Hot cracking of MG-AL-CA alloy castings

Ternary Mg-Al-Ca alloys are the base of a few new creep-resistant, light-weight Mg alloys for automobiles. Hot tearing in Mg-Al-Ca alloys was studied, including Mg-4Al-0.5Ca, Mg-4Al-1.5Ca, Mg4Al-2.5Ca, Mg4Al-3.5Ca, Mg-5Al-2.5Ca and Mg-6Al-2.5Ca, by constrained rod casting (CRC) in a steel mold. The hot tearing susceptibility decreased significantly with increasing Ca content but did not change much with the Al content. The temperature at which cracking occurred was determined by using instrumented CRC It was higher in alloys more susceptible to hot tearing. It has been widely recognized that hot tearing, also referred to as solidification cracking or hot cracking, is a major defect in alloy castings^[1]. Hot tearing occurs due to obstructed contraction of the solidifying material, often at a location of stress concentration associated with a sudden change in the transverse cross-section. Hot tearing has been studied extensively^[1-21] including binary Mg-Al alloys^{[3, 6, 22]}. The hot tearing susceptibility of some commercial Mg alloys in sand casting was also studied. Argo et al.pl and Pekguleryuz et al.^[10] studied hot tearing of high temperature creep resistant Mg-Al-Sr alloys using constrained rod casting (CRC) in a steel mold. The hot tearing tendency of these alloys, which contained about 5 wt% Al and 0.4 wt% Mn, decreased with increasing Sr content from 1.2 to 2.4 wt%. Powell et al. ^[11] studied the hot tearing in the the casting of Mg-Al-Ca-Sr alloys with about 0.28 wt% Mn and up to 0.17 wt% Sr. Alloys AXJ510 (essentially Mg-4.6Al-0.87Ca), AXJ530 (essentially Mg4.4Al-2.6Ca), AXJ530.1 (essentially Mg-5.2Al-2.6Ca-0.09Sr) and AXJ630.2 (Mg-5.9Al-2.5Ca-0.17Sr) showed significantly less hot tearing than alloy AM50 (essentially Mg-4.4Al), while alloy AXJ520 (essentially Mg-4.5A1-1.7Ca) was between the two cases. Wang et al. ^[5] detected the onset of hot tearing of alloy AZ91 in a sand mold by using a load cell and a thermocouple and found hot tearing occurred at 424°C, which was suggested to be the end of solidification where an eutectic reaction occurred. Cao and Kou^[22] have studied the hot tearing susceptibility of binary Mg-Al alloys using CRC, and the maximum cracking susceptibility was around Mg-1Al. They detected in a steel mold and detected the onset of hot tearing in a similar mold using a load cell and a thermocouple. The present paper describes some recent results on the hot tearing susceptibility of ternary Mg-Al-Ca alloys obtained by using CRC and instrumented CRC. More detailed information is available elsewhere^[23].