会社のニュース Aluminium Degassing Process

In processes such as the casting and extrusion of aluminum and its alloys, dissolved hydrogen in the melt causes porosity (gas pores) and degrades product performance. Therefore, before pouring, molten aluminum must be degassed. Degassing does not only decrease hydrogen content but also removes inclusions and improves metal density, thereby enhancing mechanical properties and surface quality of castings.

Inert Gas Purging/Rotary Degassing: This approach, well adopted in the aluminum industry, involves atomizing the inert gas into a large number of very small bubbles. The rising of the bubbles brings about the escape of hydrogen and the carrying away of some inclusions simultaneously. Rotating rotors disperse the bubbles more finely and extend the residence time, thus increasing the efficiency of hydrogen removal. ([Sohu][1])

Ultrasonic Degassing: High-intensity ultrasound generates cavitation in molten aluminum, enabling nucleation, growth, and coalescence of microbubbles. It increases the kinetics of hydrogen removal considerably and is free from environmental hazards (chemicals like chlorine). Laboratory and industrial applications have shown that ultrasonic degassing achieves comparable or better results than traditional argon rotary degassing.

Vacuum/Vacuum-Assisted: By decreasing the pressure above the surface of the melt, the diffusion and escape of hydrogen are favored. In the steel industry, vacuum degassing is used more frequently; in the case of aluminium, this process is often combined with other methods for better efficiency.

Some common types of aluminium degassers in the markets and factories are:

• mobile rotary aluminium degassers, which work well in small and medium-sized furnaces;
• large-scale online rotary systems;
• ultrasonic degassing equipment; and
• vacuum-assisted degassing systems.

Mobile and small-scale equipment uses mechanical rotors to create vortices in molten aluminum and introduce the inert gases.

Through transducers, ultrasonic equipment produces cavitation in the liquid phase.

Vacuum systems reduce local pressure in closed or semi-closed environments to facilitate degassing.

Different types of aluminium degassers vary greatly in structure, floor space, and investment cost. Selection should be made considering production capacity and process requirements.

Buying or setting up an aluminium degassing process, the following should be the focus:

• Inert gas type (argon is the primary choice).
• Gas flow rate and method of injection (single hole, porous brick, or rotating rotor).
• Rotor speed and geometry (affecting bubble diameter and dispersion).
• Processing time and molten aluminium temperature.

Rotary degassing: small, numerous bubbles and extended residence time in the liquid are preferred. Ultrasonic degassing: frequency, power, probe position, and immersion depth are the key operation parameters. Industrial practice demonstrated porous or rotary dispersion methods outperform single-hole purging.

Pre-treatment: Large inclusions and oxides are removed before degassing. Preheating and stabilizing the molten aluminum temperature.

Online Monitoring: Ensure degassing effectiveness by hydrogen content testing, density testing, or metallographic/CT testing of cast samples.

Equipment Maintenance: Rotating parts, probes, and seals shall be inspected regularly to avoid any damage to ceramics/seals or airway blockage. Good operating practices will extend the service life of the aluminium degasser and lower failure rates.

Historically, some of the degassing processes (e.g., using chlorine-containing compounds) had environmental impacts. The industry is, therefore, changing to more eco-friendly solutions, like pure inert gas and ultrasonic methods.

In addition, during the use of equipment, pay attention to the following:

• Personal protection.
• Waste gas treatment-if chemical gases are used.
• Electrical safety and high-temperature operating procedures.

National and industrial standards have clear requirements on the processes of electrolysis and flue gas treatment, and an enterprise should operate in compliance with them.

The selection of the right aluminum degasser/process combination depends upon the product quality requirements, scale of production, and environmental/safety standards:

• Prefer small mobile or ultrasonic equipment for small batches or when space is limited.
• For large continuous casts, online rotary degassing or vacuum-assisted systems are preferred.
• For high environmental requirements, use chlorine-free, residue-free physical degassing methods (inert gas/ ultrasonic).
• Only regular testing and equipment maintenance can ensure stable degassing quality.