Onset of Hot Tearing in Ternary Mg-Al-Sr Alloy Castings
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UCTION
THE Mg alloys are known to have light weight and good strength. Ternary Mg-Al-Sr alloys are the base of a few new creep-resistant Mg casting alloys. To prepare high-temperature creep-resistant alloys by casting, the castability, especially the resistance to hot tearing, must be considered. Hot tearing, also referred to as solidification cracking or hot cracking, is often a major defect in alloy castings. It occurs during solidification due to obstructed contraction of the solidifying alloy, often at hot spots where the casting solidifies last or at locations with a sudden change in cross section. Hot tearing is a complex solidification phenomenon that is still not fully understood, though various mechanisms have been proposed.[1–4] Constrained rod casting (CRC)[5–11] has often been used to evaluate the hot tearing susceptibility of Mg alloys though ring casting.[12] Attempts have also been made to determine the temperature at which hot tearing occurs during casting. Wang et al.[13] detected the onset of hot tearing in sand casting of AZ91 Mg with a load cell and a thermocouple. The curve of load vs time showed a very slight discontinuity; based on this and the cooling curve near the crack site, the hot tearing temperature was determined. To detect the onset of hot
tearing, Cao and Kou[8,9,14] developed an instrumented CRC by equipping the steel mold for CRC with a load cell and a thermocouple. The rod diameter was found to affect the shape of the load curve significantly. For a given alloy under a given casting condition, the load curve can show a clear peak at the onset of hot tearing if the rod diameter is small enough. For instance, in the case of AZ91E Mg alloy with a mold preheated to 658 K (385 C), the load curve of a 9.5-mm-diameter rod exhibited only an inflection point at the onset of hot tearing. However, with an 8.7-mm-diameter rod, the load curve showed a clear peak, and this allowed the onset of hot tearing to be detected clearly.[14] The hot tearing onset temperatures in binary Mg-Al alloys, AZ91E Mg, and ternary Mg-Al-Ca alloys were determined. More recently, Zhen et al.[15] also used a similar approach to study hot tearing in Mg-1Al alloy. In the present study, the instrumented CRC developed recently by the authors is used to investigate the hot tearing of ternary Mg-Al-Sr alloys. The purpose is to determine the temperature and fraction solid at which hot tearing occurs.
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EXPERIMENTAL PROCEDURE
A. Melting G. CAO, formerly Graduate Student, Department of Materials Science and Engineering, is Research Associate, Department of Engineering Physics, University of Wisconsin–Madison, Madison, WI 53706. I. HAYGOOD, Graduate Student, and S. KOU, Professor, are with the Department of Materials Science and Engineering, University of Wisconsin–Madison. Contact e-mail: [email protected]. edu Manuscript submitted October 5, 2009. Article published online June 4, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
Pure Mg (99.8 pct), pure Al (99.99 pct), and Sr-10Al (by wt pct) master alloy were used. About 550 g
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