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treatment provides tensile properties which far exceed those of gray iron, ap- proaching those of ferritic ductile iron.


CONCLUSION Te metallurgical study at six loca-


tions inside two identical 30.9 lb. (14 kg) aluminum A356-T6 and 206-T4 castings with local solidification times ranging between 2.8-13 minutes brought the following conclusions: For an equivalent degassing procedure,


the surface area of microvoids is higher in alloy 206 (1.66% max versus 1.26% max for alloy A356); this may be attributed to its wider freezing range (70C vs 50C for unmodified alloy A356). Microvoids are formed at grain


boundaries for alloy 206 and be- tween dendrites in alloy A356. Te maximum length of microvoids


are higher in alloy A356 (highest value of 780µm versus 450 µm in alloy 206) due to the interdendritic nature of the voids in alloy A356.


Table 1. Surface Finishes of Various Casting Processes Casting Method Die


Investment Shell Mold


Centrifugal-Perm Mold Normal Non-Ferrous Sand Normal Ferrous Green Sand 3-D Printed Molds


Within the 2.8-13 minute local


solidification time bracket, the grain size varies from 126-246µm in alloy 206 while the A356 secondary dendrite arm spacing (DAS) lies between 52 to 88µm. Te yield stress is almost inde- pendent of the metallurgical quality (i.e. the structure fineness and the microvoid level); it depends on the heat treatment applied and the amount of hardening elements (Cu, Mg). Consequently, yield stress values inside the casting are similar to that of the separately cast ASTM B26 test bar. Elongation and ultimate tensile


RMS Surface Finish 20-120 60-200 120-300 20-300


200-400 560-900 560-900+


strength are closely correlated to the metallurgical quality of the alloy; their variation inside the casting are greater in A356-T6 than in 206-T4, despite the important inverse segregation of copper observed in the 206 casting. Alloy 206 tensile properties are far


superior to those of alloy A356-T6. Elongation values inside the 206-T4 casting were three to five times those measured in the A356 casting while its yield strength was 10% higher.


PRESENTATION Critical Characteristics Affecting the


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50 | MODERN CASTING May 2017


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