| Spray deposition(SD) technology exhibits many unique advantages in fabricating large-size high-performance materials. However, its direct applications are limited due to the small amount of residual porosity in the as-deposited preforms. So, proper densification processing should be developed. But it is very difficult to densify the large-size spray deposited preforms because of the limited tonnage of the press and high processing costs. It is of practical significance to develop a feasible densification method for the fabrication of large-scale rapid-solidification structural materials as well as the applications of SD. In the present dissertation, combined with a key project supported by the National Key Program of 10th Five-year Plan, large-sized spray deposited aluminum alloys and SiCp reinforced composites tubular preforms were prepared by spray deposition process. With the assistances of hot compression tests and finite element method (FEM), the effects of processing parameters on the microstructures, mechanical properties and the densification mechanism of the compacted samples were investigated. A novel densification process termed as tube wedge pressing(WP) was developed for the first time, and the deformation and densification behaviors of the large SD tube preforms during WP were studied systematically. The main researches and conclusions are as follows.(1) 8009Al, 8009Al/SiCp and A356 alloy tubular preforms were prepared by a kind of spray deposition processing termed as multi-layer spray deposition, the effects of the main processing parameters on the microstructures and mechanical properties of the deposited preforms were analyzed, and the optimized processing parameters include, atomization pressure 0.7~0.85MPa,spray height 200~240mm,transportation pressure of SiC powder ~0.5MPa, diameter of melt stream 3.4~3.8mm for 8009Al and 3.2~3.6mm for A356, super-heat of the melt 250~300℃, rotate velocity of substrate 20~60r·min-1, scanning cycle of atomizer 30~50s. 8009Al and 8009Al/SiCp tubular preforms in dimension ofΦouter350mm×Φinner280mm×600mm and A356 alloy tubes ofΦouter820mm×Φinner760mm×550mm were successfully prepared with good shape and homogenous structure. The grain size of the as-deposited preforms was 0.3~1μm for 8009Al and 4~5μm for A356 alloy respectively, and the relative density of the preforms ranged from 83% to 88%.(2) The deformation behavior of the 8009Al/SiCp composite preforms was investigated by using high-temperature compression experiment via Gleeble-1500 thermal-mechanical simulator. The high-temperature deformation, densification and strengthening/softening mechanisms of the composite were studied based on the microstructure analysis. Densification-strengthening, Strain-hardening and particle-reinforcing effects contributed to the deformation behavior of the composite. Dynamic recovery was the dominate softening-mechanism for 8009Al matrix. The softening during the interval between the hot compression passes depended on temperature and thermal exposure duration. Effects of deformation degree and temperature on densification of porous preform were discussed. On the basis of experimental data, FEM was applied to simulate the process and the results show that the theoretical calculation and numerical simulation agree well with the practical situation. These provided necessary data for the secondary processing of the as-deposited preforms.(3) A novel densification method for processing large-sized porous tubular preforms termed as wedge pressing, was developed based on the idea of accumulating local small deformation as to integral forming. Combined with FEM, deformation and densification of porous tube during WP were systematically studied, including technical principles, processing parameters, microstructure evolution, densification behavior, and so on. 8009Al and 8009Al/SiCp composites tubes were successfully wedge pressed through a common press with low tonnage. The as-densified tube exhibited sound shape and high density, and their formability was significantly improved.The optimum hot-working temperature can be set as 460~480℃for 8009Al, at which the deformation resistance is reduced and the fine RS microstructures together with sound mechanical properties can be kept as well.The pass reduction during WP must be controlled carefully to ensure production efficiency, perfect shape and surface quality. The pass reduction was controlled at~10% at the early stage of the tube wedge pressing, and then it could be increased but less than 20% even the workpiece was densified. On the other hand, the compaction feeding should be less than the pressing plane width of the punch.A high densification rate was obtained until the total reduction was up to~30%. The reinforcement (SiC) played an important role in the densification during WP. MMCs exhibited slower densification rate than monolithic alloy at the final stage due to the possible residual pores resulted from SiC particle clusters. The main aim of WP is to eliminate /reduce pores. Metal flowing during WP was not as enough as in extrusion or rolling processing which led to inferior properties of MMCs compared with those of the matrix alloy. However, the formability was improved.The deformation during WP can be treated as plane strain with the characteristics of the thinned wall, decreased porosity, enlarged diameter and slight axial extension. The density differences in compacted tube wall were resulted from friction and densification at the early stage of WP, and it was gradually reduced during compaction processing.Encapsulation with a plastic shell helped to improve the formability of SD preforms, by which an evenly distributed temperature and proper increased hydrostatic pressure throughout the workpiece were obtained resulting in uniform deformation and improved densification.(4) For the compaction conducted on the as-deposited A356 tubular preform in dimension ofΦ800mm, WP exhibited a better application in densification of larger size deposited tube. The optimized heat treatment processing variables for SD A356 include solutionizing at 538℃for 3h before water quenching, aging at 160℃for 10h, air-cooling. During thermal exposure, the Si particles rounded and the second phases coarsened a little. The good mechanical properties benefited from the effective elimination of defects, fine-grain strengthening, age-hardening, improvement of Si shape and Si particle reinforcing. The tensile strength and elongation of the SD tube after WP and heat treatment were 330MPa and 10.5% respectively.
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