Abstract
To study the micromechanics of semisolid deformation, a modified experimental set-up is employed in Gleeble 3800 thermomechamical testing unit to achieve a uniform temperature distribution in partially remelted aluminum samples. The temperature variation was markedly reduced to one degree for a length of 4-5 mm in the middle of tensile samples. High temperature semisolid tensile tests of Al-Cu 206 cast alloys were performed at different temperatures near solidus with a strain rate of 10-3 s-1, corresponding to the solid fractions (fs) between 1 and 0.95. The stress-displacement curves with different fs were measured and analyzed. The microstructure and fracture surface of samples were examined by optical and scanning electron microscopes. The relation between the microstructural characteristics, tensile properties and fracture behavior of semisolid 206 samples at high fs were explored. Mush deformation mechanisms were discussed in term of defect nucleation and propagation at the late stage of solidification.
Highlights
Casting defects such as hot tearing and porosity are closely related to the tensile stresses arisen from thermal gradient and solidification contraction during casting process [1][2][3][4]
For solid fractions less than 0.85, the interdendritic region is fairly open and the liquid phase within the mush structure will flow without difficulty and accommodate the exposed tensile stresses
It has been generally reported that the semisolid tensile test can generate stress-strain conditions similar to the states caused during an actual solidification of aluminum alloys [4][6]
Summary
Casting defects such as hot tearing and porosity are closely related to the tensile stresses arisen from thermal gradient and solidification contraction during casting process [1][2][3][4]. For solid fractions less than 0.85, the interdendritic region is fairly open and the liquid phase within the mush structure will flow without difficulty and accommodate the exposed tensile stresses. For the solid fraction toward 0.95-0.98, the mush will be in its most critical region that the liquid phase cannot flow and feed within the mush structure spelling the formation of casting defects [5]. There is limited information on the effect of the constitutive phases such as Fe-rich intermetallics on the semisolid tensile properties of aluminum alloys during the last stage of solidification. A modified experimental set-up and a heating regime to achieve a uniform temperature distribution for the semisolid tensile tests was proposed. For the liquid fractions less than 0.1 (near the solidus), the semisolid tensile tests were conducted on two Al-Cu 206 alloys with different Fe-rich intermetallics. The crack propagation mechanisms within the mush structure at different liquid fractions have been examined and discussed
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