DCS Crystallization Analysis of AlSi17Cu5Mg Alloy After Superheating Well Above the Liquidus Temperature

Abstract This paper presents superheating of a liquid AlSi17Cu5Mg alloy to temperatures from 760°C to 960°C (in 40 increments), annealing it for a specified time and casting it into a copper die. Based on the results of mechanical properties, tensile strength (UTS), conventional yield strength (YS) and elongation (A), it was found that the optimal (in terms of measured mechanical properties) method is to superheat the AlSi17Cu5Mg alloy to temperatures from 920°C to 960°C, to anneal for about 40 min and cast it into a die. This range causes the most significant increase in mechanical properties, i.e., UTS by about 40%, YS by about 70% and A by about 60% compared to the traditionally cast alloy. The selected alloy was subjected to crystallization studies for the best superheating variant by differential scanning calorimetry (DSC). The study was carried out on a Multi HTC Setaram high-temperature calorimeter by directly determining the parameters of high-temperature processes, especially enthalpies of the occurring phase transformations, using heating and cooling rates of 20°C min −1 . Characteristic crystallization parameters ( T liq. , T E and T sol. ), enthalpies of eutectic α(Al) + β(Si), and primary silicon crystals β(Si) during endo- and exothermic reactions were determined. Based on the study, it was found that superheating the alloy increases the crystallization temperature of the primary silicon crystals ( T liq. ), reduces their size (from about 350–450 µ m to about 50–80 µ m) and changes their morphology toward compact tetrahedral and octahedral solids compared to the alloy without superheating. In addition, calorimetric studies have shown that superheating the alloy well above T liq. results in an increase in the value of specific heat capacity ( c p ), indicating a higher resistance of the alloy to thermal shocks. This is due to the separation of intermetallic phases (Al 2 Cu, Mg 2 Si), which are present in the alloy and cause its strengthening. Overheating also reduces the scatter of the results of the tested properties, which indicates a better homogeneity of the structure, especially the uniform distribution of β(Si) crystals in the α(Al) matrix. It can, therefore, be assumed that overheating is an alternative to modifying hypereutectic silumins with traditional phosphorus-based mortars.