Study On The Cracking Defects During Direct-chill Casting Of 7××× Series Aluminum Alloys

High-strength 7××× series aluminum alloys have been extensively used in aerospace industries due to their excellent properties. However, direct-chill casting of the 7××× series alloys is facing a challenge from very severe cracking defects. Hot tearing refers to the cracks formed during the solidification of the alloys, which is associated with the thermal stresses and strains generated in the mushy zone, liquid feeding as well as the microstructure and morphology of semi-solid alloy. Cold cracking is a brittle fracture which can be produced when the ingots/billets are cooled down to low temperature range. And the residual thermal stresses developed during casting and the ductility deterioration of as-cast alloys are the main causes. In this thesis, the cracking phenomenon during direct chill casting and the relevant criteria were systematically studied by various techniques:tensile experiments in Gleeble machine, constrained rod casting (CRC), thermal contraction measurement and computer simulation.The mechanical properties of as-cast 7050 alloy in the solid state and semi-solid state were measured using on-cooling and in-situ solidification approaches, respectively. The evolution of strength and ductility of the alloy with temperature and the role of strain rate were also investigated. The experimental data were fitted into the constitutive parameters needed to construct the extended Ludwik equation and high temperature creep law, by which the FEM modeling of the direct-chill casting of aluminum alloys could be carried out. The temperature field, flow field and stress field during the casting of large size round billet and flat ingot and the effects of casting parameters on the casting process were investigated in details.A modified hot tearing criterion for the direct-chill casting of 7××× series aluminum alloys was proposed based on Suyitno’s SKK criterion. Two key parameters, i.e. the fraction of grain boundaries covered by liquid phase(fGBWL), which can describe the evolution of grain coalescence during solidification, and solid energy term (Us), representing the energy needed when hot tearing propagates along coalesced grain boundaries, were introduced into the new criterion. The hot tearing susceptibility curve predicted by the modified hot tearing criterion in the mushy zone is in good agreement with casting experience.The hot tearing susceptibility and thermal contraction in the freezing range of several typical 7××× series alloys with different Zn contents and Zn/Mg ratios were measured. It was shown that the hot tearing susceptibility of an alloy not only depends on the thermal contraction in freezing range, but also the amount of nonequilibrium eutectics which can effectively accommodate the thermally induced deformation. Grain refinement could considerably reduce the hot tearing susceptibility. However, excessive addition of grain refiner may promote hot tearing susceptibility of semi-solid alloy due to the deteriorated feeding ability which is very likely to be caused by the heavy grain refinement and the formation of Ti contained primary phases at grain boundaries.A cold cracking index was also proposed by the author combining the experimental results and FEM simulation. It was found that the cold cracking tendency of the ingots during direct-chill casting of 7××× series alloys is low above 25O℃, and begins to increase dramatically when temperature is decreased below 200℃. The cold cracking tendency is basically consistent with the amount of nonequilibrium eutectics of an alloy under the same casting process. It is very likely that massive nonequilibrium eutectics located at grain boundaries greatly weaken the ductility of as-cast alloy, making the ingot prone to crack due to stress concentration. According to the cold cracking index, reducing the residual stress in the ingot by the application of water wiper can effectively decrease the cold cracking tendency.