Research On Microstructure And Mechanical Property Of Co-extruded Mg MB26/Al 7075 Composite Rod

Magnesium and aluminum are both light alloy materials,and the application of these two metals has been accelerated under the demand of structural lightweight technology and environmental protection problems.Magnesium alloy is the lightest metal structure material at present,which has many advantages such as low density,high specific strength、high specific stiffness and so on.While Aluminum and its alloys with their characteristics of low density,high strength and good corrosion resistance are the most widely used metals in industrial production,especially in automobile industry.Therefore,the weight of the product can be greatly reduced under the premise of ensuring the performance of the product,which not only greatly reduces the energy consumption,but also saves the resources if magnesium alloy can be used to replace some aluminum alloy parts.It can also reduce emissions and environmental pollution.So we came up with the idea that making Mg/Al bimetallic composites.In order to obtain bimetallic composites with high strength.High-strength aluminum alloy 7075 had been used to intensify magnesium alloy MB26.Firstly,high strength Mg MB26/Al 7075 bimetallic composite rod was successfully fabricated by co-extrusion.Then various characterization and analysis techniques such as optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD)and mechanical property test were jointly employed to investigate the microstructure,texture and mechanical properties of Mg MB26/Al 7075 bimetallic composite rod,which was prepared under different extrusion processes parameters such as extrusion speed,extrusion ratio and the number of core fiber.On the basis of the above,the effect of annealing temperature on the microstructure and properties of bimetallic composite bar was studied at last.The results in this thesis show as follows:(1)The microstructure and mechanical properties of Mg MB26/Al 7075 composite rod are significantly affected by extrusion speed.The surface defects of the prepared Mg/Al composite rod increase with the increase of the extrusion speed.The microstructure of magnesium shows a "mixed crystal" state.And the average grain size increases with the increase of the extrusion speed.Magnesium alloy would present typical extruded fiber texture after extrusion,that is,the C axis of most grains is perpendicular to the extrusion direction.With the increase of extrusion speed,the thickness of interface layer increases.However,the strength of Mg MB26/Al 7075 composite rod decreased with the increase of extrusion speed.Compared with single magnesium alloy,the co-extrusion process has a certain strengthening effect on MB26 magnesium alloy,and the strengthening effect increases with the increase of extrusion speed.(2)Extrusion ratio has obvious effect on the microstructure of Mg sleeve.It is obviously the average grain size will decrease while extrusion ratio is increasing.The Mg alloy shows typical fiber texture and does not change with the extrusion ratio.At the same time,the strength of Mg/Al composite rod increases obviously with the increase of extrusion ratio.(3)The increase of core fiber cannot obviously change the average grain size of Mg alloy,but it can obviously improve the homogeneity of the microstructure of magnesium alloy.The change of the fiber number does not change the typical extrusion texture of Mg sleeve.It has a slight increase in yield strength while the core fiber increasing.(4)After annealing at 200℃,the grain size of Mg alloy is refined and the microstructure becomes well-distributed.With the increase of annealing temperature,the crystalline grain coarsened and the homogeneity of microstructure decreased.Meanwhile,the thickness of interfacial layer increased significantly,and a new transition layer was formed between the near Mg alloy layer and Al alloy layer.Some micro-pores were formed near the Al alloy.The change trend of hardness value is not consistent as the different strengthening mechanism of the materials.The plasticity of Mg/Al composite rod firstly increases with the increase of annealing temperature,but decreases after annealing at 400℃.The strength of the composite rod decreases with the increase of annealing temperature.At the same time,the fracture mechanism of the two materials after annealing is ductile fracture.