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混合惰性气氛保护下激光沉积制造Ti-6Al-4V力学性能研究
Alternative TitleResearch on Properties of Ti-6Al-4V Manufactured by Laser Melting Deposition under the Protection of Mixed Inert Gas
候静宇
Thesis Advisor李正阳
2021-11-17
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline工程力学
Keyword激光熔化沉积技术 惰性保护气体 氧含量 无量纲工艺图 力学性能
Abstract

Ti-6Al-4V合金由于其优异的综合性能成为航空航天,汽车和生物医学行业中最常用的钛合金。传统钛合金制造技术存在生产周期长,材料利用率低,制造成本高,难以制造复杂形状零件等问题。激光沉积技术是一种将数字化模型逐层构建成实体零件的增材制造技术,已被用于生产净成形钛合金零件并降低制造成本和扩大合金的应用。目前传统激光沉积制造钛合金一般使用惰性气氛加工室,一定程度上降低了零件的加工效率,鉴于此本文提出基于随形保护罩的混合惰性气体保护的激光沉积制造方法,对Ti-6Al-4V展开实验研究,分析钛合金氧化行为,验证保护罩的保护效果。同时提出通过无量纲工艺图指导工艺参数选择,探索工艺参数对合金组织和力学性能的影响规律,发掘激光沉积制造高强度和高塑性零件的潜力。本文主要研究工作如下:

首先搭建了惰性气体保护罩装置,采用Fluent软件模拟了保护罩内氩气和氦气的流动对罩内氧含量影响,模拟结果说明混合惰性气体可迅速有效地降低罩内氧含量,静态下约11min氧含量降至125ppm,对应的理论氮含量降至500ppm

其次基于热力学原理研究Ti-6Al-4V氧化膜的产生机理,分析合金中各元素反应的条件和顺序。基于氧化动力学研究影响钛合金氧化速度的主要因素。结果表明:熔池温度低于2500K时主要发生氧化反应生成TiO2,氧化速度的控制因素是氧离子的扩散速度,氮化反应处于次要地位。激光沉积制造钛合金的化学分析结果显示平均增氧量为548ppm,平均增氮量为230ppm,符合理论计算预期。

然后通过量纲分析法推导得到无量纲等效能量密度E0*,进一步利用文献中获取的数据构建激光沉积工艺的无量纲工艺图,为比较各类型的增材制造方法、材料体系、工艺参数提供统一的框架,对参数的选取具有指导意义。对主要参数对成形的影响规律进行研究,结果表明:激光功率低于1000W时粉末易球化,恶化零件性能;搭接率在40%时具有较好的成形质量;送粉量为4.44/ming时具有较高的成形精度,最大粉末利用率为60%

在无量纲工艺图窗口内进行正交试验设计,固定其他参数,对激光功率、扫描速率和扫描间距进行优化,并研究参数对零件组织和性能的影响规律。激光沉积制造钛合金零件的沉积态组织为贯穿多层外延生长的柱状晶,晶粒主轴垂直于基底,略向扫描速率方向偏移。E0*增大,柱状晶宽度有增大趋势。柱状晶内部是针状αʹ马氏体和网篮状组织。显微硬度测试结果显示试样E0*=2.34时获得最大显微硬度为404.3HV。零件致密度呈现出E0*增大而先增大后略减小趋势,E0*=3.64时获得最大致密度为99.62%。拉伸测试结果显示所有试样的抗拉强度均达到锻件水平,但延伸率区别较大。同时本文还实现了沉积态试样性能达到文献中材料热处理后的性能水平。E0*=3.74时获得最佳力学性能,显微硬度为391.7HV,屈服强度为890MPa,抗拉强度为963MPa,延伸率为13.4%

综上,本文采用混合惰性气体保护方法进行激光沉积制造Ti-6Al-4V,成功制备出低氮氧含量的零件,具有直接制造零件和现场修复受损零件的潜力。通过无量纲工艺图和正交设计的对工艺参数进行优化,获得了性能可与锻件媲美的零件,对高强度高塑性零件的制造具有重要指导意义。

Other Abstract

Ti-6Al-4V is the most commonly used titanium alloy in the aerospace, aircraft, automotive and biomedical industries because of its excellent properties. Traditional manufacturing processes always inevitable result in material waste, high cost, long time to market and low geometries complexity. Laser melting deposition uses data contained in a 3D model to build up components by repeated layer deposition, and has been used to produce net shape titanium components by reducing the cost of fabrication and broadening the applications of this alloy. To limit thermal oxidation, conventional laser melting deposition processing is carried out under argon atmosphere within chambers, which hampers the capacity of manufacturing large parts. A new laser melting deposition processing with mixed inert gas protection was proposed to manufacture Ti-6Al-4V parts, and titanium alloys oxidation behavior was studied, verifying the protective effect of the protective shield. The normalized processing diagram was constructed to guide the selection of process parameters which was helpful to explore the influence of process parameters on the microstructure and mechanical properties of the alloy and discover the potential of laser melting deposition to manufacture high-strength and high-plastic parts. This paper is mainly as follows:

Firstly, the inert gas protective shield was built. The influence of flow of argon and helium on oxygen content within the shield was simulated by the Fluent software. The simulated results showed that the oxygen levels fell to 125ppm within 11 minutes, and the N2 levels fell to 500ppm at the same time.

Secondly, the formation mechanism of Ti-6Al-4V oxide film was studied, and the conditions of reactions were determined according to thermodynamics. Based on oxidation kinetics, the main factors affecting the oxidation rate were analyzed. The results revealed that TiO2 was the stable oxidation products when the temperature of melt pool was lower than 2500K, and the controlling factor of the oxidation rate was the diffusion rate of oxygen ions, and the nitridation was a secondary concern. The chemical composition analysis results of the as-built Ti-6Al-4V specimens showed that the average oxygen increment was 548ppm and the average nitrogen increment was 230 ppm, which was matched with expectations.

Then the normalized equivalent energy density E0* and a set of dimensionless processing parameters were derived. Using the data obtained in literatures to construct the normalized processing map of laser melting deposition provided a practical framework for comparing a range of additive manufacturing platforms, alloys and processing parameters. The influence of the main parameters on manufacturing was studied. The results showed that balling phenomenon occurred when laser power was below 1000W, which deteriorated the properties of the parts; the forming quality reached the best when the overlap was 40%; fine forming accuracy was achieved when the powder feeding rate is 4.44 g/min, and the maximum powder utilization rate was 60%.

Orthogonal design was carried out to optimize laser power, scanning velocity and hatching space distance. The influence of parameters on the microstructure and mechanical properties of parts was studied. The macrostructure of the as-built titanium alloy was columnar grains grown epitaxially across multi-layer, whose main axis was perpendicular to the substrate and slightly shifted to the direction of the scanning direction. As E0* increased, the columnar grains width tended to increase. The microstructure of the columnar grains was acicular martensite and basket-weave structure. The microhardness test results showed that the microhardness reached a maximum value of 404.3HV when E0* =2.34. The density of parts increased at first with the increase of E0* and then slightly decreased. When E0*=3.64, the density reached a maximum value of 99.62%. The results of the tensile test showed that the strength of all samples reached the level of forgings, but the elongation was scattered. The ultimate tensile strength and elongation of as-built samples made by LMD were found to be the same as those after HIP in the literature. The microhardness, yield strength, ultimate strength and elongation reached a maximum value of 391.7HV, 890MPa, 963MPa, 13.4% when E0*=3.74.

In summary, this paper proposed a method of manufacturing Ti-6Al-4V under mixed inert gas protection, and successfully fabricated parts with low nitrogen and oxygen content, and proved the potential to directly manufacture large parts and repair damaged parts on site. By optimizing process parameters through the dimensionless processing map and orthogonal design, components with performance comparable to forgings were obtained, which had an important significance for the manufacturing of high-strength and high-plasticity parts.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/87830
Collection先进制造工艺力学实验室
Recommended Citation
GB/T 7714
候静宇. 混合惰性气氛保护下激光沉积制造Ti-6Al-4V力学性能研究[D]. 北京. 中国科学院大学,2021.
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