Study On Partial Melting And Thixoforming Properties Of SiCp Particle Reinforced AZ91 Magnesium Alloy

Magnesium alloys have a wide range of applications in aerospace,automotive,biomedical and other fields because of their high specific strength,low density,good recovery and abundant resources.Due to the poor high-temperature mechanical properties of conventional magnesium alloys,their applications in structural parts operating in high temperature environments have been inhibited.The high-temperature mechanical properties of magnesium alloys are effectively improved by the addition of SiCp reinforcement phases.Although the high-temperature mechanical properties of Mg alloys are effectively improved by SiCp reinforcement phases,the plasticity of SiCp-reinforced Mg-matrix composites is not sufficiently high for conventional forging methods.Due to the adjustable viscosity and excellent fluidity of semisolid materials,semisolid forming technology(SSF)is considered to be one of the most ideal methods for realizing the near net shape manufacturing of Mg-SiCp composite products.Since 2000 s,thixoforming of SiCp-reinforced non-ferrous matrix composites have studied by researchers.Their studies mainly focused on the partial melting behaviors and formabilities of Al/SiCp-reinforced Al matrix(Al/SiCp)composites.However,there are limited investigations focusing on the relationship between morphology and thixo-formability of semisolid Mg/SiCp composite.In addition,the remelting temperature and isothermal holding time are related to the microstructure and thixoforming properties of Mg/SiCp composites.The impact is still not noticeable.Therefore,the semi-solid thixoforming process is used to realize the partial melting and semi-solid thixoforming experiments of AZ91/SiCp composite by using a multistage hot compression test machine in this paper.By changing the experimental parameters such as forming temperature and isothermal holding time and forming parts under different experimental parameters were obtained.Cylindrical specimens were cut from the extruded AZ91/SiCp composite and by wire cutting.After using these specimens as research objects,the microstructural evolution of AZ91/SiCp composite during reheating,isothermal holding,and thixo-compression was revealed.The effects of experimental parameters on thixo-formability and micro-segregation of AZ91/SiCp composite were also investigated and discussed by means of phase analysis,microscopic morphology observation.The influence of silicon carbide particles on the partial melting and thixoforming properties of induced alloy AZ91 was also discussed.Through the research in this paper,the following conclusions are obtained:(1)AZ91-SiCp composite with uniform spherical semi-solid structure can be obtained when the reheating temperature is 550 ° C and the isothermal holding time is20 s.In the reheating and isothermal holding process,the dissolution of the eutectic compounds,the partial melting of the ?-Mg matrix,the polymerization of the solid particles and the coarsening of liquid phase all took place in the ECAE-processed AZ91 and the extruded AZ91-SiCp composite.At lower temperatures,the sliding of the ?-Mg solid particles along the liquid film and its plastic deformation dominated in the semi-solid compression process,resulting in a high flow stress value.At higher temperatures,the outflow of the liquid phase and the rotation of the spherical ?-Mg solid particles produced lower values of flow stress.(2)The Polymerization of ?-Mg grains and the coarsening of the liquid phase occurred in a further isothermal holding process at a semi-solid temperature.Different volume fractions of ?-Mg,SiCp and liquid phase exhibited different forming behaviors.When the isothermal holding time extended from 20 seconds to 100 seconds,a lower flow stress was obtained.Longer isothermal holding times not only resulted in lower flow stress values,but also changed the tendency of the flow stress-true strain curve in the compression test conducted at 550 °C.(3)SiCp made the AZ91 semi-solid slurry own higher flow stress and lower liquid fraction.When the isothermal holding time was shorter,no segregation of SiCp was observed in the AZ91-SiCp composite after the compression test because SiCp did not flow outward together with the liquid phase during the thixoforming process.More and wider liquid outflow channels were formed in the semi-solid slurry that was isothermally holded for a longer period of time at higher reheating temperatures.During the compression process,SiCp flowed outward through these channels together with the liquid phase and caused the segregation of the liquid phase and SiCp in the compressed sample.