Research On Mechanical Properties And Strengthening Mechanisms Of Thixoforged C_p/AZ91D Composites

In this paper,C_p(ultrafine graphite particles)/AZ91D magnesium matrix composites were prepared by thixoforging process.The relationship between"process-microstructure-properties"is deeply studied.The effect of reheating temperature on the microstructure of semi-solid C_p/AZ91D composites and the effect of different mould temperatures on the microstructure and tensile properties of the composites were studied.The friction and wear behavior of thixoforging composites at different mould temperatures were studied,and the optimal experimental parameters were determined.The friction and wear properties of thixoforging composites under different working conditions were studied through comparative tests.Finally,the strengthening mechanism of C_p on the mechanical properties and friction and wear properties of composites was discussed.(1)When the holding temperature was 600℃and the holding time was 60min,the ideal semi-solid structure of C_p/AZ91D composite could be obtained.The holding temperature affects the grain size,shape coefficient and liquid content.During the remelting process of C_p/AZ91D composite,it went through four stages:rapid coarsening,microstructure separation,spheroidization and final coarsening.When the temperature was low,the content of liquid phase was less,and the merger between grains was intensified,which was not conducive to obtaining fine spherical semi-solid structure.Excessive temperature would lead to excessive liquid phase and coarsening of primaryα-Mg particles in semi-solid system.(2)Mould temperature mainly affected the solidification rate of tissue,and then affected the liquid content in the tissue,the number ofβphase and secondary solidified tissue,and the distribution of C_p.With the increased of mould temperature,the solidification rate slowed down,and the filling and feeding ability improved.Under the solid solution strengthening,theβphase and secondary solidification structure decreased,and the microstructure compactness gradually increased.When the mould temperature is lowed,C_p was agglomerated at the grain boundary under the action of solid-liquid interface and van der Waals force.With the increased of mould temperature,the interface reaction increased,and C_p was more likely to break through the energy barrier.At the same time,the expansion speed of solid-liquid interface was accelerated,and C_p was more likely to be swallowed up.However,when the mold temperature was too high,the system energy was higher,and the C_p in the grain was more active,which leaded to agglomeration.(3)When the mould temperature was 300℃,the tensile properties of the optimal thixotropic C_p/AZ91D composites were obtained:the tensile strength and elongation were 304.1MPa and 13.9%respectively.At low mould temperature,more brittleβphase,secondary solidification structure of pores and C_p agglomerated at grain boundaries would lead to poor mechanical properties of materials,and intergranular fracture was the main fracture of materials.With the increased of mould temperature,theβphase and secondary solidification structure decreased,C_p shifted to the interior of grains,and the fracture mode changed to transgranular fracture.However,when the mould temperature was 300℃,C_p was agglomerated in the grain,which had a certain influence on the mechanical properties of the material.(4)Thixoforging C_p/AZ91D composites showed better wear resistance at 300℃mould temperature.With the increased of mould temperature,the hardness and wear resistance of composites were improved.Changing the mould temperature did not change the main wear mechanism,but it could change the proportion of adhesive wear mechanism in the whole wear.With the increased of load,the wear rate of composites increased linearly at different mould temperatures,and the average friction coefficient decreased due to thermal softening and the difference of wear surface hardness under the action of applied load.Under different sliding frequencies,the wear rate and friction coefficient decreased with the increase of temperature.With the increased of frequency,the wear surface accelerated thermal softening,and the groove of the wear surface decreased to be smooth,the friction force and the wear rate decreased.With the increased of time,the wear mechanism changed from oxidation wear and abrasive wear to adhesion wear and abrasive wear,and finally changed to adhesion and delamination wear.(5)Compared with thixotropic AZ91D alloy under the same parameters,the wear resistance of C_p/AZ91D composites was significantly improved,and the addition of C_pdelayed its transformation to serious wear mechanism.With the increased of applied load and sliding frequency,both C_p/AZ91D composites and AZ91D alloys underwent oxidation,abrasive wear,adhesion and delamination wear.Under different wear test conditions,the composites showed better wear resistance,especially under higher load and sliding time,the wear reduction effect of C_p on AZ91D alloy was more obvious.(6)The strengthening of mechanical properties of C_p/AZ91D composites by C_preinforcement was the cooperative effect of many mechanisms.Among them,the load transfer mechanism played a leading role,and the Orowan ring mechanism and dislocation strengthening mechanism should played a secondary role.There are two main aspects that C_p could improve the wear resistance of C_p/AZ91D composites:on the one hand,C_p could improve the interfacial bonding strength;On the other hand,the formation of graphite lubricating film.