Aluminium is a metal used often for its relative strength given its light weight. When aluminum is cast, however, impurities known as inclusions can create weak points in the product. There are several causes for these inclusions, but one of the most prevalent is the presence of hydrogen. Hydrogen gas is soluble in liquid aluminium, and it can pass through molten aluminium almost as easily as it passes through air. As the liquid metal cools and hardens, the hydrogen flows from areas of high pressure to areas of low pressure. It coalesces and creates pockets of gas which, when the metal solidifies, become inclusions and weak points. Degassing is a process used to remove hydrogen from the molten aluminium.
How hydrogen is introduced.
Hydrogen can be introduced into liquid aluminium through combustion in gas furnaces, condensation on tools, fluxes, and alloy additives. Most commonly, however, hydrogen is introduced into molten aluminium via atmospheric humidity. Dissolution of hydrogen increases with an increase in atmospheric relative humidity.
The removal process.
As the demand for higher quality aluminium products increases, especially in the aerospace industry, so does the need to reduce the occurrence of inclusions in molded products. A popular hydrogen removal method involves introducing bubbled nitrogen gas into the liquid aluminium. The hydrogen is drawn to the nitrogen bubbles, then carried up through the aluminium and released on the surface. Argon is also very effective, but because of the high costs associated with this gas, nitrogen is preferred.
Nitrogen gas is introduced into the molten metal either by way of a static lance or a rotating impeller. The static lance method is less expensive, but also less efficient than a rotating impeller, especially in humid environments.
The static lance is a pipe that is inserted into the pot of liquid aluminium. Large nitrogen bubbles are released from the end of the lance and travel up through the product. When exposed to humid air, the large bubbles break the surface and attract more moisture that is dissociated into hydrogen and oxygen. The hydrogen then re-dissolves into the aluminium.
A rotary impeller works by increasing the surface area of the inert gas introduced into the metal. The smaller nitrogen bubbles created by the impeller process have much greater over-all surface area and collect more of the hydrogen. The lower surface disturbance of the smaller bubbles also allows for less hydrogen to be recaptured in the metal from the humidity in the atmosphere.
Nitrogen for aluminum degassing applications can be supplied in two ways:
A delivered supply in the form of high pressure cylinders or as a liquid in micro-bulk tanks (dewars) and bulk tanks
An on-site nitrogen generator
Disadvantages of cylinders and bulk tanks:
Dependence on outside suppliers: delivered gas is subject to price increases, rental agreements, long-term contracts, hazmat fees, inflexible delivery schedules, surcharges and taxes as well as extra administrative work.
Environmental considerations: Nitrogen is generated through fractional distillation of air. While N2 is not considered a greenhouse gas, it is important to recognize that isolation of the gas from air by fractional distillation is an energy intensive process. The energy employed to generate N2 produces a significant quantity of carbon dioxide (CO2). Also, nitrogen deliveries are made by heavy trucks that further contribute to CO2 emissions.
Safety risks: High-pressure cylinders are typically filled to 2000 psi. They must be carefully handled and stored because leaks or explosions could occur.
Benefits of an on-site nitrogen generator:
Cost effective: An on-site nitrogen generator typically pays for itself in less than 24 months. After that, the only cost associated with operation is electricity.
Continuous supply: A nitrogen generator is designed to operate automatically on a 24/7 basis. There is no need to interrupt service to change tanks.
Safety: An on-site generator eliminates the need to store and handle heavy, high pressure gas cylinders which pose risks of injury or property damage.
Improved efficiency: A nitrogen system generates nitrogen at a pressure and flow rate required for the application, on demand, so no gas is wasted.
Easy to install - Nitrogen generators are “plug and play” systems. Simply connect a standard compressed air line to the inlet of the nitrogen generator and connect the outlet to the nitrogen line.
For more information on Parker Nitrogen Generators, please contact HWL air and gas.