Influence of Methods of Producing the AlTi Master Alloy on Its Structure and Efficiency in the Grain Refinement of Aluminum Alloy

A comparative study on the influence of fabrication methods of AlTi4 master alloys on the grain size of Al₃Ti intermetallic compounds is performed. It is found that an increase in the cooling rate during the solidification from 10–15 K/s (the crystallization in a hot cast iron mold and plate 30 mm in thickness) to 60–65 K/s (the crystallization in a cold cast iron chill mold; the rod 20 mm in diameter and 170 mm in length) promotes an increase in length and thickness of needle crystals of intermetallic compounds almost twofold (from 397 × 23 to 215 × 13 μm). Herewith, a decrease in electrical conductivity and an increase in the master alloy density in the solid state are observed. The modification of the master alloy melt by the addition of magnesium in an amount of 0.5 wt % determines the formation of homogeneous fine needles of intermetallic compounds 98 × 3 μm in size. The magnesium addition insignificantly decreases electrical conductivity and density when compared with the AlTi4 master alloy crystallized at the same cooling rate (60–65 K/s). The grain refinement of aluminum of the A97 grade and AK9ch alloy (the Al–Si–Mg system) by these foundry alloys with the same amount of introduced titanium (0.01 wt %) exert hereditary influence on the density and electrical conductivity, as well as on the marcograin (A97) and aluminum dendrites (AK9ch). The maximal modifying effect is characteristic of the AlTi4 master alloy containing magnesium in an amount of 0.5 wt %. Its introduction into the alloy promotes the formation of aluminum dendrites of 10 μm in size in an amount of 1427 pieces/mm² in the alloy structure. When modifying the AK9ch alloy by the master alloy crystallized with cooling rates of 10–15 K/s, dendrites 28 μm in size in an amount of 672 pieces/mm² are formed in the alloy structure. It is proposed to use the determination procedures of density and electrical conductivity for the express evaluation of the modifying efficiency of master alloys.