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Heard about aluminum casting—the process behind lightweight yet durable components? Did you know that aluminum-silicon alloys are the key materials used for this? It’s silicon that enhances the fluidity of aluminum which ultimately allows molten alloy to fill intricate molds with precision and minimal defects.
But casting is only half the story. Once formed, these components often require machining, a step that comes with its own challenges. High silicon content can be abrasive, accelerating tool wear if not handled correctly.
This guide explains the technical aspects of aluminum-silicon alloys, covering their composition, types, casting behavior, and machining challenges. The goal is to give you a complete understanding of when and how to use them effectively.
What are Aluminum Silicon Alloys?
Pure aluminum is primarily known for its lightweight nature, corrosion resistance, and excellent thermal and electrical conductivity. However, its relatively low strength and poor wear resistance make it unsuitable for demanding applications. Silicone itself is a brittle, high-melting-point element that lacks ductility.
When the two elements are combined, we get aluminum-silicon alloys – the alloy gains improved wear resistance, and lower thermal expansion. The alloy attains a lower melting point and fluidity also improves because of silicon, making them the best candidates for casting applications.
Quantitatively, aluminum is the dominant element in the alloys. Most AlSi alloys have silicone between 3 to 25%. However, in alloys made with powder metallurgy, silicon’s percentage can be up to 50%.
Aluminum silicone alloys largely fall under the 4xxx series of aluminum alloys. That group is known for lower melting points, good wear resistance, weldability, and fluidity. Besides casting, such alloys are used in welding and brazing.
Classification Pure Al-Si Alloys
Although Al-Si alloys can include other elements like copper or magnesium, we will focus on pure aluminum-silicon alloys. In these alloys, aluminum and silicon are the main constituents.
Pure Al-Si alloys are split into three groups, based on the percentage of silicon.
Eutectic Alloys
“Eutectic” means that both aluminum and silicon solidify together at one specific temperature. This forms a fine structure where each element is present side by side.
In eutectic alloys, the percentage of silicon hovers around 11% to 13% silicon. The whole melt solidifies at once, which gives a good castability. These alloys flow well in molds, making them ideal for thin-walled parts. When refined, they have better fatigue strength and elongation.
Hypoeutectic Alloys
They have less than 12% silicon. Aluminum solidifies first, leaving a silicon-rich melt that forms the eutectic later. Slow cooling can produce large, brittle silicon plates. Refiner elements (like sodium) help create a finer structure and better strength. General automotive castings including engine blocks and housings are made from hypoeutectic alloys.
Hypereutectic Alloys
They contain more than 13% silicon. Silicon crystals form first, then the rest solidifies as a eutectic. These alloys have low thermal expansion and high wear resistance. However, the high silicon content leads to faster tool wear, so diamond tools are used for machining them. Such alloys are used for pistons, cylinder liners, and similar applications where parts need to face wear resistance.
Mechanical Properties of Aluminum Silicon Alloys
Silicon addition alters different mechanical properties of Aluminum, making it suitable for multiple applications. Here’s a look at some key properties of AlSi Alloys.
Density
Aluminum-silicon alloys do maintain a low density, similar to pure aluminum. So, they remain suitable applications where weight is a concern.
Strength
The addition of silicon enhances the strength of aluminum. Alloys with 6% to 12% silicon content exhibit optimal mechanical properties. Their tensile strength ranges between 140 to 260 MPa.
Corrosion Resistance
Pure aluminum-silicon alloys are corrosion-resistant, even in saltwater environments. The marine applications can benefit from it.
Casting Properties
The biggest change that silicon brings is a lower viscosity of molten alloy. This improves the fluidity of the material – and the alloys also enjoy a low melting point. Plus, silicon also reduces shrinkage during solidification. All these reasons explain why AlSi alloys are sometimes called casting alloys of Aluminum.
Wear Resistance
Hypereutectic alloys with more than 13% silicon, have high wear resistance due to the presence of primary silicon particles.
Machining Practices for Aluminum Silicon Alloys
As mentioned earlier, some of the alloy types are harder and wear-resistant, which makes them challenging to machine. A few tools and other machining practices that cope with this challenge are listed below:
Machining Processes
Aluminum alloys usually undergo milling and turning operations. The milling process is commonly used, especially for finishing components in the automotive industry. Face milling is often chosen for its efficiency and ability to produce quality surfaces.
Cutting Tool (Material and Types)
For machining aluminum-silicon alloys, especially hypereutectic alloys with high silicon content, tools made from cemented carbide and polycrystalline diamond (PCD) are common.
At times, cemented carbide can struggle with the adhesion of aluminum chips, leading to quicker tool wear. To combat this, diamond-like carbon (DLC) coated tools are used because they offer lower friction and enhanced hardness.
Cutting Parameters
The cutting parameters are usually specific to the grade and type of aluminum being machined. For soft aluminum alloys, cutting speeds between 100 to 500 meters per minute (m/min) are recommended. For hard silicon aluminum, cutting speeds should be reduced to 100 to 200 m/min. The feed rates are increased based on the cutting-edge diameter.
Coolants
Normally ethanol is recommended for cooling and lubricating the Aluminum machining process. However, in light of sustainability and reduced environmental impact due to hazardous coolants, dry machining is increasingly preferred. Dry machining suits well when working with DLC-casted tools.
Applications of Silicon Aluminum Alloys
Aluminum-silicon alloys are extensively used in foundries for casting vehicle parts due to their excellent casting properties and mechanical characteristics. Key applications are:
Automotive Applications
Engine blocks, pistons, cylinder heads, gear housings, wheel rims, chassis parts, intake manifolds, etc.
Aerospace Applications
Wing structures, engine components, landing gear components, satellite structures.
Conclusion
Aluminum-silicon alloys are the backbone of the casting industry, especially in manufacturing critical lightweight parts for the automotive and aerospace industries. But machining may still be required to make these Al-Si products.
If you need help with Aluminum CNC machining or other metal alloys, reach out to Rich Conn. We have a comprehensive machining setup to satisfy all your metal CNC machining needs. Whether your project demands milling, turning, or the use of 5-axis machines, we have everything.
Reach out to us and get an instant quote.
