In this paper, the process of hot crack formation has been simulated by utilizing a testing unit designed by the authors. The hot tearing temperature and the critical fracture stress have been investigated for Al-Cu alloy. It was found that both the critical fracture stress and the hot tearing temperature were closely associated with the deformation velocity. The hot tearing temperature increases with a rise in the deformation velocity. On the other hand, there appears a peak value in the critical fracture stress when the deformation velocity is increased. Further, the condition for hot crack formation has been analyzed. The hot cracks will not be formed when the condition V<(KR/Es+
β/
τ−
γR).
l is satisfied, where V, R and
l are deformation velocity, cooling rate and length of the heated zone of the specimen, respectively ; K and
γ are increasing rate of strength and decreasing rate of elongation, respectively, of the alloy in the semi-solid state region ; Es and
β are Young's modulus and elongation of the alloy at the semi-solidus temperature ;
τ is time elapsed after the beginning of contraction.
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