The Correlation between Solidification Microstructures and Mechanical and Tribological Properties of Al-Sn-Cu and Al-Sn-Si Alloys

Abstract

The comprehension of the microstructural evolution during the solidification of metallic alloys is fundamental, since by establishing correlations between the manufacture steps and the characteristics of the final product, it is possible to envisage components having better properties and higher lifetime. The search of relationships between microstructural parameters and mechanical and wear behavior of alloys is fundamental for the pre-programming of final properties of as-cast components. Binary Al-Sn and Al-Si alloys, typically used for tribological applications, particularly for bearings and components of internal combustion engines, will need to be replaced by new alternatives caused by the design of new engines, which will be subjected to higher loads and velocities and hence will demand better properties to support the operation at higher temperatures. The present study aims to contribute to the understanding of modifications caused by a third alloying addition, in particular Cu and Si, on the microstructural evolution and mechanical and tribological properties. The literature reports that Al-Sn-Si e Al-Sn-Cu alloys have a good potential for tribological applications due to the strengthening of the Al-rich matrix by Si and Cu and because the Sn particles act as a solid lubricant. However, the literature is scarce on detailed studies relating microstructure features on the mechanical and wear resistances. The way the scale of dendritic spacings is affected by alloying content and solidification kinetics in Al-Sn-Cu and Al-Sn-Si alloys will be investigated. These alloys will be directionally solidified under transient heat flow conditions for a wide range of cooling rates, both vertically upwards and downwards, with a view to permitting the effect of macrosegregation on the evolution of microstructure and properties to be also analyzed. Both theoretical and experimental approaches will be used to quantify the effects of alloying on the solidification thermal variables: metal/mold heat transfer coefficient (h_i), tip growth rate (V_L), thermal gradient (G ), and cooling rate (T ). Experimental laws relating microstructural parameters to the mechanical and wear behavior will be developed. To complement the comprehension on the microstructure evolution of these alloys, the Bridgman growth of samples will be filmed in situ using an X-ray technique in the European Synchrotron Radiation Facility (ESRF) of Grenoble, France. In order to permit the effects of higher cooling rates during solidification on microstructure modifications to be examined, laser remelting experiments will be carried out. The resulting microhardness profiles of the treated and heat affected zones will also be determined.