Although titanium alloys have very complex extraction, melting, and processing conditions, they have received significant amounts of attention for the manufacturing of aviation components, ships, and medical devices. However, their lower thermal conductivity, lower Young’s modulus, and the adhesion of titanium to tools at higher temperatures make titanium-based alloys difficult to cut. In heat-resistant alloy processing, traditional processing technologies have low efficiencies. The productivity and sustainability of conventional heat-resistant alloy processing technologies are not adequate. Environmentally friendly government regulations are also driving the manufacturing industry to adopt sustainable processing technologies, rather than inefficient traditional ones. The health and safety of workers is a key social factor that determines the sustainability of machining technology. Researchers have worked to develop alternative processing strategies to make these technologies safer, healthier, and more sustainable.
MQL systems are an effective alternative to traditional lubrication methods. These systems spray oil droplets and pressurize air through a nozzle onto the cutting area. The oil droplets provide sufficient lubrication at the tool–chip and tool–workpiece interface to reduce friction and heat generation in the cutting zone. In addition, they evaporate in a short time without leaving any residue on the workpiece or tool. Dhar et al. processed AISI-1040 steel using uncoated carbide inserts under MQL conditions. Two main performance characteristics—the chip formation and cutting temperature—were analyzed under flooding, drying, and MQL conditions. The results show that MQL is the best processing method at higher cutting speeds and lower feed rates. Khan et al. studied the effects of MQL, flooding, and dry cutting on the turning process of AISI 9310 alloy steel. Compared with flooding and dry cutting, MQL has a lower cutting temperature, lower tool wear, and favorable chip characteristics.
Although MQL seems to provide enough lubrication, the cooling effect of the processing area is insufficient under heavy processing conditions. The high heat generated during chip removal cannot be controlled, which significantly reduces the processing efficiency. Low-temperature cooling is a preferred cooling method for thermal control. In general, cryogenic cooling involves transporting a refrigerant gas in the liquid form to the tool–workpiece interface using a nozzle. LN2 is one of the most popular coolants in machining operations. It is lighter than air and can diffuse into the surroundings after use. This reduces the need for maintenance, post-processing cleaning, and processing requirements. Low-temperature cooling has been considered to be a green production technology and is one of the leading cooling methods for sustainable manufacturing. Islam et al. compared dry cooling, flood cooling, and low-temperature cooling in terms of the processing of EN24 steel. The results show that low-temperature cooling has good effects on the surface roughness, cutting force, and tool wear. Dhananchezian et al. compared low-temperature cooling with conventional cooling during the processing of Ti-6Al-4V alloys. The results show that low-temperature cooling reduces the cutting force by 35–42%. Further, the cutting temperature decreased by 61–66%, the surface roughness decreased by 36%, and the tool wear decreased by 27–39%. Previous studies have confirmed that, for titanium alloy processing, the MQL and low-temperature environments have exhibited better results than flood and dry cutting. However, the low-temperature environment only provides a cooling effect and has insufficient lubrication. In contrast, MQL mainly provides lubrication and is usually ineffective under heavy processing conditions, especially for titanium alloy processing. Therefore, a large number of researchers have explored mixed low-temperature and MQL conditions. Weinert et al. proved that cryogenic minimum quantity lubrication (CMQL) is a safe and effective cooling and lubrication method that can effectively replace conventional MQL technologies. Shokrani et al. applied CMQL to milling Ti-6Al-4V. They carried out milling experiments on a tool life model and compared the tool life under flood, MQL, low-temperature, and low-temperature micro-lubrication environments. The results show that, in a low-temperature micro-lubrication environment, the tool life is 30 times longer than that of flood cooling.
CMQL is a relatively new cooling lubrication method that has received significant research attention in recent years. It mainly provides lubrication and cooling for the processing area, improving its processing effect. This method employs two nozzles, one nozzle which sprays oil and the other sprays cryogenic liquid. However, during actual processing, the method of cooling by liquid nitrogen is the most effective. The minimum temperature of liquid nitrogen can reach −196°, which can be further improved by lubrication. Under heavy processing conditions, the continuous high temperature in the processing area leads to the instantaneous evaporation of vegetable oil, which will weaken the lubrication effect in the processing area. Similarly, the coolant may not fully penetrate the microstructure of the tool–workpiece. Nanofluids have recently been used in MQL to increase the cooling and lubrication characteristics. In nanofluid technology, when additives (called nanolubricants or nanoparticles) are added to the base fluid, the physical, friction, and thermal properties of the cutting fluid improve, depending on the properties of the solid particles. The most commonly used additives are molybdenum dioxide, boron nitride, alumina, silica, carbon nanotubes, titanium dioxide, copper oxide, graphite, etc[15–17]. In mechanical processing, although nanofluid-MQL has been studied for many years, the multi-walled carbon nanotube (MWCNTs) mixed nanofluid-MQL, which is a novel concept, has not been sufficiently researched.
In fact, few studies have focused on the surface roughness/surface morphology, tribological mechanism, tool wear, and wear mechanism of MWCNTs. In addition, the application of MWCNTs and vegetable oil combined with MQL is also rare. The main reasons for choosing MWCNTs as solid lubricants are their atomic structure and high specific surface area, along with their excellent thermal conductivity (of about 3000-3500 W· m−1 · K−1). Owing to these advantages, it is important to study the interactions between nano-cutting fluids and the tool and workpiece materials at low temperatures, and to establish a selection of sustainable processing titanium alloys (Ti-6Al-4V). Therefore, this study mainly analyzes the influence of the addition of MWCNTs in the low-temperature trace wetting (CMQL) system on the cutting performance indicators, including the tool wear and life, wear mechanism, the machining surface state (surface roughness and surface morphology), and the peak temperature of the cutting zone when turning titanium alloys (Ti-6Al-4V) with a titanium nitride coated carbide cutting blade. Therefore, five different experimental environments were designed to facilitate a comparison (Dry, MQL, LN2, LN2 + MQL, and LN2 + Nano-MQL (0.6%)). In these comparisons, the addition of 0.6% MWCNTs on the basis of low-temperature minimum lubrication is analyzed.