Effect of Ti, Sr, and B Addition on the Fluidity of A356.2 Grade Aluminum Alloy

At the present time, aluminum alloys with silicon are the most widespread construction materials. In order to increase the mechanical properties of aluminum alloys, modifying with Sr, Ti, and B is used. However, in the foundries, when using scrap and secondary aluminum alloys, the modifying elements are accumulated in alloys in the form of intermetallic particles, which can lead to a decrease in the level of castability. This is connected with the fact that the used modifiers exert a short-term effect and cannot be activated upon remelting. Hence it is necessary to add the modifiers without taking into account the intermetallic particles already contained in the melt. This paper is devoted to studies on the effect of additions of Sr, Ti, and B on the fluidity of an A356.2 grade aluminum alloy determined by means of vacuum fluidity testing. It is shown that, when AlSr10 and AlTi5B1 commercial master alloys are used (containing up to 0.3 wt % Sr and 0.5 wt % Ti), no decrease in fluidity is observed. However, adding the same amount of Ti with the use of a homemade AlTi4 master alloy leads to a considerable decrease in the fluidity. With the help of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), the microstructure and phase composition of master alloys and of an A356.2 grade alloy after adding the mentioned master alloys have been investigated. Additionally, the Thermo-Calc software package has been used to evaluate the effect of modifier addition exerted on the phase composition and phase transition temperature of the alloy. It has been established that the effect of the modifier addition on the fluidity of the A356.2 grade alloy is connected with the shape and size of crystals containing the modifying elements in the master alloy structure. When there are coarse crystals formed by such phases, it is quite possible that the crystals are dissolved incompletely, which could hinder the free flow of melt.