| In situ particulate reinforced aluminum matrix composites is regarded as one kind of high performance engineering material and has broad application prospects in highly technical area such as aerospace, national defense and advanced manufacturing industry because it has many advantages including low density, high specific strength, high specific modulus, being suitable to mass production. However, the large numbers of reinforced particulates exist in this composite material, so that its forming performance is not ideal. So it is an important and meaningful study to further improve mechanical properties of in situ particulate reinforced aluminum matrix composites, and achieve high strain rate superplasticity forming, and which will be a useful ways to improve distribution uniformity and dimensional stability of reinforced particulate and to refine matrix crystal. The article has studied the influence of different plastic deformation (forging, extrusion, rolling and FSP) on microstructure and mechanical properties of in situ particulate reinforced aluminum matrix composites, and analyzed deeply the influence of superplastic deformation behavior of composites with different deformation way and microstructure. Through the research the method and optimum superplastic deformation parameter to achieve high rate superplastic are established, and the superplastic deformation mechanism model is made. At last the friction and wear properties of the composites are characterized and the wear mechanism of different state composites is analyzed. The study of this paper provides theoretical and practical foundation for efficient forming, popularization and application of in situ particulate reinforced aluminum matrix composites.In the article it adopts Al-K2ZrF6-KBF4, Al-K2ZrF6, Al-K2TiF6-KBF4, Al-K2TiF6reaction systems, by direct melt reaction method Al-Zr-B system composite:ZrB2/6063Al, Al3Zr/6063Al, ZrB2/2024Al and Al-Ti-B system composite matiarial:TiB2/6063Al, Al3Ti/2024Al composites are synthesized under ultrasonic and magnetic coupled fields. After the calculation, the atomic lattice misfit is less than9%between particulate reinforced in situ synthesis and a-Al, and they have coherent lattice crystal face. So the particulate reinforced can be as heterogeneous nucleation core, and can effectively inhibit the matrix growing up. When reinforced particulate content is higher, the matrix grain refinement effect is more obvious.Comparable research of composites microstructure before and after plastic deformation shows that different plastic deformation will lead different microstructure change behavior of in situ particulate reinforced aluminum matrix composites:The forged matrix grains grow perpendicular to forge direction and transform into streamline fibrous tissue. And particulates migrate and rotate a little. As increasing deformation, big and long strip-like Al3Zr and Al3Ti particulates break and present rod-like or small polygonal bulk diffusing in matrix. Thus forging can improve uniformity of reinforced particulates. After rolling treatment, the matrix issues present fiber belt form with the wideness of3～5μm. And the grain orientation is preferred:consistent on the whole from chaos to the direction benefit to rolling. The agglomeration of reinforced particulates disappeared. And after extrusion treatment, matrix crystals present regular fibrous grain with the size of5～15μm. The extruded particulates are fined and blunted obviously without agglomeration. And the particulates after FSP are extremely fine less10μm and some become super fine. The casting agglomerated particulates are broken and diffused uniformly with size of0.1～5μm. The TEM analysis shows that plastic deformations don’t impair the boundary advantage of in situ composites:the boundaries are still pure and have no brittle phase. And plastic deformation also can improve even eliminate the defects in the cast issues such as holes and porosities.The XRD result shows that different degree of internal stress are aggregated in matrix after plastic processes, which resulting in the diffraction peak of (111),(220) and other direction of α-Al phase crystal face migrate toward right and widen the corresponding diffraction peak width at half height. Plastic deformations twist and deform the matrix crystal with preferred orientation and increase the dislocation density. The results of TEM demonstrate that the dislocation distribution is different generated by different plastic deformations; the dislocation in forged matrix is more than casting state and presents regular and single direction. The dislocation after FSP intertwines leading to blur and bend the crystal boundaries. After rolling, the dislocation line is neat and presents obvious slip band and the crystal boundaries is straight. The result of W-A" method shows the sort of dislocation density:FSP>rolling state>forged state>extrusion state>casting state.The results of mechanical property and mechanism research demonstrate that the influences of different plastic deformations are different on properties of composites. Conventional plastic deformations such as forge, extrusion and rolling can improve hardness, tensile strength and elastic modulus in a large extent but reduce the room temperature plastic obviously. Compared with them, FSP can improve comprehensive properties such as tensile strength, elasticity modulus and so on, especially room temperature plastic by about87%. The result also shows that only the FSP state achieves superplastic at the temperature of480℃and initial tensile rate of0.15s-1. And rolling, as-cast or extruded state don’t present superplastic under this circumstance. The improvement of properties can be explained by dislocation strengthening and fine grain strengthening assisted by Orowan strengthening mechanism.Superplastic tensile and deformation mechanism research show that superplastic of the in situ composites increases and then decreases as both tension temperature and initial strain rate increasing. And there are optimum parameters to obtain the best superplastic. The superplastic of5wt.%Al3Ti/2024Al composites processed by FSP obtain max superplastic (elongation of475.53%) at the initial strain rate of0.15s-1and tension temperature of480℃. And when the initial strain rate keeps certain, the best superplastic (640.34%) is acquired at the temperature of510℃. And the superplastic is impaired mildly with more and more reinforced particulates. Superplastic average activation energy of ZrB2/6063Al after FSP at450℃and500℃is calculated and the value is153.37KJ/mol, which is much higher than Al’s crystal boundary diffusion activation energy. This demonstrates that crystal boundary diffusion dominates. The DSC dates show that there is a little of liquid phase in ZrB2/6063Al during superplastic deformation. And the appropriate liquid phase can relax stress concentration and fill the holes in time produced by crystal boundary slide, which is benefit to improve superplastic. However when the temperature is excessive high, there are excessive liquid phase leading to decrease interfacial bonding strength extremely. As a consequence grain boundary has no ability to transmit deformation and load. Therefore the superplastic of composites will decrease obviously. Based on these dates and explanations, the superplastic mechanism of the ZrB2/6063Al in situ composites by FSP is crystal boundary sliding coordinating with mild liquid phase and dynamic recrystallization. And the superplastic fraction mechanism is that micro pores nucleate, grow then link and rupture at last.The friction and wear performance and mechanism research show that friction and wear performance of composites is overall improved by conventional plastic deformations such as forging, extrusion and rolling. After plastic deformation, the hardness of matrix is improved, reinforced particulates are blunt and diffused uniformly. The wear scar surface is more flat after friction, the concave-convex difference and the wear scar roughness decrease, which indicates that the friction property is improved obviously. And the friction coefficient and wear surface roughness increase and then decrease, the wear scar width increases and the wear scar depth decrease as deformation increasing, which shows that the composites process good wear resistance. The wear mechanisms are adhesive wear, abrasive wear and oxidation wear. After FSP, the friction and wear property decreases slightly. The friction coefficient increases on the whole and the friction surface roughness increases obviously and the increase scope rise as load increasing. The friction and wear property decrease mechanism is that matrix become soft, the hardness decreases and the reinforced particulates is extremely fine and easy to detach after FSP, which lead to change the wear mechanism from adhered wear to abrasive wear and slight oxidation wear. |
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