At NovaCast, a large proportion of our castings are produced using Aluminium alloy. The formation of Aluminium Oxide and the presence of non-metallic inclusions, collectively known as dross, is an inherent part of the casting process and one that must be managed to achieve the highest quality castings. For many years, part of the foundryman’s skill has been in the use of fluxes to reduce alloy wastage, manage dross formation and clean the alloy of impurities that would otherwise impact quality.

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Understanding Aluminium Alloys

No matter which casting process is used, certain characteristics are inherent in the alloys that are cast, and each create their own set of challenges. Aluminium alloys tend to absorb hydrogen gas and will oxidise readily when molten alloy is exposed to the atmosphere or moisture. The oxidisation process results in a continuous alumina film across the surface. While this remains unmoved, the film protects the molten alloy from further oxidisation.

In a foundry, however, movement is part of the process and this results in cracking of the surface, further oxidisation and a rapid thickening of the surface film. This ever-thickening crust also encapsulates unoxidized Aluminium resulting in a “wet dross” which wastes a considerable amount of alloy. In a typical wet dross, only 20% to 40% is Aluminium Oxide with the rest made up of encapsulated Aluminium. This can result in wastage of 5% to 10% of the total metal melted.

‘Cover’ and ‘Drossing’ fluxes are used to manage these issues by forming a molten barrier “blanket’ on the surface of the alloy to reduce oxidation and hydrogen gas adsorption, and by reducing the thickness of the dross crust. As the surface crust does not thicken as much, these fluxes can reduce the amount of suspended (wasted) liquid metal by 60%-80%.

In addition to oxidisation and gas absorption, Aluminium alloys are susceptible to metallic and non-metallic impurities that are suspended within the molten alloy. Impurities and Aluminium oxides will remain suspended because they are porous and contain some gas adhering to and trapped in the pores. These impurities can be introduced into the melt during charging, treatment or handling operations and must be removed if the quality and consistency of the final casting is to be maintained. Cleaning Fluxes are used to remove non-metallic inclusions from the melt.

Good foundry practice also involves the use of wall cleaning fluxes which help keep crucible and furnace walls above and below the melt line free of oxide build-up. This reduces both alloy wastage and another source of unwanted inclusions.

Environmental Progress

In an increasing environmentally aware world, foundries need to play their part in reducing noxious emissions – both for the good of the planet and their own workforce. Traditionally, fluxes have been a necessary evil; vital in the foundry operation but a source of pollution that needed to be addressed. Flux manufacturers are continually trying to maintain or improve performance while reducing harmful emissions, particularly of fluorides and sulphur oxide. New granular forms of flux are reducing emissions by up to 85% without significant loss of performance as they allow less flux to be used and allow it to work in a different way. It has been found that the morphology of the flux is a very significant contributor to the type and quantity of emissions.

To discuss your requirements, call a member of NovaCast’s team on +44 (0) 1225 707466, send us a message here or email sales@novacast.co.uk.

Exploring Common Types of Aluminum Casting Flux

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In the intricate world of aluminum casting, where molten metal transforms into precision-engineered components, the role of flux is nothing short of alchemical. Flux, a substance applied during the casting process, plays a pivotal role in enhancing the quality of aluminum castings. Let’s delve into the fascinating realm of aluminum casting flux and explore the common types that are instrumental in this metallurgical symphony.

Potassium Chloride Flux:

Potassium chloride flux is a stalwart in the realm of aluminum casting. It operates by reacting with oxides and impurities in the molten aluminum, facilitating their removal as dross. This type of flux is renowned for its efficiency in promoting a cleaner metal surface, thus contributing to the production of high-quality aluminum castings.

Sodium Chloride Flux:

Sodium chloride, or common table salt, might seem like an unlikely candidate for the intricate process of aluminum casting, but its effectiveness as a flux is well-established. Sodium chloride flux functions similarly to potassium chloride by reacting with impurities and oxides, allowing them to be easily separated from the molten aluminum. Its cost-effectiveness and widespread availability make it a popular choice in various casting operations.

Fluoride-Based Fluxes:

Fluoride-based fluxes, such as cryolite (sodium aluminum fluoride), are known for their ability to improve the fluidity of molten aluminum. These fluxes work by forming low melting point compounds that aid in the removal of impurities. The use of fluoride-based fluxes contributes not only to cleaner castings but also to smoother casting processes.

Fluoride-Chloride Fluxes:

Combining the benefits of both fluoride and chloride, fluoride-chloride fluxes offer a comprehensive solution for aluminum casting. These fluxes leverage the reactive properties of both elements to effectively remove oxides and impurities. The synergy between fluoride and chloride enhances the overall performance of the flux in optimizing the casting process.

Aluminum Fluxes:

Some flux formulations are specifically designed with aluminum in mind. These aluminum fluxes often contain a combination of salts and other compounds tailored to react with the specific impurities found in molten aluminum. Their composition is fine-tuned to address the unique challenges posed by aluminum casting, ensuring improved casting quality.

Borax Flux:

Borax, a versatile compound, finds its way into the realm of aluminum casting as a flux. It acts by forming a protective layer on the molten aluminum, preventing oxidation and impurity buildup. Borax flux is valued for its ability to promote a stable casting environment and enhance the overall cleanliness of the metal.

Flux Combinations:

In many casting operations, a combination of flux types may be employed to harness the strengths of each. This approach allows for a tailored solution, addressing specific challenges in the molten aluminum while optimizing the overall casting process.

The choice of flux depends on various factors, including the specific casting operation, the impurities present in the aluminum, and the desired properties of the final castings. Each type of flux brings its unique set of advantages to the metallurgical ballet, contributing to the creation of flawless aluminum components. More contact Fluxing Agents

As the aluminum industry continues to evolve, so too will the formulations and applications of casting flux. Whether it’s the reliable potassium chloride, the unassuming sodium chloride, or the intricate combinations of fluoride and chloride, these fluxes stand as unsung heroes in the transformative journey from molten metal to precision-crafted aluminum castings.

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