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A356: alloy of the nineties.

A number of this year's Aluminum Division papers concerned A356 alloy. Two researchers, W. Wang and J.E. Gruzleski, McGill Univ, studied the "Interactive Effects During Sodium or Strontium Treatment of Antimony Containing A356 Alloy" (90-19). it has been widely noted that antimony adversely affects modification treatment, leading to the formation of dense intermetallic (sludge) compounds, preventing modification.

The researchers chose the Charpy test to evaluate microstructure, since earlier work indicated that, "impact test results are a more sensitive function of aluminum-silicon as-cast microstructure than are tensile tests."

Using conventional and electron microscopy, Wang and Gruzieski focused on the compounds formed by the elements' interactions, and determined the degree of modification using a rating system developed by Apelian, et al. Results of their research indicate that an excess of the usual amount of modifier (sodium, strontium or both) reacts to form various intermetallic compounds; enough modifier still remains in solution to provide a full eutectic modification.

Tensile properties of A356 alloy, influenced by solution parameters and simplified supersaturation treatments were researched by S. Shivkumar, S. Ricci, Jr., B. Steenhoff and D. Apelian from Drexel Univ (90-180). In the first series of tests to study the influence of solution temperature on Si particle morphology, it was found that as the temperature was increased above 560C (1 040F), there was a significant coarsening of the Si particles.

At 570C (1 058F) "faceting is observed even at times as low as 25 min," compared to the 800 min required at 540C (1 004F). Tensile properties (uts and elongation) as evaluated by the Qualtiy Index (Q) were shown to reach a maximum at 550C (1022F), for both permanent and sand molded castings. But the authors cautioned that at this temperature, "it becomes imperative that the temperature in the heat treated parts be controlled precisely in order to avoid grain boundary melting."

A second series of tests involved varying the T5 temper treatment by removing castings from the mold just after the completion of solidification, then quenching in cold water and aging directly at high temperature. Their tests indicated that considerable energy savings may be available if the castings are not permitted to cool to room temperature. Their research indicates that, "the amount of Mg in solid solution may be increased appreciably," by this "simplified supersaturation" scheme.

Studies are ongoing at the Aluminum Casting Research Laboratory to determine the optimum solution parameters for maximizing fracture and fatigue properties. Preliminary indications are, however, that depending on the casting geometry, the modified T5 treatment may be an energy-saving alternative, "well suited to modified alloys which contain fine and globular eutectic Si particles in the as-cast condition."

Porosity and feeding of sodium- and strontium-modified Al-Si alloys were studied by four investigators (90-79) from Toyota Central Research & Development Laboratories, Inc. Drs. H. Iwahori, K. Yonekura, Y. Yamamoto and M. Nakamura noted that "the prevention of shrinkage porosity is as important as the removal of porosity," and that the feeding capability of various Al-Si alloys varies with the amount of silicon, copper and impurities, such as iron and manganese.

Additions of 0.2% metallic sodium and 0.05% of an Al-90% Sr alloy were made. NaCI-25% AlF3 flux additions were vigorously stirred to remove inclusions. Hydrogen content in the melt was analyzed using Ransley's Method. Radiographs were made of 15 mm cube samples before and after vacuum degassing.

The tests showed that with Na additions, vacuum degassing significantly increased casting density, while the combination of Sr modification and vacuum degassing offered little density improvement. Surface additions of sodium, however, were found to be ineffective. The researchers believe that strontium fixes the oxide-absorbed hydrogen in the melt, noting: "... when the melt without oxide is degassed in a vacuum after the addition of strontium, no porosity is found." Further analysis of the strontium and oxide inclusions at the fracture of the castings showed that "... without the addition of strontium, only oxygen was detected."

Unlike Na, Sr was found to offer no improvement in feeding capability of Al-Si alloys. Na modification "greatly improved" the feeding of high 7-8%) silicon alloys.

Two other researchers from the same facility-Dr. Y. Awano and S. Yoshihiro-presented their findings on Nonequilibrium Crystallization of AlFeSi Compound in Melt-Superheated AlSi Alloy Castings" (90-176).

Previous research has shown that by superheating the melt, the AlFeSi compound crystallized in a "Chinese script form" not in needle-like shapes known to be so deleterious to castings' strength. The common practice of adding Mg is not entirely corrective. Awano and Yoshihiro sought to investigate the relationship between the crystallographic change of an AlFeSi compound and a number of factors: the maximum superheating temperature; the effects of alloy composition; thermal history of the melt after superheating and solidification; and the period of crystallization.

The tests determined which superheat temperatures would produce the "script" crystals for various compositions of AlSiFe alloys poured in shell molds.

Radiographic examination and electron probe microanalysis (EPMA) of an A16%Si.4%Fe alloy showed that variations in the thermal history following superheating did not affect the shape of the iron compound. Superheating to the temperature at which the script-like crystals form was shown to cause an irreversible change in the melt, suppressing the formation of any needle-like formations.

X-ray diffraction determined the composition of the script formations, and showed that they had higher iron and lower silicon contents compared with T2 compounds. Interestingly, addition of Mg resulted in the formation of AlFeSi and AlMgFeSi compound of indefinite crystalline shapes.

Toughness of T6-treated, 0.5% and 0.9%Fe 356 alloy squeeze castings was increased by the elevated superheat temperature. This increase was attributed to the confirmed presence of script AlFeSi characters and the reduction of needle-like compounds.

Dr. F. Chiesa, McGill Univ, reported on his investigation in measuring the thermal conductance at the mold/metal interface of permanent molds, beginning at the onset of solidification. Chiesa devised an experimental technique permitting measurement of the interface conductance using actual foundry practices in the choice and application of foundry coatings. Experimental results were compared to a theoretical model.

Chiesa concluded that for white coatings, the insulating effect increased only marginally with coating thickness. Coatings with a high surface roughness were shown to be more insulating. Brush applications do result in rough surfaces, but the insulating efficiency is high because of the increased shell density. This suggests that a rougher surface can ... be obtained using coarser ceramic particles in the coating suspension."

A sharp increase in the heat exchanged between mold and liquid metal can be achieved by polishing an insulating coating. Further experimentation may reveal if this process persists after the onset of solidification.

Finally, Chiesa noted that the conductance of a black coating is "about twice that of a white coating of moderate (1 20 mm) thickness" during the filling process.

"Nondestructive Testing of Aluminum Composite Castings" (90-119) was reported by D.E. Hammond, Duralcan USA. Hammond developed tests that would measure the ceramic content, in the melt, and the cast product; detect particulate settling and the formation of denuded zones; and detect product defects, such as cracks, shrinkage, porosity and the foreign material contamination.

After discussing the limitations of two standard NDT methods-X-ray radiography and fluorescent penetration-Hammond focused on the promise of electrical resistivity. Using the four-terminal method to eliminate errors caused by resistance in the leads and the contacts, Hammond was able to determine the ceramic content over a range of 0-15% volume. Resistivity can also be used for the continuous measurements in molten composites, useful for on-line process control and "... for measuring the settling rate of the particles in the melt when stirring is interrupted."

Another NDT method, ultrasonic testing, also holds the promise of determining not only composition, but the size and shape of the composites. Replication of the test data showed a high degree of precision. Comparison of gravimetric measurement of the composite test bars to the resonant frequencies showed a precise linear relationship between the resonance frequency and the ceramic volume fraction. Further tests will be conducted at Texas Metal Casting to develop ultrasonic testing into a practical tool for composite quality control.
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Publication:Modern Casting
Date:Jun 1, 1990
Words:1347
Previous Article:Quality and computer modeling for the next decade.
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