Effect of Laser Process Parameters on the Microstructure and Properties of TiC Reinforced Co-Based Alloy Laser Cladding Layer
TONG Wenhui(), ZHANG Xinyuan, LI Weixuan, LIU Yukun, LI Yan, GUO Xuming
School of Materials Science and Engineering, Shenyang Aerospace University, Shenyang 110136, China
Cite this article:
TONG Wenhui, ZHANG Xinyuan, LI Weixuan, LIU Yukun, LI Yan, GUO Xuming. Effect of Laser Process Parameters on the Microstructure and Properties of TiC Reinforced Co-Based Alloy Laser Cladding Layer. Acta Metall Sin, 2020, 56(9): 1265-1274.
The severe wear and uneven wear will happen on the surface of ductile cast iron such as the traction wheel of elevator, in the long-term working under the serious wear and impact conditions. Laser cladding can be applied to reinforce its surface, which can improve the properties and life of the cladded components, save materials and manufacturing cost and raise the economic efficiency, but as to the surface of different materials, especially the ductile cast iron, the alloy powder and process parameters for laser cladding need to be chosen carefully by the experimental studies because of the rapid melting and solidification, the difference of thermo-physical properties between the laser cladding layer and the matrix, the laser absorption of the cladding layer and matrix and so on. In this work, laser cladding is employed to fabricate Co-based composite coatings reinforced by TiC particles by a 6 kW CO2 laser. The effects of the technical parameters of laser on the composition, phase and microhardness of the laser cladding layer are investigated by OM, SEM, EDS, XRD and microhardness tester, with the emphases of analyzing the changes of the distribution, morphology and size of TiC in the laser cladding layer. It is shown by the results that the cladding layer is mainly composed of γ-Co, TiC/(Ti, W)C1-x, Cr-Ni-Fe-C and a small amount of Cr7C3 phase, and its microstructure changed from the fine dendrite crystal near the cast iron matrix to the equiaxed dendrite in the middle then to the fine dendrite crystal near the surface of laser cladding layer with the dispersed distribution of TiC at the root or tip of secondary dendrite arm, even at the branch of primary dendrite arm. The number of dendrite and the dendrite arm spacing both increase in the microstructure of the laser cladding layer and the morphology of TiC is transformed from the smooth circular shape to the irregular polygon shape, and its content obviously increases with the particle size of TiC decreasing and its distribution more uniform, when the laser power is decreased from 3.6 kW to 3.2 kW or the scanning rate increased from 350 mm/min to 410 mm/min. The growth of the primary dendrite or secondary dendrite can be inhibited by the precipitation of TiC after its dissolution in the melt pool of laser cladding. In this experiment, the hardness at the surface of laser cladding layer gradually increases with the decrease of laser power or the increase of scanning rate, in which the maximal microhardness is 1246.6 HV0.2, up to increasing by 5 times of the matrix.
Table 1 Chemical compositions of nodular cast iron and Co-based alloy powder
Fig.1 SEM images of TiC particles (a) and TiC-Co-based alloy powders (b)
Scheme
P / kW
V / (mm·min-1)
1
3.2
390
2
3.3
390
3
3.4
390
4
3.4
410
5
3.5
390
6
3.6
390
7
3.6
410
8
3.2
350
9
3.2
370
10
3.2
410
Table 2 Laser cladding process parameters (Overlap ratio is 50%, spot diameter is 3 mm, thickness of cladding is 1.2 mm)
Fig.2 Microstructures of the TiC-Co-based alloy laser cladding layer on the surface of ductile cast iron (P=3.2 kW, V=410 mm/min)
Fig.3 Microstructures of cladding layer of different laser powers at V=390 mm/min
Fig.4 Effect of laser power on the dendrite arm spacing of laser cladding layer (V=390 mm/min, SDAS—secondary dendrite arm space, PDAS—primary dendrite arm space)
Fig.5 Microstructures of cladding layer of different scanning rates at P=3.2 kW(a) V=350 mm/min (b) V=370 mm/min (c) V=410 mm/min
Fig.6 Effect of the scanning rate of laser on the dendrite arm spacing of laser cladding layer (P=3.2 kW)
Fig.7 XRD spectrum of the laser cladding layer of TiC-Co-based alloy
Fig.8 SEM images (a, c, e) and corresponding EDS results (b, d, f) of the cross-sections of cladding layers (V=410 mm/min)
Fig.9 Schematic of solidified dendrite growth of laser cladding layer of TiC-Co-based alloy
Fig.10 Change of microhardness on the cross-section of laser cladding layer with the distance from the matrix surface of ductile cast iron when P=3.4 kW and V=390 mm/min
Fig.11 Effects of laser parameters on the microhardness of laser cladding layer
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