At mold temperatures higher than 220C (428F), cracks were hairline-like and were not connected. Mold temperatures above 340C (644F) were sufficiently elevated to mitigate hot tears. The thinking was that higher mold temperatures improved bulk feeding of the casting and also feeding of the intergranular and β-phase regions (microscopic feeding). In addition, elevated mold temperatures promote uniform casting contraction, result- ing in less stress concentrations and a decrease in the tendency to form hot tears in the casting.


Zhen et al. studied the effects of mold temperature in the range of 250 to 500C (482 to 932F) for binary Mg-Al alloys.44 They found that increasing mold temperature decreased hot tearing susceptibility, and higher mold temperatures led to a higher crack onset temperature and longer propagation times. The mechanism they offered was that cracks were initiated at all mold temperatures, but at higher mold temperatures cracks could be refilled by the remaining liquid and were thus healed. At higher mold temperatures one would experience a lower cooling rate, and thus coarser microstructures. A coars- er structure would lead to a thicker and more continuous re- maining liquid. This coupled with higher onset temperatures made the refilling easier. Limmaneevichitr et al. mentioned the effect of mold temperature when studying the role of grain refinement.45


They also found that cracking was more severe


in lower mold temperature experiments, e.g. 220C (428F) (compared with 250C [482F]), for the same grain refinement conditions. Sadayappan et al. studied hot tearing suscepti- bility of many Mg casting alloys, and compared them with several typical Al casting alloys.46


It was a comparative study


of hot tearing behaviors with respect to alloy type and mold temperature. The results showed that the occurrence of hot tearing was reduced when the mold temperature was 350C (662F) or higher.


Fasoyinu et al. studied the effects of grain refinement and mold temperature on hot tearing of Al alloys 206 and 535 by pouring constrained rod castings in a permanent mold.47


The


results show that hot tearing depended strongly on mold tem- perature. Fasoyinu et al. suggested that a combination of grain refinement and preheating of mold to >400C (752F) for alloy


206.0, and >300C (572F) for alloy 535.0 was necessary to prevent hot tearing. They postulated that grain refinement is related to the break down and refinement of the interdendritic structure leading to improved feeding and a decrease in the amount of residual liquid during the last stage of solidifica- tion. At higher mold temperatures, hot tearing was reduced in the casting due to improved feeding because a hot mold provides an effective thermal gradient. The strain developed in the mushy zone during the last stage of solidification was calculated using commercial simulation software and corre- lated with the fraction solid as a function of mold temperature and the potential for hot tearing. It was found that the principal strain was lower at higher mold temperatures. The reduced tendency to hot tearing in the hotter mold was associated with reduced principal strain during solidification.


In general, the literature about the effect of mold tempera- ture on hot tearing indicates that higher mold temperatures reduce hot tearing susceptibility. With respect to the effect of pouring temperature (superheat) on hot tearing, the lim- ited data that exist in the literature are contradictory.


Hot Tearing Measurements


Many techniques for studying hot tearing and assessing hot tearing susceptibility have been developed over the years. Various apparatuses were designed to induce cracking dur- ing solidification by constraining the casting to resist solidi- fication shrinkage and thermal contraction. Eskin et al. sum- marized the various measurement techniques in their recent review,5


which include ring type testing, backbone mold


testing, cold finger testing and tensile testing. This review presents and discusses additional and pertinent test methods that have been developed and used since the 2004 review.5 Generally speaking, testing methods can be classified into these three categories:43


1. Tests by observation of hot tears post solidification; 2. tests quantitatively measuring load(stresses) or dis- placement during solidification;


3. other tests including physical property tests, etc.


Figure 9. N-Tec hot tearing mold (a) and setup (b). (a)


International Journal of Metalcasting/Winter 11


(b) 31


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