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Table 1. Thermocouple Locations and Corresponding Casting Section Thicknesses


Thermocuple ID


aluminum wedge casting with a wide range of section sizes and cooling rates. Te results showed an increase in ultimate tensile strength and elongation due to a decrease in porosity of 63-84% for castings solidified under pres- sure. Density was measurably increased but did not show a strong correlation with tensile strength or elongation.


Testing the Sand Wedges Te wedge shaped castings provided


A B C D


Distance of thermocouple from bottom of the casting in mm (inches)


32 (1.25) 95 (3.75) 159 (6.25) 122 (8.75)


Section


thickness in mm (inches)


12.7 (0.5) 25.4 (1)


38.1 (1.5) 50.8 (2)


purposes. Te thin section of the wedge was 0.25 in. (6.4 mm) thick, and the thick sec- tion was 2.25 in. (57.2 mm) thick. Te top of the wedge acted as the riser. Te varia- tion in section sizes produced


directional solidification, various sec- tion sizes and a range of cooling rates for the study. Te plate was divided into two halves: “A” and “B” (Fig. 1). Side A was used for tensile specimens. Side B was saved for future testing


various cooling rates, which were faster in the thin section and decreased as section size increased. (Table 1). Te pressure vessel used in the study measured 4 x 6 ft. (1.2 x 1.8 m) (Fig. 2) and could be pressurized to as high as 10 MPa (100 atmospheres). In this study, pressure was applied via a mixture of dry compressed air and compressed nitrogen gas. To determine mechanical


properties, a 0.625-in. (15.9-mm) plate was machined from the center of the wedge, and porosity distribution was documented using x-ray radiography. Before T6 heat treatment, each plate was sectioned into square blanks. Hardness and density were determined for each blank.


Tweaking Temperature, Pressure


Te aluminum alloys in the investi- gation met ASTM A356 chemistry specification (Table 2). Brinell hard- ness ranged from 66.5-76.5 BHN after T6 heat treatment. Decreasing pouring temperature and


increasing pressure during solidifica- tion resulted in decreased porosity. At a pouring temperature of 1,300F (704C), the measured amount of porosity was 0.63% with solidification under 1 atmo- sphere of pressure (atm) and 0.23% with solidification under 10 atm, representing a porosity reduction of 63%. At a 1,400F (760C) pouring


Fig. 3. Porosity in 0.5-in. A356 castings showed reduced porosity at lower pouring temperatures and higher pressure. At top, the castings were poured at 1,300F at (from left to right) 1 atm and 10 atm. At bottom, the castings were poured at 1,400 atm at (from left to right) 1 atm and 10 atm.


Table 2. Casting Composition Si


A356 (ASTM B26) 6.5 - 7.5 704C, 1 atm 704C, 10 atm 760C, 1 atm 760C, 10 atm


6.71 6.78 6.82 6.55


Mg


Composition, wt% Fe


0.25 - 0.45 0.20 max 0.33 0.33 0.34 0.39


0.15 0.16 0.15 0.15


30 | MODERN CASTING March 2011 Ti


0.20 max 0.094 0.095 0.096 0.093


Sr


0.05 max (other) 0.094 0.095 0.096 0.093


temperature, the measured amount of porosity was 0.99% with solidification under 1 atm and 0.16% with solidi- fication under 10 atm, for a porosity reduction of 84%. Reducing the pouring tempera-


ture from 1,400F to 1,300F resulted in a 36% reduction in porosity at 1 atm (Fig. 3). When solidified at less than 10 atm, the higher pour- ing temperature yielded 30% less porosity. The results confirmed the effectiveness of applied pressure in reducing porosity, even if maximum


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