Tailoring of Microstructures and Tensile Properties in the Solidification of Al-11Si(-xCu) Brazing Alloys

Abstract

Ternary Al-11wt %Si-(xwt %)Cu alloys are highly recommended as commercial filler metals for aluminum brazing alloys. However, very little is known about the functional inter-relations controlling the solidified microstructures characterizing processes such as torch and furnace brazing. As such, we evaluated two commercial brazing alloys, which are the Al-11wt %Si-3.0wt %Cu and Al-11wt %Si-4.5wt %Cu alloys: Cu contents typically trend in between the suitable alloying spectrum. We analyzed the effects of solidification kinetics over features such as the dendrite arm spacing and the spacing between particles constituting the eutectic mixture. Also, tensile properties were determined as a function of the dendrite microstructure dimensions. The parameters concerned for translating the solidification kinetics were either the cooling rate, or growth velocity related to the displacement of the dendrite tip, or the eutectic front. The relevant scaling laws representing the growth of these brazing alloys are outlined. The experimental results demonstrated that a 50% increase in Cu alloying (from 3.0 to 4.5 wt %) could be operated in order to obtain significant variations in the dendritic length-scale of the microstructure across the produced parts. Overall, the microstructures were constituted by an α-Al dendritic matrix surrounded by a ternary eutectic consisting of α-Al + Al 2 Cu + Si. The scale measurements committed to the Al 2 Cu eutectic phase pointed out that the increase in Cu alloying has a critical role on refining the ternary eutectic. the solidification process is essential, since its influence can be noted even in the finished product [3,6]. According to Zeren et al. [7] more research efforts are necessary for a better understanding of the mechanisms responsible for strength variations in Al-Si-xCu alloys.The solidification process and the intrinsic characteristics of the alloy to be solidified have a direct influence on the microstructure formation, which determines the final properties of a casting. The obtained casting parts exhibit mechanical characteristics that depend on inherent aspects occurring during solidification, such as: grain size, the scale of the phases forming the microstructure such as dendrite arm spacings and interphase spacings, the size and distribution of such phases, chemical composition heterogeneities, inclusions, and porosity. The understanding of solidification of aluminum alloys has fundamental importance for planning of manufacturing processes, since it allows a better understanding of the factors affecting microstructure, and consequently, the product quality [8][9][10][11][12][13].For the manufacturing of castings with predefined local properties, a widely used process is foundry technology (metal/mold casting processes). The cooling rate related to the process of heat transfer from the casting into the mold is an aspect acting in parallel, which conditions the change of the size and distribution of individual components within the resulting microstructure. The kind of crystall...

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