Effect Of Cu And Mg Contents On Hot Tearing Susceptibility Of Al-6Zn-yMg-xCu Alloys

Al-Zn-Mg-Cu alloy has high specific strength,good formability and machinability,and is a good substitute for traditional steel and copper.It is widely used in aerospace exploration engineering,load-bearing structural parts,high-speed transportation and anti-corrosion materials engineering.At present,the attention of international research on Al-Zn-Mg-Cu alloy is mainly focused on the aspects of heat treatment process,deformation performance and mechanical properties of the alloy,and the hot crack defect is the bottleneck in the manufacture of complex structure,large size and net forming castings.Based on the theoretical model of Clyne-Davies,the hot tearing susceptibility of Al-6Zn-2.5Mg-xCu?x=0,1,2,3 wt.%?alloy and Al-6Zn-yMg-2Cu?y=1,3,5 wt.%?alloy is evaluated,and the hot tearing tendency is verified according to the crack size at the hot spot of"T"alloy casting with different components.Double thermocouple differential thermal analysis was also used in the experiment to collect parameters such as temperature change during the solidification process of the alloy,and to analyze the influence of the crystallization behavior of the alloy parent phase on the susceptibility to hot tearing.The microstructure,solidification path and second phase types of the alloy are observed and analyzed by optical microscopy?OM?,scanning electron microscopy?SEM?,differential scanning calorimetry?DSC?,transmission electron microscopy?TEM?and X-ray diffraction?XRD?.The results of this paper show that the formation of hot tearing in Al-6Zn-2.5Mg-xCu?x=0,1,2,3 wt.%?alloy is mainly caused by the combined action of solidification shrinkage compensation,strength theory and liquid film mechanism.It is found that with the increasing of Cu content,the hot tearing susceptibility of the alloy gradually decreased and then slightly increased,showing a V-shaped change.The hot tearing susceptibility was the smallest when Cu content was 2 wt.%.When Cu content is 2 wt.%,the hot tearing susceptibility is the lowest.At the same time,with the increasing of Cu content,the dendrite interference temperature(T_coh)decreases,and an appropriate amount of S?Al₂CuMg?phase and T?Al₂Zn₃Mg₃?phase at the grain boundary at the later stage of solidification will help to reduce the hot tearing susceptibility of the alloy to some extent,while excessive Cu?x=3wt.%?will lead to the increase of dendrite tendency of the alloy grains and hinder the local residual liquid phase shrinkage.In Al-6Zn-yMg-2Cu?y=1,3,5 wt.%?alloy solidification process,with the increasing of Mg content gradually,the temperature and precipitation of primary phase?-Al dendrite interference temperature(T_coh)gradually decreased,the second phase??MgZn₂?,S?Al₂CuMg?and T?Al₂Zn₃Mg₃?significantly increased,the number of Al-6Zn-yMg-2Cu alloy hot tearing susceptibility decreased.At the same time,due to the increasing of eutectic compounds at low melting point and the increasing of the second phase on the liquid film surface in the later cooling period of Al-6Zn-yMg-2Cu alloy,the hot tearing susceptibility of the alloy was reduced.