Solidification microstructures, mechanical and wear properties of Al-Cu-Sn alloys with various compositions

Abstract

This thesis presents two complementary studies on alloys of the Al-Sn-Cu system which, although distinct in their objectives and approaches, converge toward a deeper understanding of the mechanisms governing microstructural formation and the functional performance of these materials. In the first study, model alloys were investigated to advance the fundamental understanding of liquid phase separation phenomena and monotectic transformation. In the second study, an industrial self-lubricating alloy (SAE 783) was analyzed with the aim of correlating microstructure with mechanical properties and wear behavior. The Al-Sn-Cu system with high Cu and Sn contents exhibits pronounced complexity due to liquid phase separation during solidification. Within this context, the solidification paths of three Al-10Cu-xSn alloys (x = 5, 10, and 20wt.%) were examined using an integrated approach that combined thermodynamic calculations based on the CALPHAD method, thermal analysis by differential scanning calorimetry (DSC), and in situ observations by X-ray radiography. The results demonstrated that the mechanism of liquid phase separation strongly depends on the Sn content: in Sn-poor alloys, the process occurs progressively and culminates in a monotectic reaction, whereas Sn-rich alloys exhibit a two-step separation mechanism that is not predicted by thermodynamic calculations. The SAE 783 alloy (Al-20wt.%Sn-1wt.%Cu) was studied under transient directional solidification conditions. Microstructural characterization included automated measurements of dendritic arm spacing, X-ray microtomography, and SEM/EDS analyses, enabling the evaluation of Sn morphology and distribution within the matrix. The results revealed that the interface between the β-Sn phase and the α-Al matrix is critical for the simultaneous enhancement of strength and ductility and plays a central role in wear performance, contributing to a better understanding of the trade-off between mechanical properties and wear resistance in Al-Sn based self-lubricating alloys.