Influence of the Crystallization Rate on the Structures and Properties of Aluminum–Zirconium Alloys

The influence of a zirconium additive and a cooling rate on the structures, properties, and crystallization laws of aluminum alloys is studied. The measurement of the hardness of the formed alloys shows that the addition of 0.4 wt % zirconium to high-purity aluminum results in an increase in the hardness of the alloy by 1.5 times, and the hardness increases further with an increase in the zirconium content. According to the scanning electron microscopy (SEM) and X-ray diffraction (XRD) data, the main fraction of zirconium in the alloy is presented by intermetallic compounds from 5 to 50 μm in size with the predominant Al₃Zr composition. A possibility of using the electrolytically prepared master alloy Al–Zr for grain dividing and improving the properties of the aluminum alloys is studied. The tests are carried out at 900°C for the Al–Si–Fe alloy (AK6) to which different amounts of the Al–Zr master alloy with a zirconium content of 10 wt % have been added. The zirconium additive to the alloy in an amount of 0.1 wt % is found to divide the grain by 4–5 times without changes in the shape and structure, and the further increase in the zirconium content in the alloy exerts no effect on the average grain size. The accelerated to 10³ K/s cooling of the alloy exerts a similar effect and additionally enhances the hardness by 10 HB (Brinell hardness number). The study of the combined effect of alloying and cooling rate shows an additive effect of these factors for grain dividing, which makes it possible to achieve a decrease in the grain size to 5 μm. The absence of intermetallic compounds in the prepared samples of the AK6 alloy after the modification with the Al–Zr master alloy indicates that the phase composition of the initial Al–Zr master alloy exerts no effect on the properties of the target alloys.