自阻加热热处理对Ni60/WC涂层组织及性能的影响

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

摘要:

为改善激光熔覆涂层的微观组织及力学性能,以Ni60/WC涂层为研究对象,提出一种利用脉冲直流电对激光熔覆涂层进行自阻加热热处理的工艺方法。首先,在电流密度为3.33 A/mm²的条件下,对Ni60/WC涂层分别进行1 h和2 h的自阻加热热处理实验。其次,利用扫描电镜和X射线衍射对涂层的物相组成和微观组织的变化进行分析,并对热处理前后试样的力学性能进行测试。结果表明,在选择性加热效应的作用下,电流规避了涂层中的硬质相,产生局部高温导致WC发生二次分解；同时,γ-(Ni, Ti)固溶体中的Ti、Cr和C原子在电流的作用下发生扩散,且涂层相变朝着电导率增加的方向进行；此外,由于电流降低形核势垒并结合快速冷却作用,使形核速率显著提高,且晶粒尺寸减小约99%,最终获得细小的γ-(Ni, Ti)晶粒。对比结果显示,热处理后涂层的显微硬度、断裂韧度和摩擦磨损性能均得到了显著提升。直流电通过选择性加热效应和非热效应,规避了晶粒缺陷,增强了原子扩散能力,提高了形核速率,细化了涂层中的晶粒。

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.