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

Bruno Donadoni,José Spinelli,Leonardo Gomes,Amauri Garcia

doi:10.3390/met8100784

Bruno Donadoni, José Spinelli + Show 2 more

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https://doi.org/10.3390/met8100784

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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 + Al2Cu + Si. The scale measurements committed to the Al2Cu eutectic phase pointed out that the increase in Cu alloying has a critical role on refining the ternary eutectic.

Highlights

Over a wide range of mechanical and thermal applications, multicomponent alloys pertaining toAl–Si systems are most commonly fabricated through processes such as foundry, brazing, and welding [1].The binary Al–Si alloys present a microstructure consisting of a primary phase, aluminum, or silicon, and a eutectic mixture of these two elements [2]
The longitudinal sections could be assessed for the growth of secondary dendrite branches, whereas the cross sections along the length of the directionally solidified (DS) bodies were useful for the determination of either primary or tertiary dendritic spacing
The solidification microstructures of the Al-11wt %Si-3.0 and 4.5wt %Cu braze alloy samples were shown to be characterized by aluminum-rich dendrites enveloped by the products of the eutectic reactions; that is, the Al–Si eutectic and the Al–Si–Al2 Cu ternary eutectic

Summary

Introduction

The binary Al–Si alloys present a microstructure consisting of a primary phase, aluminum, or silicon, and a eutectic mixture of these two elements [2]. Silicon is added to aluminum alloys to promote good wear resistance, high heat transfer coefficient, and low thermal expansion coefficient. The addition of Cu to Al–Si alloys is widely used in automotive engine components, such as engine blocks, cylinder heads, and pistons, because of the good castability and fluidity of Al–Si–Cu alloys [1]. Al–Si–Cu ternary alloys are becoming increasingly important in the aerospace and automotive industries, due to their low relative weight and good mechanical strength at relatively high temperatures, and good resistance to abrasion and weldability. Al–Si–Cu ternary alloys have higher mechanical strength than Al–Si alloys and higher corrosion resistance than Al–Cu alloys [3,4,5]

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