and dendrites withdeveloped secondary dendritic arms. The distribution of Cu-rich phase gradually change fromdispersive form to agglomeration on the crystal surfaces. This change leads the arms’ growthdirection to orientate to heat flux direction. When the growth of primary arms is restrainedand those of secondary and third arms are enhanced, the equiaxed grains form.Under the condition of directional solidification (a solidification rate of30μm/s), boththe primary arm interval and secondary arm interval decrease with decreasing Al content ofZn-Al binary alloy. The preferred growth orientation is very obvious. The growth direction ofprimary arms change from <100> to <110> when the Al content decreases to40%.For the ZA27alloy, the growth direction is basically the <100> at the pouringtemperature of600℃, but that is the <110> at700℃.