【作者】 崔泽琴；

【导师】 许并社； 王文先；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2011， 博士

【摘要】 基于轻质高强和矿产资源优势,镁合金材料成为“陆、海、空、天”交通运载装备等领域的重要优势材料,被誉为“21世纪最具发展潜力和应用前景的绿色结构材料”。镁合金材料化学活性高、熔点低、导热快、热膨胀系数大等特点,使得其热加工性能较差,成为制约镁合金材料广泛应用的瓶颈。与传统热加工方法相比,激光加工具有能量密度大、加工效率高、工艺流程短、能源消耗低、绿色加工与柔性制造等特点,被称为“未来制造系统共同的加工手段”。而以高能量高频率的优势,脉冲激光成为镁合金材料热加工的一个重要途径,将有利于拓展镁合金材料的应用领域。因此,研究镁合金材料脉冲激光加工行为,将具有重要的理论意义和应用价值。针对镁合金材料吸光率低、产热少、散热快的特点,本课题与相关研究单位合作,设计研制了高能量高频率的固体Nd:YAG脉冲激光器及其加工装备。系统研究了脉冲激光作用下AZ31B镁合金材料切割、焊接、熔凝及熔覆等加工行为,并在此基础上,探索了激光加热·液氮深冷极端冷却条件下镁合金材料表面纳米化和非晶化行为。本研究取得以下结论：设计并研制成功一台单泵浦腔平均功率大于500W的固体Nd:YAG脉冲激光器。激光光束波长1.064μm,频率1-2000Hz可调,峰值功率9200W,单脉冲能量83.8J；光束参数乘积Kf为16.5mm·mrad,聚焦光斑理论直径最小可达12.7μm；并制造了与该激光器相适应的数控加工系统。探索了镁合金材料脉冲激光切割机理、切割质量影响因素和切割断面微观组织结构。研究表明：在脉冲激光作用下,镁合金材料微区熔化、气化、燃烧以及气流力的协调“挖掘”机制,使得镁合金激光切割成为可能。单脉冲能量、峰值功率、脉冲频率、脉冲宽度、离焦量和气体种类是切割质量的主要影响因素。重熔层与母材交界处没有发现明显的热影响区,这是由于脉冲激光快速加热和快速冷却的特点所致。在优化参数情况下,切割厚度可达到6mm以上。研究了镁合金材料拘束作用下脉冲激光焊接冶金微观组织和缺陷产生机理。结果表明,元素烧损、蒸发、飞溅、裂纹、气孔、夹杂是存在的主要问题。裂纹形式主要是结晶热裂纹,氢气孔是焊缝金属中主要的气孔形式。脉冲激光快热快冷可使焊接接头组织细化,平均晶粒尺寸从母材的10-30μm细化到3-10μm。晶粒细化和晶内弥散分布的细小颗粒状β-Mg17A112是焊缝金属显微硬度提高的主要原因,最大硬度值可达72HV0.05。细小晶粒对焊缝金属脆性断裂有所改善,断口局部显示出塑性断裂的形貌。采用脉冲激光对AZ31B镁合金材料表面进行熔凝试验。结果表明,熔凝层微观组织明显细化,晶粒尺寸约为2-10μm,并存在大量的胞状亚结构。TEM电镜观察发现大量纳米尺寸的β-Mg17All2相,且均匀弥散分布于胞状亚结构的内部。熔凝层显微硬度约为原始镁合金的2倍,腐蚀电位比原始镁合金正移了约106mV,这是由于晶粒细化、第二相粒子弥散强化、杂质元素固溶以及Al含量增加等综合作用的结果。利用脉冲激光在AZ3 1B镁合金材料表面成功制备了Al-Si和A12O3-TiO2复合陶瓷涂层。当A1203-TiO2陶瓷粉末的质量分数小于15%时,具有良好的熔覆工艺性能。复合涂层与基体结合区成份过渡平缓,达到了良好的冶金结合。复合涂层中陶瓷颗粒大多以“质点”形式镶嵌,同时有Mg2Si和镁铝金属间化合物生成,局部区域还发现有未熔的陶瓷颗粒。复合涂层的平均显微硬度达到225HV0.05,约是单一Al-Si涂层的1.5倍,其强化机制为细晶强化、固溶强化及弥散强化。复合涂层的摩擦磨损机制以磨粒磨损为主,氧化磨损和粘着磨损为辅。电化学腐蚀结果表明,复合涂层的抗腐蚀性都得到了改善,其原因是由于陶瓷颗粒和生成的镁铝金属间化合物提高整个复合涂层的腐蚀电位,同时晶粒细化也降低了电偶腐蚀的有效阴阳极面积。在以上研究的基础上,提出了激光加热·液氮冷却的极端快速熔凝的方法,在镁合金表面成功获得了纳米晶和非晶的混合组织,从材料凝固特性及晶体生长热力学方面探讨了纳米晶和非晶的形成机理。液氮冷却改性层显微硬度最高达148HV0.05,约为基体的3倍。电化学腐蚀试验结果表明：液氮冷却改性层的腐蚀电位为-1439mV,比空气冷却熔凝层正移了26mV,比原始镁合金正移了124mV,这是固溶度增大、非晶组织形成以及晶粒细化等综合作用的结果。更多还原

【Abstract】 High specific strength and abundant nature recourses have made magnesium alloys an ideal candidate for automobile, aerospace, navigation and electronic etc. It is known as "the most potential and prospective green structure material in the 21st century ".However, due to quantity of intrinsic properties including higher reactivity, lower melting point, bigger thermal conductivity and expansion coefficient etc, it’s very difficult for magnesium alloy to precede hot working, which limits its application greatly. Compared with traditional heat process, laser processing is called as "a common processing technique in the future", which that is characterized due to its high energy density and efficiency, short process, energy conservation and flexible manufacturing etc. Therefore, it has great significance to study the pulsed laser behavior of magnesium alloy.A solid Nd:YAG pulsed laser were designed and manufactured for laser processing of magnesium alloy materials. In this paper, laser processing behaviors of AZ31B magnesium alloy material was investigated including laser cutting, laser welding, laser surface modification etc. The behavior of nanocrystallization and amorphization of magnesium alloy surface was explored by laser heating-liquid nitrogen cooling. Some conclusions were drawn as follows:The solid Nd:YAG pulsed laser with a single pump cavity was designed and manufactured successfully with an average output power of 500W, wavelength of 1064nm, frequency of 1-2000Hz, peak power of 9200W, single of pulse energy 83.8J. Its beam parameter product Kf was 16.5mm-mrad and its minimum beam spot diameter was 12.7μm. The numerical control processing system corresponding to the laser was designed and manufactured.The laser-cutting mechanism and factors were discovered and the microstructure of pulsed laser cutting surface was observed. The results showed that it made the laser cutting possible because a micro-district was melted coordinated with the action of vaporization, burning, and gas flow. The single pulse energy, peak power, pulse frequency, pulse width, defocusing amount, and assistant gas type were the main factors that determined the cutting quality. The heat affected zone was almost indiscernible between the remelted layer and the base material. A 6mm in thickness of laser cutting could be reached with the optimized parameters.The weld metallurgy and defect behaviors of laser welding magnesium alloy were also studied. The element burning, evaporation, splash, cracks, porosity, and inclusion were the main metallurgy problems of pulsed laser welding magnesium alloy. Especially, the hot cracking was the main crack formed in the weld. Although the content of hydrogen in weld metal was higher, there was no delayed crack observed. The microhardness in the weld was improved to 72HV0.05 because of the grain refinement and dispersive distribution ofβ-Mg17Al12 particles. The crystal size was refined from 10～30μm to 3～10μm due to fast heating and quick cooling. The refined crystalline grain had positive effect on the fracture properties of the weld metal, which was characterized by ductile fracture in partial.Laser surface melting was carried out on AZ31B magnesium alloy by the pulsed Nd:YAG laser. The results showed that the microstructure of melted layer was refined obviously with the grain size of about 2～10μm, and a lot nanophase Mg17Al12 was dispersed uniformly. Microhardness of the melted layer was twice of that of the as-received magnesium alloy. The results of electrochemical corrosion showed that the corrosion resistance of laser surface melted layer had been improved due to the grain refinement, dispersion reinforcement of the second phase particle, solid solution of the impurity element and the increase of the content of Al.To improve the wear resistance and corrosion resistance of magnesium alloys, the composite coating of Al-Si and Al2O3-TiO2 on AZ31B magnesium alloy surface was prepared successfully by laser cladding. When the content of Al2O3-TiO2 was lower than 15%, the composite coating had good technological performance. The laser cladding was bonded metallurgically with the magnesium alloy substrate. Those ceramic particles were embedded by granular "particle" in the composite coating layer. Simultaneity, there were a lot of Mg-Al intermetallic and Mg2Si generated. In addition, it was found that little unmelted ceramic particles distributed in the composite coating. The average microhardness of the composite coatings was significantly improved to 225HVo.o5 compared to that of the Al-Si coating, which was 125HV0.05. The results showed that the wear resistance and corrosion resistance of the composite layer were considerably improved compared to the substrate because of the presence of Mg-Al metallic compounds, ceramic particles and grain refinement.Based on the above, a rapid melting method of laser heating and liquid nitrogen cooling was proposed. The hybrid microsturcture of nanocrystalline and amorphous on the surface of magnesium alloy were prepared successfully. The relevant mechanism was discussed in the aspects of solidification characterization and grain growth in thermodynamics. The maximum microhardness of the liquid nitrogen cooling layer reached to 148 HV0.05, which was about three times of that of the base material. The results of electrochemical corrosion showed that the corrosion potential of the melted layer cooled by liquid nitrogen was -1439mV. It shifted 26mV higher than the sample cooled in the air and 124mV higher than primitive magnesium alloys. Improved corrosion resistance can be achieved by expanding the solid solubility, forming amorphous phase, and prohibiting grain growth.更多还原