Thermal Analysis and Microstructure Investigation of Eutectic (Al-Si) and Dilute Eutectic (Al-Fe) Casting Aluminum Alloys

Abstract

The world today is experiencing a revolution in the automotive industry; internal combustion engine vehicles are being replaced rapidly by electrical vehicles. With the widespread adoption of electric vehicles, high-performance cast aluminum alloys are increasingly capturing the market held by conventional cast aluminum alloys, particularly for the manufacturing of structural components with stringent requirements. Conventional Al-Si eutectic casting aluminum alloy (ECA) – AlSi10MgMn (10 w% of silicon), as one of the specimens in this research, although possessing excellent castability and capable of demonstrating acceptable mechanical properties after appropriate heat treatment, has gradually reached their developmental limits due to complex compositions and additional heat treatment costs. The ECA alloy is a conventional Al-Si based near-eutectic composition with Mg added for solid solution strengthening, Mn for countering the harmful effects of ~0.2 w% Fe impurity in the alloy. The solidification of ECA features a sequential evolution of the phases such as the primary α(Al), Si and other intermetallic phases in that order. The novel dilute eutectic casting aluminum alloy (DECA) with (AlZnMg)-1.2Fe (1.2 w% of iron), has zinc and magnesium as solution strengtheners, exhibits excellent yield strength, ductility, and self-aging characteristics. There are only two phases that collaboratively evolve during solidification of the DECA alloy: α(Al) and AlxFey intermetallic phase, while the Zn and Mg exhibits complete solid solubility in the Al phase during solidification, and the alloy is notably castable. This research investigates the solidification (thermal and microstructure) behaviors of both the ECA and DECA alloys using a variety of methods such as simulation with the non-equilibrium Scheil-Gulliver solidification paradigm, zero thermal gradient solidification process (differential thermal analysis), and shallow thermal gradient solidification process (two-thermocouple experiment). Additionally, the grain growth restriction factor (Qtrue) and hot tearing sensitivity index (HTS) were evaluated for each solidification condition, accompanied by corresponding microstructure analyses. Although DECA castings exhibits a more desirable equiaxed rosette morphology of the primary Al phase in the microstructure, its nucleation and solidification behaviors explain why these alloys demonstrates higher hot tearing sensitivity compared to ECA. Furthermore, microstructural analysis reveals that DECA has a delayed dendrite coherency point (DCP) compared to ECA. The conventional theory that a delayed DCP is beneficial for castability does not apply in this context for the DECA system.