Controlling aluminum gas porosity.

Cast aluminum components are used for many diverse functions, from applications that do not have critical service requirements to components for aerospace and automotive applications that require high strength, ductility and fatigue properties. The melting and casting process can play a major role in the final component properties. Molten aluminum is prone to picking up hydrogen gas and oxides and is sensitive to minor trace elements. Enhanced mechanical properties can be provided by tight melt control and specialized molten metal processing techniques, including the control of hydrogen gas porosity during melting.

Hydrogen solubility is low in solid aluminum, but a change at the melt point causes molten aluminum to be vulnerable to absorbing hydrogen from moisture in the atmosphere and other sources. This hydrogen gas is not naturally expelled and will stay in the grain structure during solidification and form pores in the solid castings. Gas porosity can have a negative impact on casting quality and prevent castings from meeting high service requirements by reducing tensile, fatigue and impact properties and increasing hot tearing tendency. Additionally, gas porosity will lead to a poor surface appearance in castings with cosmetic requirements, such as castings that are polished or anodized. However, it is not always advantageous to have a completely hydrogen-free microstructure, as a certain level of hydrogen can help maintain pressure tightness and reduce macro-shrinkage.

Metalcasting melting practices typically include degassing with an inert purge gas to remove hydrogen before pouring. But taking steps to minimize hydrogen is as critical a part of the melting process as hydrogen removal. The prime source of hydrogen is moisture, and the biggest source of moisture is the atmosphere (Fig. 1). During periods of extreme humidity, metalcasters must use caution during degassing and may need to modify degassing procedures to ensure an acceptable gas level.

[FIGURE 1 OMITTED]

Eliminating Moisture

While atmospheric humidity may be the major source of hydrogen, moisture can come from many other sources, such as metal processing tools, improperly dried and cured crucibles and refractory, charge materials and furnace combustion products. All tools used in contact with molten aluminum should be fully dry. Gas or fuel oil-fired furnaces produce combustion byproducts that contain considerable moisture and should not come into contact with or be directed above the molten aluminum bath. All charge materials should be stored indoors to reduce surface moisture and oxidation and should be pre-heated before introduction into the furnace.

Another means to reduce gas porosity is careful melt temperature control. Overheating the melt will increase the absorption of hydrogen dramatically. The aluminum oxide film on the surface of molten aluminum provides some inhibitor to gas, and turbulence during handling and pouring will break that oxide skin and increase the hydrogen absorption rate.

Degassing, which is performed most often with an inert purge gas such as nitrogen, argon, freon or chlorine (or some combination), usually involves bubbling the purge gas into the bottom of the melt using a graphite lance. As the purge gas rises through the melt, it comes into contact with the hydrogen, absorbs it and carries it out to the surface.

During degassing, the purge gas is most beneficial when it is introduced to the melt in numerous fine bubbles because the increased surface area of the smaller bubbles creates more opportunity to contact the hydrogen and carry it out. Using a rotary device increases the efficiency of degassing, as the rotating impeller breaks up the purge gas bubbles, creating numerous smaller bubbles and distributing them throughout the melt.

When producing castings that demand lower hydrogen gas levels, the melt's gas level should be tested prior to pouring. A simple but effective testing method that has been used for years is the reduced pressure test (RPT). The RPT solidifies a sample of molten aluminum under reduced pressure (partial vacuum), which expands and exaggerates the gas porosity and allows visibility in the cut surface sample. The cut sample then is compared visually to a chart and assigned a numerical value that designates the gas level of the melt. An additional test involves obtaining the density of the sample; a denser sample would have less internal porosity. This provides a more quantifiable evaluation that eliminates the potential for visual inspection subjectivity.