Modification and Refinement of Aluminum-Silicon Alloys

Hypoeutectic aluminum-silicon alloys can be improved by inducing structural modification of the normally occurring eutectic. In general, the greatest benefits are achieved in alloys containing from 5% Si to the eutectic concentration; this range includes most common gravity cast compositions.
The elimination of large, coarse primary silicon crystals that are harmful in the casting and machining of hypereutectic silicon alloy compositions is a function of primary silicon refinement.

Hypoeutectic aluminum-silicon alloys can be improved by inducing structural modification of the normally occurring eutectic. In general, the greatest benefits are achieved in alloys containing from 5% Si to the eutectic concentration; this range includes most common gravity cast compositions.

Chemical Modifier

The addition of certain elements, such as calcium, sodium, strontium, and antimony, to hypoeutectic aluminum-silicon alloys results in a finer lamellar or fibrous eutectic network. It is also understood that increased solidification rates are useful in providing similar structures.

There is, however, no agreement on the mechanisms involved. The most popular explanations suggest that modifying additions suppress the growth of silicon crystals within the eutectic, providing a finer distribution of lamellae relative to the growth of the eutectic.

The results of modification by strontium, sodium, and calcium are similar. Sodium has been shown to be the superior modifier, followed by strontium and calcium, respectively. Each of these elements is mutually compatible so that combinations of modification additions can be made without adverse effects. Eutectic modification is, however, transient when artificially promoted by additions of these elements.

Antimony has been advocated as a permanent means of achieving structural modification. In this case, the modified structure differs; a more acicular refined eutectic is obtained compared to the uniform lacelike dispersed structures of sodium-, calcium-, or strontium-modified metal. As a result, the improvements in castability and mechanical properties offered by this group of elements are not completely achieved. Structural refinement is obtained that is time independent when two conditions are satisfied. First, the metal to be treated must be essentially phosphorus free, and second, the velocity of the solidification front must exceed a minimum value approximately equal to that obtained in conventional permanent mold casting.

Antimony is not compatible with other modifying elements. In cases in which antimony and other modifiers are present, coarse antimony-containing intermetallics are formed that preclude the attainment of an effectively modified structure and adversely affect casting results.

Modifier additions are usually accompanied by an increase in hydrogen content. In the case of sodium and calcium, the reactions involved in element solution are invariably turbulent or are accompanied by compound reactions that by their nature increase dissolved hydrogen levels. In the case of strontium, master alloys may be highly contaminated with hydrogen, and there are numerous indications that hydrogen solubility is increased after alloying.

For sodium, calcium, and strontium modifiers, the removal of hydrogen by reactive gases also results in the removal of the modifying element. Recommended practices are to obtain modification through additions of modifying elements added to well-processed melts, followed by inert gas fluxing to acceptable hydrogen levels. No such disadvantages accompany antimony use.

Calcium and sodium can be added to molten aluminum in metallic or salt form. Vacuum-prepackaged sodium metal is commonly used. Strontium is currently available in many forms, including aluminum-strontium master alloys ranging from approximately 10 to 90% Sr and Al-Si-Sr master alloys of varying strontium content.

Very low sodium concentrations (approximately 0.001%) are required for effective modification. More typically, additions are made to obtain a sodium content in the melt of 0.005 to 0.015%. Remodification is performed as required to maintain the desired modification level.

A much wider range of strontium concentrations is in use. In general, addition rates far exceed those required for effective sodium modification. A range of 0.015 to 0.050% is standard industry practice. Normally, good modification is achievable in the range of 0.008 to 0.015% Sr. Remodification through strontium additions may be required, although retreatment is less frequent than for sodium.

To be effective in modification, antimony must be alloyed to approximately 0.06%. In practice, antimony is employed in the much higher range of 0.10 to 0.50%.

The Importance of Phosphorus. It has been well established that phosphorus interferes with the modification mechanism. Phosphorus reacts with sodium and probably with strontium and calcium to form phosphides that nullify the intended modification additions. It is therefore desirable to use low-phosphorus metal when modification is a process objective and to make larger modifier additions to compensate for phosphorus-related losses.

Primary producers may control phosphorus contents in smelting and processing to provide less than 5 ppm of phosphorus in alloyed ingot. At these levels, normal additions of modification agents are effective in achieving modified structures. However, phosphorus contamination may occur in the foundry through contamination by phosphate-bonded refractories and mortars and by phosphorus contained in other melt additions, such as master alloys and alloying elements including silicon.

Effects of Modification. Typically, modified structures display somewhat higher tensile properties and appreciably improved ductility when compared to similar but unmodified structures. Improved performance in casting is characterized by improved flow and feeding as well as by superior resistance to elevated-temperature cracking.

Refinement of Hypereutectic Aluminum-Silicon Alloys

The elimination of large, coarse primary silicon crystals that are harmful in the casting and machining of hypereutectic silicon alloy compositions is a function of primary silicon refinement.

Phosphorus added to molten alloys containing more than the eutectic concentration of silicon, made in the form of metallic phosphorus or phosphorus-containing compounds such as phosphor-copper and phosphorus pentachloride, has a marked effect on the distribution and form of the primary silicon phase. Investigations have shown that retained trace concentrations as low as 0.0015 through 0.03% P are effective in achieving the refined structure. Disagreements on recommended phosphorus ranges and addition rates have been caused by the extreme difficulty of accurately sampling and analyzing for phosphorus. More recent developments employing vacuum stage spectrographic or quantometric analysis now provide rapid and accurate phosphorus measurements.

Following melt treatment by phosphorus-containing compounds, refinement can be expected to be less transient than the effects of conventional modifiers on hypoeutectic modification. Furthermore, the solidification of phosphorus-treated melts, cooling to room temperature, reheating, remelting, and resampling in repetitive tests have shown that refinement is not lost; however, primary silicon particle size increases gradually, responding to a loss in phosphorus concentration. Common degassing methods accelerate phosphorus loss, especially when chlorine or freon is used. In fact, brief inert gas fluxing is frequently employed to reactivate aluminum phosphide nuclei, presumably by resuspension.

Practices that are recommended for melt refinement are as follows:

  • Melting and holding temperature should be held to a minimum
  • The alloy should be thoroughly chlorine or freon fluxed before refining to remove phosphorus-scavenging impurities such as calcium and sodium
  • Brief fluxing after the addition of phosphorus is recommended to remove the hydrogen introduced during the addition and to distribute the aluminum phosphide nuclei uniformly in the melt
Refinement substantially improves mechanical properties and castability. In some cases, especially at higher silicon concentrations, refinement forms the basis for acceptable foundry results.

Modification and Refinement. No elements are known that beneficially affect both eutectic and hypereutectic phases. The potential negative consequences of employing modifying and refining additions in the melt are characterized by the interaction of phosphorus with calcium, sodium, and strontium. Strontium has been claimed to benefit hypoeutectic and hypereutectic structures, but this claim has not been substantiated.

Metal Preparation

Regardless of the types of melting and holding furnaces and the particular gravity casting process used, there is great concern for reducing or eliminating dissolved hydrogen and entrained oxides. These procedures are less frequently employed for pressure die casting, in which concerns are focused on the dominant process-related causes of casting unsoundness, namely, entrapped gas and pouring injection-associated inclusions.

Sensitivity to melt quality varies with the casting process and part design and necessitates special consideration of relevant criteria for each application. In general, the melt is processed to achieve hydrogen reductions and the removal of oxides to meet specific casting requirements. Modification and grain-refiner additions are made as appropriate to the given alloy and end product.

Different melt preparation practices are employed in die casting operations because process related conditions are more dominant in the control of product quality than those controlled by melt treatment. For this reason, degassing for the removal of hydrogen, grain refinement, and modification or silicon refinement in the case of hypereutectic silicon alloys are often intentionally neglected. The movement toward higher integrity die castings has brought into focus the importance of the same melt quality parameters established and used in the gravity casting of aluminum alloys.

In high-production die casting operations, the consumption of internal and external scrap is of primary importance in reducing base metal costs for the predominantly secondary alloy compositions that are consumed. Scrap crushing, shredding, and pre-treatment of various types precede melting, often in efficient induction systems.

Oxides entrained in the melt as a result of this sequence of operations are dealt with through the use of salts and/or reactive gas fluxing. A concern in die casting is the formation of complex intermetallics that are insoluble at melt- holding temperatures and/or precipitate under holding conditions or during transfer to and injection from the hot or cold chamber. These intermetallics (sludge) affect furnaces, transfer systems, and, by inclusion, the quality of the castings produced.

Die casters are familiar with composition limits that prevent sludge formation. A common rule is that iron content plus two times manganese content plus three times chromium content should not exceed the sum of 1.7%. This limit is arbitrary and inexact, it is often assigned values from 1.5 through 1.9%, and it is subject to the specific composition and actual minimum process temperature.

October, 2003
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