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Li, Peifeng (2023) Processing, microstructure, and tribological properties of WS2 reinforced aluminium metal matrix composite: laser powder bed fusion vs. spark plasma sintering. PhD thesis, University of Nottingham.

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Abstract

Aluminium alloys hold great promise for automotive applications due to their lightweight. However, the inferior wear resistance of aluminium alloys limited their potential for automobile parts where the tribological performances of the material are greatly concerned. In the past few decades, a variety of aluminium metal matrix composites (MMCs) with enhanced wear resistance has been developed, with introducing ceramic or solid lubricants to the matrix being the most common strategy. Self-lubricating Al-based composites reinforced with solid lubricant WS2 promise to meet the demand for greener tribological parts. However, the interfacial reactions between Al and WS2 and their effect on the tribological performances by conventional methods had not been studied previously, and the design advantages granted by additive manufacturing (AM) processes coupled with their capacity for in-situ production of composite materials are yet to be exploited in the realm of Al-transition metal dichalcogenides composites.

This work studies and compares the capability of fabricating self-lubricating materials by the AM method, laser powder bed fusion (L-PBF), and the advanced powder metallurgy method, spark plasma sintering (SPS). L-PBF was, for the first time, deployed for the in-situ fabrication of Al-WS2 composites, elucidating the process-structure-property relationships in comparison to SPS samples. The evolution of WS2 due to laser irradiation and spark plasma sintering was investigated through a holistic characterisation. Also, the effects of various processing parameters, including machine parameter sets and powder uniformity, on the as-built Al-WS2 parts fabricated by both methods, were investigated. The variations in density, part defects, microstructure evolution, and tribological behaviour realised by tweaking these parameters were explored.

For SPS parts, high density was easily achieved but the problem of reinforcement phase agglomeration may occur, whereas, for L-PBF parts, WS2 dispersed relatively homogeneously in the matrix but high porosity was one of the major challenges. These issues can be tackled by tweaking the parameters used during processing. Regarding the microstructure, the in-situ formation of new phases (W for L-PBF, Al5W and Al12W for SPS) was revealed by electron microscopes for the first time, which accounts for the enhancement in wear resistance of the composites. The chemical composition, morphology and distribution of WS2 phases in the composites fabricated by both techniques also experienced pronounced changes in relation to the parameters employed.

L-PBF Al-WS2 displayed lower friction and higher wear resistance than SPS parts at both room temperature and 200 °C. Furthermore, a novel methodology for studying the evolution of worn surfaces was developed and validated to analyse the evolution of the protective tribo-layer. It can be concluded that the tribo-layer formed at lower friction cycles for L-PBF Al-WS2 parts than SPS counterparts at the same test condition, meaning that AM will be advantageous for the performance aspect of self-lubricating materials. In general, the tribological properties of SPS and L-PBF fabricated composites depended on the fraction of Al-W intermetallic phase and parts’ porosity, respectively, while a more homogenous distribution of the reinforcement particles also played an essential in determining the tribo-layer formation and its stability for both.

This study helps to understand the response of self-lubricating materials to SPS and L-PBF process. The findings in this work provided a first step in fabricating transition metal dichalcogenides reinforced self-lubricating materials by AM, and therefore, pushing the applications of additive manufacturing in producing automotive components with critical wear properties. It also demonstrated how crucial it is to control the processing parameters for optimising the tribological performance of Al-WS2 composites.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Xu, Fang Hague, Richard Aboulkhair, Nesma Hou, Xianghui Clare, Adam
Keywords:	Self-lubricating materials; Additive manufacturing; Laser powder bed fusion; Spark plasma sintering; Aluminium metal matrix composites; Tribological performance
Subjects:	T Technology > TS Manufactures
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID:	76020
Depositing User:	Li, Peifeng
Date Deposited:	30 Jul 2024 10:38
Last Modified:	30 Jul 2024 10:38
URI:	https://eprints.nottingham.ac.uk/id/eprint/76020

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