silicon composition below 12.7 wt%. Two major components coexist in the microstructure of hypoeutectic Al-Si alloys: the primary, aluminum rich phase and the eutectic microstructure. The primary phase contains about 1.67% silicon as a solid solution and is in dendrite form. The eutectic structure, consisting of an aluminum-rich solid solu- tion and virtually pure silicon, exists in between the arms of the primary aluminum dendrites. Refinements of silicon by adding trace amount of impurities such as sodium and strontium can improve mechanical properties of resulting castings. However, current impurity-
containing hypoeutectic Al-Si cast alloys have yielded only modest improvements in ulti- mate tensile stress (UTS) (not in excess of 180 MPa) and ductil- ity (roughly 10%). Two reasons explain these moderate increases: The silicon phase in these cast
alloys is not sufficiently refined to offer a high UTS value. The eutectic point permits
the proportion of primary alu- minum and eutectic structure to promote a ductility less than 5%. T e potential exists to alter
the primary aluminum to eutectic structure ratio and refi ne silicon morphology of Al-Si alloys with the addition of barium to improve strength and ductility. Recent work on the solidifi cation of hypereu- tectic Al-Si alloys (having between 15-20% silicon) has focused on the solubility of barium in the silicon phase. T is research has established primary silicon-free hypereutectic alloys with up to 17wt% silicon can be produced by directional so- lidifi cations. A shift of the normal eutectic point (shown in Figure 2) from 12.7wt% to 17.0wt% silicon caused by the addition of Ba into the melt and related impurity modifi cation mechanisms may help develop these alloys. The same concept, which alters the ratio of the primary
aluminum to eutectic phase and refines the morphology of eutec- tic silicon, has now been used to
2
develop high strength, highly ductile hypoeutectic Al-Si alloys by conventional casting.
Procedure T e process in-
volved melting Al-Si alloys with 6-10% silicon in an argon-rich
environment. A resistance furnace was used to maintain a temperature of 1,418F (770C) for the Ba treat- ment. After which, the resulting melt was poured into a permanent mold that was preheated to 850F (454C). T e casting then cooled to room temperature before heat treat- ment. For such treatment, furnace cooled alloys were initially solution treated at 975F (525C) for 11 hours and then quenched in water. T e
quenched samples were then aged in the same furnace at 356F (180C) for 24 hours. Longitudinal and transverse section specimens taken from near the center of the samples were used to determine the microstructure. Visual inspection of the surface revealed neg- ligible amounts of porosity and other casting defects. T e samples then were subjected to tensile testing. T e microstruc- ture of the alloys was studied using scanning electron microscopy (SEM). Samples were etched to re- move surface aluminum and expose the topography and morphology of silicon phase.
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INDUSTRIES, INC. Contact: Tom Kayser Nov/Dec 2015 | METAL CASTING DESIGN & PURCHASING | 41
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