Oxy-nitriding is a widely used industrial process aiming to improve the tribological properties and performance of components. Previous studies have shown the effectiveness of the treatment with friction and wear performance, but very few have focussed on optimising this behaviour. The lubrication properties of several EP and AW additives were examined to investigate their effectiveness in improving the tribological properties of the layers formed after treatment. Previous studies showed the presence of an oxide layer on the sample could improve the effectiveness of the sulphurised olefin (SO) and tricresyl phosphate (TCP) additives. The friction and wear behaviour of oxy-nitrided samples were analysed using a tribometer and surface profiler. Scanning electron microscope, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were employed to identify the morphologies and chemical compositions of the treated surface before and after testing. No real effect on friction was observed when using the SO or TCP additives, mostly due to lack of interaction with the less reactive iron nitride layer and their roles as anti-wear additives. However, when the zinc dialkyldithiophosphatecontaining lubricant was used, a higher friction coefficient was observed. Greater improvements in anti-wear properties with the presence of additives in comparison with only using base oil were reported, with the TCP additive producing the lowest wear rates. The study effectively demonstrated that the additive package type used could impact the tribological and tribochemical properties of oxy-nitrided surfaces.
Water based metalworking fluids (MWFs) commonly used for cooling and lubrication during machining are utilised in combination with cutting tools, work materials, fixtures and machine tools. However, they are an often overlooked component of the overall machining process, despite the fact that in some reported cases MWF costs were twice that of tooling costs. During its life cycle in a machine tool, the MWF is exposed to changes due to a range of factors which impact its quality and longevity. The key process variables (KPVs) reviewed in this study are MWF concentration, hydraulic (tramp) oil, solid particulates, water quality, MWF pH and microbial contamination. The aim of the present work is to highlight these KPVs which impact machining quality and health and safety, and to present industrially applicable measurement, monitoring and control (MMC) methods and techniques. This review is supported by a machining case study which demonstrates the impact of a single KPV—hydraulic (tramp) oil on MWF quality and machining output, and the need for applying MMC methods. Continuous hydraulic (tramp) oil contamination into the cutting fluid can cause tool life and wear to vary by 70%. A novel quantification methodology with gas chromatography was developed in this study to quantitatively measure hydraulic (tramp) oil contamination present within MWF and verified through experiments. The study overall highlights the need to apply a strict maintenance programme to increase the MWF lifetime and maintain performance for improved production, experimental process control and operator health and safety.
Metalworking fluids (MWFs) can greatly improve the machinability of materials and increase cutting tool life. There are a range of MWF products available on the market, however there are very few reliable low cost machining based fluid screening tests which can help select the most suitable candidate. This study developed a novel and rigorous single point milling (SPM) procedure carried out under controlled conditions, which would provide fluid performance differentiation for a range of typical aerospace alloys. The use of a single insert with a controlled geometry reduced machining variance and ensured performance repeatability. Tool life curves were used to determine optimum machining surface speeds for Inconel 718 (In718) of 80 m/min and Ti-6Al-4V (Ti64) of 160 m/min. Carrying out trials using five different cutting fluid products within a controlled tool life window clearly demonstrated that the SPM machining test was able to differentiate performance on both In718 and Ti64 material. Overall a 65% and 53% performance difference in tool life behaviour was observed between the best and worst performing fluids for In718 and Ti64, respectively.
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