Aluminum Processing Molten aluminum has sev-
eral characteristics that can be controlled to maximize cast- ing properties. It is prone to picking up hydrogen gas and oxides in the molten state as well as being sensitive to mi- nor trace elements. Although some decorative or commer- cial castings may have quality requirements that can be met without additional processing, tight melt control and special- ized molten metal processing techniques can help provide enhanced mechanical proper- ties when needed. Alloy Chemistry—During
grains will grow until they impinge on another grain or the mold wall. Grain refining is a treatment process in which nucleating sites (in the form of titanium and boron master alloys) are added to the molten metal to aid the growth of ad- ditional grains. This leads to the creation of more grains, which causes the grains to remain smaller. A sodium or strontium
Fig. 1. The cost of aluminum casting varies with the casting process, as can be seen in the chart above.
molten aluminum processing, the percentages of alloying elements and impurities must be controlled carefully. If they are not, characteristics such as sound- ness, machinability, corrosion resistance, mechanical properties and conductivity are affected adversely. Molten aluminum alloys are prone
to chemistry changes that can be con- trolled during melting and holding. The most significant of these changes is the potential to lose magnesium and pick up iron, which can alter the mechanical properties significantly. If the service requirements of the cast component demand high material properties, these reactions must be controlled through
100 series alloys 200 series alloys 300 series alloys 400 series alloys 500 series alloys 700 series alloys
800 series alloys
facility melting and holding practices. Grain Refining & Modification—
Molten aluminum is sensitive to trace elements, but this sensitivity can be used as an advantage by adding trace amounts of material to create beneficial changes in the casting microstructure. Both grain refining and silicon modification can im- prove mechanical properties in the final component. They also can act as useful tools to optimize properties and heat treatment response to meet specific com- ponent service requirements and aid the development of certain casting properties. During solidification, aluminum freez- es in long columnar grain structures. These
Table 2. Aluminum Alloy “Pink Sheet” Classification System Alloy Series
Principal Alloy Element 99% minimum aluminum Copper
Silicon + Magnesium, Silicon + Copper, or Silicon + Magnesium + Copper Silicon
Magnesium Zinc
Tin (600 series and 900 series are not currently in use.) Fe Cu Mn Mg Zn
addition can be made to mol- ten aluminum to change the morphology (shape) of the silicon crystal. Molt en Me tal Han-
dling—Molten aluminum is prone to absorb hydrogen
from moisture in the atmosphere and other sources, which can lead to de- fects. Hydrogen gas can form pores in the solid castings, and aluminum oxide and other intermetallic impurities can solidify in the 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 prac- tices typically include degassing with an inert purge gas to remove hydrogen and fluxing to clean the molten alu- minum of oxides and other inclusions prior to pouring.
Alloy Selection A number of aluminum alloys are
available to satisfy individual require- ments. Once the casting method is de- termined, the alloy choice is limited be- cause not all alloys can be used with all casting methods. Sometimes, the alloy that shows the best properties on paper may have production characteristics that make it less desirable on an overall basis than other eligible alloys. Therefore, it is best to consult with a metalcasting
Table 3. Aluminum Association Standard Chemical Composition Limits for 356 Aluminum Alloy AA # Product Si
Ti
356.0 356.1 356.2
S&P 6.5-7.5 Ingot 6.5-7.5
A356.0 S&P 6.5-7.5 A356.1 Ingot 6.5-7.5 A356.2 Ingot 6.5-7.5 B356.0 S&P 6.5-7.5 B356.2 Ingot 6.5-7.5 C356.0 S&P 6.5-7.5 C356.2 Ingot 6.5-7.5 F356.0 F356.2
S&P 6.5-7.5 Ingot 6.5-7.5
0.6 0.50
0.25 0.25
Ingot 6.5-7.5 0.13-0.25 0.10 0.20 0.15 0.12 0.09 0.06 0.07 0.04 0.20 0.12
0.20 0.20 0.10 0.05 0.03 0.05 0.03 0.20 0.10
2010 Casting sourCe DireCtory
0.35 0.20-0.45 0.35 0.35 0.25-0.45 0.35 0.05 0.30-0.45 0.05 0.10 0.25-0.45 0.10 0.10 0.30-0.45 0.10 0.05 0.30-0.45 0.05
0.25 0.25 0.20 0.20 0.20 0.20
0.05 0.05 0.05 0.05 0.05
0.05 0.25-0.45 0.05 0.04-0.20 0.05 0.03 0.30-0.45 0.03 0.04-0.20 0.03 0.05 0.25-0.45 0.05 0.04-0.20 0.05 0.03 0.30-0.45 0.03 0.04-0.20 0.03 0.10 0.17-0.25 0.10 0.04-0.20 0.05 0.05 0.17-0.25 0.05 0.04-0.20 0.05
Others
Each 0.05
Total
0.15 remainder 0.15 remainder 0.15 remainder 0.15 remainder 0.15 remainder 0.15 remainder 0.15 remainder 0.10 remainder 0.15 remainder 0.10 remainder 0.15 remainder 0.15 remainder
Metal Casting Design & PurChasing 29 Aluminum
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