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实验基材选用管模专用材料21CrMo10热作模具钢,属于低碳马氏体钢,尺寸为120 mm长×40 mm宽×15 mm厚的钢板。激光熔覆材料选择天津铸金股份有限公司生产的高纯度In718高温合金粉末,粒度为50 μm~150 μm,粉末颗粒为球状,流动性良好,化学成分如表 1所示。
表 1 In718粉末成分(质量分数)/%
Table 1. Compositions of In718 alloy powder(mass fraction)/%
Ni Cr Mo Nb Al Ti Fe 50.00~55.00 17.00~21.00 2.80~3.30 5.00~5.50 0.30~7.00 0.75~1.15 balance -
激光加工系统包括Laserline LDM 3000-100激光器、YASKAWA机械臂、水冷机、双筒送粉器、同轴送粉激光喷嘴等,选用氮气作为保护气和输送粉末的载气。实验前用磨光机对基材表面进行打磨,除去表面夹杂物和氧化层,然后用丙酮与无水乙醇进行清洗并烘干。把合金粉末放置于120 ℃的真空烘干炉2 h左右,除去水分。如表 2所示,制备涂层分两步走:(1)激光熔覆。激光功率为2000 W,扫描速率为5 mm/s,送粉速率为8 g/min,搭接率为40%,每层抬升量为0.8 mm,共熔覆4层;(2)激光重熔。除涂层A外,每制备一层In718熔覆层随即进行激光重熔,扫描速率为单因素变量,分别是5 mm/s、15 mm/s、25 mm/s。
表 2 激光熔覆与激光重熔工艺参数
Table 2. Process parameters of laser cladding and laser remelting
No. step 1: laser cladding step 2: laser remelting laser power/W scanning speed/(mm·s-1) powder rate/(g·min-1) overlap rate/% laser power/W scanning speed v/(mm·s-1) whether to remelt A 2000 5 8 40 — — no B 2000 5 8 40 2000 5 yes C 2000 5 8 40 2000 15 yes D 2000 5 8 40 2000 25 yes 用电火花线切割制备4组尺寸相同的金相试样(长10 mm×宽10 mm×高8 mm),然后对金相试样进行研磨(依次选用80#、200#、500#、1000#、1500#砂纸)、抛光(选用自动研磨抛光设备)、腐蚀(选用100 mL HCl+100 mL C2H5OH+5 g CuCl2腐蚀液)处理。用电火花线切割制备4个拉伸试样,切割后的拉伸试样用角磨机把熔覆层顶部打磨平整光滑。图 1a为激光熔覆实验现场;图 1b为离心球墨铸管模具重熔再制造过程; 图 1c为拉伸试样示意图与尺寸。图中,HAZ(heat affection zone)表示热影响区。
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采用光学显微镜(optical microscope,OM)和扫描电镜(scanning electron microscope,SEM)观察熔覆层内部的显微结构、枝晶间的生长状况;采用SEM与能量色散光谱仪(energy dispersive spectrometer,EDS)配合检测相的体积分数、相分布、元素的质量分数及成分偏析程度;采用MATLAB软件二值化处理统计出Laves相的体积分数;采用维氏显微硬度仪测量熔覆层横截面的显微硬度,所用载荷为500 g,加载时间为15 s,从熔覆层底部热影响区至熔覆层顶部依次选取9个点,每个点进行3次测试,取平均数;采用万能拉伸试验机检测试样的拉伸强度和延伸率。
激光重熔对In718熔覆层组织与性能的影响
Effect of laser remelting on microstructure and properties of In718 cladding layer
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摘要: 为了解决激光熔覆In718合金力学性能差的问题, 选择不同的扫描速率对In718合金熔覆层进行了激光重熔处理, 采用光学显微镜、扫描电镜和能谱仪观察显微组织结构和特征, 检测不同物相的成分并分析微观偏析对显微组织的影响, 通过力学检测设备对涂层的显微硬度与抗拉强度进行检测。结果表明, Laves相主要由Nb与Mo元素的偏析所导致, 与未重熔相比, 重熔后的涂层气孔明显减少, 且不同的重熔扫描速率对组织与性能的影响也不同, 4个涂层的Laves相体积分数由34.1%减少至24.6%、16.7%和19.6%, 平均硬度由250.3 HV提高至261.5 HV、276.9 HV和268.0 HV, 抗拉强度由678 MPa提高至728 MPa、879 MPa和808 MPa, 而重熔对涂层延伸率的影响不明显; 最优的重熔扫描速率为15 mm/s, Laves相的体积分数最低, 平均显微硬度和抗拉强度最高; 激光重熔能有效改善熔覆层的形貌、降低孔隙率、减少或抑制Laves相的析出, 并且减少Laves相有助于提高In718合金的力学性能。此研究为后续的离心球墨铸管模具的再制造奠定了理论基础。Abstract: In order to solve the problem of poor mechanical properties of laser cladding In718 alloy, different scanning speeds were selected to carry out laser remelting treatment of the cladding layer. Optical microscope, scanning electron microscope and energy dispersive spectrometer were used to observe the microstructure and characteristics and to detect the composition of different phases. The influence of microsegregation on the microstructure was then analyzed. The microhardness and tensile strength of the coating were tested by mechanical testing equipment. The results show that the Laves phase is mainly caused by the segregation of Nb and Mo elements. Compared with the non-remelted coating, the pores of the remelted coating are significantly reduced, and different remelting scanning speeds have different effects on the structure and properties. The Laves phase volume fraction of four coatings respectively decreased from 34.1% to 24.6%, 16.7%, and 19.6%, the average hardness respectively increased from 250.3 HV to 261.5 HV, 276.9 HV, and 268.0 HV. The tensile strength respectively increased from 678 MPa to 728 MPa, 879 MPa, and 808 MPa. However, the effect of remelted coating on elongation is not obvious. The optimum remelting scanning speed is 15 mm/s, which has the lowest Laves phase content and the highest average microhardness and tensile strength. Laser remelting can effectively improve the morphology of the cladding layer, reduce the porosity, reduce or inhibit the precipitation of Laves phase. Reducing the Laves phase to improve the mechanical properties of In718 alloy. This research establishes a theoretical foundation for the subsequent remanufacturing of centrifugal cast ductile iron pipes molds.
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Key words:
- laser technique /
- laser remelting /
- In718 alloy /
- scanning speed /
- Laves phase /
- mechanical property
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表 1 In718粉末成分(质量分数)/%
Table 1. Compositions of In718 alloy powder(mass fraction)/%
Ni Cr Mo Nb Al Ti Fe 50.00~55.00 17.00~21.00 2.80~3.30 5.00~5.50 0.30~7.00 0.75~1.15 balance 表 2 激光熔覆与激光重熔工艺参数
Table 2. Process parameters of laser cladding and laser remelting
No. step 1: laser cladding step 2: laser remelting laser power/W scanning speed/(mm·s-1) powder rate/(g·min-1) overlap rate/% laser power/W scanning speed v/(mm·s-1) whether to remelt A 2000 5 8 40 — — no B 2000 5 8 40 2000 5 yes C 2000 5 8 40 2000 15 yes D 2000 5 8 40 2000 25 yes -