Abstract
The present study was carried out on three Al‐Si cast alloys viz., 319, 356, and 413 alloys, solidified at 8°C/s. Samples from 319 and 413 alloys were solution heat‐treated at 510°C, whereas samples from 356 alloy were solutionized at 550°C, for up to 1200 h. The results reveal that complete spheroidization of eutectic Si particles in terms of achieving individual spherical particles cannot be achieved in most Al‐Si‐Cu‐Mg alloys even after a solutionizing time of 1200 h which contradicts with the existing theory. Addition of Sr to Cu‐free 356 alloy could lead to complete spheroidization after 1200 h at 550°C if the alloy was solidified at 8°C/s. Besides the dissolution theory of Ostwald, coarsening of Si particles can as well take place by impingement, fusion, and agglomeration. Increasing the Si content makes it difficult to achieve spheroidization, i.e., fragmentation and coarsening. Results obtained from observations of deeply etched samples (3D) contradict those obtained from polished samples (2D).
Highlights
A study of Tables 2–5 shows that the average surface area of the silicon particles of alloy 319 increases with solutionizing treatment, both for modified and unmodified samples. us, there is a growth of the thickness of the silicon particles which may be accompanied by an increase in length for the unmodified samples
For Srmodified samples, the silicon particles undergo fragmentation, followed by coarsening. us, particle density is increased in these samples
Solution heat treatment has the effect of reducing the density of the Si particles in the samples. erefore, there is dissolution of certain silicon particles which feed the growth of other silicon particles
Summary
In the case of unmodified Al-Si alloys (for the majority of alloys), coarsening of Si particles takes place by a combination of particle fragmentation mechanism along with the silicon particle dilution mechanism, similar to that proposed by Ostwald. In nonmodified Al-Si-Mg alloys, the coarsening rate of eutectic silicon was found to follow the Lifshitz–Slyozof–Wagner (LSW) theory, which corresponds to a zero volume fraction approximation. E coarsening rate can be calculated using equation (1) following the diffusion-controlled growth model [7, 8], known as the LSW model: r3 − r03 KLSW ∗ t,. E equivalent circular diameter is the diameter of a circle that has the same area (A) as the Si particle and may be calculated using the following formula (Figure 3): .
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