金属钨具有高熔点（3420?C）、高硬度、高热导以及良好的耐热和耐离子辐照性能，广泛用于核聚变、军事、电子等领域。但是钨在室温下表现为高脆性，使用传统的加工方法很难制备出复杂形状的钨部件。本文使用选区激光熔化(Selectiva laser melting, SLM)的方法开展钨增材制造研究，目的是解决复杂形状钨部件难加工的问题。针对SLM成形后钨块体中的残余孔洞和裂纹缺陷，本文首先通过粉体调控实现残余孔洞的控制，然后阐明了成形后块体中的微裂纹萌生和扩展机理，并通过合金化的方法初步实现裂纹控制。 针对残余孔洞缺陷，研究发现粉体性能对SLM成形后的残余孔洞缺陷有重要影响。在同样的激光参数下，使用不规则钨粉会引起严重的球化现象，导致成形后致密度不足；使用球形钨粉则可以得到连续的熔化道，抑制球化现象的产生。这是因为球形粉体具有更好的流动性、更高堆积密度和激光吸收率，从而更容易实现熔体铺展过程。经过工艺参数优化后，使用球形纯钨粉进行激光增材制造可以获得致密度为96.0%的纯钨块体。此外，使用喷雾造粒结合等离子体球化的方法可以将粒径2μm的钨粉制备成粒径约30μm的粉体，以满足增材制造要求。 在成形的致密钨块体中发现有网状裂纹，裂纹形成机理可以简述为纳米孔洞偏聚造成的晶界开裂。因为钨的熔点非常高，甚至高于钨氧化物的沸点（WO3沸点：1837?C, WO2沸点: 1730?C），所以在激光熔池内存在氧化物气泡。凝固过程中气泡被挤到晶界处，形成了晶界偏聚现象，降低了晶界强度。在SLM成形后残余内应力的作用下，纳米孔洞偏聚的晶界处成为裂纹萌生源和扩展通道。在逐层堆积成形三维结构的过程中，块体中的裂纹不断扩展并互相连接，最终形成网状结构，而且裂纹分布与微观组织有直接关联。 针对裂纹缺陷，研究发现向钨中添加少量钽再后进行激光增材制造可以明显降低裂纹密度（减少80%），主要原因是由于钨钽合金中出现亚微米尺度的晶内胞状组织。首先，胞状组织减少了纳米孔洞在晶界的偏聚，降低了裂纹的萌生几率；其次，胞状组织的出现使晶粒内部出现大量的位错，解决了钨中位错难形核的难题，提高了晶粒塑性。钨钽合金裂纹尖端的能量释放速率比纯钨高52%，即钨钽合金中的裂纹将会面临更大的扩展阻力。因此，钨钽合金中裂纹较少。
Possessing high melting point (3420 ?C), high hardness, high thermal conductivity, and excellent heat and ion radiation resistanceis, tungsten metal is widely used in nuclear fusion, military, electronical fields. However, it is difficult to prepare tungsten parts with complex shapes via conventional processing methods because tungsten exhibits high brittleness at room temperature. In this paper, the selective additive laser melting (SLM) method was applied to additively manufacture tungsten parts. The purpose was to solve the problem of difficult machining of complex-shaped tungsten parts. From the perstives of the residual pore and crack defects, this paper systematically studies the formation rule and mechanism of the two defects. We realizes the control of residual pores through powder regulation and obtains the initial control of cracks through the alloying method. For the residual pore defects, it was found that the powder properties have an important influence on the residual pore defects after SLM forming. Under the same laser parameters, the irregular tungsten powder can cause serious balling, resulting in insufficient density after forming. The use of spherical tungsten powder can produce continuous molten tracks and suppress the occurrence of balling phenomena. Spherical powders have better flowability, higher bulk density, and laser absorption, making it easier to achieve the melt spreading process. After optimization of the process parameters, the use of spherical pure tungsten powder can result in a density of 96.0%. In addition, the use of spray granulation combined with plasma spheroidizing method can prepare tungsten powder with a particle size of 2 μm into a powder with diameter of about 30 μm to meet the additive manufacturing requirements. The network-shaped cracks are observed in the built dense tungsten block. The crack formation mechanism can be summarized as the grain boundary cracking caused by the segregation of nanopores. Since the melting point of tungsten is very high, even higher than the boiling point of tungsten oxide WO3 boiling point: 1837 ?C, WO2 boiling point: 1730 ?C), there are oxide bubbles in the laser melting pool, and the bubbles are pushed to the grain boundary during solidification, which may degrad the strength grain boundary. Under the action of the residual internal stress after SLM forming, the grain boundary where the nanopores segregate becomes a source of crack initiation; therefore, the cracks are distributed along the grain boundary. During the layer-by-layer scanning process, the cracks continued to grow and connected with each other, finally forming a network structure. For crack defects, the use of tungsten-tantalum alloy for laser additive manufacturing can significantly reduce the crack density (80% reduction). The crack reduction is mainly due to the presence of intragranular cells in tungsten-tantalum alloys. First, cellular structures reduce the segregation of nanopores at grain boundaries, reducing the possibility of crack initiation. Moreover, a large number of dislocations appear within the grains in the cell structure, which solves the difficult nucleation of dislocations in the tungsten and improves the grain plasticity. The energy release rate of the tungsten-tantalum alloy crack tip is 52% higher than that of pure tungsten. In other words, cracks in the tungsten-tantalum alloy will meet higher growth resistance. As a result, the crack density in the tungsten-tantalum alloy is lower than that of pure tungsten.
