Preparation And Properties Of AZ91D Mg Alloy-Cenosphere Composites

Magnesium matrix composites (MMCS) can improve the heat resistant and wearresistance properties of Mg alloy and are promising materials in the automotive andaerospace industries and electron products because of the combination properties ofhigh specific strength and specific stiffness. The cost of MMCs depends mainly onreinforcements and preparation process. It is still primary jobs at present to researchand develop MMCs with the low cost, high performance and easy to scale productionaccording to the requirement of the different working conditions. The researchindicated that the use of cheap reinforcement was the most effectual way to reduce thecost of MMCs. So, it was of important practical value to seek the lowest costreinforcements and prepare the particle reinforced MMCS in order to meet thedemand in different application conditions. A new type of AZ91D Mg alloy-flyashcenosphere (FAC/AZ91D) composites was developed by adding the flyashcenospheres (FAC) which were rich in natural resources and low cost, by means ofcompo-casting technology, and the room quasi-static compression, dynamiccompression and hot compression behaviour and friction and wear properties of thecomposites were studied. Optical microscopy, scanning electronic microscopy (SEM)and transmission electronic microscopy (TEM) were employed to study themicrostructure and interface structure of the as-cast composites; in the mean time,FAC were used as reactants, in situ Mg2Si/AZ91D composites were prepared bycontrolling the fabrication prarameters.In the course of the study, a low carbon steel mold with bottom pouring systemwas designed and used for pouring the molten Mg alloy, which saved efficiently theraw material of magnesium alloy and avoided the oxidation of the Mg alloy in thecourse of pouring and made the preparation of specimens simple and available.The main results were as follows:(1) The Gibbs free energy of the possible reactions was thermodynamicallycalculated between the various components in FAC/AZ91D Mg alloy systems. The kinetic model of the reaction of this system was established,and the sedimentationvelocity equation of the cenospheres was deduced. The FAC/AZ91D Mg alloycomposites were successfully prepared by controlling the process parameters, and theeffects of the related parameters on the reaction kinetics were obtained.(2) The effects of the stirring temperature, time and rate on the microstructures ofthe FAC/AZ91D Mg alloy composites were systematically studied in the semi-solidtemperature range, and the best techniques was obtained about this composite; thephysical properties (the density and thermal expansion coefficient) and themechanical properties (the hardness and tensile strength) were investigated. Theresults showed the densities of FAC/AZ91D Mg alloy composites increased andthermal expansion coefficient reduced with increasing cenospheres mass fraction; thelarger the size of cenospheres, the smaller the thermal expansion coefficient. TheBrinell hardness of the FAC/AZ91D Mg alloy composites increased with increasingthe mass fraction of the cenospheres, and the Brinell hardness began to reduce whenthe mass fraction of the cenospheres exceeds12wt.%. The ultimate tensile strength ofthe FAC/AZ91D Mg alloy composites reduced owing to adding the cenospheres, itsyield strength was enhanced with decreasing size of the cenospheres and increasingcenospheres mass fraction, however, its the elongation appeared the contrary trends toits yield strength. The elastic modulus increased due to the increase in the massfraction of the cenospheres.(3) The properties of quasi-static compression and dynasmic compression of theFAC/AZ91D Mg alloy composites were systematically studied by MTS810-22Mtesting machine and Split Hopkinson Press Bar (SHPB) system, the variation rules ofthe compressive properties under the conditions of the quasi-static compression anddynasmic compression were obtained. Under the conditions of this experiment, theroom quasi-static compression fracture strength of the FAC/AZ91D Mg alloycomposites was higher than that of the matrix Mg alloy and increased with increasingmass fraction of cenospheres or decreasing size of cenospheres or decreasing strainrate. The dynasmic compression test results showed that FAC/AZ91D Mg alloycomposites had obviously strain rate effect and small size effect, namely, thedynasmic compressive strength of the composites increased with increasing strain rateor decreasing size of cenospheres. The room quasi-static or dynasmic compressivefracture was brittle one.(4) The hot compression deformation behaviors of the FAC/AZ91D composites were studied by means of Instron-5500type thermal simulation machine. Theminimal value of the true activation energy of the hot compression deformation of thecomposites was199.33kJ/mol and the maximum value was292.45kJ/mol bycalculation according to the power law equation. The microstructures investigationshave demonstrated that the full dynamic recrystallization (DRX) was found in thiscomposite, and size of DRX increased with increasing strain rate at lower strain rateand high temperature. Lots of brand structures and twins were observed in matrix andaround cenospheres, and the brand structures consist of the lots of DRX grains. DRXand high dislocation density were comfirmed by TEM in hot compressive deformationcomposites.(5) The wear behavior of FAC/AZ91D composites was studied under theconditions of dry friction and wear. The effect of the load, wearing time and the massfraction of cenospheres on the wear behavior of FAC/AZ91D composites wereobtained. The results showed that there existed a large number of grooves on the wornsurface of the composites, which were of the furrow features. The wear mechanism ofthe composites was mainly abrasive wear under the low load, and the wearmechanism transformed into adhere abrasion and oxidation wear under high load.(6) The in situ Mg2Si/AZ91D composites were prepared by the means ofseparation pouring process after the AZ91D-cenosphers melt was isothermally keptfor a long time at high temperature. The effects of such as isothermal temperature,time and stirring time of the AZ91D-cenosphers melt and the solution treatment onthe mophorlogy of Mg2Si were investigated. The tensile strength of the compositeswas improved after solution treatment, and up to184MPa. The tensile srength of theextruded Mg2Si/AZ91D composites was302MPa,2.45times more than that of the ascast composites (123MPa). The as cast composites had better tensile strength at hightemperature, which was132MPa at200oC, and about increased24%than that thematrix. The tensile fracture of Mg2Si/AZ91D composites showed brittle one, and thatof the extruded composites was ductile fracture.