casting 101 C
be surprising that small variations in the process can lead to signifi cantly dif- ferent properties in a cast part. The specification of a particular
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alloy is based on the mechanical properties it can achieve. Looking specifically at aluminum castings, properties result from three primary factors: casting alloy, melting and casting operations, and thermal treatment. Focusing on melting and casting
properties, molten aluminum has several characteristics that can be controlled. It is prone to picking up hydrogen gas and oxides in the mol- ten state, as well as being sensitive to minor trace elements. Although some decorative or commercial cast- ings may have quality requirements that can be met without additional processing, tight melt control and specialized molten metal process- ing techniques are often needed to provide enhanced mechanical properties. Alloy Chemistry: During molten aluminum processing, the percent- ages of alloying elements and impu- rities must be controlled carefully. If they are not, characteristics like soundness, machinability, corrosion resistance, mechanical properties and conductivity are aff ected adversely. Molten aluminum alloys are
prone to chemistry changes that can be controlled during melting and holding. T e most signifi cant of these changes is the potential to lose magnesium and pick up iron, which can alter mechanical properties. If the service requirements of the cast component demand high mate- rial properties, these reactions must be controlled through melting and holding practices. Chemistry prob- lems arise when holding the metal at temperature, causing intermetallics to form in the alloy and resulting in
Casting Properties & Molten Aluminum AMERICAN FOUNDRY SOCIETY TECHNICAL DEPARTMENT
onsidering the lengths to which metalcasters go to control the melt- ing and pouring of mol- ten metal, it shouldn’t
“
Tight melt control and specialized molten metal processing techniques are often needed to provide enhanced mechanical properties.”
a more brittle casting with hard spots detrimental to machining. Grain Refining & Modifica-
tion: Molten aluminum is sensitive to trace elements, but this sensitiv- ity can be used as an advantage by adding trace amounts of materials to create benefi cial changes in the casting microstructure. Both grain refi ning and silicon modifi cation can improve mechanical properties in the fi nal component. T ey also can act as useful tools to optimize properties and heat treatment response to meet specifi c component service require- ments and aid the development of certain casting properties. During solidification, alumi-
num freezes in long columnar grain structures. These grains will grow until they impinge on another grain or the mold wall. Grain refining is a treatment process in which nucleat- ing sites (in the form of titanium and boron master alloys) are added to the molten metal to aid the
growth of additional grains (shown in the photos). This leads to the cre- ation of more grains, which causes the grains to remain smaller. With modification, a sodium or strontium addition is made to the molten alu- minum to change the shape of the silicon crystal.
Molten Metal Handling: Molten aluminum is prone to absorb hydro- gen from moisture in the atmo- sphere and other sources, which can lead to defects. Hydrogen gas can form pores in solid castings, and alu- minum oxide and other intermetallic impurities can solidify in castings as inclusions. Both gas porosity and inclusions have a negative impact on casting quality and will prevent castings from meeting high service requirements. Melting practices typically include degassing with an inert purge gas to remove hydrogen and fl uxing to clean the molten aluminum of oxides and other inclu- sions prior to pouring. ■
Grain refi nement involves the addition of nucleating sites to the molten metal to grow additional grains (right) and improve the component’s mechanical properties.
48 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2015
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