| Aluminum matrix composites have broad application prospects in aviation,aerospace, weapons, cars, electronics and other fields, owing to its high specificstrength, specific stiffness, wear resistance, fatigue, low thermal expansioncoefficient and excellent high temperature mechanical properties. In general,according to sources of reinforcing phase, the method of preparation process ofaluminum matrix composites can be divided into two categories: ex-situ method andin-situ method. Compared with the ex-situ method, the reinforcing phasessynthesized by in-situ method have many advantages，such as a smaller size, a goodcombination with matrix alloy interface, the interface is clean and non-polluting,high wear resistance, high-temperature creep resistance and other advantages. Thisallows in-situ synthesized technology has broad prospects for further development.In this paper, Al-TiO2-H3BO3-Na3AlF6is selected as the reaction system. Byanalyzing the reaction thermodynamics and kinetics, TiB2/7075aluminum matrixcomposite is successfully prepared by direct reaction synthesis. Meanwhile, theprocess of preparation of composite (reaction temperature, stirring speed, the massratio of cryolite to TiO2), heat treatment (homogenization annealing, solutiontreatment, aging treatment) are also studied to obtain the optimal parameters ofpreparation and heat treatment process. Finally, the tensile behavior and abrasivewear behavior of composites are tested, and both of wear and tensile fracturemorphology are also analyzed. The results of the whole experiment are concluded asfollows:(1) In situ TiB2/7075composite are successfully prepared by direct reactionsynthesis. The optimum process parameters are as follows: reaction temperatureis950℃~1000℃, stirring speed is45020rad/min, the mass ratio of cryolite toTiO2is1:1. Through XRD analysis, the TiB2exists in microstructure ofcomposites, showing a morphology of polygonal or block. There is also a small strip, rod-like Al3Ti. The mean size of reinforcing phases is about2~4μm, andreinforcing phases are equally distributed.(2) Comparison of the different state of materials,it is found that compared with7075alloy as-cast, the Brinell hardness of composite as-cast improves by25.5%,and the composite after hot extrusion increases by6.19%compared withcomposite as-cast.(3) After TiB2/7075composite has been heat treated, the optimum heat treatmentprocess of composite are found: homogenization annealing is455°C/24h,solution treatment is465°C/1.5h and aging treatment should be130°C/21h.Meanwhile, when study on the change of Brinell hardness of both7075alloy andcomposite followed by the different aging time under the same aging temperature,it is found that the aging peak of TiB2/7075composite appears earlier than that of7075alloy by3h.(4) About tensile properties, the tensile strength of the composite extruded barreaches633MPa, which increases by about7.3%compared to the7075matrixalloy. The tensile strength of the composite after aging treatment up to665MPa,and increases by12.7%and5%respectively compared with the7075matrixalloy and composites. The SEM analysis shows that the7075alloy is givenpriority to with ductile fracture，composite materials and ageing state ofcomposite materialsˊplastic have declined.(5) Both of7075alloy and TiB2/7075composite in as-cast and aging states are usedto be samples in abrasive wear experiment. The final results show that the wearresistance of the composite is much higher than7075alloy, and nearly the samewith that of the7075alloy after aging treatment; the wear resistance of thecomposite after aging treatment are highest among the four samples. Wearmorphology analysis shows that compared with7075matrix alloy and compositeas-cast, the composite after aging treatment presents more thinner and finer wearplough grooves, there is little wear dust on the surface, which shows the wearresistance of the composite after aging treatment is best。... |
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