Processing molten aluminum - part 1: understanding silicon modification.

Processing Molten Aluminum -- Part 1: Understanding Silicon Modification

Shipments of aluminum castings by U.S. foundries have risen by some 25% in the last decade alone. Spurred by increasing use in the automotive and aerospace industries, aluminum casting production is expected to continue growing well into the 1990s.

Because of their excellent casting characteristics, wear and corrosion resistance, wide range of mechanical properties and high strength-to-weight ration, [1] the aluminum-silicon (Al-Si) alloys have led the way in expanding the use of cast aluminum components.

The increased use of aluminum also has given rise to more stringent demands for improved metal quality and predictable casting properties. To meet these growing requirements, much of the aluminum casting research carried out during the past decade or more has been aimed at developing and improving molten metal processing techniques that satisfy the casting producer and user alike. Silicon modification as well as the control of gas and inclusions that can adversely affect casting quality have been at the forefront of much of this work.

These processing techniques and practices were the focus of the 2nd International Conference on Molten Aluminum Processing held Nov 6-7 in Orlando, FL. Sponsored by the American Foundrymen's Society and organized by the AFS Molten Metal Processing committee, the meeting feature several top aluminum experts from around the world. Following is a review of some of the material presented at the two-day conference.

Silicon Modification

The modification of the silicon in Al-Si alloys is not a new practice. Pacz was granted a U.S. patent in the early 1920s for the production of aluminum alloys treated with alkali-fluoride fluxes, [2] and is often credited with the development of silicon modification. Research conducted since that time has demonstrated that modification can, indeed, improve the mechanical properties of Al-Si alloyw by altering the silicon structure of the alloy.

Since Si is a major constituent of these alloys -- typically ranging between 5-13% -- it plays a significant role in the processing and final properties of the casting. Modification is essentially a method of altering the Si structure (morphology) during solidification, thus helping to better control casting properties.

Modification can be achieved by treating the melt with certain elements known as modifiers or by subjecting the melt to a fast cooling rate. [1] The most common modifiers in use today are Sodium (Na), Strontium (Sr) and Antomony (Sb), which is used primarily in European and Japanese foundries.

The difference between a modified and unmodified casting can be significant. According to F. C. Dimayuga, Timminco Metals, Haley, Ontario: "When an alloy is not modified, the silicon phase is present as coarse, acicular plates which cause the casting to exhibit poor mechanical properties. Ductility of unmodified alloys is usually no more than a few percent and fracture is predominantly brittle. Modification causes the silicon to assume a fine, interconnected, fibrous morphology increasing both tensile strength and ductility of the casting. Impact resistance is also enhanced." [2]

Table 1 shows a comparison of the tensile properties of an unmodified A356 alloy and those of the same alloy modified by Na, Sr and Sb.

"Differences in microstructure are ultimately translated into differences in the mechanical properties of the castings," Dimayuga said. "It is clear that improvement is tensile strength and elongation of as-cast alloys occurs as the eutectic changes from acicular to lamellar to fibrous. It is also shown that a fine lamellar structure exhibits properties comparable to those of a fibrous structure." [2]

As shown in the rating chart in Fig. 1, "Modification changes the growth morphology of the eutectic Si phase from sharp angular crystals to more rounded fibrous particles that are well distributed in the aluminum matrix and appear as small globules in the microstructure," according to V. Rauta, Technical Research Centre of Finland. [3] The tensile strength and especially elongation increase considerably, as do machinability and soundness resulting from improved feeding characteristics. The benefits of modification are most pronounced for castings or portions of castings that solidify at relatively low rates. A similar kind of effect on eutectic structure is achieved with very high solidification rates without modification."

Despite the general concensus that modification can improve the mechanical properties of Al-Si alloys, important questions still need to be answered about the practice. One is the apparent tendency of modified castings to exhibit more porosity than their unmodified counterparts. The second involves the comparative effectiveness of the three major modifying agents, Na, Sr and Sb.

Increased Porosity

The increased porosity of modified aluminum castings, according to J. E. Gruzleski, McGill University, Montreal, "may offset any advantage in improved mechanical properties which is gained by modification, as well as leading to reduced pressure tightness in castings which are required to be leak proof." [4]

While it is generally accepted that modification can lead to increased levels of gas porosity in an aluminum casting, the reason for the occurrence is not clear.

"It is supposed that hydrogen gas is added to the liquid alloy along with the modifier or that the alloy has an enhanced tendency to absorb hydrogen if it containes a modifying element," Gruzleski explained. "There is, however, a growing body of evidence to indicate that this may not be the case and that increased porosity is due to a change in the shrinkage pattern of the alloy when the modifier is added." [4]

If the latter is the case, Gruzleski offers two suggestions to avoid or minimize hydrogen gas absorption into the molten aluminum bath:

* Since most porosity is due to a combination of gas and shrinkage, gas levels should be maintained as low as possible by using the most efficient degassing units, and by exercising extreme care in all melt handling operations to minimize the opportunities for regassing.

* Since modified alloys are likely to be more difficult to feed, risering systems should be re-examined and the generous use of chills should be considered, particularly in critical casting sections.

"Finally, it must be remembered that a certain level of widely dispersed microporosity is desirable in some castings in order to combat macroshrinkage," Gruzleski explains. "This is often achieved by maintaining a high gas level in the melt. Modification can be used for the same purpose."

Modifying Agents

For many aluminum foundries, the question of whetehr to modify their melts remains a moot issue. For those that have consciously taken the modification route, the question of which modifier to use has become the central question. Sodium (Na), strontium (Sr) and antimony (Sb) are the primary choices.

Each is recognized for its ability to alter the silicon phase of Al-Si alloys and improve their mechanical properties. However, questions remain about each's overall effectiveness and longrange impact.

Accoring to B. Closset, Timminco Metals: "Sodium has been commonly used to modify the eutectic silicon since its first introduction in 1920. More recently the use the use of strontium has increased dramatically and has been widely accepted as the preferred modifier of Al-Si alloy parts, especially in the low pressure casting industry. Sr, like Na, changes the eutectic silicon from acicular to fibrous but also offers a more permanent modifying effect." [5]

While the effects of Na and Sr are comparable, the levels of Na needed to produce well-modified structures are lower than those of Sr. Generally, 0.005% to 0.01% Na will produce desired Si structures compared with 0.01% to 0.02% sr. [2] However, the recovery rate or yield of Na is very low and it oxidizes rapidly from the melt, thus requiring that larger amounts of Na be added.

Another significant difference between Na and Sr when used as a modifying agent is that overmodification and its resultant detrimental effects on mechanical properties are a major concern with Na but not with Sr. Rauta reports that "Excessive sodium additions produce an overmodified structure characterized by bands of coarse constituents. In the microstructure, scalloped shaped grains of aluminum, AlSiNa compunds, coarse silicon crystals and the fibrous eutectic occur (Fig. 1). Excessive additions of Sr do not produce aluminum banding or silicon coarsening. However, undesirable SrSi and SrAl.sub2.Si.sub2 have been observed.

"It is difficult to add Na accurately owing to its strong tendency to become oxidized. The oxidation rate of Sr is much less severe. With the loss of Na or Sr, the modified structure reverts back to the unmodified structure reverts back to the unmodified condition. The rate of change is directly proportional to the Si content, temperature of the melt and holding time, and indirectly proportional to the size of melt." [3]

Sb Refining

Introduced more recently, Sb is used extensively as an alternative to Na and Sr in Europe (France, Italy, West Germany) and the Far East (Japan, Taiwan) to produce automotive and motorcycle wheels by the permanent mold process. [5]

Its use has become the source of controversy due to a variety of reasons including its lasting effects, interaction with othet modifying agents and environmental concerns. Whereas Na and Sr are added to the molten metal bath in the foundry, Sb is usually added to the metal by the ingot producer before shipment to the foundry.

Compared with Na and Sr, Sb acts more likel a silicon refining agent thatn it does as a modifier and produces a different effect on the Al-Si alloys.

"Sb additions do not result in a fibrous silicon phase as is the case with Na and Sr but in a lamellar structure which is intermediate between acicular and fibrous," according to Closset. "This structure is also obtained if an insufficient amount of Na or Sr is present in the alloy. The effect of Sb works best when combined with rapid freezing rates. Typical levels found in Sb-treated alloys range from 0.12% to 0.30% depending on the Si content of the alloy." [2]

Unlike Na and Sr, which are subject to fade (that is, their ability to modify the Si structure dissipates over time), Sb's effects are permanent and, unless dilluted with on-Sb containing alloys, will stay with the part forever. This has led to serious questions about the potential environmental effects of parts made with Sb which will eventually end up in the scrap cycle, as well as its interaction with other modifying agents, most notably Sr.

Thermal Analysis 

As research led to a better understanding and improved application of Si modification techniques, it became increasingly evident that better methods were needed to evaluate the effectiveness of the treatment before pouring the molten aluminum. Thermal analysis techniques were subsequently developed to answer the need and to provide a rapid, on-line process control tool for foundries.

Traditionally, metallographic and fracture tests were used to determine that the appropriate degree of modification had been achieved. These measurements were time-consuming and the results available after the modification treatment faded or the castings were already poured.

Chemical analysis of the molten metal obtained at that point provided the elements present in the metal before casting but were not necessarily indicative of the microstructure of the final castings. Introduction of thermal analysis (analyzing the cooling curve of a melt sample) offered foundries the capability of determining the state of melt before casting. [3]

Tuttle [5] defines thermal analysis as "the measurement of changes in a physical property of a material as that material is heated through a phase transformation temperature range. The temperature changes in the material as a function of the heating or cooling time are recorded in some manner to detect phase transformations."

In the case of Al-Si alloys, "certain characteristics of the cooling curves developed in thermal anaysis have been related to the degree of modification," Tuttle reports.

According to Rauta, [3] "Grain refinement can be indicated as a function of undercooling magnitude and undercooling time during the liquidus arrest. As shown in Fig. 2, the modification of Al-Si eutectic alloys can be related to the depression of the eutectic arrest temperature. The depression is defined as the difference between the eutectic arrest temperature of the unmodified alloy and the eutectic arrest temperature of the modified alloy."

Modification Today

The effectiveness of Si modification for improving the mechanical properties of Al-Si alloys has been demonstrated in both the laboratory and on the foundry floor in production situations. As the demands for even more improved and predictable properties increases, it appears that molten metal processing techniques like these will take on additional importance.

References

[1] R.M. Pillai, et al, "Microstructure, Mechanical Properties and Electrical Resistivity of Eutectic Al-Si Alloy Treated with Different Modifiers and Solidified Under Varying Cooling Rates," Proceedings of the 2nd AFS International Conference on Molten Aluminum Processing (Nov 1989)

[2] F. C. Dimayuga, "Modification of Al-Si Alloys Using Sr, Na and Sb," Proceedings of the 2nd AFS International Conference on Molten Aluminum Processing (Nov 1989)

[3] V. Rauta, "A Computer Aided Quality Control Method for Modification of Al-12% Si Melts," Proceedings of the 2nd AFS International Conference on Molten Aluminum Processing (Nov 1989)

[4] J. E. Gruzelski, "Modification and the Porosity Problem," PRoceedings of the 2nd AFS International Conference on Molten Aluminum Processing (Nov 1989)

[5] B. L. Tuttle, "Definitions in Thermal Analysis," modern casting, vol 77, p 39-41 (Nov 1985)