| Rheoforming technology and its applications are the focuses of many researches. But pioneering studies on the rheoforming of aluminum alloy aimed mainly at cast aluminum alloys, only a few studies focused on the wrought aluminum alloys.7075 wrought aluminum alloy has been widely used in aerospace and automotive applications due to its high mechanical properties. But 7075 wrought aluminum alloy is very difficult to use conventional rheoforming methods due to its coarsen dendrite grains. In this paper, TiB2/7075 aluminum matrix composites were synthesized by in-situ reaction. Then TiB2/7075 aluminum matrix composites were rheoformed by serpentine tube pouring technique, which was a simple and economical method. The influence mechanisms of TiB2 particles on semi-solid slurry microstructure and apparent viscosity were analyzed. The numerical simulation of semi-solid slurry preparation and wheel casting process were done by ProCAST software. The main results are as follows:1. TiB2/7075 aluminum matrix composites were synthesized by in-situ reaction. The morphology of the TiB2 particles was rectangular. The large particles were segregated at the grain boundaries, and the size of them were 1～2μm. Most particles were submicron and distributed in the grain. The grains of TiB2/7075 composites were near-spherical after hold at 630℃ for lOmin, but that of 7075 aluminum alloy were rosette after hold at 630℃ for 30min. The influence mechanism of TiB2 particles on the microstructure of semi-solid slurry was analyzed. The results indicated that the TiB2 particles could become crystallization nuclei and restricted the growth of the primary α-A1 phase. Those were conducive to the transition from dendritic to near-spherical microstructure of 7075 aluminum alloy. Because the agglomeration of the TiB2 particles was increased with increasing TiB2 content, the grain size and growth rate of the 9.0%TiB2/7075 composites was higher than those of other TiB2 content composites.2. The influence mechanism of TiB2 particles on the apparent viscosity of 7075 aluminum alloy was investigated. The results indicated that the TiB2 particles were effective in optimization of microstructure and preventing contact of α-A1 grains. Apparent viscosities of TiB2/7075 aluminum matrix composites semi-solid slurry were significantly lower than that of the 7075 matrix alloy under the same conditions. The viscosity decreased rapidly at low shear rate after adding TiB2 particles. By the effect on microstructure, the apparent viscosity of 4.5%TiB2/7075 composite was the lowest. Based on the results, the viscosity models of semi-solid 7075 aluminum alloy and TiB2/7075 aluminum matrix composites were formulated.3. According to the numerical simulation results, the solid fraction of the melt after it flowed through serpentine tube was between 0.19～0.40 when pouring at 670℃ and the temperature of serpentine tubes was 400℃. A self-stirring can be produced when the melt flowed through the serpentine tube, which could be beneficial to get fine and spherical α-Al grains. The effect of self-stirring was increased with increasing curve number of serpentine tube. On the basis of numerical simulation, the semi-solid slurries were prepared successfully. The results of the rheo-squeeze casting experiments showed that the microstructure morphology of TiB2/7075 aluminum matrix composites was near-spherical. The average grain sizes of specimens were 23～32μm, and the shape factors were 0.85～0.96 after heat treatment. The grain size of 4.5%TiB2/7075 composite was the smallest and the shape factor of it was the highest.4. The influences of process parameters on the filling process, porosity and stress field of 7075 aluminum alloy and TiB2/7075 aluminum matrix composites wheels were studied by numerical simulation. For rheocasting, the casting temperature should be 625～635℃, the casting pressure should be 1.0MPa and the mold temperature should be 400 ℃, respectively. The effective stress of rheocasting wheel was lower than that of liquid casting. The thermal fluctuation of the mold was reduced during rheocasting process, which could effectively prolong the service life of the mold. The mold life was 12523 times when casting temperature was 680℃, and which was 14324 times when casting temperature was 630℃.5. The effects of TiB2 particles and forming method on the wear property were investigated. Comparing to the 7075 aluminum alloy, the grain size and hardness of 4.5%TiB2/7075 aluminum matrix composite were more refined and higher. The wear rate of 4.5%TiB2/7075 aluminum matrix composite was lower than that of 7075 aluminum alloy at 0.15m/s sliding speed and 30～120N applied loads for 30min. The wear resistance of rheoforming specimens was better than that of liquid casting specimens. |
PDF Full Text Request