Microstructure Characteristics Of Thin-walled Tubes Of AZ80+0.4%Ce Magnesium Alloy Prepared By Extrusion-Stretching

Magnesium alloys are the lightest structural materials in the metal field and have many advantages.Because of this,magnesium alloys have good prospects in aerospace,weapons,medical and chemical industries.Magnesium alloys have good biocompatibility.There are many reports about the research and application of vascular stent processing methods at home and abroad.However,there is a big gap between the size and corrosion resistance of magnesium alloys and the clinical application of cardiovascular stents.Therefore,further research is necessary.In this paper,the extruded AZ80+0.4%Ce magnesium alloy was used as the experimental material,and thin-walled tubes with different thicknesses were fabricated by back extrusionmulti-drawing process based on different deformation temperatures(240°C,270°C,310°C,350°C and 390°C).By means of optical microscopy(OM),X-ray diffraction(XRD),microhardness testing,scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD),the effects of different temperatures and deformation passes on the load,microstructure,grain orientation,texture evolution and microhardness of the tube before and after deformation were observed and analyzed.The effects of deformation mechanism,grain refinement,second phase transformation and dynamic recrystallization on the microhardness of magnesium alloy thin-walled tubes during extrusion-tension deformation were investigated.The results show that:(1)When the thin-walled tube is formed by the extrusion-stretching method in a wide temperature range of 240 °C to 390 °C,the tube having an inner diameter of 6 mm and a thinnest wall thickness of 0.6 mm can be successfully produced.With the deformation pass increase,the wall thickness of the pipe becomes more and more uniform,and the maximum wall thickness difference is reduced from 0.4 mm to less than 0.1 mm.The load required for forming is continuously reduced as the temperature increases and the number of deformation passes increases.The maximum load required for the reverse extrusion molding is reduced from 15 kN at 240 °C to 5.5 kN at 390 °C.(2)Due to the effect of dynamic recrystallization,the grain size is refined compared to the average grain size of the original extruded state of 47 ?m,and the average grain size after only four times of stretching at 350°C is only about 8 ?m.With the increase of deformation temperature,the dominant deformation mechanism changed from twinning below 270°C to sliding above 310°C.The mechanism of recrystallization behavior also changes from twin dynamic recrystallization and rotational dynamic recrystallization below 270°C to continuous dynamic recrystallization at 310-350°C,and finally to discontinuous dynamic recrystallization above 390°C.(3)Below 310 °C,with the increase of deformation,the morphology of the second phase in the original grain changes from lamellar to thin strip and granular,and the second phase of the recrystallized region remains basically granular.The morphology of the second phase at 350°C is mainly granular,and the second phase of the granular phase at 390 °C dissolves with deformation and eventually dissolves completely into the matrix.Both the rare earth phase and the aluminum-manganese phase are broken and thinned and evenly distributed in the structure.Affected by work hardening,recrystallization softening and phase distribution,the microhardness decreases with the increase of deformation temperature.After four times of tensile deformation,the hardness of the pipe at 240 °C reaches 100 HV,and only about 70 HV at 390 °C.(4)The original extruded billet has a strong base texture and its polar density reaches 28.9.As the deformation temperature decreases,the texture is weakened more significantly,and the anisotropy of the material is improved.The polar density of the base surface texture after backward extrusion at 390 °C is 24.7,and the density of the texture after backward extrusion at 240 °C is only 10.Except for 270 °C,the density of the polar decreased first and then increased with the deformation,and the density of the base texture at 270 °C increased first.The grain rotation caused by local distortion and twinning and activation of more slip systems caused by elevated temperature and the random orientation of dynamic recrystallized new grains all weaken the texture of the base surface,while the large increase of texture at the later stage of deformation at 390 °C is related to the selective growth of grains.