Table 1. Chemical Composition of the Alloys Used in the Present Machinability Study Element (wt%)
Alloy Alloy Code Si G2 G3
396 B319.2 G12 Fe
10.82 10.84 7.533
0.53 0.88 0.35
Cu Mn Mg Zn Sn 2.23 2.42 3.59
0.55 0.70 0.29
Table 2. Summary of Four Drills Used in Testing Solid
Drill Type
Material
Diameter (mm) Point Type
Overall Length (mm) Flute Length (mm) Point Angle
Drill Profi le
Carbide Drill Solid Carbide 6.5
Standard 90 53
120°
0.26 0.29 0.24
0.04
0.13 0.04
Ti
0.19 0.06 0.18
0.22
0.26 0.24
Bi
0.01 0.03 0.00
Sr
0.009 0.009 0.009
Al Mn/Fe S.F. Bal Bal Bal
1.04
0.82 0.83
1.65 2.35 0.93
Special Solid Carbide Drill
Solid Carbide 6.5
Self-Centering Point 106 66
140°
Cobalt Grade Drill
Solid Carbide 6.5
Self-Centering Point 91 53
130°
Solid Carbide High Precision Drill
Solid Carbide High Precision 6.5
Self-Centering Point 91 53
140°
(coded G2 and G3) and for the B319.2 alloy (coded G12). T e compositions represent average values taken over three spark measurements made on each chemical analysis sample. Melting was carried out in a
264.4-lb. (120-kg) capacity silicon- carbide crucible using an electrical resistance furnace with a melt- ing temperature of approximately 1,382F (750C). All alloys were grain refined and modified by adding 0.15% titanium and 90 ppm stron- tium. Iron and manganese were added in the form of Al-25%Fe and Al-25%Mn master alloys, while tin was added as pure metal. The melts were degassed for 15-20 minutes
G2 and B319.2-G12 alloys, containing 10.8% and 7.5% silicon, respectively. T e tin precipitates in the form of fi ne black reticulate particles or free machining inclusions for both alloys. T ese β-Sn particles always solidify at the interfaces of α-Al/Si or α-Al/Fe- rich intermetallics. Table 3 summarizes the eutectic
3
Results and Conclusions
Figures 2a and 2b show
the eff ect of tin’s addition on the microstructure of the 396-
with a rotary graphite impeller using pure dry argon. Prior to pouring, surface oxides and inclusions were skimmed off thoroughly. The melt was poured at 1,364F (740C) into a waffle-plated, graphite-coated metallic mold preheated to 842F (450C). Specimens (Fig. 1) were cut from the casting with dimen- sions of 11.8 in. (300 mm) by 6.9 in. (175 mm) by 1.18 in. (30 mm), with 0.98-in. (25-mm) ribs, separated by 0.63-in. (16-mm) gaps. T e test blocks were heat treated at 914F (490C) for eight hours, quenched in warm water at 149F (65C) and then artifi cially aged at 356F (180C) for fi ve hours. Drilling experiments were
silicon particle characteristics obtained from quantitative measurements of the alloys, investigated with respect to additions. T e addition of 0.15% tin to the 396-G2 and B319.2-G12 alloys results in a slight coarsening of the eutectic silicon particles. T e addition of tin appears to elicit similar behavior from both 396 and B319.2 alloys. As seen in Figs. 2a-2b, the eutec-
tic silicon particles are not uniformly distributed, but tend to be concen- trated at the interdendritic boundaries. Iron- and copper-containing phases
carried out on a high-speed, high- precision vertical machining center. T e experimental setup consisted of a dynamometer with four sensors, charge amplifi ers and an A/D con- verter. T e drilling speed was 11,000 rpm, the cutting depth was 1.25 in. (31.75 mm) and the feed rate was 44 in. per minute. A synthetic metalwork- ing fl uid concentrate was used to avoid heat during machining. A total of 144 holes were drilled
into each test block’s fi ve ribs, with each hole having a 0.26-in. (6.5-mm) diameter. T e center rib was not used because it tended to show shrinkage porosity. T e four special drills used in this study are shown in Table 2.
are precipitated mainly in the form of α-Al15
not in evidence because of the higher manganese-iron ratio in the 396-G2 alloy of approximately 1.04. T is ratio promotes the formation of the α-Fe
T e platelet-like β-Fe5 (Fe,Mn)3 Si2 and Al2
Cu particles. FeSi phase was
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January 2014 MODERN CASTING | 49
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