Influence Of Complex Modification And Refinement On Microstructure And Properties Of ZL109

Modifying primary and eutectic silicon is an effective method to improve themechanical properties of Al-Si casting alloys. In this paper, the changes ofmorphology and size of primary silicon, eutectic silicon and aluminum were studiedafter complex addition of P, RE, Ti to ZL109alloys. And the relevant mechanismswere analyzed. Characteristics temperatures of the various solidification stages weremeasured by DSC, and the tensile strength of these alloys were tested. Thecorresponding changes were also discussed combined with refinement andmodification mechanism.The results of the study to the morphology and size of microstructure show thatprimary Si can be refined significantly by adding P to ZL109alloy and the optimizedaddition level under the experimental conditions is0.8wt.%AlP4.5. Moreover, P canalso reduce the ratio of length to thickness of eutectic silicon and the secondarydendrite arm spacing of aluminum. The role of RE on primary and eutectic siliconcan be reflected only when it reaches a given concentration, but it has only weakmodification role of RE for primary silicon. When it reaches0.7wt.%, RE can makeeutectic silicon transform into complex morphologes which contain shorter flakes,small particulates and developed fibrous branches. In addition, RE decreasessecondary dendrite arm spacing of aluminum. Ti increases the size of primary siliconand the ratio of length to thickness of eutectic silicon. Secondary dendrite armspacing of aluminum is decreased with increasing Ti content, but Al-Ti-Si phases willbe generated with excess Ti, and resulting in refinement of aluminum reduced.DSC results show that P can improve precipitation temperature of primarysilicon TP, combined action of P and RE makes TPreduced, composite effect of P andTi can make TPincreased, and the combined effect of P and RE makes initialcrystallization temperature of eutectic (α-Al+Si) decreased significantly. The changesof characteristics temperatures also lead to the changes of characteristics temperatureranges of solidification process, which have closely relation with the morphologychanges of primary silicon, eutectic silicon and aluminum. Tensile test results show that the size and morphology of primary silicon andeutectic silicon have decisive effects on tensile strength of ZL109alloy, and secondarydendrite arm spacing of aluminum has limited impact on the tensile strength ofZL109alloy. When content of RE is0.9wt.%, the combined effects of REmodification on eutectic silicon with solution strengthening to aluminum matrixinduced the improvement of tensile strength.The results of tensile fracture observation show that the amount of biggercleaved facets in the fracture of ZL109alloy with P modification significantlyreduced compared with that of the alloy unmodified, and the cleavage facets ofparallel in the former alloy are also more uniform and finer than that in the latter alloy.A large number of features small shadow dimple was observed in the ZL109alloywith addition of P and0.7wt.%~0.9wt.%of RE. However, the tendency to brittlefracture of the alloy increase when P and Ti were put in simultaneously. The tensilefracture of the alloy with0.3wt.%Ti has shown a clear cleavage fracturecharacteristics.