电子束熔化技术制备多孔Ti-6Al-4V合金及其力学性能研究

IMR OpenIR

	电子束熔化技术制备多孔Ti-6Al-4V合金及其力学性能研究
	程旭莹
学位类型	硕士
导师	郝玉琳 ; 杨锐
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	医用钛合金多孔ti-6al-4v 电子束熔化压缩性能疲劳性能 Biomedical Titanium Alloys Cellular Ti-6al-4v Electron Beam Melting Compressive Properties Fatigue Properties
摘要	"多孔金属材料具有密度小、孔隙率高、比表面积大、模量低、良好的强度和能量吸收性等优点，在人体硬组织修复和替代材料领域具有潜在的应用前景。本文利用添加加工（Additive Manufacture: AM）的电子束熔化（Electron Beam Melting: EBM）制备技术，成功制备出随机泡沫结构和规则网格结构两类高孔隙率的多孔Ti-6Al-4V合金，并对其微观组织、孔结构和力学性能进行了研究。通过计算机断层扫描技术（Computed Tomography: CT）获得商用铝合金泡沫的随机泡沫结构，通过Materialise/Magics商用软件设计菱形十二面体密堆积结构的规则网格结构，建立AM-EBM加工所需的CAD模型。通过模型尺寸的线性变化，获得两种结构孔隙率和孔尺寸的变化。采用低间隙Ti-6Al-4V合金粉末，通过CAD模型的程序控制，利用Arcam A1型AM-EMB设备，成功制备出孔隙率为90%~92%的泡沫结构和62%~86%的网格结构的多孔合金。制备的多孔合金均为针状组织，以α′马氏体相为主体，此外还含有极少量的β相。针状马氏体组织的形成说明合金的冷却速度较快，无法通过高温β相的扩散相变产生α相，这与电子束的微区快速熔化和快速冷却特点相一致。α′马氏体相的尺寸随着孔隙率的减小而细化。这是由于多孔结构孔棱的几何尺寸随着孔隙率的减小而减小，冷却固化速度增加。显微硬度的测量结果与组织观测结果一致，即组织细化，硬度增加。多孔Ti-6Al-4V合金的动态杨氏模量和压缩强度随孔隙率的增加而降低，其中动态杨氏模量为0.2~6.3GPa，压缩强度为4~113MPa，与人体骨组织的生物力学性能基本匹配。其相对模量、相对强度与相对密度的关系基本符合Gibson-Ashby模型。与其他传统金属泡沫比较，在比模量相当的条件下，该方法制备的多孔Ti-6Al-4V合金具有比强度高的特点。多孔Ti-6Al-4V合金的压缩变形特征为典型的脆性断裂，这主要与快速冷却形成的马氏体相具有较低韧性相关联。对比两种结构的多孔合金的压缩变形行为，发现孔结构特征对压缩变形行为有显著影响：规则网格结构的断裂优先发生于孔棱结点处，而不规则泡沫结构的初始断裂则多发生于孔棱中部；前者的变形带总是沿着加载轴45°方向形成，而后者在45~90°方向随机产生。通过应力与结构的模拟分析，对规则网格结构变形带的产生进行了较好的解释。规则网格结构Ti-6Al-4V合金的疲劳失效机制为孔棱循环蠕变和疲劳裂纹的萌生与扩展共同作用，其中前者是决定疲劳寿命的主要因素。在循环加载过程中，位错通常沿着孔棱中α′的相界面产生，并且随着相对密度的提高这种现象愈加明显。这一位错密度的增加有利于对多孔材料循环蠕变效应的阻滞，从而提高其疲劳强度。规则网格结构Ti-6Al-4V合金的相对疲劳强度和相对密度可按照Gibson-Ashby模型的线性关系拟合，但是其指数项n值约为2.7。
其他摘要	Porous metallic materials have advantages of low density, high porosity, large specific area, low elastic modulus, good strength and excellent energy absorption. These merits make them great potential for biomedical applications to repair or replace dysfunctional hard tissues. In this study, cellular Ti-6Al-4V alloys with the stochastic foam and the reticulated mesh structures were designed and manufactured successfully by electron beam melting (EBM) technique using the initial powders with extra low interstitials (ELI). The microstructure, cell structure and mechanical properties of cellular alloys were investigated. The main conclusions are summarized as: The stochastic foam structure was obtained by computed tomography (CT) scan of commercial aluminum alloy foam with open cellular structure while the reticulated mesh structure was generated on the basis of a build lattice unit cell with a rhombic dodecahedron shape using Materialise/Magics software. Both kinds of CAD models were established to control automatically the manufacture procedure of the EBM equipment produced by Sweden, Arcam A1. To obtain the structures with different porosity and cell size, the cell units were dimensionally scaled. The results showed that, after above adjustment of the CAD models, the porous Ti-6Al-4V materials with the stochastic foam and the reticulated mesh structures were manufactured successfully with high porosity in the range of 90~92% and of 62~86%, respectively. The cellular Ti-6Al-4V alloys showed that they have acicular microstructure consisting mainly of thin α′ martensite, and a little amount of β phase. Such morphology of microstructure is due to rapid cooling in solidification occurring in Ti-6Al-4V alloy manufactured by electron beam melting with small dimensions. The α′ martensite become finer with the decrease of porosity and Vickers hardness increases accordingly, which may be correlated with the decrease of cell strut size and the increase of cooling rate in the manufacturing process. The dynamic Young’s modulus and compressive strength of cellular Ti-6Al-4V alloy decrease with the increase of porosity, in the range of 0.2~6.3 GPa and 4~113 MPa respectively, which matched with biomechanical properties of human bone. The correlations between relative modulus, relative strength and relative density accord with the Gibson-Ashby model. Under the condition of similar specific modulus, the EBM-fabricated cellular Ti-6Al-4V alloys possess higher specific strength than other conventional metallic foams. During the deformation, these EBM samples have a brittle response due to the poor ductility of the acicular martensite. Comparing the compressive deformations behavior of two different cellular structures, it is found that cell structure would significantly affect the compressive deformation behaviors: the cracking of regular meshes usually initiated at the cell nodes while the cracks of stochastic foams arose in the middle of cell ligaments mostly; the meshes exhibit the invariable angle of 45°between the deformation band and the compression axis whereas the angle of foams is 45~90°. This phenomenon was reasonably explained by analysis of the micromechanical model. The underlying mechanism of fatigue failure of Ti-6Al-4V regular mesh structure appears to be the interaction between the cyclic ratcheting and the fatigue crack initiation and propagation in the cell struts while the former plays a dominant role on fatigue life. During the fatigue process, dislocations are generated along the interface of α′ phase and their generation becomes more evident with the increase of the relative density. This would contribute to the retardation of the cyclic ratcheting and the improvement of the fatigue strength. The relative fatigue strength increases with the relative density increasing, which can be evaluated by the Gibson-Ashby model with the exponential factor n being 2.7, which is higher than the reported data of aluminum and nickel foams, and almost double the idealized value (n=1.5) of the stochastic open cellular foam."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64515
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	程旭莹. 电子束熔化技术制备多孔Ti-6Al-4V合金及其力学性能研究[D]. 北京. 中国科学院金属研究所,2012.