The Microstructure, Properties And Fatigue Characteristics Of Al-Mg-Mn-Sc-Zr Alloy

Abstract:The Al-Mg based alloys are medium-strength alloy and are widely used due to good formability, better corrosion resistance and toughness. However, this kind of alloy belongs to non-heat-strengthened alloys, and strain strengthening is the most important way to improve its strength. When the alloys are in a higher temperature, the strain hardening effect is partially or completely lost. To further meet the requirements of high-performance materials, scholars of former Soviet Union first proposed small amount of rare earth element Sc could significantly improve the overall performance of aluminum alloys, and they had developed a series of Al-Mg-Sc alloys. Compared to the conventional Al-Mg series alloys, the alloys containing Sc not only possess better corrosion-resistant and higher toughness property, but also a high strength and good weldability. The Al-Mg-Sc alloys require stabilizing annealing before using in order to eliminate internal stress and maintain a good mechanical property. Besides, the mechanical properties, especially the fatigue properties, of the Al-Mg-Sc alloys lead to a number of limitations to the use of it, production process also requires to control annealing in order to meet the requirements of the anisotropy in alloys. Both the effects of stabilizing treatment on microstructures and mechanical properties of Al-Mg-Mn-Sc-Zr alloy as well as the effects of annealing on the fatigue properties in alloys were studied. This could provide basis for the heat treatment process and its industrial application.Al-Mg alloys with Sc and Zr were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The alloy ingots were finally rolled into2mm thin sheets after hot rolling, intermediate annealing and cold rolling processes. The effects of stabilizing treatment on microstructures and mechanical properties of Al-Mg-Mn-Sc-Zr alloy were studied by means of Vickers hardness tests, tensile testing at room temperatures, corrosion tests, optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), respectively. The results show that the strength and hardness of Al-Mg-Mn-Sc-Zr alloy decrease, while the elongation increases with increasing the stabilizing annealing temperature. With the increasing of stabilizing annealing time the strength and hardness of the alloy drop slightly but its ductility exhibits no change. In consideration of the strength and ductility of the alloy, the suitable stabilizing annealing temperature and time of the alloy sheet should be300～350℃and1h respectively. The corrosion resistance of the alloy annealed at150℃for1h is the worst. When the annealing temperature higher250℃, the alloy has the better corrosion resistance and no obvious exfoliation corrosion and intergranular corrosion appear. Adding minor Sc and Zr to Al-Mg-Mn alloys make the recrystallization temperature increased to500℃, this is because the present of highly disperseded Al3(Sc,Zr) particles which has a strong pinning effect on dislocations and sub-grain boundaries. And sub-grain mergers and sub-grain growth of these two mechanisms constitute recrystallization nucleation mechanism of the alloy. Al-Mg-Mn-Sc-Zr alloy simultaneously a large number of Al3Sc、Al3Zr and Al3(Sc,Zr) particles dispersedly distribute in the matrix, which have function of strongly pinning sub-grain boundaries and dislocations. As the annealing temperature, the precipitation amount of Al3(Sc,Zr) particles increases, and the particle diameter somewhat increased.The effect of annealing temperature and time on the fatigue performance were investigated by fatigue testing machine and scanning electron microscopy. The results show that the fatigue crack growth areas of Al-Mg-Mn-Sc-Zr alloy is also divided into three typical parts, namely low-expansion area, stable expansion area and high-speed expansion area. In the stable crack growth area, we can see fatigue striation, fatigue level and secondary cracks. Fatigue striation is perpendicular to the stress loading direction, whereas secondary cracks are nearly parallel to the stress loading direction. For annealing at340℃, the fatigue striation is the narrowest, extending the annealing time, the fracture surface of alloy has no obvious change. Besides, the fatigue crack growth rate has a relationship with the stability of the annealing temperature, when ΔK is30MPa·m1/2, the best annealing process is340℃/1h which has the min fatigue crack growth rate. When the annealing temperature is340℃, it has little effect on fatigue crack growth rate although the annealing time increasese. When the alloy were annealed at130℃for1h and at500℃for1h, the crack path relatively flat, and a large crack bend occurs after annealed at340℃for1h. Meanwhile, annealing time has little effect on the crack propagation. Considering the fatigue performance of the alloy, the alloy annealed at340℃has the best fatigue properties.