Influence of microstructure on cavitation erosion resistance of laser-surface-melted Ni-WC composites

Abstract

Cavitation erosion (CE) is a type of wear that frequently occurs to the components operating in a fluid, such as propeller, rudder, impeller, pump, and pipeline. Failures caused by CE can result in significant economic losses in civil fields and can also pose a risk to national security in other sensitive fields, such as nuclear engineering and coastal defence.
Therefore, research into materials with effective cavitation erosion resistance (CER) has become imperative. Over the past two decades, WC-based cermet composites have been extensively utilised for resisting CE. However, further improvements in CER are still necessary. Many studies have focused on enhancing mechanical properties to achieve effective CER, but these studies have unintentionally neglected the effect of microstructure on CER.
In this study, cold-sprayed Ni-WC metal matrix composites (MMCs) were post-treated by laser surface melting (LSM), resulting in excellent CER compared to other common engineering materials such as WC-based cermet composites and 316L stainless steel. Scanning electron microscopy (SEM) images revealed that the LSM cold-sprayed Ni-WC MMCs had a hypoeutectic microstructure, where the eutectic Ni-WC network divided the primary Ni grain into many cells. Examination of post-mortem samples indicated that the eutectic Ni-WC network could restrict microcrack growth within the cells, providing a damage-control effect. Additionally, the experimental results suggested that mechanical properties may not be a reliable predictor of CER.
The excellent CER of the LSM Ni-WC MMCs warranted further exploration and optimisation. The microstructural evolution of the LSM sintered Ni-WC MMCs was investigated by taking a series of SEM images from the same positions after different CE test intervals. The results demonstrated that the eutectic Ni-WC network was effective in constraining CE. Numerical simulation results also indicated that the hierarchically layered WC lamellae in the eutectic Ni-WC region could mitigate cavitation impacts. Hence, the exceptional CER of the LSM Ni-WC MMCs was attributed to the altered microstructure.
To optimise the LSM Ni-WC MMCs and further enhance CER, CeO2 and Cr were introduced in this study. The LSM Ni-WC-CeO2 MMCs exhibited refined Ni grains and significantly higher microhardness than the LSM Ni-WC MMCs, and the distribution and morphology of the WC lamellae were also altered. However, the addition of CeO2 did not guarantee an increase in CER. SEM observations suggested that the microstructure played a dominant role in determining CER, and only MMCs with a continuous eutectic Ni-WC network and hierarchically interlocked WC lamellae exhibited the best CER. Numerical simulation results also indicated that the morphology and distribution of the WC lamellae could influence the stress and energy in the material when subjected to cavitation impacts.
For the LSM NiCr-WC MMCs, they exhibited either a hypoeutectic structure similar to the LSM Ni-WC MMCs or a fully eutectic structure depending on the Cr content. Meanwhile, the addition of Cr significantly enhanced the CER. The LSM NiCr-WC MMCs showed 10-h CE that was only 10-20% that of the LSM Ni-WC MMCs. SEM observations indicated that a lamellar eutectic with finely and densely distributed carbide lamellae may be the most optimal microstructure for resisting CE among the ones presented in this study.

Date of Award	Jul 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Hao Chen (Supervisor), Hua Li (Supervisor) & Xianghui Hou (Supervisor)