Crack susceptibility of binary aluminum alloys during solidification

Abstract Cracking can occur during solidification in welding and casting. The present study attempted to deal with some fundamental questions regarding the susceptibility to cracking during solidification. How does the phase diagram affect the crack susceptibility? Can back diffusion occur in welding? How is the dihedral angle related to bridging between grains? To help answer the questions, the maximum |dT/d(fS)1/2| (T is temperature and fS fraction solid) was calculated, whose validity as the crack susceptibility index was verified recently. To deal with curved solidus and liquidus lines (i.e., variable equilibrium segregation coefficient k), fS was calculated as a function of T by numerical integration instead of by a simple analytical equation for constant k. Based on the secondary dendrite arm spacing λ2 in common Al arc welds and the λ2−tf data of Al alloys (tf is local solidification time), it was shown that back diffusion can occur in Al arc welding. Binary Al alloy systems were investigated over wide ranges of the eutectic temperature TE (229–548 °C) and k (about 0–0.87 at TE). It was shown that with a very low TE the crack susceptibility can still be relatively low if k is very high at TE to increase the effect of back diffusion on crack susceptibility reduction, consistent with the lower crack susceptibility of Al-Mg and even Al-Zn than Al-Cu. With both a very low TE and a very low k at TE, however, the crack susceptibility can be very high, consistent with the severe crack susceptibility of Al-Sn. The effect of the curvature of the liquidus and solidus lines on the crack susceptibility was shown. A possible link between the dihedral angle and bridging between grains was also shown.