2
Procedure T e ingots were cut
into smaller pieces, cleaned, dried and melted in 220.5- lb. (100-kg) charges to pre-
pare the required alloys. Table 1 provides the chemical composition for the 396 alloys (coded G2 and G3) and for the B319.2 alloy (coded G12). T e composi- tions 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 melting temperature of approximately 1,382F (750C). All alloys were grain refi ned and modifi ed by adding 0.15% titanium and 90 ppm strontium. 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. T e melts were degassed for 15-20 minutes with a rotary graphite impeller using pure dry argon. Prior to pour- ing, surface oxides and inclusions were skimmed off thoroughly. T e melt was poured at 1,364F (740C) into a waffl e- plated, graphite-coated metallic mold preheated to 842F (450C). Specimens (Fig. 1) were cut from the casting with dimensions 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. Drill- ing experiments were carried out on a high-speed, high-precision vertical machining center. T e experimental setup consisted of a dynamometer with fours sensors, charge amplifi ers and an A/D converter. 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 metalworking 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.
Sept/Oct 2013 | METAL CASTING DESIGN & PURCHASING | 41
3
Results and Conclusions
Figures 2a and 2b
show the eff ect of tin’s ad- dition on the microstruc-
ture of the 396-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 al- ways solidify at the interfaces of α-Al/ Si or α-Al/Fe-rich intermetallics. Table 3 summarizes the eutectic
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
are precipitated mainly in the form of α-Al15
(Fe,Mn)3 Si2 and Al2 Cu particles.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60