The film thickness is considered to be much more important than surface tension since film thickness varies to a greater extent than surface tension with the change of grain size. This explains why alloys with fine grains are more resistant to hot tearing.


Sigworth considered liquid metal embrittlement to inves- tigate hot tearing.4


Griffith’s crack theory was used to ex-


plain liquid metal embrittlement. In the model, the strain energy stored in the deformed material contributes to cre- ate a new surface when cracks grow. In ductile materials most of the fracture energy is consumed at the root of the growing crack tip during plastic deformation. How- ever, when certain liquid metals are present, the ductility nearly vanishes, the fracture stress decreases remarkably, and the cleavage energies calculated are very close to the measured surface free energy. Sigworth concluded4 that liquid metal embrittlement (and thus hot tearing) is caused by a small level of surface-free energy between liquid and solid phases, that cause “cracks” or disconti- nuities in the liquid.


Rosenberg, Flemings and Taylor19 pointed out that hin-


dered feeding of the solid phase by the liquid was the main cause of hot tearing. Hot tears would not occur as long as there was sufficient feeding during solidifica- tion. In a quantitative study of the solidification, and an


evaluation of cracking in aluminum-magnesium alloys, Clyne and Davies20


focused on the solidification time in


the mushy state, and considered that hot tearing was the result of uniaxial tension. They confirmed that the latter stage of solidification is critical to hot tearing; during this stage the grains are no longer able to move freely, and that applied strain would cause hot tearing. Based on this theory, a CSC (crack susceptibility criterion) hot tearing criterion was established.2


Though the subject of hot tearing is not a simple one, we can generally summarize the theories into two groups:


• One group of theories is based on stress, strain and strain rate, and these are related to thermo-mechan- ical properties of the alloy.


• The other group of theories is based on liquid film and lack of feeding which is related to metallurgi- cal factors.


From this review, it can be concluded that hot tearing is a complex phenomenon, which combines metallurgical and thermo-mechanical interactions. Though the basic phenomena involved in hot tear formation are understood, there is still no agreement on what (controlling factor) causes hot tears.


Figure 1. Hot tear formation based on interdendritic liquid film concept.18


International Journal of Metalcasting/Winter 11


25


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