| Magnesium alloys are attractive candidates for structural applications mainly because of their low density, high specific strength and easy recyclability. As a result of the increasing global energy demand and high awareness of environmental protection in the recent years,therefore, there is a rapid growth in the application of Mg alloys in aviation, aerospace,lightweight, energy saving, environmental protection and other fields. However, magnesium alloy components are often subjected to fatigue loads in these fields, so it is inevitable to appear fatigue failure in engineering practice. With the development of heat treatment technology, researchers found that the fatigue performance by proper heat treatment process of magnesium alloy can be improved. Therefore it has far-reaching practical significance for ZK60 magnesium alloy to study effect of heat treatment on microstructure and fatigue behavior.ZK60 Magnesium alloys were treated at T4 solution in box-type resistance furnace in the conditions of 450℃ and different time of solution treatment, by means of hardness test to determine a set of parameters of the optimal solution treatment(T4, 450℃×12 h).Then ZK60 Magnesium alloys were treated at T6 heat treatment to determine a set of the optimal parameters of solution treatment plus artificial aging(T6, 450℃×12h+175℃×25 h).By using metallographic microscope, X-ray diffraction(XRD) device and transmission electron microscopy(TEM) of the alloy in three states,the changes of composition and microstructure were measured. In condition, scanning electron microscopy(SEM) was used to observe the fatigue fracture morphology of the three alloys. The materials of three states include rolling state(As-Rolled), solution treatment(T4 treatment) alloys and solution plus aging treatment(T6 treatment) alloys. By using tensile specimen machine and Vickers hardness tester,mechanical properties were measured. And the fatigue performance of test specimens was tested by high cycle fatigue test machine. Effects of the heat treatment on the mechanical properties, fatigue behavior and microstructure of the ZK60 alloys were studied, and its mechanism was discussed.The research showed that the main components of the ZK60 magnesium alloy were α-Mg,MgZn and MgZn2, while there was a great deal of microstructure structure made up of α-Mg and MgZn. There were only a few MgZn2 phase after the solution for 12 h at450℃.However, after solution treatment plus artificial aging, there was second phase precipitation. The results showed that T4 and T6 heat treatments can improve the fatigue behavior of the as-rolled ZK60 alloy. The T4-treated alloy achieved the highest fatigue strength, 112.4 MPa, being 17.9 MPa and 11.1 MPa greater than those of the as-rolled and T6-treated alloy, respectively. The average fatigue life of the heat-treated alloys is longer than that of the as-rolled alloy for a given stress amplitude. For the distribution of fatigue life, a fatigue life gap spanned from106 to 107 can be observed in the as-rolled and T4-treated alloy.Such a gap is absent after the alloy received artificial aging. And the formation mechanism is also analyzed in this paper. |
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