Abstract
A quantitative study on the kinetics of nucleation of primary Si particles (PSPs), especially under the effect of P inoculation, during isothermal melt solidification of hypereutectic Al–Si(–Cu) alloys has been realized for the first time by using a unique micro-focus in situ X-radiography method, which is impossible by synchrotron X-radiography or tomography methods. The nucleation undercooling and nucleation rate of PSPs have been measured. Besides, TP-1 type solidification test has been carried out. It is found that nucleation undercooling of PSPs is reduced, while nucleation rate and number density of PSPs is increased significantly by P inoculation. Moreover, the influence of cooling rate on the nucleation kinetics of PSPs in the P inoculated alloy was investigated in situ. It is observed that higher cooling rate has the influence of increasing the peak nucleation rate and extending the nucleation temperature ranges of PSPs, in terms of earlier nucleation at lower undercooling and nucleation stopping at higher undercooling, which results in higher number density of PSPs. The decrease in minimum nucleation undercooling with increasing cooling rate in the P inoculated alloy is unexpected, which has been attributed to the extremely slow growth rate of faceted Si crystal on AlP substrates under low undercooling.
Highlights
Hypereutectic Al–Si alloys are widely used to produce engine parts due to their low density, hightemperature resistance, high wear resistance, and low coefficient of thermal expansion [1,2,3,4]
The mechanical and physical properties of these alloys are strongly influenced by the size and morphology of the primary silicon particles (PSPs), which can have different morphologies, including octahedral, polyhedral, feathery-like, star-like containing two to five branches, and hexagonal-shaped plates [1, 3, 5, 6]
Ullah et al [11] investigated the influence of silicon content (17–38 wt.%) on the morphologies of silicon crystals formed during solidification of Al–Si alloys, and showed that the Si particles have a fish-bone or star-like morphology at lower silicon contents, but change to large plates at higher Si contents
Summary
Hypereutectic Al–Si alloys are widely used to produce engine parts due to their low density, hightemperature resistance, high wear resistance, and low coefficient of thermal expansion [1,2,3,4]. For hypereutectic Al–Si alloys without modification/inoculation, the morphologies of PSPs are strongly influenced by cooling rate and silicon contents in the alloys. The number density of Si–Si clusters determines the size of PSPs after casting This was supposed to be the reason that an over-heating of the aluminium melt before casting can significantly change the morphology of primary Si particles, from star-like and other irregular shapes to octahedral shapes [3, 7, 9,10,11,12], and at the same time reduce the size of Si particles [7, 10]
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