金属增材制造技术由于其独特的逐层增加制造理念，自问世之初，便受到广泛的关注。镍基高温合金作为一类中高温性能优异的合金，被广泛用于制造航空航天发动机的热端部件，而这些零件通常具有复杂的几何形状和减重的要求，由于金属增材制造技术在复杂零部件制造上有着得天独厚的优势，高温合金的增材制造一直是增材制造领域的重点之一。对于高温合金的送粉式激光增材制造过程，主要存在着两方面的问题，一是最终成形构件存在着的气孔、裂纹、低精度等问题，二是对成形构件的微观组织难以精确调控，尤其是对枝晶生长形态和晶粒尺寸的调控，针对上述的增材制造中存在的问题，本文主要进行了如下三方面的研究。
1. 通过进行不同工艺参数即激光功率、扫描速度和送粉率下的单道单层实验，尽可能地获得了激光送粉式增材制造中会出现的所有熔覆道类型。通过对得到的熔覆道几何形貌进行了三维的、量化的研究，提出了更加全面、准确的分类准则：在表面，提出了波纹间隔d和无量纲化平均粗糙度RaS来表征表面轮廓的稳定性；在纵截面，提出了无量纲化平均纵向粗糙度RaL来表征纵截面轮廓的稳定性；在横截面，提出了量化指标稀释率θ来表征成形的效率和熔覆道与基体的结合程度。依据提出的量化指标，可以将熔覆道分为5类：未熔、欠熔（RaS>110）、最佳（RaS<110、d~1μm、0.2<θ<0.7）、表面粗糙（d~100μm、RaL较高）、稀释率欠佳（θ<0.2或θ>0.7）。依据从中得到的最佳状态，构建出相应的工艺窗口，并对工艺参数对5种状态的影响进行了研究，研究表明，送粉率主要影响5种状态的分布，激光功率和扫描速度主要影响5种状态的分布范围，m/√v、P/√v可以很好地区分能量不足和能量充足两种状态。在工艺窗口内，对熔池尺寸和形状的影响因素进行了探讨。无量纲数ηP/((T_l-T_0)πρC_p √(αV_s r_0^3 ))（Di数）与无量纲化熔池面积A*呈现强线性正相关。对熔池形状而言，即使熔池面积相同，其形状差异较大，包括纺锤状、指状、帽沿状等，因为对熔池内部而言，有传导和对流两种机制起作用，同时还有活性元素对对流的影响，导致熔池的形状呈现不同形貌。
2. 在工艺窗口内，通过调控凝固条件，实现了通过工艺参数调控枝晶生长模式和晶粒尺寸的目标。通过控制光斑直径和功率密度来对熔池形状即固液界面形状进行调控，进而获得不同的枝晶生长模式。在低功率密度、大光斑直径的工艺条件下，熔池宽且浅，呈现纺锤状形貌，枝晶生长模式为典型的平面晶-柱状晶-等轴晶分层结构；当功率密度有所提高时，熔池变深、变宽，且带有两个锐利的拐点，枝晶生长由偏转的柱状晶区和断续的短轴柱状晶区组成；在高功率密度、小光斑直径下，熔池窄且深，呈现指状形貌，枝晶生长模式分为四个区域：随机取向晶粒组成的等轴晶区、近乎平行生长且沿中线对称的(110)柱状晶区、往扫描方向偏转的柱状晶区、平行于扫描方向生长的长轴柱状晶区。随着功率密度的提高，熔池内的微观组织织构发生了由强度强的(001)向强度弱的(110)转变的趋势。固液界面的曲率和固液界面处的G/R是决定枝晶生长形态的关键因素。随后，对包括扫描速度、激光功率、送粉率在内的工艺参数对晶粒尺寸的影响进行了实验和模拟研究。研究表明，随着扫描速度从2mm/s增加到10mm/s，冷速(G×R)的增加，导致晶粒尺寸从8.7μm细化到4.7μm；随着激光功率和送粉率的增加，冷速下降，导致晶粒变大。对枝晶生长模式而言，熔池形状是其决定性因素，而对熔池形状而言，功率密度又是影响其的关键因素；对晶粒尺寸而言，冷速是其决定性因素，而在影响冷速的主要参数中，扫描速度的作用最大。这表明可以通过不同的工艺参数控制同时调控枝晶生长模式和晶粒尺寸。
3. 在工艺窗口内，针对不同的枝晶生长模式，设计了单向和往复两种扫描方案，成形出了块体试样，进行了粗糙度测量，并在TD和SD方向进行了拉伸实验。对粗糙度而言，影响表面粗糙度的两大因素为：熔覆道的搭接高度和部分熔化粉末颗粒的粘接，这两种现象可以通过选择小曲率熔覆道和提高粉末捕获效率来进行缓解。对拉伸性能而言，通过组织调控，本文得到了拉伸强度优于合金锻造性能的试样，抗拉强度为1012.4MPa，延伸率为30.5%，对其断口进行分析，宏观断口由纤维区、放射区、剪切唇区组成，在纤维区和放射区，其断口微观形貌由小、深的韧窝组成，为典型的塑性断裂。通过对提前断裂的试样进行分析发现，气孔和裂纹的存在极大程度上恶化了力学性能，适当提高能量输入可以缓解未熔合气孔的出现，通过轨迹设计增加散热时间（如同向轨迹方案的散热时间就比往复轨迹方案时间长）可以缓解裂纹的产生。
本文通过对熔覆道三维几何轮廓进行研究，提出了更加全面、准确、量化的判别准则，构建了工艺窗口，并通过对工艺参数与状态之间的关联研究，提出了调节方案。通过无量纲化的方法，对影响熔池尺寸的影响因素进行了探讨。通过对熔池形状和凝固参数的调节，实现了对枝晶生长模式和晶粒尺寸的调控，最终获得了性能优异的增材制造试样。通过对增材制造过程中“工艺-组织-性能”的研究，对工程实际中，成形无缺陷、高性能的增材制造构件具有重要指导意义。

The metal additive manufacturing is based on a novel philosophy, which is the incremental layer-by-layer manufacturing method. The extensive attention has been paid to this novel manufacturing technique since its inception. Nickel-based superalloys are widely used to manufacture hot-end components of aerospace engines due to its excellent medium-temperature and high-temperature performance and these parts usually have complex geometries and require weight reduction. Due to its advantage over fabricating components with complex shapes, metal additive manufacturing of superalloys has been one of the focuses in this field. For laser powder-feed additive manufacturing of superalloys, there are mainly two problems. One is the defects such as pores, cracks and low precision, and the other is that it is difficult to precisely control the dendrite growth morphology and grain size of the fabricated components. In order to solve the above-mentioned problems in additive manufacturing, this paper mainly carried out from the following three aspects. 
1. Single scan track experiments were conducted under various combinations of processing parameters including laser power, scanning speed and powder feeding rate. Scan tracks had been thoroughly investigated through three-dimensional geometrical characteristics including surface, longitudinal section and transverse section, which were correlated to each other in showing track morphological characteristics in directed energy deposition. Several quantitative indexes, which were dimensionless mean roughness RaS and ripples’ interval d in surface, dimensional roughness in longitudinal section RaL and dilution rate θ in transverse section, were used to evaluate the forming quality. Five states which were no fusion, lack of fusion (RaS>110), optimum (RaS<110, d~1μm, 0.2<θ<0.7), rough surface (d~100μm, high RaL) and undesired dilution rate (θ<0.2 or θ>0.7) were distinguished based on the proposed quantitative indexes. A processing window based on these indexes was proposed for the laser powder-fed additive manufacturing of Inconel alloys. It is found that powder feeding rate affects the distribution of these five states and laser power, scanning speed affect the range of these five states. It was found that m/√v and P/√v could distinguish two states which were insufficient energy and sufficient energy well. In the processing window, it is found that the dimensionless number ηP/((T_l-T_0)πρC_p √(αV_s r_0^3 ))   (Di number) had a strong linear positive correlation with the dimensionless melt pool area A*. Although the melt pool area had a linear relationship with the Di number, the shape of the melt pool was quite different even if the melt pool area was the same, including spindle shape, finger shape, brim shape, etc. There were two mechanisms in the melt pool which were thermal conduction and convection as well as the effect of active elements on convection, resulting in the different melt pool morphologies.
2. Within the processing window, by adjusting the solidification conditions, the goal of controlling the dendrite growth mode and grain size through processing parameters was achieved. Here, small spot diameter was used for DED-L of Ni-based alloys and various crystal growth patterns were obtained by changing power density. Samples processed under low power density were shown to exhibit wide and shallow spindle-like melt pool, along with apparently hierarchical planar-columnar-equiaxed microstructure. While samples prepared under relatively high power density were shown to exhibit narrower and deeper melt pool with two sharp turning points, exhibiting inclined columnar grains and several discontinuous central axial columnar crystals. When highest power density and small spot diameter were applied, crystal growth with weak texture was achieved. Under this deep and narrow melt pool, the crystal growth could be separated into four regions: nearly-equiaxed grains with random grain orientations; horizontally symmetrically grown crystal grains; axial columnar in the center; columnar grains grew approximately vertical to the boundary of melt pool. Correlations were investigated between melt pool shape, solidification parameters and microstructure. With the increase of power density, the texture in the melt pool had a tendency to change from strong (001) crystal direction to weak (110) crystal direction. Further analysis of the melt pool shape and solidification parameters showed that the curvature of the solid-liquid interface and the G/R at the solid-liquid interface were key factors determining the growth morphology of dendrites. In order to control grain size in laser powder-fed additive manufacturing, experiments and simulations were conducted to investigate the effects of processing parameters including scanning speed, laser power and powder feeding rate on grain size of the solidified track. The experimental and simulated results indicated that cooling rate increased and grain size decreased from 8.7 μm to 4.7 μm with the increase of scanning speed from 2 mm/s to 10 mm/s. Contrarily, cooling rate decreased and grain size increased with the increase of laser power and powder feeding rate. As for the precisely control of microstructure in additive manufacturing, it is found that the shape of the melt pool is the decisive factor for the dendrite growth mode, and the power density is the key influencing factor for the morphology of the melt pool; for the grain size, the cooling rate is the decisive factor, and among the main parameters affecting the cooling rate, the scanning speed has the greatest effect. This gives researchers the possibility to simultaneously control the dendrite growth pattern and grain size by controlling different processing parameters.
3. By studying the state of scan tracks in additive manufacturing, potential defects could be eliminated. By studying the influence of melt pool shape and solidification parameters on the microstructure, the microstructure could be precisely regulated. Then, the properties in "process-microstructure-properties" relationship could be studied. In this paper, in the processing window, according to different dendrite growth modes, unidirectional and reciprocating scanning strategies were designed and the block samples were fabricated. The roughness measurement and tensile experiments in TD and SD directions were carried out. As for the roughness tests, the results showed that there were two major factors affecting the surface roughness which were the hatch distance of scan tracks and the bonding of partially melted powder particles. These two phenomena could be alleviated by selecting the scan track with small curvature and improving the powder catchment efficiency. As for tensile properties, through microstructure control, the obtained samples had comparable or even better mechanical performance than conventionally forged samples. The obtained highest tensile strength was 1012.4MPa and the elongation was 30.5%. The macroscopic fracture of this sample was composed of fiber zone, radial zone and shear lip zone. In the fiber zone and radial zone, the microscopic topology of the fracture was composed of small and deep dimples, which was typical signs for plastic fracture. Through the analysis of samples which were fractured prematurely, it was found that the existence of pores and cracks had greatly deteriorated the mechanical properties. Appropriately increasing the energy input could alleviate the emergence of pores caused by lack of fusion. Increasing the heat dissipation time through the scanning path design (as the heat dissipation time of unidirectional scanning strategy was longer than that of the reciprocating scanning strategy) could alleviate the generation of cracks.
In this paper, a more comprehensive, more accurate and quantitative criterion was proposed by studying the three-dimensional geometrical characteristics of scan tracks. The processing window was constructed, and the regulation strategy was put forward through the investigation of the relationship between processing parameters and five states. The factors affecting the size of melt pool were discussed through dimensionless analysis. By controlling the shape of melt pool and solidification parameters, the dendritic growth mode and grain size were regulation, and finally the samples with excellent properties were fabricated through laser powder-fed additive manufacturing. The research on “process-microstructure-properties” in laser powder-fed additive manufacturing can be a guide for forming defect-free components with excellent performance in engineering.