| Aluminum matrix composites reinforced with SiC particles are characteristic of high specific strength, high specific toughness and excellent abradability, but they exhibit much lower elongations than the matrices and the non-uniform distribution of SiC particles in the matrix. Plastic processing can elevate the compactability of the composites and promote the uniform distribution of SiC particles, contributing to higher strength, better plasticity and toughness. There are many reports on the topic mentioned above, while there is almost no systematic report on the regularities of the motion and distribution of SiC particles under different stess and strain states.In this dissertation, 7090Al/SiC_p (the volume fraction of SiC_p (2) The distribution and the motion of SiC particles in the PM 7090Al/SiC is 15%) composite billets were prepared by multi-layer spray co-deposition technique and powder metallurgy respectively and were successively deformed by different plastic processings such as hot extrusion, equal channel angular pressing and wedge pressing repectively. The compaction behavior, the fragmentation, the motion and the re-distribution of SiC particles in the as-deposited composite were systematically studied. The conclusions are drawn as follows:(1) SiC particles adhered mainly to the surface of the alloy droplets during deposition, leading to more SiC particles at the surface of the droplet and less in the inner. Thus, the distribution of SiC particles in the composite was characteristic of the layered feature. This layered feature of SiC particles was not removed by the following hot extrusion. The SiC particles were distributed like the streamline in the longitudinal direction, while the ring-like feature of the SiC particles was observed in the transverse direction. A higher extrusion ratio resulted in an more uniform distribution of SiC particles.composite billet during hot extrusion were also investigated detailedly and compared with the as-deposited billet. The more uniform the distribution of SiC particles in the PM billet, the more uniform of SiC particles in the as-extruded rod. There was still an order and directional distribution of SiC particles in the longitudinal direction, but there was no ring-like feature of SiC particles in the transverse direction in the as-extruded PM billet. The former was related to the stress state associated with hot extrusion. In addition, a more uniform distribution of SiC particles was observed in the as-extruded PM billet and no SiC segregation streamline occurred. With a small extrusion ratio (11), many obvious interfaces between powder and powder remained existent since the deformation stress was not large enough to achieve the metallurgical bond. Metallurgical interfaces were achieved with the extrusion ratio of 17.(3) There were a large amount of short rod-like MgZn2 and spherical CuA12 particles in the matrix of the as-extruded 7090Al/SiC_p composite. The MgZn2 particles were mainly distributed within the grains with the length of about 200 nm, and the width of about 60 nm, while the CuA12 particles were mainly distributed at grain boundaries or the near areas with the diameter from 420 nm to 40-60 nm.(4) The solid solution and artificial aging processes of the as-extruded 7090Al/SiC_p aluminum alloy matrix composite were systematically investigated. The best solid solution process was solutionized at 475℃for 1h. The particles of MgZn2 and CuAl2 were dissolved after the solid solution treatment and the composite in the solid solution state was characteristic of the ultimate strength and elongation of 610 MPa and 2.0% respectively, while that in the solid solution plus aging state exhibited the ultimate strength and elongation of 765 MPa and 1.5%.(5) Temperature had an obvious effect on the plastic behavior of the spray-deposited composite during equal chnnel angular pressing. The hard SiC particles were broken by the shear stress and many cavities between the broken SiC fragments occurred with the the pressing temperature of 300℃, which could not be filled with the matrix. With the pressing temperature of 350℃and 400℃, the cavities derived from the SiC breakup were filled with the matrix and flew with the matrix in a certain range. The optimal pressing temperature was 400℃.(6) Equal channel angular pressing was performed on the spray-deposited 7090Al/SiC_p composite at 400℃with the routes of A, Bc and C and the channel angular of 90°. The 7090Al/SiC_p(7) On the basis of the conception that a large deformation could be obtained through the accumulation of many times of small deformation, the distribution of SiC particles and the deformation of the pores during the wedge pressing of the large dimension spray co-deposited 7090Al/SiC composite experienced by Bc had higher mechanical properties and equiaxed grains with the average grain size of 400 nm were obtained from the fourth pass. The tensile strength, the yield strength, the modulus and the elongation of the composite after peak ageing treatment were 769MPa, 574MPa, 106GPa and 7.4% respectively. p composite were investigated. Wedge pressing resulted in the local deformation, which led to the shear deformation and the closure of pores. Ultimately the pores were eliminated and the densification was realized. The SiC particles rotated under the pressing force and exhibited an order distribution with the long axis perpendicular to the pressing direction.
|
PDF Full Text Request