Effect Of Heat Treatment On Microstructure And Mechanical Properties Of AZ80Mg Alloy

AZ80Mg alloy are likely to be widely used magnesium alloy, which for its excellent mechanical properties and it without expensive Zr and rare earth elements. In this paper, through metallographic microscope(OM)、scanning electron microscope(SEM)、EDS、 transmission electron microscopy (TEM)、XRD、energy spectrum analysis (DSC)、extension test、hardness test and other experimental methods, effect of heat treatment on microstructure and mechanical properties of AZ80Mg alloy were investigated, the spreading activation energy and the activation energy were investigated. The main conclusions are presented as follows:(1) Severe dendritic segregation exists in AZ80Mg alloy ingot. There are many massive secondary phases in grain boundary and Al elements are unevenly distributed. The main secondary phase are a-Mg and β-Mg17Al12. The over-burnt temperature of AZ80Mg alloy is430℃. And the suitable homogenization treatment is410℃×25h, which is consistent with the results of homogenizing kinetic analysis.(2) During the homogenizing treatment the spreading activation energy is121.7KJ/mol. The AZ80Mg alloy comprehensive mechanical properties improved greatly, the mechanical properties of as-cast AZ80UTS is154MPa and EL is3.5%, after homogenizing treatment UTS become198MPa and EL become6%.(3) The peak hardness of T5and T6are27h and24h respectively. The peak hardness, room temperature mechanical properties and high temperature mechanical properties of T5are higher than T6. T5treatment is superior to T6treatment for the forging AZ80Mg alloy.(4) High temperature tensile test and fracture analysis shows that high temperature fracture is ductile fracture. There are a great deal of Mg17Al12phase after aging treatment, it precipitated mainly for rod-shaped, granular and edge shape in T5processed; it precipitated mainly for edge shape and rod in T6processed.(5) The number of twins and dislocation increases with the increasing pre-deformation strain. As pre-deformation strain increases from0to4%, a few grains showed the twins; when cold pre-deformation strain rises to8%, most of the larger grains contain twins. The tensile strength increases with the level of pre-deformation, on the contrary, the elongation decreases with the level of pre-deformation. The8%pre-deformation specimens had the maximum increase after aged, tensile strength increased22MPa, yield strength increased58MPa and elongation increased35%.(6) As pre-deformation level increases, time needed to reach peak hardness can be evidently shortened. Only19h is needed for8%and12%strained sample to reach peak hardness compared to27h for unstrained one and23h for4%strained one. As pre-deformation strain rises from0to4%, the peak hardness increases from88.7HB to95.9HB; however, the peak hardness value of8%and12%strained are only94.3HB and93.6HB which are less than the sample4%strained.(7) The tensile strength increases with the level of pre-deformation, on the contrary, the elongation decreases with the level of pre-deformation. Thus, there is a significant increase in strength with increasing level of pre-deformation. The suitable thermo-mechanical treatment is8%pre-deformation for19h in170℃, the tensile strength is352MPa, yield strength is281MPa and elongation decreased to5.5%. Compared with the unstrained alloy, the strained alloy present more precipitations and the size of precipitations is smaller.(8) Activation energy required for Mg17A112phase formation decreased from101.98to93.66kJ/mol in the AZ80alloy with4%pre-deformation, further, it decreased from101.98to88.27kJ/mol with8%pre-deformation in the AZ80alloy. The decreasing trend is indicative of the fact that the activation energy decreases with increasing level of pre-deformation, which is in line with the results obtained in the ageing behavior of pre-deformation alloys.