Abstract
Hardness of dissimilar laser clad samples of NiCrBSi alloy on a Cu substrate was investigated, with the aim of optimizing the manufacturing process for high-durability continuous casting molds for steelmaking. The performance of the clads is compared with that of samples prepared by thermal spray coating, and an optimal process is proposed. Dissimilar laser cladding between NiCrBSi alloy and Cu was achieved with a hardness of ~450 HV using a high-power diode laser, and no cracks and pores were observed. Post-clad heat treatment performed below the melting point of the Cu substrate (1357 K) using a furnace (1223 K for 500 min) resulted in a decrease in the hardness to 142 HV, which was attributed to the dilution of the alloy with the soft Cu substrate and changes to the microstructure; the solidification microstructure was almost homogenized during the heat treatment, and secondary boride phases were formed and almost dissolved in the matrix phase. Laser surface heat treatment of the clad metal at 1323 K resulted in a decrease in the hardness (to 359 HV near the surface), with a heat treatment depth of ~1.3 mm. In contrast, the hardness of the as-sprayed coatings was 730–750 HV, which drastically increased to ~1200 HV after laser fusing because of the formation of finely distributed secondary phases. Therefore, to achieve high-durability continuous casting mold components, minimization of Cu dilution is preferentially recommended for the laser cladding of NiCrBSi alloy on Cu substrate. Furthermore, when it is difficult to minimize the Cu dilution during the laser cladding, thermal spraying in conjunction with laser fusing treatment appears to be sufficiently applicable for high-durability continuous casting molds.
Highlights
To extend the service life of high-performance materials exposed to extremely demanding environments, such as the high temperatures and pressures present in steelmaking, aerospace, and power plant applications, optimization of the local microstructure, and of the mechanical and chemical properties at the surface, is crucial
In other words, minimized dilution of Cu is recommended for the laser cladding of NiCrBSi alloy on Cu substrate
Successful laser cladding between NiCrBSi alloy and Cu was achieved using a highpower diode laser, and a sound clad metal without cracks or pores was obtained, with a hardness of ~450 HV
Summary
To extend the service life of high-performance materials exposed to extremely demanding environments, such as the high temperatures and pressures present in steelmaking, aerospace, and power plant applications, optimization of the local microstructure, and of the mechanical and chemical properties at the surface, is crucial. Zhao et al explored the laser cladding of high-entropy alloys on carbon steel, and reported enhanced wear resistance in NaCl solution as well as in air [17] Most of these studies commonly emphasized that the advantage of surface modification using lasers is that it allows precise control of the microstructure and mechanical properties for only the required area, with effective productivity. Previous studies by Chun et al intensively investigated the surface coating of heat-resistant Ni alloys (NiCrBSi, NiWCrBSi) and Co alloys (Stellite) on a Cu substrate by thermal spraying to manufacture a high-durability continuous casting mold [2,4,5,6] These studies evaluated the applicability of subsequent laser-assisted fusing treatment of the coating to further improve the surface performance and interface bondability.
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