In-situ Fabrication Of Spinel Whisker Reinforced Al-Mg Alloy Based Composite And Its Aging Characteristics

Aluminum matrix composite （AMCs）, with a low density, high specific strength,good wear and corrosion resistance, excellent size stability etc., is becoming one ofthe main researching directions of high performance structural materials and hasbroad application prospects. MgAl₂O₄spinel whisker, as a new kind of hard ceramicreinforcement, exhibits excellent mechanical properties and thermal stability such ashigh melting point, low defect density and high strength etc, becoming an idealpractical reinforcement for AMCs. The key to improving the properties of thecomposites lies in the fabrication of MgAl₂O₄whiskers with good size stability anddispersion in the matrix. In this work, pure Al and Al alloy were used as matrix inorder to explore fabrication techniques of in-situ generation of MgAl₂O₄spinelwhisker in the matrix and investigate microstructure and mechanical properties of thecomposite. Moreover, by studying the aging process, the optimal technologicalparameters were obtained and the effects of in-situ generated whiskers on agingkinetics and strengthening mechanism of the composites were also discussed.Al and Mg powder was used as raw material; the fabrication process wasoptimized through adjusting ball milling pretreatment and hot extrusion parameterswith boric oxide addition. The effects of different boric oxide content on thegeneration of whiskers and mechanical properties of composites during sinteringprocess were also investigated. The results show that the mechanical properties of thecomposites could be improved with an optimized ball milling and hot extrusionparameter and a proper content of boric oxide addition could enhance both thehardness and compressive strength of the composites, while the tensile strength andthermal stability were decreased on the contrary.With in-situ processing techniques, the MgAl₂O₄spinel whisker was synthesizedin6061Al alloy matrix by cold pressure sintering-hot extrusion and hot pressmethods. Through the characterization of microstructure and mechanical propertytests, the influences of sintering conditions on the generation of whiskers werestudied and the relationship between whisker content and mechanical propertieswere also explored. The results indicate that during cold-pressure sintering process,the hardness and tensile strength of the composites increase with an increasingcontent of whiskers but the ductility decreases at the same time; and during hotpressing process, it was proved that sintering pressure is one of the necessaryconditions of the generation of whiskers. The solution-artificial aging treatment was performed on MgAl₂O₄/6061Alcomposites under different temperatures and time. The changing of mechanicalproperties before and after aging was investigated and an optimized aging parameterwas determined. The effects of in-situ generated whiskers on the composites’ agingkinetics were discussed. It is concluded that after aging treatment, the composites’hardness can reach HV195and tensile strength,400MPa. Compared with6061Alalloy, the time to reach peak aging value in the composites was reduced evidently,which is mainly due to the more favorable places for second phase’ s nucleation andgrowth provided by the large amount of dislocations generated from the growth ofwhiskers during the molding and aging process of the composites, thus the agingkinetics of the composites was accelerated.