Manufactural Investigations on Dissimilar Laser Cladding and Post-Clad Heat Treatment Processes of Heat-Resistant Ni Alloy on Cu Substrate

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.