Degassing and Refining Processes in Aluminum Alloy Casting

Degassing and Refining Processes in Aluminum Alloy Casting

For buyers, porosity and inclusion defects in castings are key pain points that affect product performance and business stability. As a core pretreatment step before aluminum alloy casting, degassing and refining processes directly determine the purity of the molten aluminum, thereby preventing cracking and leakage during subsequent processing. This article systematically analyzes the degassing and refining process from four perspectives: process principles, core methods, key control points, and practical application value, helping buyers accurately assess a supplier's technical capabilities.

I. Basic Understanding: Why Degassing and Refining Are the "Lifeline" of Aluminum Alloy Casting

During the aluminum alloy smelting process, two types of harmful impurities are inevitably introduced into the molten aluminum. If not removed through process removal, they will directly lead to scrapped castings or substandard performance.

Gaseous Impurities: Hydrogen, primarily an "invisible killer," reacts with water vapor and oil contaminants in the furnace gas at high temperatures to produce hydrogen. The solubility of hydrogen in molten aluminum decreases sharply with decreasing temperature. When molten aluminum solidifies, hydrogen that isn't promptly expelled forms pores. These pores weaken the mechanical properties of the casting, reducing tensile strength and fatigue life. They also affect the casting's airtightness, leading to leakage in products like hydraulic components and radiators.

Solid Inclusions: The main culprits that disrupt casting uniformity. Inclusions primarily originate from scale in the raw materials, sloughed furnace linings, and oxidation products (such as Al₂O₃) during the smelting process. These solid impurities act like "breakpoints" within the casting, reducing its machinability (such as accelerated tool wear during cutting) and causing stress concentrations under load, leading to cracking during use. This is particularly problematic for high-strength applications such as aerospace and automotive structural parts.

II. Degassing Process: The Core Method for "Extracting" Hydrogen from Molten Aluminum

The core principle of the degassing process involves introducing an inert or reactive gas into the molten aluminum. The bubbles act as an "adsorption-carrying" mechanism, drawing dissolved hydrogen to the surface. Currently, mainstream degassing methods fall into three categories, each with its own applicable scenarios and technical advantages.

1. Rotary Injection Degassing (SNIF): The efficient and stable mainstream choice
Process Principle: A rotating nozzle with air holes is inserted into the molten aluminum, and argon or nitrogen is introduced. The shear force generated by the rotation disperses the gas into tiny bubbles (1-3mm in diameter), significantly increasing the gas-liquid contact area and effectively adsorbing hydrogen.
Key Advantages: Degassing efficiencies can reach 70%-90%, with uniform bubble distribution, eliminating splashing caused by localized excessive airflow. The rotation speed (typically 300-600 rpm) and gas flow rate (5-15 L/min) can be adjusted to accommodate different grades of aluminum alloys (such as 6061 and 7075).
Applications: Continuous production lines in medium- to large-scale foundries, particularly suitable for mass-produced castings such as automotive wheels and engine blocks.

2. Vacuum Degassing: A High-End Solution for Ultimate Purity
Process Principle: Molten aluminum is placed in a sealed container and vacuumed using a vacuum pump. This reduces the hydrogen partial pressure at the surface of the aluminum, encouraging the hydrogen in the aluminum to diffuse toward the surface and escape.
Key Advantages: This process can control the hydrogen content in the aluminum to below 0.1mL/100g Al, making it the only process currently capable of meeting the purity requirements of aerospace-grade aluminum alloys (such as 2024). It eliminates the need for gas introduction, preventing secondary contamination from foreign impurities.
Applications: High-end casting production, such as aircraft structural parts and precision instrument housings, requires extremely high hydrogen content. However, this method is associated with high equipment costs and relatively low production efficiency.

3. Gas Agitation Degassing: A Low-Cost, Entry-Level Option
Process Principle: Inert gas is introduced directly through a tube inserted into the aluminum liquid. The upward pressure of the gas stirs the aluminum liquid, causing hydrogen to escape with bubbles.
Key Advantages: This method offers a simple structure, low investment costs, and convenient operation, making it suitable for small foundries or small-batch trial production. Limitations: Bubble diameter is large (5-10mm), resulting in a small gas-liquid contact area and a degassing efficiency of only 40%-50%. This can easily lead to localized overheating of the molten aluminum, affecting compositional uniformity.

III. Refining Process: A Key Method for Removing Solid Inclusions

The refining and degassing processes are typically performed simultaneously or in conjunction with each other. Their core goal is to remove solid inclusions from the molten aluminum through adsorption, dissolution, or sedimentation, achieving the purity required for casting. Currently, the mainstream refining methods are categorized as chemical refining and physical refining.

1. Chemical Refining: Impurities are Removed by a Chemical Reaction Between the Solvent and Inclusions
Chemical refining is the most commonly used method. Refining solvents (such as chlorides and fluorides) are added to the molten aluminum to react with inclusions, producing a slag that floats easily and is then removed by skimming. Commonly used solvent types:
Chloride solvents (such as NaCl-KCl mixed salts): Suitable for removing metallic impurities such as Mg and Ca from molten aluminum. The resulting MgCl₂ and CaCl₂ have low melting points and tend to form floating slag.
Fluoride solvents (such as Na₃AlF₆): Strongly adsorb Al₂O₃ inclusions, effectively removing oxide impurities caused by scale, and are commonly used in the refining of high-purity aluminum alloys.
Key Control Points: The amount of solvent added must be precisely controlled (typically 0.5%-2% of the molten aluminum weight). Excessive addition can lead to loss of molten aluminum composition. The refining temperature must be controlled between 720-760°C. Too low a temperature will result in incomplete solvent melting, while too high a temperature increases the risk of oxidation.

2. Physical Refining: Inclusions are separated mechanically or by filtration.
Physical refining does not require the addition of chemical solvents and is suitable for applications requiring extremely high purity. It primarily includes filtration refining and sedimentation refining. Filtration Refining:
Principle: Molten aluminum is passed through a porous filter medium (such as a ceramic filter plate or ceramic foam filter). Inclusions are trapped by the filter medium, achieving separation.
Advantages: High filtration precision (can retain inclusions larger than 5μm) and no chemical contamination. Currently, mainstream ceramic foam filters have a porosity of 80%-90%, resulting in high flow rates and suitable for continuous production.
Sedimentation Refining:
Principle: Utilizing the density difference between inclusions and molten aluminum (e.g., Al₂O₃ density 3.97g/cm³, molten aluminum density 2.38g/cm³), inclusions are allowed to slowly settle to the furnace bottom during quiescence, and the bottom slag is then removed.
Application: For larger inclusions (diameter > 50μm), a longer quiescence time (typically 30-60 minutes) is required. This method is less efficient and is mostly used as an auxiliary refining method.

IV. Key Process Control Points: "Critical Variables" Determining Degassing and Refining Results

Even with advanced process equipment, improper control of key parameters can significantly reduce process effectiveness. Buyers can assess a supplier's process control capabilities using the following four core parameters:

Temperature Control: The optimal temperature range for degassing and refining is 720-760°C. Temperatures below 720°C increase the viscosity of the molten aluminum, slowing the rise of bubbles and hindering hydrogen escape. Temperatures above 760°C accelerate the oxidation rate of the molten aluminum, which in turn increases the content of oxide inclusions.

Gas Purity: The purity of inert gases (argon, nitrogen) must be ≥99.99%. Oxygen or water vapor in the gas will react with the molten aluminum to form new oxide inclusions or hydrogen, leading to "secondary contamination."

Processing Time: The optimal time for rotary jet degassing is 5-8 minutes. A shorter time will result in incomplete degassing, while a longer time will increase the temperature drop of the molten aluminum. The rest time after refining should be controlled within 15-30 minutes to ensure that inclusions fully float.

Testing Frequency: Professional suppliers should test the hydrogen content (using a hydrogen analyzer) and inclusion content (using metallographic analysis) of the molten aluminum after each batch. The hydrogen content should be controlled below 0.15-0.3mL/100g Al. The inclusion grade should meet the ASTM E45 standard for Class A and B inclusions, ≤ Level 2.

V. Practical Value for Buyers: Selecting Suppliers with Mastery of Core Processes Reduces Partnership Risks

A supplier's degassing and refining process capabilities are directly related to procurement costs, production efficiency, and product quality, specifically in three key areas:

Reducing Casting Scrap Rates: A comprehensive degassing and refining process can reduce casting scrap rates due to porosity and inclusions from 15%-20% to below 5%, significantly reducing rework costs and delivery delays for buyers.
Ensuring Product Performance Stability: Precision-processed molten aluminum can maintain a ±5% fluctuation in the mechanical properties of castings, making it particularly suitable for bulk orders requiring high performance consistency (such as automotive parts and household appliance components).
Meeting High-End Market Access Standards: For products destined for high-end markets such as Europe, the US, and aerospace, the supplier's degassing and refining processes must comply with international standards such as ISO 21078 (Determination of Hydrogen Content in Aluminum and Aluminum Alloys) and ISO 16232 (Determination of Inclusions in Aluminum and Aluminum Alloys). This is a key to entering the high-end market.

VI. Our Process Advantages: Providing Buyers with Stable and Reliable Aluminum Alloy Castings

As a company specializing in aluminum alloy casting exports, we understand the importance of degassing and refining processes to our customers. We ensure the quality of every batch of castings through the following three key advantages:

Equipment Configuration: We have introduced three German-imported rotary jet degassing lines (SNIF method) and two vacuum degassing systems, meeting the needs of both mass production and high-end custom orders. We can control hydrogen content to as low as 0.1mL/100g Al.
Testing System: Each furnace is equipped with a Japanese HORIBA hydrogen content analyzer and metallographic microscope. Test data is uploaded to the cloud in real time, allowing buyers to access process inspection reports for each batch at any time, achieving transparent traceability.

Proven in Use: We have supplied castings to over ten global automotive parts manufacturers (such as Bosch in Germany and Delphi in the United States). Our castings have passed rigorous ASTM E45 inclusion detection and fatigue testing, maintaining a consistently low defect rate below 3%.