Casting Answers & Advice.

Q: The melt procedures in our aluminum foundry generally provide us with clean metal, but we occasionally have some problems with inclusions and oxides. What equipment or techniques are available to assess molten aluminum melt cleanliness prior to pouring?

Background: Some aluminum foundries use the standard Reduced Pressure Test (RPT) to monitor melt cleanliness. Although the RPT is used primarily to quantify hydrogen (H) porosity in the melt, the number and size of H pores is increased by the presence of nonmetallic microinclusions (oxides), so inclusion contamination in the test sample can change the test parameters. A "dirtier" melt will bring the first bubble to the surface of the specimen during solidification faster than a "cleaner" melt, and the sample will have a darker surface appearance.

Beyond this test, there are several other methods available that measure melt cleanliness:

* fracture testing uses a small step block casting to quantify the number of inclusions visible on the surface of the casting;

* one form of filter testing measures the amount of molten metal that flows through a ceramic filter before clogging;

* another filter-based test records the filtrate weight against time as the melt is drawn into a specially designed crucible. The shape of the filtration weight vs. time curve correlates with the melt inclusion content;

* a third filtration method involves filtering a known amount of molten aluminum through an ultra fine filter under pressure to produce a concentration of solid inclusions on the filter surface that permits the quantitative metallographic measurement of melt cleanliness

* an electrical analysis method uses a heat resistant tube that draws molten aluminum through a small hole under vacuum, looking to detect inclusions as a drop in electrical resistance across the hole. The amplitude of the resistance and the number of pulses correspond to the size and the number of melt impurities.

Research is being done with the use of sound waves to measure inclusion content in aluminum melts.

Recommendations: These techniques are generally available as commercial products. The most appropriate procedure for your shop probably depends on many factors, including your shop size, melt volume, desired cleanliness level and equipment cost. Literature has been published documenting these technologies and is available through the AFS Library or visiting the AFSearch website at www.afslibrary.com.

Q: Our iron foundry has been having trouble with silica refractory life in the lower lining in our coreless furnaces. Although the refractory material has not changed, there is a new shakeout system in molding. Could there be a connection?

Background: Not only is it possible, it is quite likely. Silica is an acid refractory and demands an acid (high silica) slag. A basic slag will chemically attack the lining. The major source of silica in most systems is the sand sticking to the sprue. If you installed better, more efficient sprue handling equipment, you are likely sending a cleaner sprue back to your furnace. A typical iron shop runs 50-80% [SiO.sub.2] in its slag. Most shops produce 25-50 lb slag/ton of iron (with some as high as 70 or more). In order to maintain an acid slag (50% minimum silica), there must be at least 15-25 lb of sand/ton of iron coming along with the sprue. It is not unusual to see shops deliberately add new sand to the furnace to help maintain an acid slag.

Recommendations: Even if your final slag contains a relatively high silica content, it may not have come from the charge, but from erosion of the furnace lining. If there is doubt, try adding extra silica sand to the furnace over a period of several weeks. If the volume of the slag and the percent [SiO.sub.2] go up, then you probably didn't need the addition. If the slag volume and chemistry do not change, you have successfully substituted new sand in the charge for the silica in your lining.