Hot Tearing Susceptibility of Mg-Ca Binary Alloys
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HOT tearing is a major casting defect and has a significant impact on the quality of cast products.[1] The tears can compromise a cast component’s structural integrity, lead to a loss of pressure tightness, and act as stress raisers aiding in the propagation of fatigue cracks or catastrophic failure. Thus, tear-free castings are highly desirable for the subsequent processing and service.[2] Hot tearing is a complex solidification phenomenon which is still not fully understood.[3] It occurs above the effective solidus temperature due to the obstructed contraction of solidifying alloys, often at hot spots where the liquid metal solidifies last or at areas with sudden changes in cross sections.[4] Industrial and fundamental studies of this phenomenon show that hot tear occurs when the liquid flow through the mushy zone becomes difficult and insufficient to fill the initiated cavities at the far end of the dendrites. Hot tearing has been extensively studied on steel and aluminum alloys in the past decades. However, only few Mg alloy systems, Mg-Al,[3,5,6] Mg-Zn,[7] Mg-Y,[8] and JIANGFENG SONG, Ph.D. Student, YUANDING HUANG, FELIX BECKMANN, and NORBERT HORT, Scientists, and KARL ULRICH KAINER, Professor, are with the Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany. Contact e-mail: [email protected] ZHI WANG, Scientist, is with the Institute of Materials Research, Helmholtz-Zentrum Geesthacht, and also with the School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China. AMIRTHALINGAM SRINIVASAN, Scientist, is with the Institute of Materials Research, Helmholtz-Zentrum Geesthacht, and also with CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum 695019, India. Manuscript submitted on March 5, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
Mg-Gd,[9] have been subjected to hot tearing studies. Cao et al.[6] reported that the alloys containing 1 to 2 wt pct Al were the most susceptible to hot tearing, while Mg-8 wt pct Al alloy was the most tear resistant. Hot tearing behavior of commercial Mg alloys such as AZ31,[10] AZ91,[10,11] and AM60[10,12] have also been studied. Their findings showed that the additions of rare elements (REs) increased the hot tearing susceptibility of AZ31, AZ91, and AM60, while Sr addition decreased hot tearing tendency of AM60. A study by Powell et al.[13] showed that 2 wt pct Ca addition was the optimum level for best castability for AXJ alloys. Effects of Ca, Sr, and Zn additions on hot tearing behavior of Mg-Al alloys were also studied earlier.[14–16] In Mg-Al-Ca ternary alloys, hot tearing tendency of Mg-4 wt pct Al alloys decreased with increasing the Ca content (0.5 to 3.5 wt pct), while hot tearing tendency of Mg-2.5 wt pct Ca did not change much with the addition of Al (4 to 6 wt pct).[14] Hot tearing resistance was found to enhance with increase in both Sr and Al contents in Mg-Al-Sr ternary alloys.[4] Wang et al.[11] reported that addition of Zn (up to 1 wt
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