【作者】 陈志钢；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2009， 博士

【摘要】 喷射沉积技术在制备大尺寸高性能材料方面具有独特的优越性,但沉积坯中含有一定的孔隙度使直接应用受到限制,须经适当的后续致密化加工才能体现其优异性能。目前由于受到设备吨位及成本的制约,对大尺寸喷射沉积坯件的塑性成形难以实现。因此,立足于工程实际需要,探索此类多孔坯实用可行的致密化方法,对解决大尺寸快速凝固结构材料的制备与后续塑性加工等难题乃至喷射沉积技术的应用与发展具有十分重要的意义。本论文结合“十五”科技攻关重点项目,采用喷射沉积技术制备大尺寸铝合金8009Al、8009Al/SiCp和A356管坯;辅以热/力模拟实验和有限元数值模拟等手段研究热变形工艺参数对管坯组织与性能及其致密化过程的影响;首次采用大尺寸喷射沉积管坯楔形压制新工艺,从工艺原理、致密化机制及压制力等方面系统地研究大尺寸喷射沉积管坯的楔压变形与致密化规律。主要研究内容和结论如下:(1)通过多层喷射沉积技术研究了大尺寸铝合金8009Al、8009Al/SiCp和A356管坯的制备工艺,分析了主要工艺参数对管坯组织和性能的影响,得到优化的工艺参数:雾化气体N2压力0.7~0.85MPa,喷射高度200~240mm,SiCp输送压力~0.5MPa;对于8009Al和A356,熔体液流直径分别为3.4~3.8mm和3.2~3.6mm;熔体过热度250~300℃;基体转速20~60 r·min-1;雾化器扫描周期30~50s。在此基础上成功制备出尺寸为Φouter350mm×Φinner280mm×600mm的8009Al及其复合材料管坯,其晶粒尺寸为0.3~1μm,复合材料中SiCp分布均匀; A356管坯尺寸为Φouter820mm×Φinner760mm×550mm,晶粒尺寸为4~5μm。管坯形状规整,致密度~83%-88%,为后续致密化加工研究提供了合适的材料。(2)通过Gleeble-1500热/力模拟试验研究了喷射共沉积8009Al/SiCp复合材料的热压缩流变行为,结合对显微组织的分析,研究了该材料高温塑性变形致密化及材料变形中的强/软化机制:复合材料的变形强化包括因致密化引起的增强和基体金属加工硬化以及SiCp增强作用;动态回复是基体软化的主要方式;道次间保温阶段的软化程度与温度和保温时间有关。分析了变形量、温度等参数对多孔材料致密化效果的影响,并基于实验数据对该致密化过程进行了有限元数值模拟,理论预测、数值模拟和实验结果三者有较好的一致,为合理制定喷射沉积管坯的后续致密化加工提供了必要的材料数据和工艺依据。(3)基于多道次局部小变形累积致整体成形的思路,采用一种新颖的大尺寸喷射沉积管坯致密化加工方法-楔形压制工艺。从工艺原理、工艺参数、楔压过程中多孔坯微观组织的演变、致密化过程及压制力等方面,结合有限元数值模拟研究了喷射沉积多孔管坯的楔压变形与致密化规律。通过该工艺在较小吨位压力机上成功实现了对Φ300mm的8009Al及其SiCp增强复合材料管坯的致密化加工,得到的管坯形状规整,致密度和成形性能有效改善。8009Al的适宜热加工温度为460~480℃,以利于降低变形抗力并保持材料的快凝组织特征和优异的力学性能;道次压下量应兼顾加工效率和管坯的形状及表面质量:管坯楔压初期,道次压下量宜在10%左右,管坯基本致密后可适当提高但不应超过20%,同时压制步进量应小于楔形压头施压面宽度。变形程度30%前,管坯经历了较快的致密化过程;增强颗粒对管坯楔压中的致密化过程产生影响:SiCp的局部团聚并增加孔隙使复合材料管坯的致密化速率在楔压后期较未增强材料缓慢。楔压工艺的主要目的是消除/减少孔洞,楔压过程中管壁的剪切流动不如挤压或轧制充分,SiCp的局部团聚使复合材料的力学性能低于未增强材料,但致密后其可加工性提高。管坯楔压过程近似平面应变,其结果是管壁变薄,孔隙度减小,伴随着扩径与较小的轴向伸展。楔压初期的摩擦与致密化引起变形区壁厚方向的密度差异,这种差异随变形过程的继续而减小。在管坯外表面包覆可塑性包套有利于保持管坯整体温度的均匀,并适当增加压制中的静水压力,促进管坯变形均匀和致密化,改善了喷射沉积低塑性难变形多孔管坯楔压中的可加工性。(4)对尺寸达Φ800mm的喷射沉积A356合金管坯楔压致密化加工结果进一步表明,楔压工艺在大尺寸喷射沉积管坯的后续致密化方面有较好的实用性;喷射沉积A356合金的最佳热处理工艺参数为:538℃固溶3h,160℃时效10h;热处理后合金中的Si球化且其它第二相无明显长大;良好的力学性能得益于孔隙等缺陷的有效消除、喷射沉积细晶组织、时效强化、Si颗粒的球化及其颗粒增强效应;经楔压致密并热处理的管坯试样拉伸强度和延伸率分别为330MPa和10.5%。更多还原

【Abstract】 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.更多还原