Microstructure and dynamic fracture behaviors of 17-4PH stainless steel fabricated by selective laser melting
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摘要: 通过激光选区熔化(selective laser melting, SLM)技术制备了17-4PH不锈钢，采用电子背散射衍射(electron backscattered diffraction, EBSD)和透射电子显微镜(transmission electron microscope, TEM)等方法对沉积态和固溶态试样微观组织结构进行了分析. 通过示波冲击试验确定了裂纹萌生扩展的特征阶段和动态裂纹扩展阻力曲线(J−R曲线)，研究了微观组织与动态断裂性能之间的关系. 结果表明，沉积态试样主要由<100>择优且沿增材方向拉长的δ铁素体柱状晶、取向随机的细小马氏体，以及少量奥氏体组成，不同截面具有显著的组织各向异性；大尺寸δ铁素体柱状晶与细小晶粒的结合面作为薄弱环节，使其脆性增加，J−R曲线的撕裂模量较低，以准解理方式断裂. 固溶热处理明显弱化组织各向异性，微观组织由尺寸细小、均匀的马氏体组成，其冲击吸收能量提升1倍，动态断裂韧性优良，属于韧性断裂. 大尺寸δ铁素体柱状晶与周围细小马氏体晶粒界面结合较弱是沉积态17-4PH不锈钢动态断裂性能较差的主要原因.Abstract: 17-4PH stainless steel was fabricated by selective laser melting (SLM). The microstructure of the as-built and solution heat treated 17-4PH was analyzed by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The relationship between microstructure and dynamic fracture behavior was investigated by performing instrumented impact test. Absorbed impact energies related to crack initiation, stable and unstable propagation were calculated and the dynamic J−R curves were estimated.The results demonstrate that as-built 17-4PH stainless steel mainly consists of coarse columnar δ ferrite grains growing along the building direction with <100> texture and fine martensitic grains with random orientation. A small amount of austinite can also be found in the as-built sample. As-built 17-4PH stainless steel displays low resistance to crack initiation and propagation, resulting in marginally rising J−R curve and quasi-cleavage fracture. After solution heat treatment, the retained ferrite transforms into martensite and microstructural anisotropy can be eliminated. The impact toughness is 1 times higher than that in as-built conditions and the dynamic J−R curve rises steeply, indicating superior dynamic mechanical properties. Fracture surfaces revealed that the inferior dynamic fracture toughness of as-built 17-4PH stainless steel can be attributed to the weak boundaries between the coarse δ ferrite grains and surrounding fine martensite grains.
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Keywords:
- selective laser melting /
- stainless steel /
- microstructure /
- mechanical properties
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图 2 沉积态和固溶态17-4PH不锈钢的XRD图谱
Figure 2. XRD patterns of as-built and solution heat treated 17-4PH stainless steels
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图 3 SLM成形17-4PH不锈钢的BS值统计信息
Figure 3. Statistics of BS obtained from 17-4PH stainless steels fabricated by SLM
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图 4 沉积态SLM成形17-4PH不锈钢xOy和yOz面BS图
Figure 4. BS maps of as-built 17-4PH stainless steel fabricated by SLM. (a) xOy plane; (b) yOz plane
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图 5 沉积态SLM成形17-4PH不锈钢TEM明场像与SAED图
Figure 5. TEM bright-field image of as-built 17-4PH stainless steel fabricated by SLM and SAED patterns
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图 6 SLM成形17-4PH不锈钢的反极图
Figure 6. Inverse pole figures of 17-4PH stainless steel fabricated by SLM. (a) xOy plane of as-built sample; (b) yOz plane of as-built sample; (c) xOy plane of solution heat treated sample; (d) yOz plane of solution heat treated sample
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图 7 SLM成形17-4PH不锈钢极图
Figure 7. Pole figures of 17-4PH stainless steels fabricated by SLM. (a) ferrite of as-built 17-4PH sample; (b) martensite of as-built 17-4PH sample; (c) martensite of solution heat treated 17-4PH sample
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图 8 示波冲击过程中断裂不同阶段与柔度变化率法示意图
Figure 8. Schematic diagram of different stages of fracture during an instrumented impact test and variation of compliance changing rate
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图 10 SLM成形17-4PH不锈钢的J−R曲线
Figure 10. Dynamic J−R curves of 17-4PH stainless steels fabricated by SLM
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图 11 SLM成形17-4PH不锈钢冲击断口形貌
Figure 11. Fracture surfaces of 17-4PH stainless steels fabricated by SLM. (a) as-built sample; (b) solution heat treated sample
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表 1 17-4PH不锈钢粉末的化学成分(质量分数，%)
Table 1 Chemical compositions of 17-4PH stainless steel powder
C Cr Ni Cu Nb Si Mn N Fe 0.011 15.92 4.66 4.2 0.30 0.62 0.33 0.01 余量 
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表 2 示波冲击试验裂纹扩展不同阶段冲击吸收能量
Table 2 Absorbed impact energies associated with different stages of instrumented Charpy impact tests J
试样类型 裂纹萌生功Win 稳定扩展功Wstable 失稳扩展功Wunstable 总冲击吸收能量Wtotal 沉积态 4.81 ± 0.32 4.29 ± 0.45 4.74 ± 0.28 17.20 ± 1.01 固溶态 6.84 ± 0.46 15.59 ± 1.21 4.48 ± 0.36 34.86 ± 0.98
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