Casting Al-Si Alloys (ENGS 24)

Casting Aluminum–Silicon Alloys

ENGS 24: Science of Materials · Final Project · Summer 2025

Sand Casting · SEM · Optical Microscopy · Hardness Testing

Overview Methods Results: Microstructure Results: Cooling Rate Analysis Deliverables

Project Overview

This project investigated how silicon composition and cooling rate affect the microstructure and hardness of sand-cast aluminum alloys near the eutectic composition (~12.6 wt% Si).

By systematically varying silicon content from 0–25 wt% and analyzing the resulting microstructures, we explored structure–property relationships central to alloy design and manufacturing.

Experimental Approach

1) Casting

Sand molds
0–25 wt% Si
900 °C melt

2) Sample Preparation

Precision sectioning
Polishing to 0.3 µm
NaOH etching

3) Characterization

Optical microscopy
SEM imaging
Rockwell hardness

Sand casting and melt preparation of Al–Si alloys.

Sample preparation workflow for cast Al–Si alloys — Sample preparation workflow: sectioning, polishing, and etching prior to microscopy.

Microstructure vs Silicon Content

6% Si microstructure — 6 wt% Si — dendritic Al matrix

12.6% Si microstructure — 12.6 wt% Si — eutectic structure

25% Si microstructure — 25 wt% Si — primary Si crystals

Analysis: As Si content approached eutectic composition (~12.6% Si), more and more eutectic structures can be seen. As we created compositions that are hypereutectic (> 12.6% silicon), silicon crystals can be seen appearing.

Effect of Cooling Rate

Fast Cooling
Dendrite-dominated structure

Slow Cooling
Eutectic-dominated structure

Fast cooling: directional and uneven; Al dendrites solidify first
Slow cooling: more uniform; eutectic solidification produces interwoven Al–Si

Conclusions/Results: Structure–Property Relationship

Note: the orange and blue points represent samples that went through two seperate sample preperation steps as one of the machines broke during the term. Although a difference can be seen in the hardness values, the overall trend still holds true.

Hardness increased with silicon content up to the eutectic composition, where fine interwoven phases impede dislocation motion. At higher silicon levels, large primary Si crystals increased brittleness. (See Microstructure vs Silicon Content Effect of cooling rates.)