Thixoforming of an Fe-Rich Al-Si-Cu Alloy—Thermodynamic Characterization, Microstructural Evolution, and Rheological Behavior

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.

Distribution and Morphology of α-Al, Si and Fe-Rich Phases in Al–Si–Fe Alloys under an Electromagnetic Field

“…Flemings [4] discovered metallic alloys with special rheological properties caused by the formation of a non-dendritic structure in the semisolid state, e.g., in aluminum alloys, which leads to a transformation of the dendritic phase into the α-Al phase, shaped as rosettes or spheroids (globular) [5]. Non-dendritic structures exhibit rheological properties [6] that improve the mechanical properties of composites [7] and alloys [8], thereby allowing for the production of advanced engineering parts [9] in rheocasting [10], thixowelding [11], semisolid metal processing (SSM) [12,13], thixoforming [14], through the use of cooling slopes [15], by magnetohydrodynamics (MHD) [16] or simply by mechanical stirring [17]. Melt flow induced by rotating magnetic fields (RMF) in a technology called electromagnetic stirring (EMS) [18,19], transforms the microstructure [20] and improves the properties of billets [21].…”

“…Two main motivations spurred the current study. Firstly, due to the continuous development of forced-flow manufacturing technologies [5][6][7][8][9][10][11][12][13][14][16][17][18][19][20][21][22][23][24], the modification of the microstructure was investigated on the whole sample and in detail on individual Cylindrical samples were solidified with and without an induced rotating magnetic field (RMF) during the controlled slow cooling in insulated crucibles. Microstructures from optical metallography were measured using the ImageJ 1.51a software.…”

“…Two main motivations spurred the current study. Firstly, due to the continuous development of forced-flow manufacturing technologies [5][6][7][8][9][10][11][12][13][14][16][17][18][19][20][21][22][23][24], the modification of the microstructure was investigated on the whole sample and in detail on individual precipitates. Secondly, in terms of deleterious iron precipitates, the possibility of iron removal or its intermetallics separation was of interest.…”

Determination of the SSM Processing Window

“…The method combines the principles of differential calculus and features of the DSC curves to identify the liquidus, solidus, knee, and SSM working window on the fs vs. T, (or fl vs. T) curves of candidate alloys for SSM applications. This methodology has already been published in a partial and dispersed way in different works in the past [12][13][14][15], and the aim here is to present it in a more cohesive and didactic way, synthesizing the presented data and comparing them.…”

Effect of Rotating Magnetic Field on Microstructure in AlCuSi Alloys

“…In the presence of intensive melt stirring, non-dendritic structures may occur, with the primary α-Al phase shaped as spheroids (globular) or rosettes [5]. Non-dendritic structures exhibit rheological properties [6] improving the mechanical properties of alloys [7] and composites [8], making semisolid metal processing (SSM) [9], thixoforming [10], rheocasting [11] and thixowelding [12] unique for the production of advanced engineering parts [13]. Semisolid slurries [14] with globular solid particles may be produced by magnetohydrodynamic (MHD) [15] or mechanical stirring [16].…”