【作者】 张丹青；

【导师】 魏伯康； 蔡启舟；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2011， 博士

【摘要】 难熔金属及其合金具有熔点高、高温强度高、低蒸汽压、低的膨胀系数以及在许多介质中的耐蚀性好等一系列优良特性,广泛应用于武器装备、医疗器械和通讯发射装备等领域。但是难熔金属的成形性能差,限制了其应用范围的扩大。目前,难熔金属大多采用粉末冶金成形,这种成形工艺需要昂贵的工装模具、工艺过程复杂,难以成形三维结构复杂的零件。因此,开发难熔金属的先进成形技术已成为研究热点之一。选择性激光熔化成形作为一种新型的成形技术具有制造过程柔性程度高、所制造的零件具有优异的力学性能和化学性能、可实现组分连续变化的梯度功能材料的制造、产品的尺寸大小和复杂程度对加工难度影响很小等特点,因此,选择性激光熔化成形技术在难熔金属成形中具有广阔的应用前景。本文以难熔金属钨及钨合金为研究对象,采用选择性激光熔化成形技术制备钨及钨合金试样,系统地研究了工艺参数(激光功率、扫描层厚、扫描速度、扫描间隔等)对钨及钨合金成形性和微观组织的影响；探讨了激光熔凝过程中的组织转变和生长机理。通过强化烧结改善钨及钨合金的成形性能,研究了添加稀土氧化物对钨合金微观组织的影响及其作用机制。此外,探索了难熔金属Mo和Nb的选择性激光熔化成形工艺。取得了如下研究结果：采用选择性激光熔化成形了纯W的试样,成形过程在高能量密度的激光束作用下发生了部分颗粒的熔化,其成形机制为液相烧结机制。激光熔化-凝固成形纯W试样表面烧结道出现了择优取向生长的针状组织,随着激光功率的增加针状组织尺寸逐渐减小。烧结试样的显微硬度值随扫描层数的增加呈下降趋势,当扫描层厚由第一层增加至第六层时,试样的显微硬度由826HV降低为353HV。研究了添加Ni对W基金属粉末激光熔化成形的影响规律。结果表明,选择性激光熔化W-Ni合金的成形机制为激光能量输入引起Ni颗粒及部分W颗粒熔化-凝固粘结未熔W颗粒的液相烧结过程。在相同激光作用条件下,Ni含量为10wt.%、20wt.%、40wt.%的烧结试样的微观组织分别为条状、枝晶状和蜂窝状,Ni含量的增加能有效降低熔体粘度,提高烧结体组织的均匀性。在分析W-Ni合金烧结试样显微硬度变化的基础上,考虑由于松散粉末收缩引起的实际粉层变化,建立了粉层收缩模型。基于该模型的分析发现,成形过程中实际粉层厚度随加工层数的变化在开始5层内的变化剧烈；随着加工层数的增加,粉末的实际厚度趋于定值,这个值可以由切片厚度和相对松装密度的比值来确定。基于实验数据建立W-Ni-Cu合金选择性激光熔化成形的工艺窗口。基于该工艺窗口选择工艺参数,成形的W-Ni-Cu合金零件表面平整、没有明显的裂纹与孔洞缺陷。在较高的激光能量输入时,W-Ni-Cu合金的成形机制由传统的液相烧结转变为液相烧结与W颗粒熔化/凝固的综合作用机制。研究了稀土氧化物对钨基合金的选择性激光熔化成形的影响。结果表明,在W-Ni-Cu合金中添加1.0wt.%La2O3,具有改善固液润湿性、提高形核率、细化晶粒和净化晶界的作用,并能抑制微裂纹的形成,显著改善W-Ni-Cu合金粉体激光熔化成形性能、提高烧结体的显微硬度。采用有限元法建立了选择性激光熔化成形过程温度场的三维模型。以90W-7Ni-3Fe合金体系为例,结合实验研究获得了不同工艺参数下温度场的变化及烧结体的组织演变规律与成形机制。在激光功率一定时,当扫描速度较高(110mm/s)时,W-Ni-Fe合金的组织为液相凝固粘结未熔化颗粒,其成形机制为液相烧结。随着扫描速度的减小,组织中的枝品逐渐增加,说明熔池温度升高,钨颗粒发生了熔化。当扫描速度为20mm/s,扫描间隔为0.05mm时,合金的组织为柱状品,其成形机制为合金粉末熔化-凝固的成形机制。研究了难熔金属Mo和Nb的选择性激光熔化成形工艺及组织演化过程。研究发现,成形试样中出现了Mo-Ni固溶体相,Mo-Ni固溶相有利于促进烧结的致密化过程,成形试样显微组织中粉末颗粒和粘结相分布较均匀。相对于纯Mo的成形试样,90Mo-Ni成形试样的表面孔隙率明显下降。选择性激光熔化Nb粉末形成的显微组织为呈方向性、规则排列的层片状组织。更多还原

【Abstract】 Because of its good performances such as high melting point, high temperature strength, low vapor pressures, low expansion coefficient and outstanding corrosion resistance, refractory alloys has been one of the irreplaceable materials which can be widely used in industry fabrication, military field, medical device and communication equipment. However, due to refractory alloy’s bad forming characteristics, the scope of its application was limited. Until now, refractory alloys were usually fabricated through powder metallurgy technology which needs expensive and dedicated tools. Therefore, the developments on new forming methods for refractory alloys become one of the most important focuses. Selective laser melting (SLM), as a new forming method, can directly fabricate parts with complex shapes by melting loose metal powder. The SLMed final components are excellent in mechanical and chemical property which can meet the requirements for direct usage. Due to its flexibility in materials and shapes, SLM method exhibits a great potential for processing refractory alloys’parts with complex shapes.In this paper, selective melting technique was chosen to fabricate refractory alloys (tungsten and its alloys). The effects of processing parameters (such as laser power, layer thickness, scan velocity, scan interval) on forming characteristics and microstructure were systematically investigated. The microstructure transformation and forming mechanism in laser melting-solidification process were also studied. Enhanced sintering technology was applied to optimize the forming property rare earth oxide was added in order to obtain the optimal performance of final parts. Further more, the forming process of refractory alloys (Mo, Nb) was also addressed. The overall results were as follows.Using W as raw material, SLM method was applied to forming three dimensional specimens. Part of W metal particles melted under large energy density of laser beam with the mechanism of liquid phase sintering. The microstructure of SLMed W specimens appeared the acicular dendrite growing along the preferred growth direction. With the enlargement of laser power import, the dimension of acicular dendrite reduced accordingly. When the layer number increased from one to six, the micro hardness of SLMed specimens was decreased from 826HV to 353HV.The effective principle of adding Ni element during SLM process was studied. The forming mechanisms of W-Ni alloys are coexisted liquid phase sintering and melting/solidification of part W particles by laser energy input. Under the equal laser condition, the microstructure of SLMed specimens with the Ni element content of 10wt.%, 20wt.%,40wt.% showed bar shape, dendrites and alveolar, separately. Adding Ni element can decrease the melt viscosity and increase the homogenize microstructure.On the basis of the variation of W-Ni hardness value, a powder shrinkage model can be established according to the thickness of powder layer shrinkage and forming layer number. The variation of real layer thickness and the relevant mathematical explanations are discussed. The results show that the total shrinkage of metal powder layer sharply increases in the initial five layers, and then reaches to a plateau value with the increased processing layers. This value is defined by the ratio of sliced layer thickness (h) to relative density (k) during selective laser melting process.The processing window was established according to experiments of W-Ni-Cu alloys Based on the processing window, final parts was fabricated showing harmonious sintering surface with no cracking and obvious porosity. The sintering metallurgical mechanism transform from LPS to melting/solidification of W particles with the enhancement of laser energy input.To further optimize the microstructure and forming property, rare earth oxide La2O3 was added to W-Ni-Cu alloy powder. Suitable content of La2O3(0.5wt.%-1.0wt.%) can enlarge the laser absorption of powder system, refine the microstructure, decrease roughness of SLMed specimens, reduce microcrack, leading to a better forming characteristics and a higher micro hardness value. Using 90W-7Ni-3Fe as raw material, finite element analysis method was applied to analysis the three dimension temperature field of powder bed under different forming parameters. Experiments on different processing parameters were carried out, and the change of temperature field and microstructure evolution was investigated. Fixing the laser power as a constant value, with a high laser scan velocity(110mm/s), the microstructure of W-Ni-Fe alloys showed un-melted W particles bonding of melting Ni, Fe powder through LPS. Decreasing the laser scan velocity, an increase of dendrites can be obtained, showing an enlargement of molten pool temperature with melting of W particles. Under the laser scan velocity of 20mm/s combined with scan interval of 0.05mm, the microstructure of specimens showed columnar grains with the melting-solidification forming mechanism.The SLM forming process and microstructure evolution of Mo, Nb was investigated. Mo-Ni solid solutions appeared in SLMed specimens with homogenized particulate dispersion and bonding coherence, which were benefit for the density of sintering specimens. Compared to SLMed Mo metal powder, the SLMed mixture 90Mo-Ni powder can fabricate higher relative density and lower surface porosity under the equal laser condition. The microstructure of SLMed Nb specimens showed layered structure with regular arrangement.更多还原