Effect of Titanium Levels on the Hot Tearing Sensitivity and Abnormal Grain Growth After T4 Heat Treatment of Al–Zn–Mg–Cu Alloys

In this paper, the castability of Al–6wt%Zn–2wt%Mg–2wt%Cu alloy with different titanium levels was investigated with a constrained rod casting (CRC) mold. With the increasing level of titanium, the hot tearing sensitivity (HTS) index decreased from the value of 17 (without Ti) to the value of 7 (0.06 wt% of Ti). But further increasing of titanium weight percentage will deteriorate the castability of the alloys. The HTS index increased from the value of 7 (0.06 wt% of Ti) to the value of 13 (0.24 wt% of Ti). The grain sizes and grain morphologies of as-cast as well as solid solution-treated samples with different Ti levels were measured and analyzed. For the CRC mold castings, the grain size decreased with the increasing of Ti levels in the as-cast samples, and the grain morphology was changed from dendrite to globular. The Ti level required for minimum grain size decreased as the cooling rate of solidification increased. After solid solution treatment, abnormal grain growth occurred only in samples with less than 0.1 wt% of Ti. The tensile samples with different Ti levels were cast in the tilt pouring cast machine with permanent mold. The tensile test results of heat-treated samples show that the samples with grain growth during heat treatment did not affect the tensile properties.