Microalloying and Modification of Cast Aluminum Alloys for Increasing their Level of Exploitation Properties at Elevated Temperatures. A Review

Abstract

The work is devoted to the analysis of the most effective microalloying additives and modifiers influence on increasing the mechanical properties of Al-Si-based alloys (silumins) during their operation at elevated temperatures. It is shown that cast aluminum alloys based on the Al-Si system belong to a number of cheap and widely used heat-resistant aluminum alloys, but their level of mechanical properties is quite low, and operating temperature limits are mostly determined by 250 °C. Modification and microalloying is widely used to increase the level of operational properties of this type of alloys. In recent years, complex multicomponent modification of silumins by such elements as chromium, manganese, nickel, cobalt, titanium, zirconium and vanadium is considered to be more and more effective in strengthening, grain refinement, shape shifting of iron-containing phases, etc. Triple addition of these elements in a total amount up to 0.25 wt. % in many cases increases the efficiency of modification, compared with the addition of a single element. It is shown, that the addition of vanadium, molybdenum and tungsten helps to increase the hardness of alloys in the cast state. In this case, after hardening and two-stage aging, for alloys with molybdenum there is an increase in yield strength by 10% while maintaining the level of strength. Hafnium is considered as a promising nucleating element, the addition of which also significantly increases the resistance of recrystallization. Its addition to heat-resistant aluminum alloys can provide stabilization of mechanical properties up to 400 0 C. It is necessary to ensure the maximum possible grinding of hafnium intermetallics, especially in the presence of silicon in the alloy. Modification of aluminum alloys with scandium with the addition of titanium, zirconium or hafnium promotes the formation of Al 3 (Sc,Zr/Ti/Hf) dispersoids with a cubic crystal lattice of favorable symmetry L1 2 and a stable layered «core-shell» structure. The content of silicon in the alloy should be minimal due to the formation of harmful silicides. The addition of rare earth metals has a similar effect, but without the formation of layered structures. In this case, REM can form silicides, and can modify eutectic or primary silicon. In both cases, the addition of transition metals or REM, simultaneously with the modification of scandium alloys, increases the high-temperature stability of cast Al-Si-based aluminum alloys mechanical properties of through the formation of less active and diffusion-moving reinforcing dispersed phases.