Fracture behavior of thixoformed 357-T5 Al alloys
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2/12/04
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Fracture Behavior of Thixoformed 357-T5 Al Alloys CHUL PARK, SANGSHIK KIM, YONGNAM KWON, YOUNGSEON LEE, and JUNGHWAN LEE The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary . An almost threefold increase in impact energy value was, for example, observed with increasing volume fraction of primary from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary in the present study appears to be related to the magnitude of stress concentration at the interface between primary and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary in thixoformed 357-T5 alloy was discussed.
I. INTRODUCTION
SEMISOLID metals and processing techniques are entering into the commercialization stage for various industrial applications, including automobile and electronic industries, due to the extensive research efforts in the last 10 years.[1–5] The recent interest in the semisolid metal research area encompasses the mechanical and corrosion properties of final products as related to the microstructural characteristics.[6–9] Previously, the authors reported that the mechanical and corrosion properties of thixoformed 357 alloy with different reheating temperatures, and the results were compared with those of permanent mold cast (PMC) 357 alloy.[8] The effects of microstructure, including the volume fraction and the size of primary phases, on the tensile and corrosion behaviors of thixoformed 357-T5 alloy were also reported.[10–15] It was also found that tensile and corrosion properties of thixoformed 357-T5 alloy were greatly affected by the microstructural features. The tensile elongation of 357-T5 alloy, for example, increased exponentially from 3.1 to 8.6 pct with increasing volume fraction of primary from 55 to 68 pct.[12] In the course of previous studies, it was speculated that the fracture behavior would also be affected by the microstructural features. At present, the detailed microstructural characteristics have not yet been correlated with the fracture behaviors of thixoformed 357 alloy.
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