The benefits of grain refining and grain modification in sand castings

Published - 29th Aug 2024

Optimising the performance of aluminium-silicon alloys

Grain refining and grain modification are important in producing the optimum sand casting for your requirements. Understanding and controlling grain structure is crucial in metal manufacturing to achieve the desired mechanical properties and ensure the quality and performance of the final product.

So, what exactly is the grain, and how can this be changed to improve the performance of metal in sand castings?

In this blog, we’ll look at the importance of grain refining and modification in casting aluminium silicon alloys.

What is the ‘grain’ of metal?

• Metals are crystalline, made up of repeating patterns of metal atoms.
• As metal solidifies after heating, the atoms begin to cling together. This happens at different locations in the molten metal, called ‘nucleation points’.
• Each of these nucleation points will grow into a grain.
• The speed at which these nucleation points form is called the ‘nucleation rate’.
• The size and shape of the grains depends on factors that affect the nucleation rate, including the cooling rate and the presence of grain refiners.
• Many metals change their grain structure when heated, even without melting. The temperature at which this occurs is called the ‘recrystallisation temperature’ – when the metal crystals change structure and new grains grow.
• A microscope is usually needed to see grain structure, but in some metals the grains can be visible to the naked eye.

Why does grain size matter?

The size and structure of grains significantly affect the metal’s strength and ductility. For example, finer grains create a tighter and more uniform structure, reducing the chance of porosity that can cause cracks in the casting. Controlling grain size can therefore play an important role in achieving the desired mechanical properties of your casting, ensuring the quality and performance of your final product.

By implementing grain refinement techniques, Haworth Castings can significantly improve the quality and performance of our cast products for demanding industries, including for performance-critical applications such as Defence and Aerospace.

Enhancing the performance of aluminium-silicon alloys in casting

Aluminium is commonly alloyed with silicon for ease of casting. This is because it increases the fluidity of the melt, reduces the melting temperature, decreases the contraction associated with solidification and is a cost-effective raw material. Silicon also has a low density, which may be an advantage in reducing the overall weight of the cast component. Silicon has a very low solubility in aluminium, so it precipitates as virtually pure silicon, which is hard and therefore improves the abrasion resistance.

Aluminium-silicon alloys (Al-Si alloys) are critical in many applications due to their excellent castability, corrosion resistance and mechanical properties. However, the inherent microstructure of these alloys can limit their performance. This is where effective grain refining and modification help, providing significant benefits that elevate the quality and capabilities of Al-Si castings.

There are two principal ways to enhance the performance of the metal alloy in Al-Si castings: grain refining and grain modification.

Grain refining in sand casting

Grain refinement promotes the growth of finer grains, which merge more tightly to form a more homogeneous microstructure.

Grains size can be refined by introducing nucleating agents such as Titanium and Boron into the molten alloy to promote fine, uniformly distributed nucleation points.

Grain refinement can also be achieved through controlling the cooling rate of the alloy. Thermal grain refinement through rapid cooling results in the growth of finer grains.

Grain modification in sand casting

Grains can also be modified by adding certain elements or compounds which transform the coarse, plate-like silicon structures in the Al-Si alloy into fine, fibrous forms. This change enhances the alloy’s mechanical properties and overall performance.

Key benefits of grain refining and modification

(1) Improved mechanical properties

• • Enhanced strength and ductility: Fine-grained structures significantly improve tensile strength and ductility. This is crucial for applications requiring a high strength-to-weight ratio, such as in the automotive and aerospace industries.
  • Superior fatigue resistance: Smaller grain sizes reduce the likelihood of crack initiation and propagation, so enhance fatigue resistance. This is particularly beneficial in components subjected to cyclic loading.  For safety-critical components, grain refinement can be vital to achieve the required fatigue properties and overall performance.

(2) Enhanced castability and machinability

• • Reduced porosity: Fine grains help minimise the formation of gas porosity during solidification, leading to denser and more robust castings.
  • Improved fluidity: Grain refining improves the fluidity of the molten alloy, facilitating the casting of complex geometries with thin walls and intricate details.
  • Better machinability: A refined microstructure results in smoother surfaces and less tool wear during machining operations, lowering production costs and improving surface finish quality. Better machinability is important for the precise manufacturing of complex automotive and aerospace components.

(3) Optimised thermal and electrical conductivity

• • Uniform thermal expansion: Refined grains provide a more uniform thermal expansion coefficient, reducing the risk of thermal stress and distortion in high-temperature applications.
  • Enhanced electrical properties: Grain refinement can improve the electrical conductivity of Al-Si alloys, making them suitable for electrical and electronic applications where efficient thermal and electrical conduction is critical.

(4) Corrosion resistance and durability

• • Uniform properties: Grain refinement creates a more uniform microstructure, ensuring consistent properties throughout the casting, which is essential for parts with varying section thicknesses common in automotive and aerospace applications.
  • Uniform corrosion resistance: A refined and modified microstructure promotes a more uniform corrosion rate, enhancing the alloy’s durability in harsh environments.
  • Improved oxidation resistance: Modification processes can increase the alloy’s resistance to oxidation, extending the service life of components exposed to oxidative conditions.

(5) Cost efficiency and sustainability

• • Reduced alloy additions: Efficient grain refining and modification reduce the need for excessive alloying elements, lowering material costs.
  • Energy savings: Improved castability and reduced defect rates translate to lower energy consumption during casting and subsequent processing stages.
  • Recycling and reusability: Enhanced material properties contribute to longer component lifespans, supporting sustainability through reduced waste and more efficient recycling processes.

Expert grain refining and grain modification at Haworth Castings

Effective grain refining and grain modification of aluminium-silicon alloys present numerous advantages that extend beyond improved mechanical properties. By optimising the microstructure of the Al-Si metal alloy we can achieve superior castability, enhanced durability and greater cost efficiency. These benefits not only elevate the performance of Al-Si castings but also contribute to the broader goals of sustainability and innovation in the foundry industry.

Please get in touch to find out how Haworth Castings can optimise the manufacture and performance of your components.