Influential Effects of Follow-up Feed Pulse Current on a Laser Clad Ni-based Alloy Coating
D-S. Wang, Y-Y. Zhu, G. Qu and X-H. Zhou
A new follow-up feed pulse current-assisted laser cladding process with a fibre laser was proposed. The conventional laser cladding and follow-up feed pulse current-assisted laser cladding were comparatively studied, and the influences of follow-up feed pulse current on the morphology, porosity, microstructure and cracks of the Ni60A Ni-based alloy coating prepared on the substrate surface of a 42CrMo structural alloy steel were analysed. Results showed that most current would be brought into the molten pool via the skin effect of pulse current when the follow-up feed electrode was used, thereby improving the current utilization efficiency. In addition, the approximate electrode distance could greatly increase the current density, enhance the improvement effect, and facilitate the preparation of a cladding coating with few defects and high quality. Compared with conventional laser cladding, the follow-up feed pulse current-assisted laser cladding could not only improve the cladding efficiency but also refine the grains at the cladding layer, reduce its porosity, and homogenize the microstructure of cladding layer; moreover, the dendritic crystal formed by the conventional laser cladding was transformed into isometric crystals. In the conventional laser cladding, the porosity was 0.62%, and the average grain size was approximately 6 μm. The two figures for the specimen obtained through the follow-up feed pulse current-assisted laser cladding were 0.39% and 4 μm. Meanwhile, the residual thermal stress and cracking sensitivity of the cladding layer could be reduced. Especially, online self-healing of cracks at the cladding layer could be realized by virtue of the flow-around effect and Joule heating effect formed by the high energy density pulse current, which was exclusive for the small electrode distance of follow-up feed electrodes at crack tip of the cladding layer and, thus, a crack-fee cladding layer was formed, and the easy cracking problem of laser cladding layer was solved.
Keywords: Fibre laser, Ni60A Ni-based alloy, 42CrMo structural alloy steel, laser cladding, pulse current, follow-up feed, cracks, online self-healing