Abstract
Laser surface melting followed by rapid solidification is an effective means to produce very fine microstructures with desirable surface properties because of the high rates of cooling associated with it. In the present study, the effect of rapid cooling on the silicon particle size, distribution, and morphology of hypereutectic Al–17wt.%Si and Al–20wt.%Si alloys have been investigated. A continuous-wave CO2 laser of wavelength 10.6 μm and a Trumpf Yb-YAG disk laser of wavelength 1.030 μm were used with a beam diameter of 1 mm and scanning speeds ranging from 5 to 100 mm/s. Rapid solidification increased the solubility of silicon in aluminum to approximately 5wt% and induced non-equilibrium hypoeutectic microstructures comprising large volume fractions of primary α-Al dendrites and ultrafine Al–Si eutectic of lamellar morphology. Both α-Al dendrites and the silicon particle sizes were significantly reduced from micron to nanoscale level. The morphology of silicon particles is modified from massive polygonal and plate-like to a mixture of fine flakes with round corners, feathery and fibrous, or a coral-like and thread-like structure. The eutectic silicon size and the interlamellar spacing were reduced to 30 and 10 nm, respectively. Furthermore, most of the silicon crystals in the eutectic region and the aluminum dendrites contained a significant number of twins which were considered as an essential contributor to the mechanism of growth and branching. Microhardness values increased two to threefold due to the refinement of the microstructural constituent.
Highlights
Al–Si casting alloys have received considerable interest as candidate materials in the automotive and aerospace applications due to the high specific strength, relatively low coefficient of thermal expansion, good wear resistance, and excellent fluidity[1,2]
Morphology, and distribution of eutectic silicon are vital microstructural parameters determining the mechanical properties of Al–Si alloys
This investigation aims at studying the effects of cooling rate on the microstructure of hypereutectic Al–Si alloys with a view to refine and modify the silicon eutectic particles and improve surface-related properties such as wear and strength
Summary
Al–Si casting alloys have received considerable interest as candidate materials in the automotive and aerospace applications due to the high specific strength, relatively low coefficient of thermal expansion, good wear resistance, and excellent fluidity[1,2]. The microstructure of the hypoeutectic Al–Si cast alloy usually consists of a primary phase (α-Al) and eutectic mixture of Al–Si. The eutectic silicon crystallizes under conventional solidification conditions into a course, plate-like morphology. It is well known that the as-cast unmodified eutectic silicon has an undesired plate-like morphology, which is the stress concentrator reducing strength, ductility, and fatigue strength. This investigation aims at studying the effects of cooling rate on the microstructure of hypereutectic Al–Si alloys with a view to refine and modify the silicon eutectic particles and improve surface-related properties such as wear and strength
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