【作者】 张昊；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2008， 博士

【摘要】 喷射沉积技术作为一种新型的金属及金属基复合材料制备技术,在高性能材料制备方面具有独特的优越性。但喷射沉积坯料含有一定的孔隙度,直接使用性能较差,必须通过后续的致密化和塑性变形才能真正实现其高性能。由于孔隙的存在,喷射沉积材料的成形性能较差,直接使用常规的塑性加工工艺(如锻造、轧制等)对其进行加工,因为应力状态容易导致裂纹的形成。而采用挤压等工艺,由于受到设备的限制,无法实现大尺寸坯料的变形。本文针对不同形状的喷射沉积坯料,对大尺寸喷射沉积坯料和喷射沉积板坯的致密化技术分别进行了研究。对于大尺寸坯料而言,在利用有限元技术优化楔压工艺的基础上,首次提出了梯温楔压工艺。而对于喷射沉积板坯而言,对新型的陶粒轧制和外框限制工艺进行了系统的研究。本论文的主要研究内容和研究结论如下:(1)研究了喷射沉积5A06铝合金多孔材料高温压缩变形行为及显微组织的演变规律。实验结果表明:喷射沉积5A06铝合金热压缩变形的流变应力随温度的升高与应变速率的降低而减小;在较高的变形温度下,应力-应变曲线较平缓,未出现明显的屈服点;而在较低的变形温度下,应力–应变曲线呈上升趋势,未出现峰值。建立了喷射沉积5A06铝合金热压缩变形流变应力的本构方程,将流变应力模型计算值与实测值进行比较,结果较为精确,为有限元数值模拟分析提供了数学模型。并对热压缩后显微组织进行了金相分析和硬度测试,初步探明了孔洞及材料硬度同温度和应变速率的关系。建立了单轴压缩时相对密度与高向变形量之间的理论关系,并用有限元模拟了单轴热压缩过程,结果显示理论公式与模拟结果和实验结果三者吻合较好,互相佐证。(2)借助于有限元软件,针对大尺寸喷射沉积坯料研究了楔形压制实验中不同的压制温度和压制速度以及压制方式等相关工艺参数对变形与致密化的影响,结果表明:双面楔压可有效提高致密化效果,合理的楔压温度是450～500℃,合理的楔压速度是0.5～1.0mm/s。(3)在楔压工艺的基础上,提出了梯温楔压工艺,对梯温楔压过程中致密化和组织演变规律进行了研究,并与恒温楔压结果进行了比较。研究结果表明:梯温楔压工艺可以有效提高喷射沉积坯料整体的致密化效果。当高向变形量达到25%时,坯料沿高向的密度分布均匀,整体相对密度能达到95%以上。与恒温楔压工艺比较发现,采用梯温楔压能增加坯料底部的横向流动,达到消除孔隙的效果,坯料底部的相对密度有很大的提高。通过对比梯温楔压和恒温楔压后坯料的微观组织发现,梯温楔压工艺能有效改善喷射沉积多孔坯料的微观组织。当高向变形量为25%时,与恒温楔压坯料中存在较多孔洞相比,梯温楔压试样已基本致密。由于梯温楔压工艺能提高喷射沉积多孔坯料的致密化效果,因此使梯温楔压试样的力学性能也得到了大幅度的提高。梯温楔压后的断口形貌说明,梯温楔压后材料中大量的孔隙得以消除,同时SiC颗粒与基体间的结合得到了有效改善。(4)研究了陶粒轧制工艺参数诸如轧制方式和原始坯件的设计对轧制变形行为的影响,结果表明:采用道次间180°转向轧制工艺,且坯料尺寸调整为L×B×H=30×30×8mm能够满足喷射沉积铝合金板坯轧制成形性能的要求。对在陶粒轧制工艺条件下喷射沉积板坯的致密化规律和变形规律的研究表明:由于陶瓷颗粒对材料纵向和横向变形的阻碍作用使多孔材料的延伸变形量明显小于常规轧制,从而减小了促使裂纹形成的拉应变,有利于材料的致密化进程。在陶粒轧制过程中,板坯在高向变形量达到65%时仍不会形成表面裂纹,而在常规轧制变形中,当轧制高向变形量为26.5%时轧件表面就已经出现较多细密的横向裂纹。陶粒轧制工艺成功地改善了喷射沉积铝合金板坯直接轧制的加工性能,板坯在先陶粒轧制致密再直接轧制工艺下通过充分的变形,能得到较好的室温力学性能,轧制出的8009Al/SiCp复合材料板材的抗拉强度和伸长率分别为505.0MPa和6.5%。在陶粒轧制过程中喷射沉积板坯的致密化和轧制变形行为与用于传递压力的陶瓷颗粒介质特性有关。采用200目Al2O3进行陶粒轧制时,喷射沉积铝合金板坯的致密化效果和轧制成形性能最佳。(5)分析了外框限制轧制技术的工艺条件和影响喷射沉积坯料致密化效果的各种因素,对合理的外框限制轧制工艺进行了研究。实验结果表明:采用外框限制轧制技术能有效地提高喷射沉积板坯的轧制成形性能,可以避免常规轧制工艺中存在的喷射沉积坯料容易开裂的问题,合理地控制轧件与边框间的间隙尺寸有利于提高沉积坯的致密化效果。当外框和坯料的长度和宽度间隙分别为θB=4～6%,θL=7.5～10%时,板坯的致密化效果和轧制成形性能最佳。研究了在外框限制轧制工艺条件下喷射沉积板坯的致密化变形行为和孔洞、组织的演变规律,分析了轧制板材的力学性能。在轧制的初始阶段,轧件高向上的变形不均匀,高向变形量由表面层向中心层逐渐减小。当轧件充满边框内的间隙后,轧件的高向变形逐渐趋于均匀。在轧制过程中,轧件上表面的致密化速度最快。在喷射沉积8009Al/SiCp坯件的轧制过程中,SiC颗粒容易被破碎和细化,其分布也趋于均匀。喷射沉积坯料通过外框限制轧制后致密度最高可达到99.0%,轧件去除边框后可以直接进行常规轧制变形而不会出现裂纹;轧制出的5A06Al合金的抗拉强度和伸长率分别为340.0MPa和12.0%;8009Al/SiCp复合材料板材的抗拉强度和伸长率分别为450.0MPa和9.5%。更多还原

【Abstract】 As an advanced material preparation technology, spray deposition has an advantage in producing high performance alloys and metal matrix composites. However, the workability and mechanical properties of spray deposited preforms are poor owing to the porosity formed during the deposition processing. So, further densification and plastic deformation treatment are needed to prepare fully dense products with high mechanical properties. Due to the porosity, the forming property of spray deposited preforms is poor. Thereby, processed directly by tranditional plastic working, such as forging and rolling, spray deposited preforms are easy to crack during the processing, because of stress condition. Resultingly limitation of equipment, it is difficult to densify the spray deposited preforms with large dimension by extrusion. According to shapes of spray deposited preforms, different densification technologies of preforms with large dimension and sheets were investigated respectively. As for spray deposited preforms with large dimension, the process parameters of Sequential Motion Compaction (SMC) were optimized by using finite element method. Based on SMC, a novel technique named Sequential Motion Compaction with Temperature Gradient (SMC-TG) was developed firstly. And as for spray deposited sheets, the Ceramic Rolling (CR) and Frame-Confined Rolling (FCR) were studied detailedly. The main researches in this study are as follows:(1) A study on the behavior of hot compression of spray-deposited 5A06 aluminum alloy was performed. The experimental results show that the flow stress of spray deposited 5A06 aluminum alloy during the hot compression decrease with the enhancement of deformation temperature and the decrease of strain rate. At the higher deformation temperature, stress-strain curves are gentle and there are no obvious yield points on curves. With comparing, at lower temperature, stress-strain curves are ascending curves, but no peak values on curves.A constitutive equation was established by analyzing the relationships of the flow stress and the strain and the deformation temperatures and strain rates. The proposed equation gives a good agreement with the measured values, which consequently can be as the numeric-simulated flow stress model of spray-deposited 5A06 aluminum alloy during hot deformation. In this study, the changes of the microstructure and the hardness have revealed the relationships of temperatures and the strain rates.A theoretic relationship to relative density and high reduction was established in the single axial compress, the results of FEM show that the theory values and the FEM results give a good agreement with the measured values.(2) By the finite element software, the effects of the different process parameters on the deformation and densification in the SMC process were researched. The results show that the two face SMC is needed and the reasonable temperature is 450℃～500℃, the reasonable velocity is 0.5～1.0mm/s.(3) Based on SMC, the novel technique named as“Sequential Motion Compaction with Temperature Gradient (SMC-TG)”is applied. The densification and evolution of microstructure during SMC-TG were investigated. The results of SMC and SMC-TG were compared. As a result, the SMC-TG process could improve the densification of preform. When the height reduction is 25%, the distribution of relative density along height direction is homogeneous, and the unitary relative density is above 95%. Compared with SMC process, the lateral flow on bottom of preform is increase, the effect on reduction of porosity is attained, and the bottom of preform has higher relative density. SMC-TG could improve the microstructure of spray deposited preforms, by compared SMC with SMC-TG. When the height reduction is 25%, the preform processed by SMC-TG is compact basically, and there are many pores in SMC preform. Due to SMC-TG could densify the spray deposited preform effectively, the mechanical property is improve obviously. From the results of fractography, after SMC-TG, the pore spaces have been eliminated, and at the same time, the bonding between SiC particles and matrix has been reformed.(4) The effects of processing conditions during“Ceramic Rolling”technique, such as rolling method and the size of preforms, were investigated. To meet the needs of forming property, the 180°turning rolling method must be adopted, and the best size of preform is L×B×H=30×30×8mm.The densification and deformation of spray deposited sheets during CR were investigated. Comparing with traditional rolling, due to the ceramic particles serve the function of rejection of metal flow, a remarkable lower elongation is attained for the ceramic-rolled samples, and tension strain which can induce the occurrence of crack is reduced. It is in favor of the densification process of preform. During traditional rolling, transversal cracks usually occur on the surface of preform when the thickness reduction is up to 26.5%. But in the processing of ceramic rolling, no cracks appear while the thickness reduction is 60%. The CR technique could improve the rolling forming property of spray deposited preform. After CR, it could attain favorable mechanical property at room temperature by direct rolling. The tensile strength and elongation of 8009Al/SiCp is 505.0MPa and 6.5% respectively.The densification and plastic deformation behaviors and the fracture behaviors of the as-spray deposited porous performs are related with the effects of particulate characteristics during CR process. The preform could attain best effect of densification and forming property by using the Al2O3 particle with the size of about 200 meshes.(5)The processing conditions of FCR and the influence factors on the densification effect were analyzed. The experimental results show that the FCR technique can greatly improve the workability of spray deposited preforms and avoid cracks whcich easily occur during the conventional rolling processing. The best gaps size in length and width wereθB=4.0～6.0% andθL=7.5～10.0% for the densification and workability of spray deposited preforms.The densification and deformation mechanism, evolution regularities of the pores and microstructures during the FCR processing are discussed. The mechanical properties of the as-rolled sheets were examined. At the beginning of the rolling processing, the distribution of thickness reduction in the as-rolled sheets were not uniform, the thickness reduction on the surface layer of the sheet was smaller than that in the center layers. When the preforms fully filled the frame, the thickness reduction distribution will tend to be uniform. The densification rate of the surface layer of the preforms was the most fast. During rolling of the 8009Al/SiCp preforms, the SiC particles easily break up into finer pieces and the distribution state tend to be more uniform. The relative density of spray deposited preform could attain 99.0%, and the preforms can be direct rolled without crack. The tensile strength and elongation of 5A06 aluminum alloy is 340.0MPa and 12.0% respectively, and for 8009Al/SiCp composite is 450.0MPa and 9.5% respectively.更多还原