Development and Evaluation of a Novel Supply System to Reduce Cutting Fluid Consumption and Improve Machining Performance

Gariani, Salah (2019) Development and Evaluation of a Novel Supply System to Reduce Cutting Fluid Consumption and Improve Machining Performance. Doctoral thesis, Northumbria University.

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Abstract

Reducing cutting fluid consumption remains a goal of the machining industry. Despite their reported advantages such as heat dissipation, friction reduction, extended tool life, and improved surface quality, cutting fluids pose several health and environmental concerns throughout their lifecycle, in particular when conventional mineral oil-based, semi-synthetic or synthetic fluids are used. Manufacturers are encouraged to reduce the use of harmful conventional fluids. However, the usage of cutting fluid is still an unavoidable industrial practice, especially when machining titanium alloys, due to the generation of large quantities of heat. High cutting temperature is one of the main reasons for rapid tool wear and hence the poor machinability of titanium alloys. Vegetable oil (VO)-based fluids have been suggested as favourable alternatives to the conventional fluids due to their superior tribological properties and high biodegradability. Several cutting fluid supply systems have been developed to reduce cutting fluid use, such as minimum quality lubricant (MQL) and cryogenic cooling or to control the temperature in the cutting zone, for example flood, and high pressure cooling (HPC) systems, to improve productivity and increase the overall performance of machining processes. Even though process improvements are achieved by these systems, inaccuracies in estimating cutting fluid flow rates, high fluid consumption and low penetrability, as well as high set-up costs, are their technical and economic drawbacks. For these reasons, the need for an innovative supply system to deliver fluids in machining processes has become crucial.

In this PhD project, a novel controlled cutting fluid impinging supply system known as ‘CUT-LIST’ is developed to deliver an accurate quantity of cutting fluid into machining zones through precisely-oriented coherent nozzles. The design of CUT-LIST is supported by numerous fluid dynamic and metal cutting theories along with extensive experimentation. The performance of the new system is evaluated against a conventional flood system during the step shoulder milling of Ti-6Al-4V using a water-miscible vegetable oil-based cutting fluid. The effect of cutting conditions on the key measures of the process are investigated, including cutting force, workpiece temperature, tool flank wear, burr formation and average surface roughness (Ra). The effect of CUT-LIST on the micro-hardness and microstructure of the machined surface as well as chip formation are also evaluated. The study shows that the new system provides a dramatic decrease in cutting fluid consumption of up to 42% with noticeable reductions in cutting force, tool flank wear and burr height of 16.41%, 46.77% and 60% respectively. Relatively smaller surface roughness (Ra) values are also found with the use of the CUT-LIST supply system. In terms of the effect of the new system parameters on key process measures, feed rate has a major effect on cutting force, burr formation and surface roughness, with the highest percentage contribution ratios (PCRs) of 47.46%, 38.69% and 39.10% respectively. Meanwhile, the cutting speed has a major effect on workpiece temperature and flank wear, with the highest PCRs of 46.5% and 59.23% respectively. Nozzle position at a 15˚ angle in the feed direction and 45˚or 60˚ against feed direction helped in minimising workpiece temperature. An impinging distance of 55 or 75 mm is also necessary to control burr formation, workpiece temperature, and Ra. Metallurgical observation shows that both systems achieved acceptable micro-hardness values for aerospace components (386.3 to 419 HV100). However, a slight reduction in micro-hardness of ~5.5% was recorded with the use of CUT-LIST. The hardness is lower at distances < 50 μm below the machined surface as a result of thermal softening, while it becomes higher at distances <100 μm from the surface due to cyclic internal work hardening. The micro-hardness then gradually decreases until it reaches the base material’s nominal hardness.

Both systems also produce a thin, plastically deformed layer below the machined surface under all conditions investigated. Despite the noticeable reduction in cutting fluid consumption achieved by CUT-LIST, no significant disparity is found in the microstructural subsurface damage caused by the two systems. Microstructural alteration is strongly affected by cutting speed and fluid flow rate. At higher cutting speeds, the conventional system shows visible surface defects such as smearing, surface cavities and erosion in workpiece material. With both systems, desirable discontinued serrated chips are generated. However, the increase in fluid flow rate significantly influences chip morphology, while the average distance between chip segments is more pronounced and evident with the increase in cutting speed. Severe crack propagation (up to a depth of 200 μm) is observed in the chip end free surface, with the use of the conventional system.

In addition, CUT-LIST shows decreases of up to 12.5 % in saw-tooth height (hmax) and increased segment width up to 13.63 % at higher speeds, while the transition from aperiodic to periodic serrated chip formation is closely controlled by cutting speed and feed rate. Chip segmentation frequency and shear angle are also found to be sensitive to cutting speed, whilst CUT-LIST provides a larger shear angle compared to the conventional system.

Based on the results achieved by CUT-LIST, it is apparent that the new system possesses various advantages over the conventional system. Hence, CUT-LIST can be considered as a feasible, efficient, and ecologically beneficial solution, offering less fluid consumption in machining processes.

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