Both Chamberlain et al. and Sigworth et al.’s work showed that alloy interactions also affect hot tearing.23,24


Considerable


variation in hot tearing characteristics was observed in the Al- Zn-Mg system, and it depended on Mg and Zn contents and their interaction. The propensity to hot tearing decreased with increasing Mg:Zn ratio. Specifically, no hot tearing was ob- served when the Mg:Zn ratio was greater than 1.4:1.


Over the years, many studies have shown the importance of the amount of eutectic and how it affects hot tearing in Al and Mg alloys.12,13,19,25,26


The work to date has established


that hot tearing tendency is related to the amount of eutectic liquid present during the latter stages of solidification. Hot tearing resistance is lowered as the eutectic phase present is lowered. In contrast, beyond a certain critical value, hot tearing resistance increases with increasing eutectic content.


Rosenberg et al.19 considered that “pockets” or “films” of the


liquid eutectic phase reduce strength of the nearly solid cast- ing, and thus lower hot tearing resistance. While the eutectic phase completely surrounds the primary grains, resistance to hot tearing increases due to improved feeding. Pumphrey et al. emphasized13


that with increasing eutectic, stress accom- modation and healing are critical.


Eskin et al. published a comprehensive review on hot tearing susceptibility for many aluminum alloy systems,5


including


Al-Cu, Al-Mg binary alloys, Al-Cu-Mg, Al-Cu-Si, Al-Mg- Si and Al-Cu-Li ternary alloys, and AA2XXX (Al-Cu-Mg), AA6XXX (Al-Mg-Si) and AA7XXX (Al-Zn-Mg) series commercial alloys. In this review we augment the critical review; specifically, hot tearing susceptibility and effects of alloy composition are discussed below.


Clyne and Davies studied the Al-Mg system alloys using a “dog-bone” specimen.20


= 0.03%) with varying Mg content was used in their first series of tests. Maximum susceptibility was observed at about 1%Mg.


High purity aluminum (total Fe + Si Pekguleryuz and Vermette26 studied hot tearing resistance of


Mg die cast alloys using a constrained-rod mold; they rated hot tearing resistance by considering rod length, severity and position of the crack. They reported a trend between freez- ing range and eutectic fraction to hot tear tendency. Alloys with freezing range around 100°C seemed to have high hot- tear sensitivity, while alloys above and below that interval showed lower tendencies to hot tearing. Hot tear susceptibil-


The test showed that even the high purity base alloy (without addition of Mg) had a significant degree of cracking. However, when super purity aluminum was used, the crack was eliminat- ed. It was found that the onset of cracking was rapid at very low solute content. They further studied various levels of Mg at dif- ferent pouring temperatures using commercial purity aluminum (0.29%Fe, 0.44%Si). Their results are shown in Figure 4, which indicate that the maximum cracking susceptibility is dependent on both composition and pouring temperature. As pouring tem- perature increases, and Mg content lowered, the maximum in cracking susceptibility curves is raised.


Spittle and Cushway studied the Al-Cu with up to 15% Cu using dog-bone castings.27


The experiments showed that alloy


composition influences crack formation. The actual composi- tion range, in which cracking was observed, depended on al- loy grain structure and pouring temperature (superheat).


(a)


(b)


Figure 3. a) Schematic illustration of hot tearing susceptibility as a function of alloy composition for a binary alloy, shown as a lambda curve,22 ring die test.2


b) Hot tearing for Al-Cu alloys, showing a peak at approximately 0.7%Cu from the conical International Journal of Metalcasting/Winter 11 27


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