As discussed earlier in the chapter, there has been little work available on the machinability of GLARE and fibre metal laminates in general. Moreover, little work has been published on the application of cutting fluids in composite machining and no reported studies on the application of cutting fluids when machining FMLs. Machining industry continuously aims to improve the machining process by reducing the production costs and by creating a safer and healthier working environment. Tackling machining costs besides health and safety regulations can be achieved by reducing the costs incurred from cutting fluids and their disposal process. According to a survey conducted by the European Automobile Industry, the cost incurred on lubricants comprises nearly 20 % of the total manufacturing cost [204, 205]. While the cost of the cutting tool is only 7.5 % of the total cost [206, 207]. The recent advancements in coolant technologies and machining operations are driven by the high costs of coolants and machining, in general, have led to alternative routes for sustainable machining. For example, using efficient cooling techniques which require small amounts of cutting fluids which do not require further treatment or dispensing at the end of their life cycle such as minimum quantity lubrication (MQL) and cryogenic cooling using liquid nitrogen (LN2) and carbon dioxide (CO2). The current chapter provides a detailed review of the cooling methods used in machining industry with a focus on minimum quantity lubrication and cryogenic cooling of metals and composites and the recent advancements in these technologies.
2.10.1 Cutting Fluids
70 % of the functional percentage of the cutting fluids is used for chip removal while 20 % and 10 % are used in cooling and lubricating respectively [208]. Cutting fluids in machining can be classified based on their composition to Oil-based cutting fluids commonly used as a lubricant between the chip, tool and the workpiece. Or water-based fluids which are usually used for cooling and heat extraction or cryogenic gases in their liquidus phase, which is mainly used for heat extraction and reduction of dimensional changes of the machined workpiece [209]. Since most of the work done in the machining process are converted to heat, reducing the heat generated due to the friction between the tool and workpiece can increase the tool life, produce better-machined surface finish and clear away the formed chips. Moreover, allowing for higher cutting speeds to be used. Flood coolant is the most commonly used cooling method in the industry [210].
al. [211] as shown in Figure 2.18. Generally, the characteristics of the cutting fluids mainly depend on the type of the machining process and the machined material. There are two common methods for delivering the cutting fluid in a machining process: The first method delivers the coolant externally through a nozzle targeted at the workpiece/cutting tool at the cutting zone. The second method delivers the coolant internally and is commonly used in milling and drilling operations. The method is designed to deliver the coolant more efficiently by creating holes in the cutting tool such as in drilling operations where the coolant is delivered close to the centerline of the drill bit head.
Figure 2.18: Classification of cutting fluids by Yildiz et al. [211]
The second method delivers the coolant through small holes made in the cutting tool. Cakir et al. [212] previously summarised the factors for selecting suitable cutting fluids such as the nature of the machining process and the type of workpiece cutting tool materials. Water or water soluble (mixed with oil when machining metals) coolants are rarely used when machining composites [67, 185, 213]. Unless the water can easily evaporate after the ma- chining process causing no damages to the fibres, there is always a risk of moisture absorp- tion, a decrease in strength properties and degradation the fibre-matrix interfacial bonding, which could compromise the long-term durability of the composite structure. Therefore, many researchers carried out their composite machining experiments without the use of any coolants [68, 80, 214–220]. the lubricants types can be classified based on the amount of lubricant used in the machining process as shown in Table 2.16.
Table 2.16: Classification of lubricant types [208].
2.10.2 Dry Machining
Dry machining, from its name, implies that no coolant, cutting fluid or lubricant is used with the machining process. Thus, considerably saving large costs and keeping the process clean and environmentally friendly. Advantages of dry machining are both environmental and economical: it has no contamination risk or pollution hazards whatsoever on the en- vironment or the operator [125, 221]. In addition, large cost savings from those usually incurred when using cutting fluids and time-money required to dispose of them. Moreover, in some cases the wastes and pieces produced from the machining process can be recycled and used again for other operations in contrast to when coolants are used as the waste ma- terial needs to undergo a special cleaning process to eliminate the coolants materials from chemical additives to small particles in order for them to be used again.
Disadvantages of dry machining are the high temperatures associated with the cutting pro- cess in the tool-workpiece cutting zone, excessive tool wear and the difficulty of chip removal especially in drilling operations, and the requirement of special cutting tools which can resist high temperature [125, 221]. Additionally, built up edge (BUE) and adhesion can form on the cutting tool and workpiece especially at high cutting speeds. Dry machining is used when the use of coolants is not suitable or recommended. Dry machining process can be en- hanced by improving the cutting tool material, its coating and the method for chip removal. Sreejith et al. [155] recommended using coated cutting tools and specifically diamond coated tools when dry machining of nonferrous metals such as aluminium as they have no affinity for aluminium and their high thermal coefficient and heat diffusion. Additionally, improving the dry machining performance can be achieved by using high-pressure compressed air to help evacuate the clogged chips instead of using coolants.
2.10.3 Minimum Quantity Lubrication
Minimum quantity lubrication, near dry machining or simply MQL is one of the latest technologies for delivering precise quantities of cutting fluid to the targeted cutting region where cooling is needed. The idea is to use the least amount of cutting fluid or lubricant mixed with air, typically a flow rate of 50 to 500 mL/hour is used [222–224]. The amount of lubricant used can make a substantial difference in the cutting process by improving surface finish and tool life considerably. Advantages of MQL is that it produces drier workpiece and can provide lubrication when high-pressure of flood coolant cannot be used. It can also reduce the costs associated with the usage and waste disposal of similar cutting fluids used in high-pressure and flood coolants [208]. Moreover, it can reduce the thermal shock of the tool and the workpiece. Additionally, MQL can reduce the mist and spray generated and hence it can be used in non-enclosed machines. Disadvantages of MQL is that it cannot flush away chips and swarf from the cutting zone and limited ability to cool the machined surface [225]. The additives used in MQL can sometimes cause corrosion and slip accidents for operators if left on machined parts without removing them [225]. MQL can only reduce the heat generated from the cut chips but not the heat generated by the tool and workpiece, therefore, it is not ideal for use in a high production environment or applications which require high cutting speeds. MQL have been widely used in all machining applications; it can be supplied externally through one or several nozzles as shown in Figure 2.19.
The number of nozzles and their positions and orientation, as well as the spray pattern all, play a significant role in the machining performance [226]. The internal MQL cooling is supplied through holes created with the cutting tool such as drilling or milling processes. Indeed, the efficiency of external MQL is limited by its penetration to reach the cutting zone. The external supply of the MQL coolant is only practical up to length/diameter ratios of l/D < 3, for larger ratios the cooling influence becomes less effective and might require re-wetting several times which prolongs the time of the machining process [226]. MQL is compressed with air and usually sprayed from a close range from the cutting zone area. Many parameters influence the MQL efficiency such as the mist pressure and lubricant flow rate, the droplet size and nozzle distance from the cutting zone, method of supplying the MQL (internally or externally). The common fluids used are water, oil or a mixture of water and oil mixed with compressed air. The application of MQL has been used with ferrous and non-ferrous alloys such as steel, titanium and aluminium. The following section summarises some work done on the machining of ferrous alloys using MQL while the later section focuses on the near-dry machining of aluminium alloys with a focus on drilling process.
The MQL drilling of metals has been the focus study of many researchers. Most of the drilling researches were carried out using external MQL systems while few used internal ones. Milling was also investigated. The flow rate usually ranged between 5 and 250 ml/hr and air pressure between 3 and 23 bars. Generally, most of the work carried out on MQL focused on turning and grinding operations of various types of steel [223, 224, 227–233] which could be due to the better coolant penetration in those machining operations than in milling and drilling where the coolant impact could be limited when the cutting tool is engaged with the workpiece material [234, 235]. In most studies, the application of MQL in metal machining helped improve surface finish, reduce tool wear, cutting temperatures and cutting forces compared to dry and conventional cooling [223, 224, 227, 231, 233–238]. The previous studies also showed MQL improved the dimensional accuracy of the cutting process besides reductions in BUE and delay the formation of welding of chips especially at higher cutting speeds. Moreover friction and thermal distortions in the workpiece were reduced which lead to improved productivity and by allowing for higher cutting velocities and feed rates.
As discussed previously, the MQL can be supplied externally by means of nozzles attached near the cutting zone or internally by means of the tool spindle and internal cooling ducts inside the cutting tool [226], the external system is used in all machining applications while the internal MQL system was mainly developed for milling and drilling processes for better coolant impact and penetration. A drawback of external systems is that variation of the cutting tool and workpiece dimensions which require a continuous change of nozzle position
and orientation for better performance [226]. However, other studies reported that the application of MQL can be disadvantageous over flood cooling at high cutting speeds and feed rates as hole size was outside tolerance range due to the limited capacity of the MQL to remove heat from the cutting zone and the inability of MQL to minimise the adhesion of aluminium on the cutting tools [239–244]. Other studies reported that the combination of proper coating and MQL can give tool life performance which is comparable to wet machining at reduced costs [245–247].