Abstract
Thixoforming depends on three factors: (a) the thermodynamic stability of the solid-to-liquid transformation in the presence of temperature fluctuations; (b) the size and morphology of the solid particles in the liquid in the semisolid state; and (c) the rheology of the semisolid slurry during formation. In this study, these parameters were characterized for an Al-Si-Cu alloy with a high Fe content (B319+Fe alloy). Fe is usually found in raw metal produced by recycling, and its removal increases processing costs. This study is an attempt to use this lower-cost alloy for the thixoforming route. Thermodynamic analysis was performed by numerical simulation (under Scheil conditions) and the application of the differentiation method (DM) to differential scanning calorimetry (DSC) curves recorded during heating cycles up to 700 °C at 5, 10, 15, 20, and 25 °C/min. The processing window was evaluated by comparing the results of the DM and those of the analysis of open-die thixoforged samples after isothermal heat treatment at 575, 582, 591, and 595 °C for 0, 30, 60, and 90 s. The microstructural and rheological behavior of the semisolid slurry was analyzed at 591 and 595 °C for all four soak times. Isothermal heat treatment caused the refinement and spheroidization of the solid phase. Good agreement between the predicted thermodynamic behavior and the microstructural behavior of the thixoformed B319+Fe alloy samples was observed. Although the alloy exhibited a coarse microstructure, it was microstructurally and rheologically stable at all temperatures and for all the soak times studied, indicating that B319+Fe is a promising raw material for thixoforming.
Highlights
The microstructure of the as-cast B319+Fe alloy is shown in Figure 2 as a reference for the heat-treated conditions
Thixoforming at 595 ◦ C results in slightly smaller values of maximum stress and apparent viscosity (Figure 14b,d) than thixoforming at 591 ◦ C (Figure 14a,c), which agrees with the higher liquid fraction of the alloy at 595 ◦ C and consequent reduced viscosity and strength of the slurry. These results indicate that the liquid fraction is more effective in changing the rheological behavior of the B319+Fe alloy than the slight changes in microstructure caused by different soak times
The results reveal good agreement between the thermodynamic behavior predicted by the differentiation method (DM) and the data obtained from semisolid forging of samples of the alloy
Summary
It is crucial in thixoforming that (a) the solid particles in the liquid have a refined, spheroidal microstructure and (b) the solid-liquid mixture is stable. The former requirement ensures that the Metals 2018, 8, 332; doi:10.3390/met8050332 www.mdpi.com/journal/metals. A stable solid-liquid mixture, i.e., a controllable microstructure, rheology, and liquid fraction in the presence of temperature variations during formation, ensures that SSM processing is reproducible. Achieving this stable mixture requires certain thermodynamic characteristics, which in turn depend on the chemical composition of the alloy and the kinetics of the thixoforming process. An alloy possessing these characteristics is said to be thixoformable [10,11,12]
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