A modified hot tearing criterion for direct chill casting of aluminium alloys

Abstract

Using the good criteria to predict hot tearing is very important during DC casting of aluminium alloys. Among all the hot tearing criteria, a fracture-mechanics based SKK criterion proposed by Suyitno et al. has made considerable improvements in the hot tearing prediction. However, its obtained hot tearing susceptibility (HTS) evolution during solidification is also not completely consistent with real industrial production circumstances, especially when approaching the solidus temperature. In this paper, some further modifications are made based on the SKK criterion to emphasise the important effect of solid bridging/grain coalescence on hot tear propagation. It is proved that the HTS evolution in freezing range predicted by the modified hot tearing criterion is in good agreement with casting practice.

Effect of Main Elements (Zn, Mg, and Cu) on Hot Tearing Susceptibility During Direct-Chill Casting of 7xxx Aluminum Alloys

“…Its variance will obviously affect the predictions. [7,16] Also, the viscosity is needed to be determined for every composition. Thirdly, the SKK criterion always predicts an increasing hot tearing tendency during solidification, which does not agree with the casting practice.…”

“…However, it is known that certain grain bridging occurs during the last stage of solidification. [37][38][39] Recently, Bai et al [7] improved this criterion by considering the effect of solid bridging/grain coalescence, which plays an important role in hot tearing. Two important parameters were introduced into the criterion, including the fraction of grain boundaries covered by liquid and a solid energy term, representing the energy for hot tear propagation along coalesced grain boundaries.…”

“…In future, the effect of solid bridges should also be included by applying the modified SKK criterion. [7] Finally, the effect of minor elements (Fe and Si) on the hot tearing susceptibility of 7xxx alloys requires further investigations.…”

“…The length of mushy zone L is determined by the dendritic rigidity temperature and solvus assuming the thermal gradient being equal to 7500 K/m. [2] Moreover, it is difficult to measure the Young's modulus of the semi-solid materials, [31] and here 170 MPa is chosen which lies between those used by Suyitno et al [16] and Bai et al [7] Figure 8 gives the results predicted by the SKK criterion. For all compositions, the HTS increases with the increase of solid fraction.…”

“…It is well known that high casting speeds and large ingot/billet diameters can easily lead to higher hot tearing susceptibilities in 7xxx alloys. [7][8][9] Besides these factors, alloy composition is another crucial point to affect the hot tearing susceptibility. Bai et al [10,11] investigated the hot tearing and thermal contraction behavior of several commercial 7xxx alloys.…”