pouring crucible and gas protection are used to ensure a con- sistent pouring head and minimize contamination.


Hot tearing susceptibility was evaluated based on crack width, rod length and crack location. Cao correlated the load and cooling vs. time curve to hot tearing; this work shows that load and cooling curves correlate with phase for- mation and crack onset, healing, propagation and the final crack width. The crack onset time and temperature can be determined from the load curve and its first derivative. Their study found that the larger the freezing temperature range, the higher the hot tearing susceptibility. The element Ca had a significant effect on the susceptibility of ternary Mg-Al-Ca alloys to hot tearing. In Mg-4Al-yCa alloys, the crack sus- ceptibility decreases sharply as the Ca content (y) increased from 0 to 2.5 %. The susceptibility of Mg-xAl-2.5Ca to hot tearing did not change significantly as the Al content (x) in- creased from 4 to 6%.


Figure 16 shows a schematic of the experimental setup devel- oped by Zhen et al. to study Mg-Al alloys.64


It consists of a constrained rod steel mold, a contraction force measurement


system with a load cell, and a data acquisition system. The principle of the test is to measure the contraction force in- duced by solidification and thermal shrinkages and to monitor the evolution of hot tearing. When a hot tear occurs during solidification, the induced contraction force is accordingly released. A drop can be observed on the force curve. The be- havior of hot tearing can then be assessed by analyzing force development during solidification; this includes the initia- tion of hot tearing, the evolution and the final size of the hot crack and so on. Zhen’s mold design minimized the friction between the solidifying casting and the inner wall of the mold and thus eliminated any errors in measuring contraction force. The operation procedure was that once the casting started, the force measurement system was activated. The force and tem- peratures of the mold at different positions, including at the hot spot area, were recorded. Zhen used a total volume value as the measure of hot tearing severity. With such an instru- mented apparatus they studied the influence of mold tempera- ture and found that increasing mold temperature decreased hot tearing susceptibility. The contraction force curves also indicated that the liquid refilling played an important role in partially or completely healing cracks


Figure 15. Schematic diagram of Cao’s instrumented and constrained rod testing apparatus.63


Figure 16. Schematic of the experimental setup by Zhen et al.64 34 International Journal of Metalcasting/Winter 11


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