In-situ Synthesis Of MgAl₂O₄w/6061Al Composites By Reaction Hot Pressing And Its Strengthening Mechanism

Aluminum matrix composites(AMCs) have attracted intensive interests and have been successfully used in aerospace, transportation and electronic industry, owing to their light-weight, high strength, wear and corrosion resistance, and good size stability. It is important to choose excellent performance reinforcements and explore new technology to further develop the properties and realize the industrial application of AMCs. MgAl2O4 spinel whisker is regarded as a quite promising reinforcement for AMCs because of its low defect density, high strength and melting point. Recently, reaction hot pressing has attracted widespread attention, which can be applied for fabricating metal matrix composites with reinforcement uniformly distribution and good interface bonding. It is important to investigate the preparation of MgAl2O4 whisker reinforced 6061 Al composites(MgAl2O4w/6061Al) by reaction hot pressing method.In this work, 6061 Al powders, Mg powders and H3BO3 powders were used as starting materials, and then ball milling pretreated and hot-pressed method were performed to prepare in-situ MgAl2O4w/6061 Al composites. The effects of hot-pressing sintering parameters, T6 aging treatment and hot extrusion on the whiskers and composites were systematically studied. Moreover, the effects of in-situ generated whiskers on aging kinetics and strengthening mechanism of the composites were also investigated.The results indicate that the MgAl2O4 nanowhiskers, with diameters ranging from 30 nm to 200 nm, were successfully in-situ synthesized under a relatively low temperature(600 °C) during reaction hot pressing process. The whiskers, possessing aspect ratio 10~50, are uniformly distributed and firmly bonded with the matrix, and penetrate into adjacent 6061 Al particles to form a framework like bridge bearing and transferring extrinsic loads. In addition, the MgAl2O4w/6061 Al, prepared by sintering at 600 °C under the pressure of 50 MPa, exhibits the optimal performance.The 6061 Al alloy and composites have obvious aging strengthening after T6 treatment. Compared with 6061 Al alloy, the presence of MgAl2O4 whiskers accelerates the aging kinetics due to a high density of dislocations resulting from the difference in thermal contraction between whiskers and matrix, which provide the inhomogeneous sites for precipitation nucleation.Hot-extrusion plays a positive effect on the properties of composites. The brittle fracture characteristic transfers to the coexistence of both ductile and brittle fracture characteristics. During extrusion, lots of dislocations generating near the interface, and the composites experience dislocation strengthening. Additionally, the whiskers are alignment distribution and interfacial bonding with matrix is enhancing, which are beneficial to transferring and distributing extrinsic load.The tensile properties of the composites are significantly improved after extrusion and aging treatment, because of the matrix strain hardening and orientation distribution of the reinforcements. The maximum ultimate and yield tensile stress are achieved at 508 MPa and 436 MPa, which are enhanced over 2.8 and 1.1 times to the unreinforced counterpart, respectively.