The Aluminum-Silicon Alloy Connection

This connection sounds like a new hip-hop duo, but it is the driving force in the foundry industry. Silicon, the most commonly used alloying element in aluminium alloys, can be found in these alloys in varying amounts, ranging from 5% to 25%. This versatility allows them to be categorized into eutectic, hypereutectic, and hypoeutectic types, catering to a wide range of applications in the foundry industry. Most common are the hypoeutectic alloys, ranging from 7 to 12% of silicon.

But first, what is an eutectic point?

An eutectic systemis a homogeneous mixture with a melting point lower than the constituents. The eutectic temperature is the lowest possible melting point over all constituents’ mixing ratios. The melt undergoes a direct phase change from liquid to solid at that point. 

Aluminium-Silicon alloys have an eutectic point of 12.7% silicon. And silicon is added to aluminium for several reasons:

1. Improved Castability: Silicon enhances the fluidity of aluminium, making it easier to cast into complex shapes in HPDC. The silicon content in Rheocasting does not impact castability.
   
   

2. Strength: Adding silicon can increase the strength of the aluminium while sacrificing ductility. The silicon needles can be modified with strontium to round them and avoid being a crack’s starting point.
   
   

3. Corrosion Resistance: Al-Si alloys are corrosion-resistant, making them suitable for harsh environments.
   
   

4. Lower Thermal Expansion: Silicon lowers the alloy’s thermal expansion of the aluminium, reducing shrinkage porosity.
   
   

Are There Any Downsides? While Al-Si alloys have many benefits, there are a few downsides:

1. High-Temperature Strength Degradation: Al-Si alloys lose strength rapidly at temperatures higher than 250 °C.
   
   

2. Carbon Footprint: Most silicon comes from primary sources and has a high carbon footprint of 4 to 10 kg CO2/kg Si.
   
   

3. Formation of Harmful Phases: Silicon can interact with aluminium and iron to form harmful intermetallic compounds, drastically impacting the alloy’s ductility.

It’s important to note that the benefits and downsides of Al-Si alloys can vary depending on the alloy’s composition and the specific application it’s used for. Understanding the Al-Si phase diagram, a crucial tool for alloy designers and manufacturers provides a roadmap for tailoring the properties of Al-Si alloys to meet specific application requirements. Simulations are needed to account for all interactions for a deeper analysis of which phases will form.

If you find such information valuable, you can learn way more in the Process Optimisation Workshop. Schedule a Free Consultation Call to discuss your foundry’s situation.