Dynamic Precipitation And Structure Characterization Of Secondary Phases In 5052-based Alloys

The 5XXX series, non-heat-treatable aluminum alloys, possessing good combinations of low density, high specific strength and stiffness, good corrosion resistance, well weldability and easy shaping, are widely used for the transportation and aircraft industry. With the development of lightweight automobiles, the demand for aluminum alloy grows and better property is required. The dynamic precipitation duing deformation is important for the formability of the aluminum alloys.So it is necessary to conduct a research on the morphology and distribution of second phases with different conditions and the dynamic precipitation behavior duing the deformation process.5052 aluminum alloy is one of the most widely used products in 5XXX series aluminum alloys. In this paper, the test alloys are 5052 based alloy, 5052-alloyed with the joint addition of Er and Zr, and the 5052-alloyed with Sr and Cu respectively. The microstructures and properties of 5052、5052-Er、5052-Sr、5052-Cu alloy with different conditions were made a systematic analysis by OM、DSC、SEM、TEM、EDS and Gleeble-3500 etc, in order to control the precipitation of the second phases in the aluminum alloy.The results show that the microstructures of the as-cast 5052、5052-Er、5052-Sr alloys consist of α-Al and iron-phases with the shape of acicular or Chinese characters. The size of second phase is larger than 100 μm and the dendritic segregation is abvious in the cast structures. The microstructures of the as-cast 5052-Cu alloy consist of α-Al and copper-phases or iron-phases. Three kinds of alloying methods improved the as-cast microstuture. With the Er(Zr) or Cu addition, the as-cast microstutures are greatly changed. During the solidification, Er is existence as solid solution, and the other is forming Al3 Er phase, which can refine the structure and purify the melt. The morphology and composition of the second phase is transformed by the aidditon of Cu.The morphology、size and composition of the second phases are changed after homogenizing annealing. In the 5052、5052-Er、5052-Sr alloys, the second phases are refined and precipitated a small amount of granular second phase. In the 5052-Cu alloy, most particles are re-soluted in α-Al and part of Mg、Cu atoms are existence as solid solute, which could be precipitate as Al2 Cu Mg.It can be found that the 5052-Cu alloy has higher hot tensile strength than the other three alloys, when the four alloys are tested by means of isothermal tension. The results show that the true stress-true strain curves exhibit serrated yielding phenomenon during isothermal tension under conditions of 100 ℃ with a strain rate of 0.01 mm﹒s-1 and 250 ℃with 0.01 mm﹒s-1 or 0.1 mm﹒s-1 strain rates, which is resulted from a dynamic strain aging(DSA).We also studied the deformation behavior of non-isothermal tension with the Cu-added alloy. The results show that the the true stress-true strain curves exhibit serrated yielding phenomenon under condition of heating rate 0.25 ℃﹒s-1, which is resulted from a dynamic strain aging(DSA). Dynamic precipitates, Al2 Cu Mg pahse, is found to occur during non-isothermal tension under certain combination of heating rate and strain rate, which contributes to a long “yield platform” in the true stress-true strain curves.The deformation behavior of 5052-Cu alloy has been studied by isothermal compression tests. The results show that the flow stress increased with the increase of temperature and strain rate. The dynamic recrystallization occurs at 470 ℃. With the strain rate decreased, the proportion of the dynamic recrystallization grain increased and the dynamic recrystallization have been completed when the rate was 0.001 s-1. With the h igher strain rate(0.1 s-1), the dislocation density was increased and the finer particles was foremed. The rod-shaped phase was broke up, which formed the nano-particles. The higher dislocation density and finer paticles can effectively hinder the dynamic recrystallization.