phase at the expense of the β-Fe phase. As seen in Figs. 3 and 4, the high
precision solid carbide drill is best for the 396 alloy, with the low- est mean total drilling forces and torques. The high precision solid carbide drill shows the most satis- factory behavior and results for the mean total drilling force and torque with the G2 alloy. With regard to the G3 alloy, the high precision solid carbide drill continued to produce the lowest mean force and torque values. In fact, these values were the lowest among all tests on the three alloys. With regard to the B319.2-G12 alloy, the cobalt grade drill obtained the lowest force and torque. In terms of the number of holes drilled, the results indicated silicon content has little effect on tool life. While the 396 and B319.2 alloys
Fig. 2a
contain silicon, copper and magnesium as the main alloying elements, the main difference is their silicon content (10.8% in the 396 alloys and 7.5% in the B319.2 alloy). Tis difference is reflected in microstructures that show larger regions of aluminum-silicon eutectic in the 396 alloy. Te significant increase in cutting
force in the 396-G3 alloy may be a result of hard, complex intermetallic sludge. Te microhardness of sludge has been reported as 500–900 BHN, as compared to a matrix hardness of 111 BHN for the 396-G3 alloy. Te sludge phase may behave like an abrasive in an otherwise relatively soft matrix, capable of causing excessive tool wear and resulting in increased cutting force and torque. Te fluctua- tions in total drilling force and torque values during drilling of the 396-G3 alloy resulted from the distribution
Fig. 2b
and size of the sludge particles within the test block itself. Te addition of 0.25% iron and 0.25% manganese to the 396-G3 alloy caused some deterioration in drill life. Also, this alloy presented greater fluctuations in the total drill- ing forces and torque, with the drill breaking after 971 holes with the solid carbide drill. Te drill likely became encrusted with a number of sludge intermetallic particles, leading to drill breakdown. Te 396-G2 alloy displayed behavior similar to the B319.2-G12 alloy, which contains the same amount of added tin. The results of total drilling force
and torque with the solid carbide drill and special solid carbide drill show the 396-G2 and B319.2-G12 alloys, which both contain 0.15% tin, behave similarly when using either drill type. In comparison,
Fig. 2. Optical micrographs show the effects of tin on silicon morphology in grain-refined and heat treated 396-G2 alloy (2a) and B319.2-G12 alloy (2b). 50 | MODERN CASTING January 2014
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