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1、 华 中 科 技 大 学 博 士 学 位 论 文 I摘 要 用选择性激光熔化技术能直接成形金属零件，它们可被应用于生物医学、航空航天等领域。与国外相比，我国对该技术的研究起步较晚，还存在较大的差距。选择性激光熔化成形金属零件过程中，球化、裂纹、变形、翘曲、脱层等的形成机理及其预防措施是国内外研究者需要攻克的关键技术瓶颈问题。为了解决这些问题，本文作者参与研制选择性激光熔化装备，并研究选择性激光熔化金属粉末（304L、316L）的成形特征，分析成形过程的热应力、热应变，利用 X 射线衍射（XRD） 、金相显微（OM） 、电镜扫描（SEM） 、显微硬度等方法，分析成形件的组织结构和机械性能。为提高
2、零件成形效率及机械性能，提出并研究选择性激光熔化技术与热等静压技术的复合成形技术。主要研究内容及结论如下： 在择性激光熔化装备研制方面，选择波长为 1064nm、1090nm 的激光器，即固体半导体 Nd- YAG、光纤激光器。采用动态聚焦和静态聚焦方式，使激光束聚焦焦点处激光功率密度超过 106W/cm2。将装备做成较高的真空度，使之能成形钛合金和高温合金零件。 在研究金属粉末的选择性激光熔化的成形性方面，重点研究了 304L、316L不锈钢粉末的成形性。研究表明：316L 的成形性较 304L 的好；含碳量低的合金粉末成形性比含碳量高的成形性好；气雾化粉末比水雾化粉末的成形性好。 在选择性
3、激光熔化金属粉末的成形工艺方面，本文分层次进行了实验并优化工艺参数。首先，表达不同工艺参数下的单线（道）扫描成形轨迹特征，确定粉末的成形性；第二，进行单层多道扫描，确定完全致密度所需的扫描策略、工艺参数；最后，实施块体的成形，确定成形件的致密度与扫描策略、层厚等工艺参数之间的关系。扫描策略是影响致密度的一个关键因素，在已有的扫描策略（分组变向、轮廓等距偏移）基础上，提出新的扫描策略- - 变扫描矢长分块变向扫描、回填变向扫描（跳转变向扫描） 。实验研究表明：在相同条件下回填变向扫描策略比其它扫描策略更能有效的提高致密度。较优工艺参数为：激光功率 98W、扫描速度范围为华 中 科 技 大 学 博
4、 士 学 位 论 文 II30- 100mm/s、扫描间距 0.06mm、扫描层厚为 0.06- 0.08mm。 在成形过程中的变形、裂纹、脱层等的研究方面，应用 ANSYS 模拟并分析了成形过程温度场、热应变场、热应力场。分析结果表明：成形件的残余拉应力较大；在基板与成形件接触处，出现应力集中。为了减小成形件的残余拉应力，可采取预热基板、重扫描策略、退火后处理的方法。 在研究选择性激光熔化与热等静压（SLM/HIP）的复合成形技术方面，首先采用选择性激光熔化技术成形完全致密零件外壳或包套，接着经热等静压处理成形完全致密零件。通过对该复合成形技术的初步实验研究，结果表明：要求成形包套无任何缺陷
5、（包套不存在相通的气隙）且保持包套内的高真空度（10- 5Torr） 。 通过本论文的系统研究，解决了选择性激光熔化技术所碰到的球化、翘曲、脱层等问题，实现该技术直接成形完全致密度金属件，为该技术深入发展奠定了一定的基础。 关键词：选择性激光熔化; 扫描策略; 温度场; 应变场; 应力场; 热等静压 华 中 科 技 大 学 博 士 学 位 论 文 IIIAbstract Metal parts could be directly manufactured with the selective laser melting (SLM) technology. They are applied to
6、 biomedical field, aerospace field, etc. There are wide gaps in the SLM technology home, compared with abroad. In the SLM processes, some problems, such as balling effect, crack, curl, distortion, delamination and low processing efficient, might occur. To solve them, were studied the researches on t
7、he SLM machines, formabilities of metal powders such as 304L and 316L, optimum processing parameters, new scan strategies, analysis of temperature, residual stresses and residual strain fields, microstructures by XRD, optical micrograph, and SEM, mechanical properties of SLM parts by microhardness,
8、and densification of SLM hollow parts by hot isostatic pressing (HIP) for higher efficient forming processing and mechanical properties. These research results are given as follows. To develop SLM machines, the Nd- YAG laser with 1064nm wavelength and fiber laser with 1090nm wavelength were utilized
9、. To gain the laser power density of more than 106W/cm2, the focal diameter has to be small enough to the order of magnitude of micron. Two kinds of focusing means such as dynamic focusing and static focusing were used. to avoid the oxidization and nitride of metal, the building chambers of the mach
10、ines have to be airtight or protected by inert gases (argon, or helium) or in high vacuum (10- 5Torr). Thus the developed SLM machines were equipped with vacuum pumps. Powders are one of important factors having great impact on successfully forming fully dense metal parts wih the SLM technology. The
11、 single metal element and single pre- alloyed powders are better for the SLM processes. From the studies, the gas- atomized 316L powders have better formability than the waer- atomized 304L ones, and low- carbon powders do better than high- carbon ones. Thereupon the gas- atomized 316L stainless ste
12、el powder, whose size distribution of lower 20 microns is over 50% volume, is more suitable for the HRPM- II SLM machine. A special program on the SLM processing was developed, and then optimum parameters were obtained. Firstly, the features of single tracks built on a thin layer powder deposited on
13、 a substrate are given in detail. From these features for processing parameters, the powder formability and track attributes can be gained. Secondly, thin layer multi- track builds are tested, and then the optimum scan space and scan strategies can be optimized to 华 中 科 技 大 学 博 士 学 位 论 文 IVget highe
14、r surface finish quality for the full density. Thirdly, blocks are formed with parameters, and the parts densities are related to layer distances, scan strategies and other parameters. The scan strategy is one important factor on the full density of metal parts. new scan strategies such as consecuti
15、ve sub- sector scan strategy with adjustable scan length, refilling scan strategy with bi- directional scan, and spiral scan strategies were developed, based on sub- area scan strategy, contour equidistant offsetting scan strategy. The tests results show that the refilling scan strategy with bi- dir
16、ectional scan is excellent to directly manufacture fully dense parts. To directly form dense parts with the SLM technology, the above presented program could be used to optimize parameters. The obtained optimum parameters are 98W laser power, 30- 100mm/s scan speed, 0.06mm scan space, lower than 0.08mm layer distance. The temperature field, strain field and stress field in the SLM processing were simulated with finite element methods for the ANSYS software. From the simulation results, ther
