Effect Of Cu Addition On The Microstructure And Mechanical Properties Of AZ80 Magnesium Alloy

As one of the lightest metal materials,magnesium alloy has become an important metal engineering material because of its high specific strength,specific stiffness and many other advantages.However,compared with Al alloys,the widespread use of Mg alloys has been limited by some shortcomings in its properties.Therefore,the research on high performance Mg alloys has attracted more and more attention of scholars.In this paper,for the sake of improving the mechanical properties of AZ80 alloy and expand its application range,Effects of Cu content on the microstructures and mechanical properties of AZ80 alloy were systematically investigated by means of optical microscopy?OM?,scanning electron microscopy?SEM?equipped with energy dispersive spectronmetry?EDS?,X-ray diffraction?XRD?and tensile properties testing.This study mainly focus on the microstructure and room temperature mechanical properties of AZ80-xCu?x=0,0.3,0.5,1.0,2.0?alloys.The microstructures of as-cast,homogenized and extruded AZ80-xCu?x=0,0.3,0.5,1,2?alloys were investigated.The results show that the main phases of as-cast AZ80-xCu alloys are matrix phase a-Mg phase,divorced eutectic Mg₁₇Al₁₂ phase and MgAlCu phase.The divorced eutectic and precipitated phases distributed basically dissolve into the matrix,but they are not completely dissolved in AZ80-2Cu alloy and MgAlCu phase still exists after homogenization treatment.After extrusion deformation,the grain size of the alloy is obviously refined,dynamic recrystallization occurs and there is a distinct extrusion streamline along the extrusion direction.Proper addition of Cu can effectively improve the microstructure uniformity and refine the grain size of AZ80 alloy.The amount of MgAlCu phase in as-cast and extruded AZ80-xCu alloys increases with the increase of Cu content.The size of MgAlCu phase in as-extruded AZ80-xCu alloys also increases with the increase of Cu content,and the phase aggregates at grain boundaries.The microstructure and mechanical properties of AZ80-xCu alloys were studied.The results showed that continuous Mg₁₇Al₁₂ phase?CP phase?and discontinuous Mg₁₇Al₁₂ phase?DP phase?were formed after T5 heat treatment.The Mg₁₇Al₁₂ phases were basically dissolved in the metal matrix,and the MgAlCu phase was partly dissolved after solute treatment at 420°C/2 h and 440°C/2 h.The two temperatures had little effect on the solubility of MgAlCu.Finally,420°C/2 h was chosen as the solid solution process parameters.After T6 heat treatment,CP phase and DP phase were precipitated in alloys.Proper addition of Cu can promote the precipitation of Mg₁₇Al₁₂continuous phase and increase the density of CP phase.Among the five alloys,the T6aged AZ80-0.3Cu alloy has the smallest size and the highest density of CP phase.The MgAlCu phase in the alloys will inhibit the formation of DP phase.With the increase of Cu content,the number of MgAlCu phase in as-aged alloys increases,while the DP phase region decreases and the CP phase increases.In addition,the peak hardness of as-aged AZ80 alloy can be increased by adding appropriate amount of Cu.The room temperature mechanical properties of as-extruded and as-aged AZ80-xCu alloys were tested.The results show that the optimized aging treatment for AZ80 alloy is T5 treatment.The mechanical properties of AZ80-xCu alloys after T5 and T6 aging treatment are significantly higher than that of extruded alloys,and the elongation decreases.For extruded and T5 aged alloys,the mechanical properties can not be effectively improved by adding Cu,even the strength can be reduced by excessive addition.The mechanical properties of AZ80 alloy can be significantly improved by Cu after T6 treatment.By comparing the mechanical properties of the alloy,the T6 aged AZ80-0.3Cu alloy has the best comprehensive mechanical properties:the yield strength,tensile strength and elongation are 308.9 MPa,455.9 MPa and 7.3%respectively.Did the aging treatment at different temperature of as-soluted AZ80-0.3Cu alloy.The results show that different aging temperature has different influence in the microstructure and mechanical properties of AZ80-0.3Cu alloy.After aging treatment,the shorter time required for the alloy to reach the peak hardness with the increase of aging temperature.There are three morphological regions of Mg₁₇Al₁₂ discontinuous precipitates in T6 aged AZ80-0.3Cu alloy:?1?the area of granular phase.?2?the area of granular phase and fine lamellar phase.?3?the area of granular phase and coarsened lamellar phase.The DP phase in three regions can be observed at 150°C,175°C and200°C.However,when the temperature is 200°C,the area?3?is more than others,because the DP phase in the alloy coarsen with the increase of temperature.The CP phase is also affected by aging temperature.The amount and size increases with the increase of temperature.After aging treatment,the yield strength and tensile strength of AZ80-0.3Cu alloy are obviously improved,and the elongation decreases.From the results of tensile properties test,the optimized aging treatment for AZ80-0.3Cu alloy is175°C/40 h.