Nanoparticle-Induced Superior Hot Tearing Resistance of A206 Alloy
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AL-CU alloys offer excellent mechanical properties approaching some grades of ductile iron, and are significantly stronger than the premium quality cast Al-Si alloys that are the most widely used in various industrial applications.[1] Al-Cu alloys also provide excellent high-temperature strength and fatigue life. However, it is well known that the applications of cast Al-Cu alloys are limited because of their poor castability.[2] Their long solidification range makes them extremely susceptible to hot tearing.[3] Therefore, the minimization of hot tearing is critical to expand their structural applications. Hot tearing has been extensively studied, and several reviews in the field are available.[4–6] However, there are different theories of hot tearing, such as the stress-based theory, strain-based theory, and strain rate-based theory, HONGSEOK CHOI, Research Scientist, WOO-HYUN CHO, Graduate Research Assistant, XIAOCHUN LI, Professor, are with the Department of Mechanical Engineering, University of WisconsinMadison, 1035 Mechanical Engineering Bldg., 1513 University Avenue, Madison, WI, 53706. Contact e-mail: [email protected] HIROMI KONISHI, Research Scientist, is with the Departments of Geoscience and Mechanical Engineering, University of WisconsinMadison. SINDO KOU, Professor, is with the Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Avenue, Madison, WI 53706. Manuscript submitted March 4, 2012. Article published online November 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
which have been reviewed by Eskin.[7] As stated by Campbell,[8] hot tearing is one of the most serious defects that a casting can suffer, and it has remained a major problem in the foundries although it has been researched and understood in a general way. Nevertheless, it has been well known that grain refinement and thermal mold control are the two common methods to reduce hot tearing in casting processes of Al-Cu alloys.[9,10] However, the hot-tearing tendency of Al-Cu alloys is still far worse than A356 (~Al-7Si-0.3Mg), a widely used casting alloy with excellent hot-tearing resistance. It has been also reported that element impurities (e.g., Fe, Mg, and Zn) have an effect on the hot-tearing tendency of alloys.[7,11] Nagaumi et al.[11] have shown that a high Fe content, e.g., 0.25 wt pct, can increase the hot-tearing susceptibility (HTS) of Al-Mg-Si alloys. Large intermetallic particles of a(AlFeMn) can increase hot tearing by bridging growing dendrites to form the semisolid rigid prematurely and block liquid feeding to decrease a chance of crack healing. Therefore, it suggests that a formation of intermetallics and eutectics during a solidification process would correlate with the hot tearing of highly hot-tear-susceptible alloys. Recently, a new class of nanostructured materials, metal matrix nanocomposites (MMNCs), has been gaining more and more attention. MMNCs can be defined as nanoelement (e.g., nanoparticles, nanotubes, nanoplatelets, etc.)-reinforced metals. Studies have VOLUME 44A, AP
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