【作者】 陈光霞；

【导师】 曾晓雁；

【作者基本信息】 华中科技大学 ， 物理电子学， 2009， 博士

【摘要】 可摘除局部义齿支架是人体口腔修复的重要辅助工具，其形状多为复杂的三维曲面，一般用铸造工艺进行单件生产。但传统的铸造技术工序繁琐复杂，周期长，容易产生粘砂、缩孔、裂纹、冷隔与偏析等铸造缺陷。另外，对于活性高、易氧化的钛及钛合金支架，制作环境难保证，制造成本高。因此，迫切需要寻找一个新的制造技术来克服传统制造方法的不足。激光快速成型技术具有制造周期短、精度高、工艺简便等诸多优点，可以克服传统铸造工艺的不足。然而，国内外对于采用激光快速成型技术进行可摘除局部义齿支架制造的相关研究还在探索之中，缺乏相应的试验数据和理论研究。本文以激光直接制造技术DLF(Direct laser fabrication)和选择性激光熔化技术SLM(Selective Laser Melting)为基础，从设备研制、软件开发、基础工艺、加工效率、成型精度、成型件的微观组织和机械性能入手，对激光快速成型技术在可摘除局部义齿支架制造上的应用进行了深入的研究。主要研究结果如下：首先，构建了一个由Nd：YAG激光器及辅助设备、数控工作台、送粉系统、控制软件组成的DLF系统，能够方便地调节各加工工艺参数。通过相关工艺研究，获得了DLF快速成型优化的工艺参数范围：激光功率为200～500W、扫描速度为700～1400mm／min、送粉量为4．2～6．8g／min；系统地分析了DLF快速成型工艺对所加工零件的组织、机械性能的影响。通过测试，成型件的物相为奥氏体，与粉末材料相同，拉伸强度可达912MPa以上，平均显微硬度为HV261。实验证明，DLF技术虽然在微观组织和机械性能方面很优异，但由于它采用开环三轴数控与同轴送粉方式，对于复杂形状零部件的加工，在精度(尺寸误差2．5％)和效率(1．06cm~3／h)方面难以满足实际要求，特别是对于复杂三维曲面的可摘除局部义齿支架而言更加困难。因此，DLF方法不适合用于直接制造可摘除局部义齿支架。我们决定采用选择性激光熔化技术(Selective Laser Melting，SLM)解决支架直接制造问题。为此，本文构建了一个由Yb光纤激光器、扫描振镜、气体净化装置、铺粉装置、控制系统组成的选择性激光熔化系统，该系统由控制系统实现各个部分的协调工作。通过相关基础工艺研究，获得了优化的工艺参数范围：激光功率150～200W、扫描速度5～25mm／min、切片层厚0．02mm～0．04mm。实验发现，为了获得性能理想的金属零件，在激光加工过程中必须注意如下几个关键技术：(1)基板必须预先进行磨削(Ra为0．8μm)、去油污和水分、喷砂毛化处理；(2)切片层厚度在0．01～1．0mm精确可调，并且重复性好；(3)激光功率密度适中，不产生球化或飞溅现象。通过测试，SLM激光快速成型技术所制备金属零件致密度可达96％以上，拉伸强度为635MPa，平均显微硬度为HV307，成型件的物相为奥氏体，也与粉末材料相同；成型件平均误差为1．1％，制造效率为1．77cm~3／h。通过对成型件表面粗糙度的研究发现，表面粗糙度与切片层厚、激光加工参数、表面倾斜角度有关。切片层厚越小，表面粗糙度值越小；相对于XOY平面的倾斜角度越大，表面粗糙度值越小；在其它参数相同情况下，搭接率为30％时，表面粗糙值最小。实验结果表明，SLM成型件微观组织性能与机械性能良好，制造精度和制造效率比DLF要高。系统地开展了SLM快速成型加工效率的研究。结果表明，在保证成型件精度、微观组织与机械性能的前提下，要提高加工效率，应该从扫描路径规划、铺粉工艺、节约切片与扫描填充计算时间等多方面入手。在此基础上，本文提出了用于路径规划的“二分法”分区扫描算法，它可以消除扫描区域中包含内环时激光加工的跳转用时；同时，引入多线程技术，使切片、扫描填充计算与激光加工同步执行，节约了加工时间50％以上，从而提高了制造效率。利用上述设备、加工工艺和软件，采用优化的工艺参数，成功地实现了可摘除局部义齿支架的制造，特别是实现了钛合金支架的制造。通过对制造的可摘除局部义齿支架的形状精度的测试表明，利用本文SLM设备所制造的义齿支架，平均加工精度可达±0．172mm；通过支架在石膏模上的佩戴试验表明，所制造的可摘除局部义齿支架，通过一定的后续加工处理，完全可以应用于临床。更多还原

【Abstract】 The removable partial denture framework is very complex in shape with many curved surfaces, which is a very important assistant tool in prosthodontics. Generally, frameworks are fabricated by single piece basing on mould by traditional cast technology, which has many shortages of multiple steps, time cost and defects such as sand adheres, inner holes, cracks and segregation etc. Note that it is difficult for this method to be used to fabricate frameworks of titanium alloys due to their high activity. In order to achieve the proper fabrication environment, the cost of casting is very high. Hence, it is significant to find a novel manufacturing technology to overcome shortages of the traditional dental framework fabrication method.As a novel method, the laser rapid prototyping technology (LRPT) has the advantages of short manufacturing cycle, high precision and flexibility, simple fabrication arts, which can be used to replace the conventional cast technology in the areas of prosthodontics. Whereas, the experimental and theoretical researches on the fabrication of removable partial denture frameworks by LRPT are very few up to now, which retard the applications of this technology in prosthodontics. In this Ph.D. dissertation, the technologies of direct laser fabrication and selective laser melting were compared to investigate the possibility of fabricating the removable partial denture frameworks. The equipment design, software development, basic fabrication parameters, processing efficiency and precision, microstructure and mechanical properties of final parts are investigated to realize the formation of removable partial denture frameworks. The following are the main results:Firstly, a DLF (Direct Laser Fabrication) setup composed of a Nd: YAG laser, a CNC stable and a powder feeder was built up. The effects of DLF parameters on the structure and mechanical properties of parts were analyzed systemically on the basis of the DLF setup above mentioned were investigated. The optimized parameter ranges are as follows: laser power 200～500W, scan velocity 700～1400mm/min, powder feed rate 4.2～6.8g/min. The phase of the fabricated parts is austenite, which is the same as that of the starting powder material. The fabricated sample has a tensile strength over 912MPa and an average micro-hardness of HV261. But the open-loop control and coaxial powder nozzle decide the DLF technology is not suitable for denture framework fabrication with high precision (smaller than 2.5% in dimension) and high efficiency (1.06cm~3/h) despite good microstructure and mechanical properties can be obtained.Accordingly, the selective laser melting technology was used to fabricate removable partial denture frameworks. In our project, a SLM system was designed and developed by ourselves, which is consisted of a Yb-fiber laser, an optical scanner, a powder coating device, a gas purification system and a control system. By using above SLM system, the process parameters were investigated systematically and optimized. The optimized parameter ranges are: laser power 150～200W, scan velocity 5～25m/min, slicing layer thickness 0.02～0.04mm. The experimental results show that several key steps should be noticed during SLM process to obtain metal parts with better performance: (1) the substrates should be milled to Ra0.8μm and free of water before sand blasting; (2) The thickness of slice should be adjusted easily and controlled in the range of 0.01～1.0mm. (3) Proper laser power density should be adopted to prevent the emergence of balling or splash.The parts formed by optimized parameters have a density above 96%, a tensile strength of 635MPa and an average micro hardness of HV307 for stainless steel powder. It is interesting to find that the phase of the built parts is also austenite, which is the same as that of the starting material. The measurements also show that the average dimension error is about 1.1% and the manufacturing efficiency of SLM is 1.77cm~3/h. The surface roughness is decided by slicing thickness, laser process parameters and the incline angle. The smaller slicing thickness or bigger incline angle leads to low roughness surface. Importantly, the overlap is a key parameter to control the roughness, the surface with lowest roughness can be obtained at an overlap of 30% when other parameters are kept the same. Hence, the microstructure and mechanical properties of fabricated parts is better, but the manufacturing precision and efficiency is high by SLM in comparison with that of DLF technology.The manufacturing efficiency of SLM was studied systemically by using this system. The results indicate the main ways to increase the manufacturing efficiency on the basis of ensuring the precision, microstructure and mechanical properties are proper scanning path layout, higher powder coating velocity, less time of slicing and filling path calculation. In this dissertation, a partition scanning arithmetic called dichotomy was used to design the scanning paths on the basis of reducing the laser skipping time. In addition, multithread calculation was introduced to make the slicing, scanning filling calculation and laser process run synchronous, which saves 50% of all manufacturing time and increases the manufacturing efficiency.We finally fabricated removable partial denture frameworks with stainless steel and titanium materials successfully by using the SLM system under optimized parameters. The tests showed that the frameworks have an average dimension precision of±0.172mm. The wearing experiments on the plaster mouth molds demonstrated the removable partial denture frameworks fabricated by SLM can be fully applied to clinic after some postprocessing procedures.更多还原