Effect of resistance heating heat treatment on microstructure and properties of Ni60/WC coatings

Abstract:

To enhance the microstructure and mechanical properties of laser cladded coatings, this study focuses on Ni60/WC coatings and proposes a resistance heating heat treatment (RHHT) process using pulsed direct current. First, RHHT experiments were carried out on Ni60/WC coatings for 1 h and 2 h, respectively, under the current density of 3.33 A/mm² .Subsequently, SEM and XRD were used to analyze the phase composition and microstructure transformation of coatings, and mechanical properties of the specimens before and after RHHT were tested. The results show that due to the selective heating effect, the electric current bypasses the hard phases within the coating, generating localized high temperatures which cause the secondary decomposition of WC. The Ti, Cr, and C atoms dissolved in γ-(Ni, Ti) diffuse under the influence of electric current, and the phase transformation in the coating proceeds in the direction of increasing electrical conductivity. Moreover,due to the electric current reducing the nucleation energy barrier in combination with rapid cooling, the nucleation rate was significantly increased and the grain sizes were reduced by approximately 99% which resulted in fine γ-(Ni, Ti) grains after the RHHT. By comparing the mechanical properties of the coatings before and after RHHT, it was found that the microhardness, fracture toughness and wear properties were effectively improved. The direct current, through its selective heating effect and athermal effects, circumvented the grain defects, and enhanced atomic diffusion ability, increased the nucleation rate, and refined the grains in the coating.