扫描速度对球墨铸铁激光熔覆层组织的影响

Effect of Scanning Speed on Microstructure of Laser Cladding Layer on Nodular Cast Iron

目的揭示不同扫描速度下激光熔覆Fe基覆层显微结构的变化规律,以寻求高效的组织控制手段。方法使用半导体激光器,在相同送粉量和激光功率条件下,采用不同扫描速度,在球墨铸铁表面制备激光熔覆层。采用光学显微镜、扫描电子显微镜和能谱仪、X射线衍射仪表征覆层微观组织和相组成,采用洛氏硬度计对覆层硬度进行测定。结果扫描速度为7 mm/s时,稀释率为16%,覆层组织主要是马氏体和少量残余奥氏体,晶粒细小,无裂纹,洛氏硬度为55.5HRC;扫描速度为5 mm/s时,稀释率为30%,覆层中的晶粒形态变粗大,且得到更多的柱状晶和等轴晶,残余奥氏体含量大幅增多,马氏体含量明显下降,结晶裂纹倾向大,洛氏硬度为21.7HRC。结论扫描速度在球墨铸铁的覆层制备中发挥了重要作用,对覆层凝固过程、晶粒生长、相组成、裂纹敏感性、硬度有很大影响。通过适当调整扫描速度可以改变马氏体和残余奥氏体的量,获得无裂纹且硬度合适的覆层,实现覆层组织和力学性能的有效控制。

The work aims to reveal the evolutions in microstructure of laser cladding Fe-based layers at different scanning speed,and then obtain an efficient way of controlling structure.Provided with the same F/V(powder feeding rate/laser beam scanning speed)and laser power,semiconductor laser was used to prepare laser cladding layers on the surface of nodular cast iron at different scanning speed.Microstructures and phases of the cladding layers were characterized with optical microscope,scanning electron microscope,energy dispersive spectroscopy and X-ray difFactometer.Hardness of the cladding layers were measured by Rockwell hardness tester.At scanning speed of 7 mm/s and dilution rate of 16%,microstructure of the cladding layers was mainly composed of martensite and a small amount of retained austenite,the grains were small,no crack existed and Rockwell hardness was 55.5HRC.At the scanning speed of 5 mm/s and dilution rate of 30%,grain morphology of the cladding layers became more coarser,more columnar crystals and equiaxed crystals were obtained,and content of retained austenite increased and content of martensite decreased significantly,but the cladding layer presented bigger crack susceptibility and Rockwell hardness decreased to 21.7HRC.Scanning speed plays a great role in preparation of laser cladding layers on the surface of nodular cast iron,and has great effects on solidification,phase composition,grain growth,crack susceptibility and hardness.Scanning speed can be adjusted to change content of mastensite and retained austenite in the cladding layers,obtain crack-free cladding layers of reasonable hardness,and further effectively control microstructures and mechanical properties of the layers.